Hard Probes 2018: International Conference on Hard & Electromagnetic Probes of High-Energy Nuclear Collisions

30 Sept 2018, 08:45 → 5 Oct 2018, 20:00 Europe/Zurich

Aix-Les-Bains, Savoie, France

David d'Enterria (chair) (CERN), Andreas Morsch (co-chair) (CERN), Philippe Crochet (co-chair) (Univ. Blaise Pascal Clermont-Fe. II (FR))

Hard Probes 2018: International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions

Online conference Proceedings: Proceedings of Science HP18

 

 

 

 

 

 

 

 

 

 

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Hard Probes 2018 POSTER

Sunday, 30 September

09:00 → 17:40 Student Lectures

09:00 Low-x QCD

Speaker: Jamal Jalilian-Marian (Baruch College)

hp2018-school.odp

hp2018-school.pdf

09:50 Nuclear PDFs

Speaker: Aleksander Kusina (IFJ PAN)

kusina_nPDFs.pdf

10:40 Break

11:00 Hard probes in small systems

Speaker: Dennis Perepelitsa (University of Colorado Boulder)

dvp-HP18-StudentLecture-10-30-18.pdf

11:50 Electromagnetic and electroweak probes

Speaker: Alexander Milov (Weizmann Institute of Science (IL))

sasha_ew_2018_09_30.pptx

12:40 Lunch

14:00 Heavy flavor + quarkonia theory

Speaker: Pol Gossiaux (Subatech)

Gossiaux - 1609.pdf

Gossiaux - 1609.pptx

14:50 Heavy flavor + quarkonia experiment

Speaker: Barbara Antonina Trzeciak (Utrecht University)

HFQuarkonia_StudentLecture_TrzeciakHP2018_v1.pdf

15:40 Break

16:00 Jets theory

Speaker: Carlos Albert Salgado Lopez (Universidade de Santiago de Compostela (ES))

HP2018-CAS-2.pdf

16:50 Jets experiment

Speaker: Sevil Salur (Rutgers State Univ. of New Jersey (US))

HP_Salur_studentday.pdf

Monday, 1 October

08:45 → 10:20 PLENARY

Convener: Andreas Morsch (CERN)

08:45 Conference Welcome (CERN Research Director)

Speaker: Eckhard Elsen (CERN)

Hard Probes 2018.pdf

09:05 ALICE

Speaker: Torsten Dahms (Technische Universitaet Muenchen (DE))

tdahms_20181001.pdf

09:30 ATLAS

Speaker: Alexander Milov (Weizmann Institute of Science (IL))

sasha_atlasHP_2018_10_01.pdf

09:55 CMS

Speaker: Yen-Jie Lee (Massachusetts Inst. of Technology (US))

20181001-HP2018_yenjie-v24.pdf

10:20 → 10:40 Coffee break

10:40 → 12:05 PLENARY

Convener: Giulia Manca (Universita` degli studi di Cagliari and INFN, Cagliari, IT)

10:40 LHCb

Speaker: Emilie Amandine Maurice (Centre National de la Recherche Scientifique (FR))

LHCb_maurice.pdf

11:00 PHENIX

Speaker: Takao Sakaguchi (BNL)

PHENIX_Highlights_final.pdf

11:20 STAR

Speaker: Sooraj Krishnan Radhakrishnan (State University of New York (US))

HarProbes2018_ver4f.pdf

11:40 Photon and weak probes of QCD matter (theory)

Speaker: Jacopo Ghiglieri (CERN)

ghiglieri_hp2018.pdf

12:05 → 14:00 Lunch break

14:00 → 15:15 PLENARY

Convener: Jean-Paul Blaizot (CEA)

14:00 Parton Saturation

Speaker: Prof. Alfred Müller (Columbia University)

Hard-Probes talk 2018_A Mueller.pdf

14:25 Nuclear Parton Distribution Functions

Speaker: Hannu Paukkunen (University of Jyväskylä)

nPDF_Paukkunen.pdf

14:50 Early time dynamics and hard probes in heavy-ion collisions (theory)

Speaker: Soeren Schlichting (University of Washington)

SSHardProbes18-plenary.pdf

15:15 → 15:40 Coffee break

15:40 → 18:10 PLENARY

Convener: Ramona Vogt (LLNL)

15:40 Jet substructure in high-energy hadron collisions

Speaker: Felix Ringer (Lawrence Berkeley National Laboratory)

HardProbes18_FRinger.pdf

16:05 Jets in QCD matter: Monte Carlo approaches

Speaker: Liliana Apolinario (LIP (PT))

JetsMC_vFinal.pdf

16:30 Jet-medium interactions in QCD matter (theory)

Speaker: Dr Tan Luo (CCNU)

HP2018-TanLuo.pdf

16:55 Lattice and EFT approaches for hard probes in QCD matter

Speaker: Miguel Angel Escobedo Espinosa (University of Jyväskylä)

hp_escobedo.pdf

17:20 Open heavy-flavour probes of QCD matter (theory)

Speaker: Shanshan Cao (Wayne State University)

Shanshan - HP 2018.pptx

17:45 Quarkonia probes of QCD matter (theory)

Speaker: Marzia Nardi (Unknown)

Nardi.pdf

18:10 → 19:40 Welcome Drink

Tuesday, 2 October

09:00 → 10:20 Parallel 1: Initial State: Low-x QCD

Convener: Jamal Jalilian-Marian (Baruch College)

09:00 DIS structure functions at low x at NLO in the dipole factorization : including massive quarks

Deep Inelastic Scattering (DIS) is the cleanest tool available to probe the content of a fast proton or nucleus. In the regime of low Bjorken x, one enters in the nonlinear regime of gluon saturation, where the gluons are better described as a strong coherent semi-classical field (a.k.a. Color Glass Condensate) than as a collection of quasi on-shell partons. Hence, that regime lies outside the validity range of the collinear factorization, and is better described within the dipole factorization of DIS observables which allows to resum coherent multiple scattering on the target, and also to resum the high-energy leading logarithms (LL). One of the motivations to study in detail the regime of gluon saturation in proton and nuclei is that it drives the physics of the earliest stages of heavy collisions, leading to the formation of the Quark-Gluon Plasma.

So far, phenomenological studies have been performed successfully at LO+LL accuracy in the dipole factorization using HERA data for proton DIS. However, in order to reach precision, NLO corrections should be included as well as high-energy NLL resummations. This is important not only to extract as much knowledge as possible out of the HERA data, but also in prevision of future electron-proton and/or electron-nucleus colliders.

In this talk, we will present the first complete calculation of the (fixed order) NLO corrections to DIS structure functions on a dense target in the dipole factorization picture in the massless quark case, and present new results on the massive quarks contribution (which is known to be sizable in DIS) at NLO. We will also discuss issues related to the implementation of that result in practical phenomenological studies, such as new fits of the dipole-target amplitude.

Speaker: Guillaume Beuf

GBeuf_HP_2018.pdf

09:20 Deep inelastic scattering in the dipole picture at next-to-leading order

We study quantitatively [1] the importance of the recently derived NLO corrections [2,3,4] to the DIS structure functions at small x in the dipole formalism. We show that these corrections can be significant and depend on the factorization scheme used to resum large logarithms of energy into renormalization group evolution with the BK equation. This feature is similar to what has recently been observed for single inclusive forward hadron production [5,6]. Using a factorization scheme consistent with the one recently proposed for the single inclusive cross section, we show that it is possible to obtain meaningful results for the DIS cross sections.

[1] B. Ducloué, H. Hänninen, T. Lappi, Y. Zhu, Phys.Rev. D96 (2017) no.9, 094017
[2] G. Beuf, Phys.Rev. D94 (2016) no.5, 054016
[3] G. Beuf, Phys.Rev. D96 (2017) no.7, 074033
[4] H. Hänninen, T. Lappi, R. Paatelainen, arXiv:1711.08207 [hep-ph]
[5] E. Iancu, A.H. Mueller, D.N. Triantafyllopoulos, JHEP 1612 (2016) 041
[6] B. Ducloué, T. Lappi, Y. Zhu, Phys.Rev. D95 (2017) no.11, 114007

Speaker: Henri Hänninen (University of Jyväskylä)

hanninen_nlodis_HP2018.pdf

09:40 Impact parameter dependent JIMWLK evolution meets HERA data

The small-x evolution of protons is determined from numerical solutions of the JIMWLK equations, starting from an initial condition at moderate $x$ for a finite size proton. The resulting dipole amplitude is used to calculate the total reduced cross section $\sigma_r$ and charm contribution to it, as well as diffractive vector meson production.

We compare results to experimental data from HERA and discuss fundamental problems arising from the regime sensitive to non-perturbative physics. We emphasize that information on the gluonic content of the proton, gluon spatial distributions and correlations over wide ranges in $x$, which can in principle be constrained by our study, are essential ingredients for describing the initial state in proton-proton and proton-ion collisions. We demonstrate how the proton evolves as a function of $1/x$, and study the energy dependence of the event-by-event fluctuations of the proton structure.

We further demonstrate how electron-ion collisions will be essential to access the gluon saturation in a kinematical region where non-perturbative effects are suppressed.

Reference:
H. Mäntysaari, B. Schenke, to be submitted in June 2018

Speaker: Dr Heikki Mäntysaari (University of Jyväskylä)

hp18_mantysaari.pdf

10:00 Resolving the Gluon Saturation Issue - Small-x Physics with the LHeC

The Large Hadron-electron Collider LHeC is a proposed upgrade of the LHC. An energy recovery linac would provide 60 GeV electrons to collide with the proton and nuclear beams from the HL-LHC and, eventually, with those from the HE-LHC and the FCC-hh. Working concurrently with the pp, pPb or PbPb LHC modes, it will provide ep (ePb) collisions in the TeV regime with luminosities $\sim 10^{34} (10^{33})$ cm$^{-2}$s$^{-1}$ to achieve integrated luminosities $\sim 1$ ab$^{-1}$ (10 fb$^{-1}$) in ten years. With a huge extension of the kinematical coverage with respect to HERA, down to $x \sim 10^{-6}$ in the perturbative region for the LHeC, it offers the possibility to establish the existence of the novel non-linear regime of QCD expected at small x. In this talk we discuss the opportunities that non-diffractive measurements at the LHeC offer for determining: (i) the small-x partonic structure of protons; (ii) the eventual breaking of the standard picture based on fixed-order perturbation theory and; (iii) the mechanism, linear or non-linear, for such breaking. For the latter, the use of both proton and nuclear beams should be crucial. This information would be very important for understanding the small system puzzle observed in pp and pPb collisions at the LHC.

Speaker: Heikki Mäntysaari (University of Jyväskylä)

lhec_smallx.pdf

09:00 → 10:20 Parallel 2: Jets & high-pT: High-pT hadrons

Convener: Magdalena Djordjevic (Institute of Physics Belgrade)

09:00 ALICE results on the production of charged particles in pp, p-Pb, Xe-Xe and Pb-Pb collisions at the LHC

We present the measurement of charged particle production at mid-rapidity ($|\eta| < 0.9$) in pp ($\sqrt{s}$ = 2.76 TeV, 5.02 TeV), p-Pb ($\sqrt{s_{\rm NN}}$ = 5.02 TeV), Xe-Xe ($\sqrt{s_{\rm NN}}$ = 5.44 TeV) and Pb-Pb ($\sqrt{s_{\rm NN}}$ = 2.76 TeV and 5.02 TeV) collisions with ALICE at the LHC. The transverse momentum ($p_{\rm T}$) spectra are measured in the broad range $0.15 < p_{\rm T} < 50$ GeV/$c$ and in the narrow centrality bins spanning the whole 0–100% interval. The $p_{\rm T}$ spectra measured in p-Pb, Xe-Xe and Pb-Pb collisions are compared to those in pp collisions in terms of nuclear modification factors and are compared with the parton energy loss models. A detailed comparison of nuclear modification factors in p–Pb and in peripheral A–A collisions will be discussed. The results suggest that the peripheral collisions can be affected by biases caused by the event selection and collision geometry, which can lead to an apparent nuclear modification even in the absence of jet quenching.

Speaker: Jacek Tomasz Otwinowski (Polish Academy of Sciences (PL))

JacekOtwinowski_talk_HP2018.pdf

09:20 Charged particle nuclear modification factors in pPb, PbPb and XeXe collisions with the CMS experiment

The spectra of charged particles in XeXe and PbPb collisions at $\sqrt{s_{NN}} = 5.44$ TeV and $\sqrt{s_{NN}} = 5.02$ TeV, respectively, are presented in six ranges of collision centrality. The PbPb nuclear modification factor is constructed with a measured pp reference, and the XeXe nuclear modification factor is formed with an extrapolated pp reference. Both are found to be heavily suppressed in the most central collisions. The path-length and collision-energy dependence of parton energy loss are probed by comparing these two systems of differing size. The data are also compared to various theoretical models, as well as previous measurements at lower collision energies. The pPb nuclear modification factor is constructed using a measured pp reference and is seen to be slightly above unity for the highest transverse momentum probed by the measurement. This illustrates contributions due to initial-state effects, such as anti-shadowing in the nuclear parton distribution functions.

Speaker: Austin Alan Baty (Massachusetts Inst. of Technology (US))

Baty_XeXeRAA_HP2018_v20.pdf

09:40 Charged-particle production in Pb+Pb and Xe+Xe collisions measured with the ATLAS detector

Measurements of charged-particle production in heavy-ion collisions and their comparison to $pp$ data provide insight into the properties of the quark-gluon plasma. In 2015, the ATLAS detector at the LHC recorded 0.49 nb$^{-1}$ of Pb+Pb collisions and 25 pb$^{-1}$ of $pp$ collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. In addition, around 3 $\mu$b$^{-1}$ of Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV were recorded in 2017. These samples provide an opportunity to study the system size dependence of parton energy loss. The large acceptance of the ATLAS detector allows measurements of charged-particle spectra in a wide range of both pseudorapidity and transverse momentum, and differential in collision centrality. Charged-particle spectra measured in Pb+Pb and Xe+Xe collisions are compared to the analogous spectra measured in $pp$ collisions, and the resulting nuclear modification factors are scrutinized. In particular, the nuclear modification factors are found to scale approximately with the number of participating nucleons, which may be a key to predicting the behavior of even smaller collision systems.

Speaker: Petr Balek (Weizmann Institute of Science (IL))

PetrBalek_charged_hadrons.pdf

10:00 Quenching of Hadron Spectra in Heavy Ion Collisions at the LHC

The $p_\perp$ dependence of the nuclear modification factor $R_{AA}$ measured in PbPb collisions at the LHC exhibits a universal shape, which can be very well reproduced in a simple energy loss model based on the BDMPS medium-induced gluon spectrum. The scaling is observed for various hadron species ($h^\pm$, D, $J/\psi$) in different centrality classes and at both colliding energies, √s=2.76 TeV and √s=5.02 TeV. Results indicate an 10-20% increase of the transport coefficient from √s=2.76 TeV to √s=5.02 TeV, consistent with that of particle multiplicity. Based on this model, a data-driven procedure is suggested, which allows for the determination of the first and second moments of the quenching weight without any prior knowledge of the latter.

Speaker: Francois Arleo (Laboratoire Leprince-Ringuet)

arleo_hp18.pdf

09:00 → 10:20 Parallel 3: HF & QQbar: Heavy flavour production

Convener: Elena Gonzalez Ferreiro (Universidade de Santiago de Compostela (ES))

09:00 PHENIX results on charm and bottom quark yields in p+p and Au+Au Collisions at $\sqrt{s_{\rm NN}}$=200 GeV.

The PHENIX Experiment measures electrons from heavy flavor decays at mid-rapidity |y|<0.35 in a range of 1<pT<7GeV/c. Separated charm and bottom yields are obtained by unfolding distance of the closest approach distributions. Charm and bottom quark yields are measured in p+p and Au+Au collisions obtained from the high luminosity RHIC runs taken in 2015 and 2014 respectively. The results in p+p collisions pose a tight constraint to the pQCD estimate of heavy flavor production. The nuclear modification of charm and bottom yields in Au+Au collisions can reveal how the energy that hadrons lose in the QGP medium depends on the quark mass, especially at low pT. This presentation will report on the status of these measurements.

Speaker: Timothy Thomas Rinn (Iowa State University (US))

hp2018_trinn.pdf

09:20 Measurements of open bottom hadron production via displaced J/$\psi$, $D^0$ and electrons in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV at STAR

Recent RHIC and LHC results show that the nuclear modification factors of open charm hadrons at high transverse momenta as well as their elliptic flow are similar to those of light flavor hadrons, indicating that charm quarks also interact very strongly with the Quark Gluon Plasma (QGP). During interactions, charm quarks suffer from substantial energy loss and gain significant elliptic flow. It is then imperative to measure bottom production in heavy-ion collisions to study the mass dependence of parton-medium interactions in the QGP suggested by QCD.

In this talk, we will report on STAR measurements of open bottom hadron production through their displaced decay daughters ($B \rightarrow J/\Psi,~D^{0},~e$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. These measurements are made possible with the high-precision vertexing and tracking provided by the Heavy Flavor Tracker. With a factor of 2 times more data recorded in year 2016 and a successful implementation of a supervised machine learning method in offline reconstruction, signal precisions of non-prompt $D^0$ from bottom hadron decays are greatly improved compared to previous preliminary results. Centrality dependence of nuclear modification factors for non-prompt $D^0$ will be presented. These new results will be compared to those of open charm hadrons at RHIC and those of open charm and bottom hadrons at the LHC as well as to theoretical calculations. Physics implications will be discussed.

Speaker: Mr Xiaolong Chen (University of Science and Technology of China and Lawrence Berkeley National Laboratory)

OpenBottom_xlchen_v4.pdf

09:40 Studies of beauty suppression via measurements of nonprompt $D^0$ mesons in PbPb collisions at 5.02 TeV

The transverse momentum spectra of $D^0$ mesons from b hadron decays are measured in pp and PbPb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV with the CMS detector at the LHC. The measurement is performed in the $D^0$ $p_T$ range of 2–100 GeV /c and in the rapidity range of |y| < 1. The $D^0$ mesons from b hadron decays are distinguished from prompt $D^0$ mesons by their decay topologies. In PbPb collisions, the $B \rightarrow D^0$ yield is found to be suppressed in most of the measured $p_T$ range compared to pp collisions. The suppression is weaker than that of prompt $D^0$ mesons and charged hadrons for $p_T$ around 10 GeV /c. While theoretical calculations incorporating partonic energy loss in the quark gluon plasma can successfully describe the measured $B \rightarrow D^0$ suppression at higher $p_T$, the data show an indication of larger suppression than the model predictions in the range of 2 < $p_T$ < 5 GeV /c.

Speaker: Hao Qiu (Purdue University (US))

BToD_HardProbes2018.pdf

10:00 $B_s$ and $B^+$ meson nuclear modification factors in PbPb collisions at 5.02 TeV with the CMS detector

Beauty quark production in heavy-ion collisions is considered to be one of the key measurements to address the flavour-dependence of in-medium energy loss in heavy-ion collisions. On the other hand, the measurement of the production of strange beauty mesons can provide fundamental insights into the relevance of mechanisms of beauty recombination in the quark-gluon plasma. In this talk, CMS will present the nuclear modification factor, $R_{AA}$, of fully reconstructed $B_{s}$ and $B^{+}$ mesons as a function of transverse momentum at $\sqrt{s_{NN}}=5.02$ = 5.02 TeV in PbPb collisions for the first time. The $R_{AA}$ double ratio between $B_{s}$ and $B^{+}$ is also presented.

Speaker: Guillaume Falmagne

talkBsHardProbes.pdf

09:00 → 10:20 Parallel 4: Electroweak: Photon-induced processes (UPC)

Convener: Alexander Milov (Weizmann Institute of Science (IL))

09:00 Exclusive Charmonium production in PbPb and pp collisions at LHCb

At the LHC, the highly boosted electromagnetic field of the beam
particles represents a source of quasi-real photon. Vector meson
photo-production measurements in pp/Pb-Pb collisions are sensitive to
the gluon parton distribution functions in the proton/nucleus. LHCb
results on charmonium production in ultra-peripheral Pb-Pb collisions at
5.02 TeV and in pp at 13 TeV will be presented.

Speaker: Samuel Belin (Universita e INFN, Cagliari (IT))

Talk_aix_les_bains.pdf

09:20 Low-$p_{\rm T}$ $e^+e^-$ pair production in Au+Au collisions at $\sqrt{s_{\rm NN}}$= 200 GeV and U+U collisions at $\sqrt{s_{\rm NN}}$ = 193 GeV at STAR

In high-energy heavy-ion collisions, dileptons can be produced via photon-photon and photonuclear processes, induced by the strong electromagnetic field accompanying highly charged relativistic heavy ions. These two processes were conventionally and extensively studied in ultra-peripheral collisions (UPCs). Recently, a significant excess of $J/\psi$ yields at low transverse momenta ($p_T$) was observed by the ALICE and STAR collaborations in peripheral heavy-ion collisions, qualitatively explained by coherent photonuclear production mechanisms. If indeed coherent photonuclear production was the underlying mechanism for the observed $J/\psi$ excess, the energetic and high-density photon flux would be there and coherent photon-photon production would also contribute to the $e^{+}e^{-}$ pair production. Measurements of $e^+e^-$ pair production at low $p_T$ for different collision systems and energies thus are important to verify and further constrain the existence of strong electromagnetic field and its possible impact on emerging phenomena in hadronic heavy-ion collisions.

In this talk, we will present centrality and invariant mass dependences of inclusive $e^{+}e^{-}$ pair production at $p_T<$ 0.15 GeV/$c$ in Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $\sqrt{s_{NN}}$= 193 GeV. The observed excess of $e^{+}e^{-}$ yields with respect to the known hadronic sources will be presented as a function of centrality and $p_T^2$. Physics implications will be discussed together with model comparisons.

Speaker: Dr Shuai Yang (Brookhaven National Laboratory)

HP2018_ShuaiYang_final.pdf

09:40 Coherent J/$\psi$ photo-production in Pb-Pb collisions with nuclear overlap, studied with ALICE at the LHC

The photo-production of heavy vector mesons in the electromagnetic interactions
of ultra-relativistic nuclei is thought to be sensitive to the gluon parton
distribution functions in the nucleus and thus to cold nuclear effects like shadowing
or gluon saturation. Besides the well known observations of vector meson
production in ultra-peripheral collisions (UPC), recently there were reports on
J/ψ photo-production in peripheral and semi-central nucleus-nucleus collisions
both at LHC and RHIC energies, observed as an excess over the expected normal
hadronic production at very low $p_T$ ($p_T$<300 MeV/$c$).

The ALICE Collaboration published J/$\psi$ $p_T$-integrated coherent photo-production
cross sections in peripheral and semi-central Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76~TeV
at forward rapidity ($2.5 has presented preliminary results in peripheral Pb-Pb collisions at $\sqrt{s_{NN}}$=5.02 TeV
at mid-rapidity ($|y|<0.9$). Additionally, thanks to the very good tracking resolution
of the central barrel, the extraction of the $p_T$-differential cross section
was also possible, confirming the photo-production origin of the J/$\psi$ excess.
The quantitative understanding of this low-$p_T$ excess poses significant theoretical
challenges since the J/$\psi$ photo-production depends on the collision dynamics as well as on the photon-flux and
the photo-nuclear cross section.

In this talk we will present the latest ALICE measurements on $p_T$-integrated
and $p_T$-differential coherent J/$\psi$ photo-production cross section
in peripheral Pb-Pb collisions at the available LHC energies,
in comparison to the UPC measurements. These results will be discussed and compared to several model calculations of J/$\psi$ photo-production in Pb-Pb collisions with and without nuclear overlap.

Speaker: Laure Marie Massacrier (Centre National de la Recherche Scientifique (FR))

HP2018_v3.pdf

10:00 Measurement of exclusive Upsilon photoproduction in pPb collisions with CMS experiment

The aim of this study is to measure exclusive photoproduction of three resonant particles, $\bf \Upsilon(1S)$, $\bf \Upsilon(2S)$ and $\bf \Upsilon(3S)$ mesons, in their $\mu^{+}\mu^{-}$ decay modes, in ultraperipheral pPb collisions. The $\bf \Upsilon(1S)$ photoproduction cross section are measured, over the rapidity range $| y | < 2.2$, as a function of the photon-proton center-of-mass energy, $W_{\gamma p}$, which provides valuable information of the gluon distribution at small values of parton fractional momenta $x$. The slope of the squared transverse momentum ($p_{T}^2$) dependent differential cross section is measured to determine the size of the production region. The results are compared to other measurements and to theoretical predictions.

Speaker: Kousik Naskar (Bhabha Atomic Research Centre (IN))

kousik_HP18_v3.pdf

10:20 → 10:45 Coffee break

10:45 → 12:25 Parallel 1: Initial State

Convener: Xin-Nian Wang

10:45 Inclusive photon production in DIS and p+A collisions at small x

We perform a first computation of inclusive photon production in DIS to NLO accuracy in the impact factor and to NLLx in QCD evolution at small x. We also discuss the corresponding formalism in p+A collisions, and numerical results for the same. We outline an efficient framework to perform computations in the Regge limit of QCD to higher orders. This will be important for quantitative comparison of CGC results to collider data and to matching to collinear QCD at high pT.

Speaker: Dr Raju Venugopalan (BNL)

Hard-Probes2018_raju.pdf

11:05 Forward photon measurements with ALICE at the LHC as a probe for low-x gluons

The low-x gluon density in the proton and, in particular, in nuclei is only very poorly constrained, while a better understanding of the low-x structure is crucial for measurements at the LHC and also for the planning of experiments at future hadron colliders. In addition, deviations from linear QCD evolution are expected to appear at low x, potentially leading to gluon saturation and a universal state of hadronic matter, the color-glass condensate. However, these effects have not been unambiguously proven to date. Fortunately, data from the LHC can be used directly to provide better constraints of the PDFs. In this context, a Forward Calorimeter (FoCal) is proposed as an addition to the ALICE experiment, to be installed in Long Shutdown 3 (2024-2026). The main goal of the FoCal proposal is to measure forward (3.5 < y < 5) direct photons in pp and p–Pb collisions to obtain experimental constraints on proton and nuclear PDFs in a new region of low x ($10^{−5} − 10^{−6}$). Based on the current knowledge from DIS experiments and first results from LHC, we will discuss the physics case for this proposed detector. While open charm measurements do provide important constraints, a photon measurement would provide additional unique information due to the cleanness of the dominant physics processes. In addition it provides measurements of neutral mesons, two-particle correlations, and jets in pp, p–Pb and (partially) in Pb–Pb. The direct photon measurement requires a new electromagnetic calorimeter with extremely high granularity. We will also present the corresponding innovative design principle of a high-resolution SiW sandwich calorimeter and results from the ongoing R&D program with test beams.

Speaker: Thomas Peitzmann (Nikhef National institute for subatomic physics (NL))

hard-probes-focal-final.pdf

11:25 Measurement of long-range correlations in $pp$ collisions characterized by presence of a Z boson with the ATLAS detector

Measurements of correlations between two particles separated in pseudorapidity and azimuthal angles have shown striking similarities between results obtained in $pp$, $p$+A and A+A collision systems. In $pp$ collisions, unlike in $p$+A and A+A, the strength of the correlations, quantified by the anisotropy parameter $v_2$, shows little dependence on the observed charged-particle multiplicity. Recent theoretical models suggest that this can result from an intrinsically weaker correlation between the charged-particle multiplicity and the impact parameter of the $pp$ collision. An independent handle on the impact parameter can be obtained in principle by requiring the presence of a hard-scattering process in the collision. This talk presents the first measurement of two-particle correlations in $pp$ collisions with a Z boson identified via its dimuon decay channel. The analysis uses ATLAS data recorded with nominal $pp$ luminosity, with high pileup, and is analyzed using a new procedure developed to correct for the contribution of tracks arising from pileup vertices. The multiplicity and transverse momentum dependence of the inclusive charged-particle $v_2$ measured in Z-tagged events at $\sqrt{s}$ = 8 and 13 TeV is compared to the $v_2$ measured in minimum-bias collisions. They are found to be of a similar magnitude to each other and to that measured in typical events.

Speaker: Adam Trzupek (Institute of Nuclear Physics Polish Academy of Sciences (PL))

HP2018_AdamTrzupek.pdf

11:45 2+1D simulations of pre-equilibrium stage with QCD kinetic theory

The produced matter in the high energy nuclear collisions reinteracts and form a plasma which ultimately equilibrates and exhibits collective hydrodynamic flow. The connection between the early gluon production in classical field simulations and hydrodynamic expansion at later times is given by the QCD kinetic theory. In this work we construct a set of non-equilibrium Green functions calculated in QCD kinetic theory to provide a practical tool to propagate energy and momentum perturbations from the early initial state to a time when hydrodynamics becomes applicable. We demonstrate with a realistic simulation of a heavy ion collisions that such linearized kinetic propagator (called KøMPøST) provides the smooth transition from the classical fields to hydrodynamics. KøMPøST can be easily incorporated into existing hydrodynamic simulations improving the early time description of heavy ion collisions.

References:
1. A. Kurkela, A. Mazeliauskas, J.-F. Paquet, S. Schlichting and D. Teaney
Matching the non-equilibrium initial stage of heavy ion collisions to hydrodynamics with QCD kinetic theory, arXiv:1805.01604
2. A. Kurkela, A. Mazeliauskas, J.-F. Paquet, S. Schlichting and D. Teaney
Effective kinetic description of event-by-event pre-equilibrium dynamics in high-energy heavy-ion collisions, arXiv:1805.00961

Speaker: Aleksas Mazeliauskas (Universität Heidelberg)

Mazeliauskas_HP18_v3.pdf

12:05 Transport of Heavy Quarks Across Glasma

The quark-gluon plasma, which is produced at an early stage of ultrarelativistic heavy-ion collisions, is expected to be initially strongly populated with chromodynamic fields. We address the question how heavy quarks interact with such a turbulent plasma in comparison with an equilibrated one of the same energy density. For this purpose we derive a Fokker-Planck transport equation of heavy quarks embedded in a plasma of light quarks and gluons. We first discuss the equilibrium plasma and then the turbulent one applying the same approach, where the heavy quarks interact not with the plasma constituents but rather with the long wavelength classical fields. We first consider the three schematic models of isotropic trubulent plasma and then the simplified model of glasma with the chromodynamic fields only along the beam direction. The momentum broadening and collisional energy loss of a test heavy quark are computed and compared to those of equilibrium plasma of the same energy density. It is argued that in spite of its short lifetime the glasma can provide a significant contribution to the collisional and radiative energy loss of heavy quarks.

Speaker: Stanislaw Mrowczynski (Jan Kochanowski University)

Mrowczynski.pdf

10:45 → 12:25 Parallel 2: Jets and High-pT Hadrons

Convener: Helen Caines (Yale University (US))

10:45 Neutral meson production in ALICE

Neutral mesons, namely neutral pions and $\eta$-mesons, are abundantly produced in AA collisions and can be reconstructed and identified via two-photon decays in wide range of transverse momenta. This makes them excellent probe of the parton energy loss in heavy ion collisions.

Photons in ALICE can be reconstructed via several complementary methods, using either the central tracking system identifying photons converted to $e^+e^-$ pairs in the material of the inner barrel detectors or the electromagnetic calorimeters. Thus we used the respective advantages of the detectors, i.e. the excellent momentum resolution of the conversion photons down to very low transverse momenta and the high reconstruction efficiency and triggering capabilities of the calorimeters. This approach provided reliable cross-check of results, allowed to reduce statistical and systematic uncertainties and to measure the neutral meson spectra in wide range of transverse momenta.

In this talk we will report a measurement of the nuclear modification factors of light neutral mesons in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV and 5.02 TeV and in Xe-Xe collisions at $\sqrt{s_{\rm NN}} = 5.44$ TeV. We compare our results to the measurements of other experiments and to model calculations.

Speaker: Dmitri Peresunko (National Research Centre Kurchatov Institute (RU))

Mesons.pdf

11:05 Constraining energy loss from high-pt azimuthal asymmetries

The nuclear modification factor, $R_{AA}$, has been satisfactorily described by various jet quenching models. Nonetheless, all these formalisms seemed to underpredict the high $p_T$ ($>10$ GeV/c) elliptic flow, $v_2$. Recently, it has been argued that by the inclusion of event-by-event fluctuations in the soft sector the $R_{AA} - v_2$ problem may be solved. In order to confirm that this is indeed the answer to the puzzle, one should consider the effects of changing the energy loss implementation. We will show, within the ASW Quenching Weights and EKRT hydrodynamics, that the description of the high-$p_T$ harmonics is still a challenge and discuss the opportunities that it offers to constrain the detailed mechanism of energy loss.

Speaker: Dr Carlota Andres (Jefferson Laboratory)

HP2018.pdf

11:25 Systematic studies of di-jet imbalance measurements in STAR

STAR has previously reported significant transverse momentum imbalance of a specific set of di-jets selected with "hard cores", i.e. with a constituent cut of 2 GeV/$c$. After reclustering these same di-jets with a lower constituent cut of 200 MeV/$c$, the di-jet balance is restored to the level of p+p collisions within the original cone size of $R=0.4$.

The interpretation of these results as resulting from tangential bias with restricted in-medium path lengths promised Jet Geometry Engineering of jet production vertices through systematic variations of parameters such as centrality, the constituent $\mathrm{p_T}$-cutoff, and the initial imbalance between the hard cores.

Taking advantage of the tenfold increase statistics from recent data taking runs, we will present a systematic exploration of jet quenching effects on di-jets sampled from a larger parameter space and explore in detail the possibilities and limitations of using them to constrain the influence of geometry on energy loss in the quark gluon plasma created in heavy-ion collisions.

Speaker: Nicholas Elsey (Wayne State University)

hard_probes_elsey_v6.pdf

11:45 Effects of jet-medium interactions on angular correlations of jet-particle pairs at different energy scales

Energetic heavy quarks passing through the hot and dense medium of a quark-gluon plasma (QGP), represented by the resulting mesons, are viewed as a suitable probe for the interactions inside of the QGP, in particular the mechanisms of energy loss, as they are less likely to thermalize within the medium and are mostly created at early stages of the medium evolution.
However, models of both, purely collisional energy loss as well as combinations of collisional and radiative energy loss are equally successful for reproducing the nuclear modification factor $R_{AA}$ and the elliptic flow $v_2$ [1]. In an attempt to discriminate between the two different energy-loss mechanisms, an alternative observable, the angular correlations between two mesons were investigated. Azimuthal correlations between pairs of heavy mesons, like $D$-$\bar{D}$ pairs, allow for distinguishing the energy-loss scenarios [2].

We continue these studies by investigating the angular correlations between pairs of heavy and light mesons ($D$ and $\pi$), originating from a heavy quark jet. This is motivated by the fact that the emitted gluon in radiative collisions hadronizes and these hadrons are correlated to the emitting heavy quark.

We created a Monte-Carlo code for the parton splitting in the vacuum together with an effective medium model. This program represents a consistent framework to study the influences of either collisional or radiative processes on parton propagation, and the resulting two-particle correlations. As experimental data for angular correlations of soft particles has become available [3,4], we extended our studies to angular correlations of charged particle pairs in various $p_T$ ranges. We particularly focused on describing angular jet-broadening effects in the collisional and radiative models simultaneously for correlations in longitudinal as well as azimuthal directions.

[1] P. B. Gossiaux, J. Aichelin, T. Gousset and V. Guiho, J. Phys. G 37 (2010)
094019 doi:10.1088/0954-3899/37/9/094019 [arXiv:1001.4166 [hep-ph]].
[2] M. Nahrgang, J. Aichelin, P. B. Gossiaux and K. Werner, J. Phys. Conf. Ser.
509 (2014) 012047 doi:10.1088/1742-6596/509/1/012047 [arXiv:1310.2218
[hep-ph]].
[3] ALICE Collaboration, Phys. Rev. Lett. 119 (2017) 102301.
[4] ALICE Collaboration, Phys. Rev. C96 (2017) 034904.

Speaker: Martin Rohrmoser (Jan Kochanowski University Kielce)

rohrmoser_talk.pdf

12:05 Jet-associated deuteron production in pp collisions at 13 TeV with ALICE at the LHC

The production of deuterons in high-energy collisions is particularly sensitive to the space and time evolution of the system, as well as the baryon production and transport mechanisms. Recent ALICE measurements of spectra and anisotropic flow of (anti-)deuterons provide insight into the production mechanisms of particles in heavy-ion collisions thanks to a critical comparison with the available theoretical approaches: coalescence and hydrodynamic models. We present new preliminary results on deuteron-jet correlations from pp collisions at 13 TeV to complement these findings. The measured jet-associated yields of (anti-)deuterons will be also compared to theoretical predictions in the context of a baryon coalescence model.

Speaker: Brennan Schaefer (Oak Ridge National Laboratory - (US))

BSchaefer_HP_2018_Deut_in_Jets.pdf

10:45 → 12:25 Parallel 3: HF & QQbar: Heavy flavour flow

Convener: Andrea Dainese (INFN - Padova (IT))

10:45 Measurement of directed flow of $D^0$ and $\bar{D}^0$ mesons in 200 GeV Au+Au collisions at RHIC using the STAR detector

Charm quarks, owing to their large mass, are produced predominantly during the initial hard scatterings in heavy-ion collisions and therefore can be a valuable tool to study the early time dynamics of these collisions. The rapidity odd directed flow of particles produced at mid-rapidity in heavy-ion collisions originates from a tilt in the reaction plane of the thermalized medium caused by the asymmetry between number of participants from projectile and target nuclei as a function of rapidity. Recently, it has been predicted that the slope of the directed flow at mid-rapidity for $D^0$ mesons, arising from the transport of charm quarks in the tilted medium, can be several times larger than that of light flavor hadrons. The magnitude of the slope is expected to be sensitive to the magnitude of the tilt and the charm quark drag coefficient in the medium. It has also been predicted that the transient electromagnetic field generated at early time can induce a much larger directed flow for heavy quarks than for light quarks. In this talk, we will report on the first measurement of the directed flow for the $D^0$ and $\bar{D^0}$ mesons as a function of rapidity in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV using high statistics data collected with the Heavy Flavor Tracker in 2014 and 2016 RHIC runs. The results will be compared to those of light flavor hadrons and model predictions, and the physics implications of these measurements will be discussed.

Speaker: Liang He (Purdue University)

D0-v1-HP2018-v2.5.pdf

11:05 Heavy Flavor Azimuthal Correlations in Cold Nuclear Matter

It has been proposed that the azimuthal distributions of heavy quark-antiquark pairs may be modified in the medium of a heavy-ion collision. This assumption is tested through next-to-leading order (NLO) calculations of the azimuthal distribution, $d\sigma/d\phi$, including transverse momentum broadening, employing $\langle k_T^2 \rangle$ and fragmentation in exclusive $Q \overline Q$ pair production [1]. The differences between NLO calculations and heavy $Q \overline Q$ pair production in event generators are also discusssed.

First, single inclusive $p_T$ distributions calculated with the exclusive HVQMNR code are compared to those calculated in the fixed-order next-to-leading logarithm approach. Next the azimuthal distributions are calculated and sensitivities to $\langle k_T^2 \rangle$, $p_T$ cut, and rapidity are studied at $\sqrt{s} = 7$ TeV. Finally, calculations are compared to $Q \overline Q$ data in elementary $p+p$ and $p + \overline p$ collisions at $\sqrt{s} = 7$ TeV and 1.96 TeV as well as to the nuclear modification factor $R_{p {\rm Pb}}(p_T)$ in $p+$Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV measured by ALICE. While these studies were done for $p+p$, $p + \overline p$ and $p+$Pb collisions, understanding azimuthal angle correlations between heavy quarks in these smaller, colder systems is important for their interpretation in heavy-ion collisions.

The low $p_T$ ($p_T < 10$ GeV) azimuthal distributions are very sensitive to the $k_T$ broadening and rather insensitive to the fragmentation function. The NLO contributions can result in an enhancement at $\phi \sim 0$ absent any other effects. Agreement with the data was found to be good.

The NLO calculations, assuming collinear factorization and introducing $k_T$ broadening, result in significant modifications of the azimuthal distribution at low $p_T$ which must be taken into account in calculations of these distributions in heavy-ion collisions.

[1] R. Vogt, to be submitted.

Speaker: Ramona Vogt (LLNL and UC Davis)

HP18_talk_small.pdf

11:25 Measurement of D-meson nuclear modification factor and flow in Pb-Pb collisions with ALICE at the LHC

Open heavy-flavour mesons are a unique tool to investigate the properties of the Quark-Gluon Plasma (QGP) formed in heavy-ion collisions at high energy. Given their large masses, heavy quarks are produced in the initial stages of the collisions and thus witness the whole system evolution, losing energy interacting with the medium constituents. The measurement of the nuclear modification factor (RAA) of open heavy flavours can provide important information about the colour-charge and parton-mass dependence of the energy loss. The measurement of the elliptic flow ($v_{\rm 2}$) at low $p_{\rm T}$ can give insight into the participation of the heavy quarks in the collective expansion of the system and their thermalization in the medium. At high $p_{\rm T}$, it allows us to investigate the path-length dependence of parton energy loss. These two observables can also help us to understand possible modifications of heavy-quark hadronization in the medium. In particular, the role of the recombination mechanism can be studied for charm via the comparison of D mesons with and without strange-quark content. Finally, it has been shown that very strong electric and magnetic fields created at early times can affect the charm quarks dynamics. Their effect can be investigated by measuring charm-particle directed flow, $v_{\rm 1}$.

In this talk, the latest results on the $p_{\rm T}$-differential $R_{\rm AA}$ and $v_{\rm 2}$ of $\rm D^0$, $\rm D^+$, $\rm D^{∗+}$ and $\rm D^+_s$ mesons measured at central rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with ALICE will be presented for different centrality classes. The comparison with predictions from theoretical models and the constraints set to charm spatial diffusion coefficient in the medium will be discussed. The final results on the Event-Shape Engineering (ESE) technique applied to the D-meson elliptic flow in semi-central Pb-Pb events will be also presented, to investigate the influence of initial geometry fluctuations to heavy-flavour production. Finally, the status of the measurement of $\rm D^0$ $v_{\rm 1}$ will be presented.

Speaker: Fabrizio Grosa (Politecnico di Torino (IT))

Dmesons_PbPb_HP18_Grosa.pdf

11:45 Charm $v_2$ is more hydrodynamic than light quark $v_2$

Heavy quarks are produced by hard scatterings at early times and experience almost the entire history of the collision evolution in relativistic heavy ion collisions. Their azimuthal anisotropy $v_{2}$ is a very useful tool for the study of the properties of the quark-gluon plasma (QGP). Recent studies with transport models suggest that the majority of the light quark $v_{2}$ at RHIC energies comes from anisotropic escape of partons, not from the hydrodynamic flow [1-2]. So a natural question is whether the charm quark $v_{2}$ mainly comes from the anisotropic escape or hydrodynamics.

To address this question we report in this talk our recent study [3-4] using a multi-phase transport (AMPT) model, which has been very successful in describing experimental data for the bulk matter [5]. We study the charm $v_{2}$ in heavy ion as well as small system collisions by tracking the evolution history of quarks of different flavors at both RHIC and LHC energies. The charm quark $v_{2}$ is studied as a function of the number of collisions the charm quark suffers with other quarks and compared to the $v_{2}$ of lighter quarks. We find that the common escape mechanism is at work for both the charm and light quark $v_{2}$. However, contrary to the naive expectation, the hydrodynamics-type flow contributes much more to the charm $v_{2}$ than the light quark $v_{2}$. This could be explained by the smaller average deflection angle the heavier charm quark undergoes in each collision, thus charm quarks remember better the accumulative anisotropy than the light quarks. Our finding suggests that the charm $v_{2}$ is a better probe for studying the hydrodynamic properties of the quark-gluon plasma.
[1] L He, T. Edmonds, Z.-W. Lin, F. Liu, D. Molnar, and F. Wang, Phys. Lett. B 753, 506 (2016).
[2] Z.-W. Lin, L. He, T. Edmonds, F. Liu, D. Molnar, and F. Wang, Nucl. Phys. A 956, 316 (2016).
[3] H.L. Li, Z.-W. Lin and F. Wang, arXiv:1804.02681 (2018).
[4] Z.-W. Lin, H.L. Li and F. Wang, EPJ Web of Conferences 171, 19005 (2018).
[5] Z.-W. Lin, Phys. Rev. C 90, 014904 (2014).

Speaker: Hanlin Li (Wuhan University of Science and Technology)

HP2018_lihl_v2.pdf

12:05 Event-multiplicity and event-shape dependence of open heavy-flavour production in pp collisions with ALICE at the LHC

Heavy-quarks are sensitive probes for investigating the properties of the high- density medium formed in heavy-ion collisions. Their investigation in proton- proton (pp) collisions at the LHC, besides furnishing the necessary baseline for measurements in nucleus–nucleus collisions, provides precise tests for perturba- tive QCD (pQCD) calculations based on the factorization approach down to very low Bjorken-x values. The analysis of heavy-flavour production as a function of the multiplicity of charged particles produced in the collision and of event-shape variables, like spherocity, which classify events according to their topology, can give insight into multiple-parton-interaction phenomena. These studies provide an handle to understand the interplay of hard and soft processes and to search possible connections between small and extended interacting systems.

The excellent tracking and particle-identification capabilities of the ALICE detector allow to fully reconstruct hadronic decays of open-charm mesons at central rapidity and to study leptons from charm- and beauty-hadron decays at central and forward rapidities. In this contribution, the latest results on the production of D mesons and open heavy-flavour hadron decay electrons at mid-rapidity (|$y$| < 0.8) and open heavy-flavour hadron decay muons at forward rapidity (2.5 < $y$ < 4) in pp collisions at various collision energies will be presented. On top of showing precise measurements of the $p_{\rm T}$- and $y$-differential cross sections, which provide stringent constraints for pQCD calculations, studies addressing the dependence of open heavy-flavour production on the event-multiplicity will be discussed, along with comparison with model expectations. Finally, the dependence of the $\rm D^0$ production on the event multiplicity and spherocity in pp collisions at $\sqrt{s}$ = 7 TeV will be reported.

Speaker: Shreyasi Acharya (Department of Atomic Energy (IN))

HPSlides2018_Shreyasi.pdf

10:45 → 12:25 Parallel 4: Electroweak: Photon-induced processes (UPC)

Convener: Axel Drees

10:45 Tale of coherent photon products: from UPC to HHIC

The coherent photon-nucleus and photon-photon interactions has been studied in detail to probe the gluon distribution in nucleus and to test QED via relativistic heavy-ion collisions. These kind of interactions are traditionally thought to be only exist in ultra-peripheral collisions (UPC), where there is no hadronic interactions. Recently, significant excess of $J/\psi$ yield and dielectron pair production at very low transverse momentum ($p_{T} <$ 0.3 GeV/c) were observed by the ALICE and STAR collaborations in peripheral A+A collisions, which points to evidence of coherent photon products in hadronic heavy-ion collisions (HHIC). The possible survival of photoproduced $J/\psi$ and electron pair merits theoretical investigations, which are currently absent on the market.

In this talk, we report on calculations of $J/\psi$ yield from coherent photon-nucleus interactions and dilepton production from photon-photon interactions in HHIC at RHIC and LHC energies. The model used to calculate the cross section is discussed and the expected yield are compared with experimental results from RHIC and LHC. We predict the coherent production contribution of $J/\psi$ and dielectron from isobaric collisions (Ru+Ru, Zr+Zr) for the further experimental test at RHIC.

This talk is based on:
1) Wangmei Zha etal., Phys. Lett. B781 (2018) 182; 2) Wangmei Zha etal., Phys. Rev. C97 (2018) 044910; 3) paper in preparation.

Speaker: Wangmei Zha (USTC/BNL)

HP2018_wangmei.pdf

11:05 Measurements of photon-photon and photon-nucleus collisions with the ATLAS detector

In ultra-relativistic heavy-ion collisions the intense electromagnetic fields of the nuclei provide a large flux of equivalent photons. This flux leads to photon-photon and photon-nucleus reactions at high center-of-mass energies. In ultra-peripheral collisions (UPCs), the nuclei have large impact parameter, and the dominant interaction mechanism is through these photon-induced processes. This talk presents a series of measurements of such processes performed by the ATLAS Collaboration. It includes new measurements of exclusive UPC dimuon production, which provide detailed constraints on the nuclear photon flux and its dependence on impact parameter and photon energy. This production mechanism may also occur in events with smaller impact parameter, resulting in dimuons produced in the same events in which a hot nuclear medium is formed. Measurements of $\gamma+\gamma\rightarrow\mu\mu$ in non-UPC collisions are also presented; the dimuons exhibit a centrality-dependent broadening of their azimuthal angle correlations suggesting that such muons provide a new probe of the medium. Finally, measurements of jet production in photon-nucleus collisions will be presented; such processes provide a clean probe of the nuclear parton distributions.

Speaker: Peter Alan Steinberg (Brookhaven National Laboratory (US))

Steinberg_HP2018_v1.pdf

11:25 Evidence for light-by-light scattering and limits on axion-like-particles from ultraperipheral PbPb collisions at 5 TeV

A measurement of light-by-light scattering, γγ → γγ, in ultraperipheral PbPb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is reported. The analysis is conducted using a data sample corresponding to an integrated luminosity of 390 μb − 1 recorded by the CMS experiment at the LHC. Light-by-light scattering processes are selected in events with two photons exclusively produced, each with transverse energy ET > 2 GeV, pseudorapidity |eta| < 2.4, diphoton invariant mass m(gamgam) > 5 GeV, diphoton transverse momentum p T < 1 GeV, and diphoton acoplanarity below 0.01. After all selection criteria are applied, 14 events are observed, compared to expectations of 11.1 ± 1.1 (th) events for the signal and 3.8 ± 1.3 (stat) for the background processes. The significance of the light-by-light signal against the background-only hypothesis is 4.1 standard deviations. The measured fiducial light-by-light scattering cross section, σ fid ( γγ → γγ ) = 122 ± 46 (stat) ± 29 (syst) ± 4 (th) is consistent with the standard model prediction. The present results allow also to place new competitive limits, reported for the first time, on the production of pseudoscalar axion-like particles, produced in the process γγ → a → γγ, over the mass range m a = 5–50 GeV.

Speaker: Jeremi Niedziela (CERN)

light_by_light_hard_probes_2018.pdf

11:45 Ultra-peripheral-collision studies in the fixed-target mode with the proton and lead LHC beams

We discuss the physics case related to the studies of ultra-peripheral $p$+(H,$^3$He) and Pb+(H,$^3$He) collisions in the fixed-target mode at the LHC. In particular, we show how one can measure the gluon GPD $E_g(x,\xi)$ in exclusive $J/\psi$ photoproduction with a transversely polarised hydrogen target.

Speaker: Nodoka Yamanaka (Riken)

2018.10.02_yamanaka.pdf

12:05 Observing the Higgs boson in ultraperipheral heavy-ion collisions at the LHC and FCC

We present a study the two-photon production of the Higgs boson, gamma-gamma->H, at the LHC and Future Circular Collider (FCC) in ultraperipheral PbPb and pPb collisions at sqrt(s_NN) = 5.5, 8.8, 39 and 63 TeV. Signal and background events are generated with MADGRAPH 5, including gamma fluxes from the proton and lead ions in the equivalent photon approximation, yielding sigma(gamma-gamma->H) = 18 pb, 0.17 pb at the LHC and 1.75 nb and 1.5 pb at the FCC in PbPb and pPb collisions respectively. We analyse the H->b-bbar decay channel including realistic reconstruction efficiencies for the final-state b-jets, showered and hadronized with PYTHIA 8, as well as appropriate selection criteria to reduce the dominant exclusive gamma-gamma->b-bbar continuum background. Observation of the Higgs boson is achievable in the first FCC year with the expected PbPb integrated luminosities.

Speaker: Patricia Rebello Teles (CBPF - Brazilian Center for Physics Research (BR))

HardProbesTalk.pdf

12:25 → 14:00 lunch break

14:00 → 16:00 Parallel 1: Jets & high-pT: Photon-jet studies

Convener: David Morrison (BNL)

14:00 PHENIX measurement of direct photon-triggered two-particle correlations in heavy ion collisions and its implication of the medium induced energy loss.

Direct photon-hadron correlations are an excellent probe for QCD effects, including parton energy loss in the Quark-Gluon Plasma. At leading order, direct photons balance the pT of the away-side jet. In addition, as a colorless probe, direct photons do not interact strongly with the colored medium providing a less biased trigger than a single high-pT hadron. PHENIX has measured direct photon-triggered two-particle azimuthal correlations in a variety of collision systems at 200 GeV. In d+Au collisions, no modification of the per-trigger pair yields compared to p+p collisions was observed constraining the amount of cold nuclear matter effects in such collisions. In A+A collisions, direct photons have been identified statistically as well as using an isolation cut. Combining data sets from different collision systems allows us to quantify the transition from suppression at high zT (=pT,h/pT,gamma) to the enhancement of low zT particles relative to p+p, and to study this transition as a function of trigger pT. Integrating per-trigger yields in different ranges of the away-side gives insights on the redistribution of energy within the jet. The implication for our understanding energy from these measurements will be discussed.

Speaker: Alexandre Lebedev (Iowa State University (US))

A.Lebedev_TuesdayAfternoon.pdf

14:20 Energy loss and modification of photon-tagged jets with ATLAS

Events containing a high-transverse momentum ($p_T$) prompt photon offer a useful tool to study the dynamics of the hot, dense medium produced in heavy ion collisions. Because photons do not carry color charge, they are unaffected by the medium, and thus provide information about the momentum, direction, and flavor (quark or gluon) of the associated hard-scattered parton before it begins to shower and become quenched. In particular, the presence of a high-$p_T$ photon can be used to select $pp$ and Pb+Pb events with the same configuration before quenching, limiting the effects of selection biases present in other jet measurements. This talk reports results from the large statistics $pp$ and Pb+Pb data delivered by the LHC in 2015, including measurements of the overall parton energy loss (via the photon+jet $p_T$ balance) and the modification of the component of the shower which remains correlated with the initial parton direction (via the longitudinal fragmentation function in cone). Since all results are fully corrected for detector effects with unfolding procedures, the talk also includes direct, systematic comparisons to state of the art theoretical models.

Speaker: Dennis Perepelitsa (University of Colorado Boulder)

dvp-HP18-ATLAS-10-2-18.pdf

14:40 Probing jet medium interaction via dijet and photon-jet $p_{\rm T}$ imbalance

In the recent two papers [1,2], we develop the first systematic theoretical approach to dijet and photon-jet $p_T$ imbalance in hadron-hadron collisions based on the perturbative QCD (pQCD) expansion and the Sudakov resummation formalism. We find that the pQCD calculation at next-to-leading order is indispensable to describe the experimental data, while the Sudakov resummation formalism is vital near the end points where the pQCD expansion fails to converge due to the appearance of large Sudakov logarithms. Utilizing our resummation improved pQCD approach, we obtain good agreement with the most up-to-date fully corrected ATLAS data on dijet and photon-jet $p_T$ imbalance in $pp$ collisions. Combining with the BDMPS jet energy loss formalism, we extract the value of jet transport coefficient $\hat{q}_0 \sim 2$-$6~\textrm{GeV}^2/\textrm{fm}$ for the quark-gluon plasma created in $PbPb$ collisions at 2.76A TeV and $\hat{q}_0 \sim 2$-$8~\textrm{GeV}^2/\textrm{fm}$ for that created at 5.02A TeV. This work paves the way for a more complete and deeper understanding of the properties of strongly-coupled QCD medium via the studies of dijet and photon-jet $p_T$ imbalance in relativistic heavy-ion collisions.

[1] L. Chen, G.Y. Qin, S.Y. Wei, B.W. Xiao and H.Z. Zhang, Phys. Lett. B (2018), arXiv:1612.04202 [hep-ph].

[2] L. Chen, G.Y. Qin, L. Wang, S.Y. Wei, B.W. Xiao, H.Z. Zhang and Y.Q. Zhang, Nucl. Phys. B (2018), arXiv:1803.10533 [hep-ph].

Speaker: Dr Shu-yi Wei (Ecole Polytechnique)

pT-imbalance.pdf

15:00 Measurement of the semi-inclusive distribution of jets recoiling from direct photon and $\pi^0$ triggers in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV with the STAR experiment

We present a measurement of semi-inclusive recoil jets for both direct-photon and $\pi^{0}$ triggers, within various trigger transverse momentum ($\rm{p_{T}}$) ranges between 9 GeV/c and 30 GeV/c, in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, using 13 $\rm{nb}^{-1}$ of data recorded in the year 2014 by the STAR collaboration. Details of the analysis will be discussed, including correction for the large uncorrelated background contribution to the recoil jet distribution using a mixed-event technique. Finally, we report a comparison between the direct-photon and $\pi^{0}$ triggered recoil jets in central Au+Au collisions and their suppression with respect to p+p collisions and PYTHIA reference for different trigger $\rm{p_{T}}$ ranges.

Speaker: Dr Nihar Sahoo (Texas A&M University)

HP2018_nihar_v5.pdf

15:20 Jet-induced medium responce and γ-jet fragmentation function

We use the state of art CoLBT-hydro to investigate the importance of jet-induced medium excitation in calculating γ-jet fragmentation function. CoLBT-hydro model is Linear Boltzmann Transport model cocurrently coupled with 3+1D hydrodynamic model in real time. It is designed for simultaneous event-by-event simulations of jet propagation and hydrodynamics evolution of the bulk medium including jet-induced medium excitation. In this model, the lost energy-momentum of energetic partons propagating through the medium is considered as a source term in the hydrodynamic, which will induce medium excitation. We carry out the first study with CoLBT-hydro of medium modification of γ-jet fragmentation function in heavy-ion collisions at both RHIC and LHC. CoLBT-hydro describes well the suppression of leading hadrons due to parton energy loss and predicts an enhancement of soft hadrons due to jet-induced medium excitation. Similarly, the calculation of medium modification of γ-jet fragmentation function indicates the enhancement of soft hadrons and the suppression of the leading hadrons inside the jet cone. There are two unique features of jet-induced medium excitation: the onset of soft hadron enhancement at a constant p^h_T and depletion of soft hadrons in the γ direction. We will also discuss the hadron flavor dependence of the soft hadron enhancement due to jet-induced medium excitaiton.strong text

Speaker: WEI CHEN (CCNU)

HP2018_chenwei_v2.pdf

15:40 Photon-tagged jet fragmentation functions and jet shapes in pp and PbPb collisions with the CMS detector

One can impose constraints on theories for parton-medium interactions by measuring jet substructure observables, such as fragmentation functions or jet momentum density profiles. Tagging jets with an associated photon helps to reduce the ambiguities in such measurements, since the photon, not interacting with the QCD matter, gives precise information about the parton's momentum before traversing the medium. Furthermore, the parton produced in association with a photon is often a quark, providing a pure sample for quark-jet modification. The results of photon-tagged jet fragmentation functions and the first measurement of photon-tagged jet shapes in pp and PbPb collisions at sqrt(s_NN) = 5.02 TeV collision energy using data collected by CMS will be reported.

Speaker: Kaya Tatar (Massachusetts Inst. of Technology (US))

181002_HP2018_KayaTatar.pdf

14:00 → 16:00 Parallel 2: Jets & high-pT: Jet quenching

Convener: Urs Wiedemann (CERN)

14:00 Jet splitting function in the vacuum and QCD medium

The two-prong substructure of the leading subjets inside a reconstructed jet opens new windows on precision constraints on the in-medium modification of parton showers. We present the first resumed calculation of the groomed soft-dropped subjet momentum sharing distribution in heavy ion collisions, and demonstrate that both the STAR data at RHIC and the CMS results at LHC can be understood in the unified framework of soft-collinear effective theory with Glauber gluon interactions. Recent advances in understanding mass effects on the QCD splitting functions enable us to apply this method for the first time to heavy flavor tagged jets, the main topic of this presentation. Theoretical predictions for the momentum sharing distribution modification of jets tagged by single and two in-jet heavy mesons will be presented. We find that in the kinematic region that will be accessed by sPHENIX in the future, or by studying jets of lower transverse momenta than currently explored at the LHC, there is a unique reversal of the mass hierarchy of jet quenching effects. Namely, the momentum sharing distribution of b-tagged jets is more strongly modified in comparison to the one for light jets. This unique feature provides a handle on mass corrections that are at present difficult to constraint using inclusive heavy meson production.

Speaker: Haitao Li (Los Alamos National Laboratory)

haitao_HP_2018.pdf

14:20 Sorting out energy loss for medium-modified jets

Most studies of medium-induced jet modification rely on the comparison of jet properties measured in heavy ion collisions with a proton-proton baseline of the same reconstructed jet $p_T$. Migration of jets from higher to lower $p_T$ due to energy loss, together with the steepness of the jet spectrum, lead to a heavy ion jet sample with a given $p_T$ range which is dominated by the jets that were least modified. We introduce a new strategy to compare heavy ion jet measurements to proton-proton baselines which views energy loss as being monotonic in $p_T$. In this strategy, the jets in a heavy ion collision ordered by $p_T$ can be viewed as modified versions of the same number of highest energy jets in proton-proton collisions. We validate, at MC level, the correlation between the $p_T$ of the parton that initiates a heavy ion jet with the $p_T$ of the vacuum jet which corresponds to it via our novel binning procedure. We show that this strategy mitigates the effect of bin migration and provides a complementary way to study jet modification in heavy ion collisions.

Speaker: Jasmine Brewer (Massachusetts Institute of Technology)

JasmineBrewer-HardProbes-2018_upload.pdf

14:40 Higher-order corrections to jet quenching

The phenomenon of jet quenching in ultra-relativistic heavy ion collisions reveals the effect of substantial final state interactions that cause QCD jets to lose energy to the quark-gluon plasma (QGP), mainly by induced gluon radiation. In standard analytic approaches to energy loss, jets are approximated by single partons and thus higher-order effects in the strong coupling constant are neglected. This may prove insufficient to reliably extract QGP properties at high pT, where a significant jet suppression was recently reported by the ATLAS collaboration in PbPb collisions at the LHC. In this work we explore higher-order contributions to the inclusive jet spectrum which may be sizable owing to the fact that the probability for a highly virtual parton to split in the medium increases with the jet pT. As the effective number of jet constituents increases, jets are expected to lose more energy than a single color charge. This translates into a logarithmic enhancement of higher-orders in the perturbative series that need to be resummed. As a result we obtain a Sudakov-like suppression factor which we investigate in the leading logarithmic approximation. We note, however, that the phase space for higher-order corrections is mitigated by coherence effects that relate to the fact that, below a characteristic angular scale, the medium does not resolve the inner jet structure. In this case, the jet lose energy coherently as a single color charge, namely, the primary parton.

References:
[1] Y. Mehtar-Tani and K. Tywoniuk, “Quenching of high-pT jet spectra,” arXiv:1707.07361 [hep-ph]
[2] Y. Mehtar-Tani and K. Tywoniuk, “Radiative energy loss of neighboring subjets,'' arXiv:1706.06047 [hep-ph].

Speaker: Yacine Mehtar-Tani (INT, University of Washington)

hp2018.pdf

15:00 Nuclear modification of full jets and jet structure in relativistic nuclear collisions

With our coupled jet-fluid model [1, 2, 3], we study the nuclear modifications of full jets and jet structures for single inclusive jet, dijet and gamma-jet events in relativistic heavy-ion collisions. The evolution of full jet shower is studied via a set of coupled transport equations including the effects of collisional energy loss, transverse momentum broadening and medium-induced splitting process. The dynamical evolution of the bulk medium is simulated via hydrodynamic equation with source terms which describe the energy and momentum deposited from hard jet to soft medium. Our detailed analysis indicates that collisional absorption (energy loss) tends to narrow the jet shape function while transverse momentum kicks and medium-induced radiations broaden the jet transverse profile. Also, jet-induced flow plays a significant contribution to jet shape function and dominates at large angles away from the jet axis. The final nuclear modification pattern for the jet shape function is a combined effect from various jet-medium interaction mechanisms. Our detailed studies for single inclusive jets, dijets and gamma-jets for various kinematics indicate that the nuclear modification of jet shape has strong jet energy and flavor dependence: the inner core of lower energy (gluon) jets is easier to be modified by jet-medium interaction than higher energy (quark) jets.

Reference:
[1] Ning-Bo Chang, Guang-You Qin, Phys.Rev,C94,024902 (2016)
[2] Yasuki Tachibana, Ning-Bo Chang and Guang-You Qin, Phys.Rev,C95,044909 (2017)
[3] Ning-Bo Chang, Guang-You Qin, Yasuki Tachibana, in preparation

Speaker: Dr Ning-Bo Chang (Xinyang Normal University)

changnb-HP2018_v4.pdf

15:20 Jets in non-equilibrium quark-gluon plasma

In this talk we study jets in non-equilibrium quark-gluon plasma using new theoretical tools developed for photons [1]. Jets broaden through radiation which is suppressed because of coherent interaction with the medium. We analyze this radiation when the medium is not in thermal equilibrium. Jet observables can then be used to gain direct access to non-equilibrium properties of the QGP, such as its shear viscosity and other transport coefficients. As an example we consider the anisotropic plasma formed in early stages of heavy-ion collisions [2]. Momentum broadening is reduced in the direction of the anisotropy which affects the rapidity dependence of jet observables. Finally, we discuss how to generalize our calculation to higher anisotropy where momentum broadening becomes singular because of rapid growth in gluon density. This requires us to incorporate background chromomagnetic fields.

[1] S. Hauksson, S. Jeon, C. Gale, Phys. Rev. C 79, 014901 (2018)
[2] P. Romatschke, M. Strickland, Phys. Rev. D 68, 036604 (2003)

Speaker: Sigtryggur Hauksson (McGill University)

hp2018_hauksson.pdf

15:40 Automated discovery of jet substructure analyses

Jet substructure analysis has the promise to reveal the details of the QCD shower beyond existing jet cross section and coincidence measurements in hadron collider QCD and heavy-ion. Traditional jet substructure variables have been constructed using expert knowledge, and are largely transplanted, unmodified, from the high-energy physics to heavy-ion, thus inhibiting its performance as a QCD shower and fragmentation model discriminant. A novel neural network architecture is described that is capable to bootstrap the entire chain of statistical analysis on its own, and further allows the extraction of closed-form algebraic expressions from the learned result — allowing the automatically constructed jet substructure analysis to be subsequently understood and reproduced by humans. The type of jet substructure variables and analyses is discussed, when the neural network constructs is tasked to distinguish jets undergoing energy loss in different medium temperatures, demonstrated using both JEWEL and the Linearized Boltzmann Transport Monte Carlo, and at the presence of a realistic heavy-ion underlying event. The resulting analyses is shown to reliably distinguish different initial temperatures, a performance beyond existing, manually designed analyses.

Speaker: Yue Shi Lai (University of California Berkeley (US))

hp-20181002.pdf

14:00 → 16:00 Parallel 3: HF & QQbar: Heavy flavour in the QGP

Convener: Roberta Arnaldi (Universita e INFN Torino (IT))

14:00 D-meson production in jets in pp and PbPb collisions with the CMS detector

The measurement of D-meson production in jets can provide important insights into the interactions of heavy-flavour quarks with the quark-gluon plasma created in heavy ion collisions. In particular, the role of gluon splitting processes in the production of heavy flavour, which is fundamental for a complete understanding of the quenching mechanisms for both light and heavy quarks, can be explored. Large datasets for proton-proton and PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV were collected with the CMS detector during the 2015 LHC run. These data enable measurements of D-meson production as a function of the radial distance between the jet axis and the D meson in different intervals of D-meson transverse momentum. The ratio of the results for PbPb and pp collisions will be compared to similar measurements of jet radial profiles using light particles from the CMS experiment at the same center-of-mass energy.

Speaker: Michael Peters (Massachusetts Inst. of Technology (US))

Peters_HP18.pdf

14:20 Effects of drag induced radiation and multi-stage evolution on heavy quark energy loss

Heavy quarks serve as ideal probes of the QGP properties produced in energetic nuclear collisions, and provide a unique opportunity to study the mass effects on parton energy loss. We develop a multi-stage approach [1,2] for heavy quark evolution inside the QGP, in which heavy quarks first undergo a rare-scattering multiple-emission evolution at momenta large compared to their mass (sensitive only to the transverse diffusion coefficient $\hat{q}$), and then evolve through a single-scattering induced emission (Gunion-Bertsch) stage at momenta comparable to their mass [sensitive to not only $\hat{q}$, but also the longitudinal drag $\hat{e}$ and diffusion $\hat{e}_2$ coefficients]. This multi-stage approach is coupled to a (2+1)-D viscous hydrodynamic model for a quantitative investigation of charm vs. beauty quark energy loss inside the QGP. Based on this approach, we find that drag induced radiation has a considerable impact on the energy loss of intermediate $p_T$ massive beauty quarks. This effect increases the suppression of $B$ mesons and narrows the difference between the $R_{AA}$ of $B$ and $D$ mesons. Our results are consistent with the experimental data at the LHC and contribute to a more quantitative understanding of the transverse momentum dependence of the mass hierarchy of parton energy loss inside the QGP.

[1] S. Cao, A. Majumder, G.-Y. Qin and C. Shen, arXiv:1711.09053.

[2] S. Cao, et. al., Phys. Rev. C96 (2017) no.2, 024909

Speaker: Abhijit Majumder (Wayne state university)

majumder_HP18.pdf

14:40 Extract heavy quark transport properties using a linearized-Boltzmann--Langevin model

Recent progress in open-heavy-flavor measurements and future experimental upgrades are bringing heavy-flavor physics into the precision era, allowing for strong quantitative constraints on the transport properties of heavy quarks in the quark-gluon plasma. On the theory side, it will be imperative to understand the impact of model uncertainty regarding the extraction of heavy-quark transport properties from experimental measurements. A model that suits this purpose is the Lido model, which combines two widely used kinetic approaches: a matrix-element based linearized-Boltzmann transport and diffusion based on the Langevin equation. Heavy quarks undergo both perturbative QCD scattering and non-perturbative diffusive interaction with each of these being a tunable component of the model. We extract the heavy-quark transport coefficients by conducting a Bayesian model-to-data analysis at LHC energies. The impact of modeling uncertainties on the results from the Bayesian calibration are discussed and the extracted transport coefficients are compared to previous calibrations using a radiation-improved Langevin equation.

We also investigate the possibility of absorbing pQCD processes with small momentum transfers to the medium into the diffusion part of the Lido model by restricting the use of pQCD matrix elements processes with large momentum transfers. This study allows us to construct a model that smoothly interpolates between a pure pQCD based approach and a radiation-improved Langevin equation by tuning a single scale parameter, and therefore helps us to further study heavy quark modeling uncertainties in a generalized setup.

Speaker: Weiyao Ke (Duke University)

weiyao-ke.pdf

15:00 Measurements of heavy-flavour decay leptons production in Pb--Pb and Xe--Xe collisions with ALICE at the LHC

Charm and beauty quarks are expected to form on a shorter time scale with respect to the strongly interacting matter, Quark-Gluon Plasma (QGP), produced in high-energy heavy-ion collisions. Therefore, witnessing the full evolution of the collision, they are effective probes to study the mechanisms of parton energy loss and hadronisation in the hot and dense medium, giving insight on the QGP evolution and its transport coefficients. The heavy flavour nuclear modification factor ($R_{\rm AA}$) and the elliptic flow ($v_{\rm 2}$) allow investigating the interaction strength of heavy quarks with the constituents of the expanding medium. The comparison of the $R_{\rm AA}$ of charm, beauty and light-flavour hadrons provide information about the colour-charge and parton- mass dependence of parton energy loss. At low $p_{\rm T}$ the $v_{\rm 2}$ is expected to give insights into the degree of thermalisation of heavy quarks in the deconfined medium, and at high $p_{\rm T}$ it carries information on the path-length dependence of in-medium parton energy loss.

In this talk, measurements of $R_{\rm AA}$ and $v_{\rm 2}$ of open heavy-flavour hadrons via semi- leptonic decays to electrons at mid-rapidity and muons at forward rapidity in Pb--Pb collisions at LHC energies will be discussed. The progresses on the analysis of the production and anisotropy of electrons from beauty-hadron decays will be also discussed. In addition the $R_{\rm AA}$ of heavy- flavour hadron decay leptons in Xe--Xe collisions will be presented, along with the prospects for measuring the total charm cross section in this collision system. Comparisons with model calculations including the interaction of heavy quarks with the hot, dense, and deconfined medium will be also shown.

Speaker: Zuman Zhang (Central China Normal University CCNU (CN))

ALICE_open_HF_lepton_HP2018_zuman.pdf

15:20 Studies of Heavy-Flavor Jets Using $D^0$-Hadron Correlations in Azimuth and Pseudorapidity in Au+Au Collisions at 200 GeV at the STAR Experiment

Heavy flavor (HF) quarks (charm, bottom) are important probes of the medium produced in relativistic heavy-ion collisions because they are formed in the early stage and propagate throughout the lifetime of the system. HF-meson spectra and azimuthal anisotropy ($v_{2}$) measurements have been reported by experiments at RHIC and the LHC, and they suggest strong interactions of HF quarks with the medium. $D^{0}$-meson + charged hadron correlations on relative pseudorapidity and azimuth ($\Delta\eta$, $\Delta\phi$) provide a method for disentangling correlation structures on ($\Delta\eta$, $\Delta\phi$) – allowing for separation of structures related to jets and bulk phenomena directly, with the $D^{0}$-meson serving as a proxy for a heavy-flavored jet. The $D^{0}$-meson is reconstructed via its hadronic decay channel using the Heavy Flavor Tracker (HFT) in STAR, which allows for rejection of background via identification of secondary decay vertices.

In this talk, we present two-dimensional $D^{0}$-hadron correlations as a function of centrality in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. These data reveal a jet-like, peaked structure at ($\Delta\eta$, $\Delta\phi$) = (0, 0) (near-side), and a $\Delta\eta$-independent azimuthal harmonic modulation similar to that reported for light-flavor dihadron correlations. Here, we focus on the evolution of the near-side peak's yield and widths on ($\Delta\eta$, $\Delta\phi$) as a function of centrality. Finally, the evolution of all of the jet-peak parameters will also be compared to various light-flavor hadron correlations.

Speaker: Alexander Jentsch (UT Austin, STAR)

Hard_Probes_2018_Alex_Jentsch_v5.pdf

15:40 ALICE results on quarkonium production in p-Pb collisions

The study of quarkonium in proton-nucleus collisions is an important tool to investigate how cold nuclear matter effects can influence the J/ψ, the ψ(2S) or the Υ production. Mechanisms as the modification of the parton distribution functions in nuclei, the presence of a color glass condensate or the coherent energy loss of the QQ ̄ pair in the medium have been shown to be the relevant processes to describe the production of the most tightly bound states, as the J/ψ and the Υ(1S). On the contrary, final state mechanisms, possibly related to the presence of a dense medium, are required to explain the stronger suppression observed for the loosely bound ψ(2S) state.

ALICE has measured quarkonium production in p-Pb collisions over a wide kinematic range, covering the backward (−4.46 < $y_{\rm cms}$ < −2.96), the mid (−1.37 < $y_{\rm cms}$ < 0.43) and the forward (2.03 < $y_{\rm cms}$ < 3.53) rapidity regions, down to zero transverse momentum.

Measurements of the nuclear modification factor ($R_{\rm pA}$) obtained at mid- rapidity in p-Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV and $\sqrt{s_{\rm NN}}$=8.16 TeV, for prompt and non-prompt J/ψ, will be presented. Results on the transverse momentum, rapidity and centrality dependence of the J/ψ and Υ(1S) $R_{\rm pA}$, measured at forward and backward rapidities at $\sqrt{s_{\rm NN}}$=8.16 TeV, will also be shown. Finally, the production of the ψ(2S) and Υ(2S) resonances in p-Pb collisions at √sNN=8.16 TeV, at forward and backward rapidities, will be discussed in comparison to the production of the most tightly charmonium and bottomonium states. All the results will also be compared to those obtained at lower energies and with the available theoretical calculations.

Speaker: Dr Antoine Lardeux (University of Oslo (NO))

HP2018_Lardeux.pdf

14:00 → 16:00 Parallel 4: Novel theoretical developments

Convener: Francois Arleo (Centre National de la Recherche Scientifique (FR))

14:00 Higgs boson suppression in quark-gluon matter

The final-state interactions of a Higgs boson with a dense quark-gluon medium are studied. The dominant Higgs-parton scattering processes, $g\,H\to\,g\,g,Q\,\bar{Q}$, have a total cross section $\sigma_{Hg}\approx $1-10$ \mu b$, in the kinematical range of relevance at current and future hadron colliders. Such medium-induced enhancement of the Higgs decays into a pair of jets leads to an effective depletion of its final visible yields in the standard $H\to \gamma\gamma,ZZ^*(4\ell)$ discovery channels, compared to the accurately known theoretical predictions for its production and decay in the absence of final-state interactions. By embedding a Higgs boson, with transverse momenta spectra computed at NNLO+NNLL accuracy, in a quark-gluon medium described by a 2D+1 viscous hydrodynamics expansion with the lattice-QCD equation of state, we present realistic estimates of the expected Higgs boson suppressed yields as a function of its transverse momentum $p_{\rm T}$, and the total particle
density in pp, pPb, and PbPb collisions at LHC and FCC energies. Yields suppressions of up to 15\%
are expected, mostly in the region $p_{\rm T}$ below $50$ GeV.

Speaker: Constantinos Loizides (ORNL)

higgs-loizides-v3a.pdf

14:20 Bayesian extraction of $\hat{q}$ with a multi-stage jet evolution approach

The jet energy loss coefficient $\hat{q}$ serves as one of the most important transport coefficients that characterize the modification of hard jets in a QGP. A quantitative extraction of $\hat{q}$ requires sophisticated jet quenching theory, precise and extensive experimental data, as well as an advanced statistical framework that compares theory to data. Within the JETSCAPE collaboration, we have developed a multi-stage approach of jet evolution, where the medium-modified parton showers at high virtuality scale are described using the DGLAP evolution and simulated with the MATTER event generator, while the in-medium elastic and inelastic scatterings of partons at low virtuality scale are described using a transport theory implemented with the LBT event generator.

The transition from the DGLAP phase to the transport phase of jet modification, and its dependence on properties of the jet and the local medium, has so far only been approximately estimated. The goal of this work is to use the advanced statistical framework to determine this transition. To this end, a simplified version of the state-of-the-art JETSCAPE generator is embedded into a Bayesian analysis framework for a simultaneous calibration on jet quenching data at various centrality bins in 200~GeV Au-Au, 2.76~TeV Pb-Pb and 5.02~TeV Pb-Pb collisions, from which $\hat{q}$ is systematically extracted as functions of both jet energy and medium temperature. The dependence of $\hat{q}$ on the medium induced virtuality scale is quantitatively explored for the first time. This virtuality, in combination with the local temperature and parton energy, serves as a crucial separation scale between the DGLAP dominating region and the transport dominating region for jet quenching and is found to vary from 1.25~GeV at RHIC to 2.5~GeV at the LHC.

Speaker: Ron Soltz (Lawrence Livermore Nat. Laboratory (US))

bayesian_qhat_hp18_jetscape_soltz.key

bayesian_qhat_hp18_jetscape_soltz.pdf

14:40 Hybrid Hadronization

We have developed a hybrid hadronization model with two well-defined limits: string fragmentation for dilute systems, and quark recombination for dense sytems of partons. The model smoothly interpolates in between those limits by evaluating the probability of quark wave packets to coalesce with each other into hadronic wave functions as a deciding factor. We will discuss a new implementation in terms of a Monte Carlo hadronization code that can work with existing Monte Carlo event generators like PYTHIA and the JETSCAPE framework. Its main application is the hadronization of jets in a variety of different systems, with and without a thermal medium present. We will demonstrate the capabilities of our code by, e.g., studying the hadronization of full p+p events, including the underlying event, and by looking at jet and high momentum hadrons in A+A collisions. We will discuss how known medium effects, like an increased baryon/meson ratio, depend on the system size.

Speaker: Rainer Fries (Texas A&M University)

HP2018_HH_Fries_noanim.pdf

HP2018_HH_Fries.pdf

HP2018_HH_Fries.pptx

15:00 Heavy flavor energy loss from AdS/CFT: A novel diffusion coefficient derivation and its predictions

We derive a diffusion coefficient of quarks propagating through a strongly coupled plasma from the AdS/CFT correspondence. This diffusion coefficient interpolates between light and heavy flavour. In contradistinction to past results, this diffusion coefficient does not increase with quark velocity. Taking our diffusion coefficient derivation as fundamental, we use the fluctuation-dissipation theorem to predict a strong-coupling heavy quark drag that is slightly different from the original calculations of Gubser and Herzog et al.

From this result, we compute the suppression, angular, and rapidity distribution of single open heavy flavour and the momentum, angular, and rapidity correlations for pairs of open heavy flavour at RHIC and LHC from an AdS/CFT-based energy loss model. We quantitatively compare these strongly-coupled QGP predictions to the weakly-coupled QGP predictions of Nahrgang et al., PRC90 (2014) [arXiv:1305.3823].

We find that the strongly coupled correlations of high transverse momentum pairs (>4GeV) are broadened less efficiently than the corresponding weak coupling based correlations, while low transverse momentum pairs (1−4GeV) are broadened with similar efficiency, but with an order of magnitude more particles ending up in this momentum class. The strong coupling momentum correlations we compute account for initial correlations and reveal that the particle pairs suppressed from initially high momenta to the low momentum domain do not suffice to explain the stark difference to the weak coupling results in momentum correlations for 1−4GeV. From this, we conclude that heavy quark pairs are more likely to stay correlated in momentum when propagating through a strongly coupled plasma than a weakly coupled one.

We also compare our predictions with experimental data from ALICE and CMS. We find good agreement with both B and D suppression data, unlike earlier AdS/CFT based calculations.

Speaker: Robert Hambrock (University of Cape Town)

Heavy_flavor_energy_loss_from_AdSCFT-_A_novel_diffusion_coefficient_derivation_and_its_predictions.pdf

Heavy_flavor_energy_loss_from_AdSCFT-_A_novel_diffusion_coefficient_derivation_and_its_predictions.pptx

15:20 Bottomonia Suppression from AdS/CFT

We compute for the first time the suppression of bottomonia in a strongly coupled QGP and compare the results to those from a weakly coupled QGP and to data. Using imaginary time techniques we numerically determine the real and imaginary parts of the ground state binding energy of the $\Upsilon$(1S) in a $p_T$-independent potential computed from AdS/CFT and another computed from pQCD. We implement the complex binding energies in a suppression model to determine the $\Upsilon$(1S) nuclear modification factor in $\sqrt{s}_{NN}=2.76$ TeV Pb+Pb collisions. This simplest strong-coupling, $p_T$-independent potential leads to a significant oversuppression of $\Upsilon$(1S) compared to data while the results from the pQCD-derived potential are consistent with data. We will then investigate the validity of using complex heavy quark potentials from AdS/CFT for all quark separation $r$ by comparing our imaginary time binding energies to those binding energies derived independently from AdS/CFT. Time permitting, we will compare independent calculations of the bottomonia spectral functions from AdS/CFT and then present results based on a more sophisticated $p_T$-dependent quarkonia potential also derived from AdS/CFT.

Speaker: William Alexander Horowitz (University of Cape Town (ZA))

181002(HP)F.pptx

15:40 Introducing HIJING++: the Heavy Ion Monte Carlo Generator for the High-Luminosity LHC Era

Beyond 2020 we will enter the High-Luminosity era of the LHC, right after the upgrades of the second Long Shutdown of the Large Hadron Collider (LHC). The ongoing state-of-the-art experimental instrument upgrades require high-performance simulation framework in the background, which is modern, robust, and comes with long term support.

Here we present the HIJING++ (Heavy Ion Jet INteraction Generator, C++ version), which is the successor of the original FORTRAN version, has been used intensively since almost three decades by the heavy-ion community.

HIJING++ contains all the (improved and fine tuned) physics that it's predecessor have. Embedding specific Pythia8 features we adopted also some effects such as color reconnection (CR) and multiparton interaction (MPI), that proved to be crucial recently in the understanding of the experimental results. Moreover, the nuclear medium effect modules like jet quenching and induced gluon radiation were implemented. Development of the HIJING++ framework is based on solid C++ foundations, which became a well established standard among the high energy physics community. Thanks to the modular nature of the language and utilizing it's best features the HIJING++ is highly customizable and very flexible: it can be extended with new features and with new physics in a convenient, easily maintainable way. During the development we keep in mind the requirements of the high-energy heavy-ion community: it has a native thread based parallelism, an easy-to-use analysis interface and a modular plugin system, which makes room for possible future improvements like GPU acceleration.

In this talk, I will present the structure of the HIJING++ and its application indeed. The key aspects of the improved physics is presented, with emphasis on the new jet quenching module based on the Gyulassy-Lévai-Vitev formalism will be reviewed. I will also show qualitative and performance studies in comparisons to other heavy-ion Monte Carlo code developments and to recent experimental results.

Speaker: Mr Gábor Bíró (Wigner RCP of the HAS)

GaborBiro_HardProbes18.pdf

16:00 → 16:25 coffee break

16:25 → 18:25 Parallel 1: Jets & high-pT: Jet-medium interactions

Convener: Abhijit Majumder (Wayne state university)

16:25 Isolated photon + hadron and jet correlation in p-Pb and pp collisions with ALICE

The photon-tagged correlation of jets and jet fragments is a promising channel for the study of parton energy loss in nuclear collisions. While existing measurements in pp and nuclear collisions have used high energy photons and jets, we focus on an unexplored kinematic range given by $10 < p_{\rm T} < 20$ GeV/$c$ photons and the corresponding low jet $p_{\rm T}$. We present results obtained using 5.02 TeV pp and p-Pb collisions. A combination of isolation and electromagnetic shower-shape variables is used to reduce the large background from meson decays and fragmentation photons. We show how the access to this kinematic range of hard probes was achieved with a novel combination of high rate and low-momentum tracking using the electromagnetic calorimeters and the Inner Tracking System of the ALICE experiment.

Speaker: Miguel Ignacio Arratia Munoz (Lawrence Berkeley National Lab. (US))

ARRATIA.pdf

16:45 Jet energy loss in a flowing plasma

We present new results on the energy loss of light partons traversing a highly dynamical strongly coupled quark-gluon plasma. As QGP has large gradients in both temperature and the fluid velocity, it is crucial to study energy loss without assuming a homogeneous plasma, especially as it is known that energy loss depends on the virtuality of the partons, which evolves in the plasma. In a holographic description, we consider several subsequent improvements of the hydrodynamic background by keeping increasing orders in the gradient expansion. Varying temperature and velocity profiles in ideal hydrodynamics considerably modifies the energy loss, with further corrections from viscous effects. We then perform a numerical analysis of jet energy loss of an ensemble of jets in a boost-invariant and transversely-expanding droplet of QGP, for both small and large systems. Including flow increases the nuclear modification factor and reduces the narrowing of the jet shapes. We end with the modifications of the jet width and mass distributions, both for a flowing and non-flowing medium.

Speaker: Wilke van der Schee (MIT)

Talk_Hard_Probes2018.pdf

Talk_Hard_Probes2018.pptx

17:05 Jet quenching parameter in an expanding QCD plasma

We study the phenomenon of transverse momentum broadening for a high-$p_T$ parton propagating through a weakly-coupled quark-gluon plasma undergoing boost-invariant longitudinal expansion. We propose a boost-invariant description for this phenomenon, in which the broadening refers to the angular variables $\eta$ (the pseudo-rapidity) and $\phi$ (the azimuthal angle) --- the same variables as generally used to describe particle distributions in the experiments. The jet quenching parameter $\hat{q}$, which is the only property of the medium to enter this description, depends upon the proper time alone: it decreases with time due the dilution of the medium via expansion.

We furthermore consider radiative corrections to $\hat q$. As in the case of a static medium, we find potentially large corrections enhanced by a double logarithm. But unlike for the static medium, these corrections are now local in time: they depend upon the local (proper) time characterizing the expansion, and not upon the overall path length. We identify and resum such corrections to all orders into a renormalized jet quenching parameter. The main effect of this resummation is to slow down the decrease of $\hat q$ with increasing time. We argue that the same (renormalized) value for $\hat q$ should also enter the calculation of medium-induced radiation in the expanding medium.

Speaker: Bin Wu (CERN)

hp2018.pdf

17:25 Dynamical quenching weights in an expanding medium

In this work, we extend the re-summation of multiple medium-induced emissions to apply to dynamically expanding media. This is done by recasting the quenching weight as the solution of a rate equation with medium-induced partonic splitting functions that are sensitive to the expansion. We perform the calculations in the framework of Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPSZ) formalism for multiple soft scatterings with a time-dependent transport coefficient. Furthermore, we discuss the validity of a dynamical scaling law that relates the spectrum in an expanding medium to the equivalent static case with re-scaled medium parameters [1] and test the size of energy loss fluctuations in a realistic medium.

References:
[1] Carlos A. Salgado and Urs Achim Wiedemann "A Dynamical Scaling Law for Jet Tomography", Phys Rev Lett. 89.092303.

Speaker: Souvik Priyam Adhya (Institute of Particle and Nuclear Physics Faculty of Mathematics and Physics, Charles University)

hardprobes18_spa_v1.pdf

17:45 Data-driven constraints on the drag and diffusion of light partons

The momentum exchanged by hard partons with the quark-gluon plasma ranges from frequent soft interactions to rare large angle scatterings. The larger number of soft interactions makes possible an effective stochastic description of parton-plasma interactions in terms of a small number of transport coefficients, drag and diffusion~[1]. In this work, we study jet energy loss in heavy ion collisions using a combined stochastic and large angle scattering parton energy loss model, building on the systematic division between soft and hard parton-plasma interactions derived for a high temperature plasma in Ref. [2]. We use a Bayesian analysis to perform a first systematic extraction of the transport properties of light partons propagating in a quark-gluon plasma, constraining the temperature dependence of the drag and diffusion coefficients by comparison with data. We further study the dependence of this result on the separation of scale between soft and hard parton-plasma interactions.

[1] Moore and Teaney, Phys.Rev.C71:064904 (2005)
[2] Ghiglieri, Moore and Teaney, JHEP1603:095 (2016)

Speaker: Tianyu Dai (Duke University)

Hard_Scatteringd_and_Langevin_Reformulation_of_Parton_Energy_Loss.pdf

16:25 → 18:25 Parallel 2: Jets & high-pT: Jet-medium interactions

Convener: Henner Buesching (Johann-Wolfgang-Goethe Univ. (DE))

16:25 Hard probe radiative energy loss beyond soft-gluon approximation

One of the widely used kinematic assumptions in calculating hard probe radiative energy loss within QGP, is the soft-gluon approximation, which considers that energy loss of the parent parton via gluon's bremsstrahlung is small compared to its initial energy. However, diverse theoretical formalisams obtained a notable energy loss of high $p_{\perp}$ particles, implicitly suggesting inadequacy of this approximation.

To address this issue, we relaxed the soft-gluon approximation within DGLV formalism [1]. The obtained analytic expressions are quite more involved compared to the soft-gluon case. The effects of this relaxation on numerical predictions for the first order in opacity fractional radiative energy loss and number of radiated gluons are small, although of opposite signs. Moreover, a joint effect for these two variables results in an insignificant suppression alternation. Consequently, the results presented here provide confidence that, regardless of the concerns mentioned above, surprisingly, the soft-gluon approximation can reliably be applied within DGLV formalism. Finally, we discuss generalizing the relaxation in the dynamical medium as well, which ensures broader relevance of the conclusions obtained here.

[1] B. Blagojevic, M. Djordjevic and M. Djordjevic, arXiv:nucl-th/1804.07593 (2018).

Speaker: Bojana Blagojevic (Institute of Physics Belgrade)

HP18BojanaB.pdf

16:45 Event-plane dependent away-side jet-like correlation shape in 200 GeV Au+Au collisions from STAR

A strongly coupled quark-gluon plasma (QGP) is created in relativistic heavy-ion collisions. Jets are a good probe of the energy loss properties of the QGP experienced by hard partons traversing the medium. Experimentally we take high transverse momentum ($p_{T}$) trigger particles as proxies of jets. The jet-like yield of high-$p_{T}$ particles found in correlations with respect to highly energetic back-to-back partons is significantly suppressed. In-medium path length that the recoil (away-side) parton traverses is expected to depend on its emission azimuthal angle with respect to the event plane (EP) in non-central Au+Au collisions. The purpose of this analysis is to investigate the EP dependence of jet energy loss.\

Measurements of jet-like correlations are complicated by the large underlying anisotropic flow background. In this analysis, we require a large recoil ($P_{x}$) within a specific pseudo-rapidity ($\eta$) range from a high-$p_{T}$ trigger particle to enhance in-acceptance population of away-side jet-like correlations. We subtract the flow background by taking the difference of the two-particle correlation in the close-region and far-region with respect to the $\eta$ region of $P_{x}$. The close and far-region are symmetric about mid-rapidity. We analyze the correlation shape as a function of the trigger particle azimuthal angle relative to the EP reconstructed from the beam-beam counters (BBCs) which are displaced by several units in $\eta$ from the mid-rapidity region. The large $\eta$ gap can effectively eliminate the correlation between trigger particles and EP.
We correct for the relatively large resolution effect from the BBC EP determination via an unfolding procedure.
EP dependent away-side jet-like correlation shapes and their implications for jet-medium interactions will be discussed.

Speaker: Li Yi (ShanDong University)

HP2018_YILi_for_LiangZhang.pdf

17:05 Transverse Momentum Balance and Angular Distribution of $b\bar{b}$ Dijets in Pb+Pb collisions

The productions of inclusive b-jet and $b\bar{b}$ dijets in Pb+Pb collisions have been investigated by considering the heavy quark and the light quark in-medium evolution simultaneously. The initial hard processes of inclusive b-jet and $b\bar{b}$ dijets productions are described by a next-to-leading order (NLO) plus parton shower Monte Carlo (MC) event generator SHERPA which can be well matched with the experimental data in p+p collisions. The framework combines the Langevin transport model to describe the evolution of bottom quark also its collisional energy loss and the higher-twist description to consider the radiative energy loss of both bottom and light quarks. We compare the theoretical simulation of inclusive jet and inclusive b-jet $R_{\rm AA}$ in Pb+Pb collisions at $\sqrt{S_{\rm NN}}=2.76~$TeV with the experimental data, and then present the theoretical simulation of the momentum balance of the $b\bar{b}$ dijet in Pb+Pb collisions at $5.02~$TeV with the recent CMS data for the first time. A similar trend as that in dijets has been observed in $b\bar{b}$ dijets, the production-distribution shifted to smaller $x_J$ due to the jet quenching effect. At last, the prediction of the normalized azimuthal angle distribution of the $b\bar{b}$ dijet in Pb+Pb collisions at $5.02~$ TeV has been reported. The medium induced energy loss effect of the $b\bar{b}$ dijets will overall suppress its production, but the near side ($\Delta \phi \to 0$ region) suffers more energy loss than away side ($\Delta \phi \to \pi$ region), therefore lead to the suppression on the near side and the enhancement on the away side in the normalized azimuthal angle distribution.

Speaker: Sa WANG (Central China Normal University)

SW_HP2018.pdf

17:25 The LPM effect in a partonic transport approach

When traversing hot and dense QCD matter energetic partons lose energy by both elastic collisions and medium-induced gluon radiation. Several analytic calculations of radiative jet energy loss demonstrated the importance of the non Abelian Landau-Pomeranchuk-Migdal (LPM) effect, a coherence effect resulting from the finite formation time of emitted gluons. While in these calculations it is possible to calculate gluon emissions including the LPM effect by rigorously resumming diagrams to any order in the opacity, it is still not straightforward to consider such coherence effects in dynamical transport simulations. Therefore we will revisit in this talk the implementation of the LPM effect in the partonic transport approach BAMPS. By using Debye screened leading-order pQCD matrix elements for the elastic and an improved Gunion-Bertsch approximation for the inelastic processes, BAMPS simulates both the jet and medium evolution in ultra-relativistic heavy-ion collisions. We will present a comparison between an effective modeling of the LPM effect via a theta function in the radiative matrix elements and a stochastic Ansatz for the suppression of gluon radiations. Furthermore, we will confront these results with the spectrum calculated via the AMY formalism for gluon emission that was recently implemented into BAMPS. Finally, we will discuss possible consequences of the different scenarios for the resulting experimental observables.
​

Speaker: Mr Florian Senzel (Goethe University Frankfurt)

HP2018_Senzel2.pdf

17:45 Finite formation time effects for in-medium parton splittings

We present a novel setup of an antenna splitting inside the medium taking into account the finite formation time of the dipole, which turns out to be an important scale. In particular, we consider the splitting of a color-singlet dipole, studying in detail its formation and subsequent propagation through the medium. We discuss the role of coherence and the relevant time scales which control this scenario, while also providing theoretical support for vacuum-like emissions early in the medium. We also discuss the generalization to arbitrary dipole color charge. Finally, by mapping the spectrum of in-medium splittings through the corresponding kinematical Lund diagram, we elicit regimes of a close correspondence to a semi-classical description and regimes where this description breaks down.

Speaker: Víctor Vila (Universidade de Santiago de Compostela)

VilaV_HP18Talk.pdf

16:25 → 18:25 Parallel 3: Jets & high-pT: High-pT correlations

Convener: John William Harris (Yale University (US))

16:25 PHENIX results on nuclear modification of $\pi^0$-hadron correlations in small and large collision systems

Two-particle correlations using high-pT particles allow jet-like measurements at low Q2 where jet reconstruction is challenging. Therefore, such correlations provide critical input to our understanding of how partons interact in a colored medium. PHENIX has measured 0-hadron correlations in multiple collisions systems. The away-side widths in p+Au collisions are broader than those in p+p collisions. In Au+Au collisions both the shape and the yield of the away-side distributions are modified compared to those in p+Au and p+p collisions. The implications of these results to our understanding of energy loss in cold and hot nuclear matter will be discussed.

Speaker: Megan Elizabeth Connors (Yale University (US))

HP2018_Connors_PHENIXpi0hadronsv2.pdf

16:45 Parton energy loss effect on Z+jet production in high-energy nuclear collisions

Gauge boson tagged jet production has long be regarded as a "golden channel" to study the jet quenching effect. Recently, the back-to-back azimuthal alignment $\Delta\phi_{jZ}$, $p_T$ asymmetry $x_{jZ}$ distribution and its mean value, as well as the average number of jet partners per Z boson $R_{jZ}$ of Z associated jet production have been reported in both pp and Pb-Pb collisions by CMS.
In the talk, we report the theoretical calculations of Z/W+jet production in pp and Pb-Pb at the LHC, which are confronted with all available data and very nice agreements between theory and data are observed for all four observables of Z+jet in both pp and Pb-Pb collisions. In the model, a very good description of Z+jet in pp is achieved by utilizing Sherpa, which combines the NLO with resummation by a matched parton shower (PS). To compute observables of Z+jet in Pb-Pb we consider the parton energy loss in hot/dense QCD medium, which is simulated by Linear Boltzmann Transport(LBT) model.
Our calculations of distributions of $R_{jZ}$ can give excellent descriptions of CMS measurements both in pp and Pb-Pb. Compared to pp collisions, $R_{jZ}$ is suppressed because large fraction of jet loss energy and fall below transverse momentum threshold in Pb-Pb collisions. We evaluate the transverse momentum imbalance $x_{jZ}$ simultaneously which is broadened and shifted to lower value due to jet jet quenching, and we find a very good agreement with LHC data for both $x_{jZ}$ distribution and its mean value as well as the relative shift between pp and Pb+Pb collisions.
The distribution of $\Delta\phi_{jZ}$ at large azimuthal angle in Pb-Pb is moderately suppressed relative to that in pp, and it is the suppression of multi-jets lead to the modification of Z+jet azimuthal angle correlations.
To investigate in more detail where the energy is gone and how the energy is transported inside and outside the jet-cone, we also calculate the modification of the substructure and energy flow of Z boson tagged jet. The energy flow within the jet is modified by shifting the momentum away from the jet axis out to large relative angular distances.

Speaker: Mr Shan-Liang Zhang (Central China Normal University)

zjet_zsl _hp2018.pdf

17:05 Scaling properties of high $p_{\rm T}$ light hadrons from small to large systems by PHENIX

PHENIX has performed measurements of light hadrons (0, , and other hadrons with masses up to ~1GeV) in a broad set of projectile-target combinations including p+Au, d+Au, 3He+Au, Cu+Cu, Cu+Au, Au+Au, and U+U. This rich collection of data sets allows for detailed studies of parton energy loss in large systems, exploring various scaling behaviors from RHIC to LHC. We also explore the evolution of the hadron spectra with system size. In particular, the multiplicity dependence of hadron production in small systems is examined for signs of energy loss at high pT, and collective effects at low pT.

Speaker: Yuri Mitrankov (Peter the Great St. Petersburg University, St. Petersburg, 195251 Russia)

mitrankov_HP18_final.pdf

17:25 Hadron+jet measurements in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with ALICE

The measurement of jets recoiling from a trigger hadron in heavy-ion collisions can be used to understand the properties of the Quark Gluon Plasma. Jet-medium interactions cause jets to lose energy in the medium, and may modify the jet structure. Jet deflection towards large angles may also occur - multiple soft scatterings in the medium broadens the overall azimuthal correlation, while possible Moliere scatterings off quarks and gluons is expected to broaden the tail of the correlation. This can be studied through a measurement of the hadron-jet acoplanarity. While a measurement from Run 1 showed no evidence of large-angle jet broadening with respect to vacuum expectation within experimental uncertainties, recent theoretical work has highlighted that low hadron trigger $p_{\rm T}$ and low recoiling jet $p_{\rm T}$ configurations show more sensitivity to in-medium modifications to the acoplanarity.

To explore these phenomena, we present a new measurement of the semi-inclusive distribution of charged jets recoiling from a trigger hadron in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. This technique experimentally allows to suppress the combinatorial background and thus explore jet substructure and acoplanarity down to low $p_{\rm T}$ and large $R$. A study of jet quenching through the $p_{\rm T}$-differential comparison of central and peripheral collisions, jet acoplanarity through the angular distribution of the hadron and recoil jet, and jet energy redistribution through comparison of different jet cone radii will be shown.

Speaker: Jaime Norman (Centre National de la Recherche Scientifique (FR))

Talk_v8.pdf

17:45 Using di-hadron correlations to investigate jet modifications in Pb-Pb collisions with ALICE

The comparison of jets measured in heavy-ion collisions with jets measured in pp collisions is a rich source of information on jet-medium interactions. These medium-induced modifications can be prominent at low $p_{\rm T}$, where traditional jet reconstruction tools are difficult to use. Measurements of di-hadron correlations provide an alternative means to study jets in this $p_{\rm T}$ regime. With this technique, one calculates the pseudorapidity ($\Delta \eta$) and azimuthal angle ($\Delta \varphi$) differences between trigger and associated particles, and observes the manifestation of the jet fragmentation as a peak around $(\Delta \eta, \Delta \varphi) = (0, 0)$. The modification factor $I_{\rm AA}$ is the yield of the jet-like peak in Pb-Pb divided by the corresponding yield from pp collisions at the same energy. In this talk, we will present the latest ALICE measurements of $I_{\rm AA}$ with charged hadrons from collisions with a center of mass energy per nucleon-nucleon pair of 2.76 TeV. We observe that the $\Delta \eta$-dependent $I_{\rm AA}$ shows a narrowing in pseudorapidity in central collisions for trigger particles with a high $p_{\rm T}$. We also investigate the path-length dependence of jet modification by measuring $I_{\rm AA}$ as a function of the relative angle between the trigger particle and the event plane. These measurements will be compared to various model calculations, and are expected to place strong constraints on energy loss models.

Speaker: Hyeonjoong Kim (Yonsei University (KR))

20181002_HJKIM_HardProbes_.pdf

16:25 → 18:25 Parallel 4: HF & QQbar: Heavy flavour flow

Convener: Silvia Masciocchi (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

16:25 Charmonia production in large and small systems measured by ATLAS

The experimentally observed dissociation and regeneration of bound charmonium states in heavy-ion collisions provide a powerful tool to probe the dynamics of the hot, dense plasma. These measurements are sensitive to the effects of color screening, color recombination, and possibly to other, new phenomena affecting dynamics of heavy quarks in the QCD medium. In the large-statistics Run 2 lead-lead and proton-lead collision data, these phenomena can be probed with unprecedented precision. Measurements of the ground and excited charmonium states, as well as their separation into prompt and non-prompt components, provide further opportunities to study the dynamics of heavy quark energy loss. In addition, charmonium production rates, and excited to ground states ratios, in small systems are a valuable probe of cold nuclear matter effects. In this talk, the latest ATLAS results on charmonia production will be presented, including new, differential measurements of charmonium suppression and azimuthal anisotropy in lead-lead and proton-lead collisions.

Speaker: Sebastian Tapia Araya (Federico Santa Maria Technical University (CL))

Stapia_HP2018_v5.pdf

16:45 Quarkonium production as a function of charged particles multiplicity in pp and p-Pb collisions measured by ALICE at the LHC

In pp collisions such as the ones delivered by the LHC since 2010, the production of heavy quarks proceeds dominantly from the hard scattering of two gluons. These quarks then hadronize in either open heavy-flavor hadrons or quarkonia (e.g. $J/\psi$, $\Upsilon$). The study of quarkonium production as a function of the charged particle multiplicity therefore naturally links soft and hard processes that occur in the collision and allows one to study their interplay. While a linear increase of quarkonium production as a function of the charged particle multiplicity can be reasonably well understood in the context of multi-parton interactions, the observation of deviations with respect to a linear increase requires a more detailed description of the collision and the inclusion of additional mechanisms such as collective effects, color reconnection or percolation. In addition, further insight on the interplay between soft and hard processes can also be gained experimentally by measuring correlations between the quarkonium and the other hadrons produced in the collision as a function of both pseudo-rapidity and azimuthal angle.

The ALICE detector at the LHC measures quarkonium production down to zero transverse momentum in two rapidity ranges: at mid-rapidity in the di-electron decay channel using the central barrel and at forward rapidity in the di-muon decay channel using the muon spectrometer. In this presentation we will discuss the production of quarkonia as a function of the charged particle multiplicity as measured by ALICE in pp collisions for center-of-mass energies ranging from 2.76 TeV to 13 TeV in these two rapidity ranges and for several intervals of the quarkonium transverse momentum. Similar measurements in p-Pb collisions at center of mass energies per nucleon-nucleon collision of 5.02 and 8.16 TeV will also be presented. These results will be compared to corresponding measurements performed for D mesons, as well as to model calculations. Finally, $J/\psi$-hadron correlations at mid-rapidity in pp collisions at $\sqrt{s} = 13$ TeV will also be discussed.

Speaker: Dhananjaya Thakur (Indian Institute of Technology Indore (IN))

dthakur_HardProbes_v5.pdf

17:05 Open-heavy-flavour production and elliptic flow in p--Pb collisions at the LHC with ALICE

Measurements of open heavy flavour production in p--A collisions offer a way to study so called Cold Nuclear Matter (CNM) effects, which include gluon shadowing, gluon saturation, $k_{\rm T}$ broadening, and energy loss in CNM, that can affect the production rate and the kinematic properties of charm and beauty hadrons in nuclear collisions.
In addition, in recent years, in pp and p--Pb collisions one has abserved effects like long-range $v_{2}$-like angular correlations in high-multiplicity events or the enhancement of baryon production, typically observed in nucleus-nucleus collisions and ascribed to the collective expansion of the Quark-Gluon-Plasma (QGP). Therefore, one of the possible explanations advanced was the formation of a small-size QGP. The p--Pb measurement of angular correlations of heavy-flavour particles with charged particles, at central and forward rapidities, provide an important extension of studies performed with light-flavoured hadrons to further characterise the $v_{\rm 2}$-like behavior, understand its initial- or final-state nature, constrain its development in the partonic and/or hadronic stages, and better investigate its mass dependence. The study of open heavy flavour production in high-multiplicity p--Pb collisions can also add important information to address the expected connection between collective-like and energy-loss-like effects. Moreover, production studies as a function of the multiplicity of charged particles produced in the collision can give insight into multiple parton interactions and into the interplay between hard and soft processes
In this talk, we will present production measurements of D$^{0}$, D$^{+}$, D$^{*+}$ and D$_{s}^{+}$ mesons reconstructed with the ALICE detector via their hadronic decays at mid-rapidity in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, collected during the LHC Run-2. In particular, we will report cross sections down to $p_{\rm T}$ = 0, nuclear-modification factors as well as multiplicity- and centrality-dependent studies.
The measurement of the production of heavy-flavour hadron decay electrons at central rapidity at $\sqrt{s_{\rm NN}}$ = 5.02 and 8.16 TeV will be also presented. In this regard, we will discuss the measurement of $v_{\rm 2}$ of heavy-flavour hadron decay electrons in high-multiplicity p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 and the measurement of $v_{\rm 2}$ of muons from heavy- flavour hadron decays at forward rapidity in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV. The data will be compared to light flavour ones and theoretical calculations. The production results will be compared with predictions from theoretical models including CNM effects, as well as assuming the formation of a QGP in p--Pb collisions.

Speaker: Cristina Terrevoli (Universita e INFN, Padova (IT))

HardProbesTerrevoli2018_HFpPb.pdf

17:25 Heavy flavor production and flow in large and small systems with ATLAS

Heavy flavor production and collectivity in A+A and $p$+A collisions provide insight into the energy loss mechanism and transport properties of heavy quarks in QGP regions of varying sizes and temperatures. This talk presents a comprehensive overview of the latest ATLAS heavy flavor results in different collision systems. Final results on the production of muons from heavy flavor decays in 2.76 TeV Pb+Pb and $pp$ collisions are presented. These feature a strongly suppressed nuclear modification factor and large azimuthal anisotropies that are compared to a variety of theoretical predictions. Then, to better understand the details of heavy quark-QGP interactions and the nature of small collision systems, these measurements are extended to 8.16 TeV $p$+Pb collisions. No significant modification of the heavy flavor production, as measured with prompt D0 and D* mesons, is observed. However, a significant azimuthal anisotropy of heavy flavor particles in $p$+Pb is observed via two-particle correlations for a broad set of heavy flavor signatures, including heavy flavor decay muons, prompt D mesons and J/Psi particles. These new results are compared to the azimuthal anisotropies for similar particles in Pb+Pb collisions, to that for light-flavor particles in p+Pb collisions, and to state-of-the-art theoretical predictions. Together, the lack of a large suppression in the presence of a strong azimuthal anisotropy may help provide strong constraints to models attempting to describe possible QGP formation and dynamics in small systems.

Speaker: Mirta Dumancic (Weizmann Institute of Science (IL))

hp2018_dumancic_final_ver2.pdf

17:45 PHENIX measurements of heavy quark anisotropic flow in Au+Au and d+Au collisions at $\sqrt{s_{\rm NN}}$=200 GeV

The measurement of heavy quark collective motion in heavy ion collisions is a powerful tool to reveal key QGP properties, such as shear viscosity and quark diffusion. PHENIX has new measurements of the second Fourier harmonic, v2, of separated charm and bottom quarks at mid-rapidity |y|<0.35 in Au+Au collisions at sqrt(sNN)=200 GeV. The technique utilized in these measurements makes use of the distance of the closest approach of electrons from the semi-leptonic decays of charm and bottom mesons. In addition, measurements of v2 of inclusive heavy flavor particles was performed in the small d+Au system using muon decays at forward and backward rapidities covering the range 1.2<|y|<2.2. The experimental study in small systems will help the understanding of the origins of particle collectivity in hadronic collisions. This presentation will show these results and discuss them in the view of the current theoretical calculations.

Speaker: Kazuya Nagashima (Hiroshima University)

PHENIX_HFv2_HP18.pdf

18:25 → 20:00 Poster Session: Poster session (wine & cheese)

Poster_list_Hard_Probes_2018.pdf

Wednesday, 3 October

09:00 → 10:20 Parallel 1: Initial State: Nuclear PDFs

Convener: Carlos Albert Salgado Lopez (Universidade de Santiago de Compostela (ES))

09:00 Towards a Neural Network determination of nuclear parton distribution functions

The QCD factorization theorems allows one to write any hard-scattering cross section involving hadrons in the initial state as a convolution between perturbative matrix elements and non-perturbative parton distributions functions (PDFs), which need to be extracted from experimental data by means of a global QCD analysis. Thanks to recent progress from both experiment and theory, the PDFs of free nucleons are currently well known over a wide kinematic range. However, the situation is rather different for the case of bound nucleons inside heavy nuclei, where a number of effects lead to differences between the nuclear PDFs (nPDFs) and their free nucleon counterparts. Improving our understanding of nPDFs is of crucial importance for the modeling of the initial state in heavy ion collisions. In this talk I will present the progress towards a first determination of nPDFs based on the NNPDF framework, extensively used for the cases of unpolarised and polarised free nucleon PDFs.

Speaker: Mr Rabah Abdul Khalek (PhD Student at Nikhef)

nNNPDF1.0 - HardProbes2018.pdf

09:20 nPDF studies with electroweak bosons in pPb at 8.16 TeV with the CMS experiment

Yields of W and Z bosons can be used to probe the nuclear parton distribution functions of quarks and antiquarks. Final results on W boson and Drell-Yan production in pPb collisions at a nucleon-nucleon center-of-mass energy of 8.16 TeV using the CMS detector will be presented. The muon decay channel is used to study both positive and negative W bosons as a function of muon pseudorapidity. Rapidity and charge asymmetries in the W yield are studied. The Drell-Yan cross section is extracted as functions of the dimuon mass for the first time in pPb collisions, between 15 and 600 GeV, and both as a function of dimuon transverse momentum and rapidity, in the Z boson mass region. Comparisons to theory calculations show that these data are sensitive to the presence of nuclear modifications to the parton distributions in the lead nucleus, and can help improve and constrain theoretical calculations.

Speaker: Hyunchul Kim (Chonnam National University (KR))

20181003_HyunchulKIM_HardProbes2018.pdf

09:40 Measuring electroweak boson production in p--Pb and Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with ALICE at LHC.

W and Z bosons are electroweak probes and, therefore, insensitive to the strong interaction, they are clean observables of the initial state of heavy-ion collisions. Despite their low production rates, their relatively clean signatures in the leptonic decay channels allow their study in heavy-ion collisions at the LHC. W and Z bosons measurements in p--Pb and Pb--Pb collisions provide constraints on the nuclear parton distribution functions (nPDFs) of (anti)quarks. In particular, the W and Z rapidity-differential production cross-sections and the decay lepton charge asymmetry as a function of rapidity provide stringent tests of nPDFs. Electroweak boson measurements in heavy-ion collisions also constitute a reference for medium- induced effects on other probes.

In this contribution, the measurement of W and Z boson production in p-Pb and Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV at forward rapidity (2.5 < $y_{\rm lab}$ < 4.0) with ALICE at the LHC will be presented. These measurements are complementary to those by ATLAS and CMS at central rapidity, and more precise than LHCb ones with similar rapidity coverage. Rapidity differential measurements of W and Z, as well as of the charge asymmetry of W-decay leptons, in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV will be discussed.

Final results on Z production cross-section in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV will be shown together with the first measurement of W production at forward rapidity. Results will be compared with model calculations including nPDFs. In addition, the centrality dependence of W yields in p--Pb collisions and of Z production in Pb-Pb collisions will be discussed as a test of binary scaling.

Speaker: Dr Amal Sarkar (iThemba LABS, National Research Foundation (ZA))

HP_2018_Amal_Sarkar.pdf

10:00 Heavy electroweak boson production in Pb+Pb collisions with ATLAS

Electroweak bosons produced in Pb+Pb collisions are an excellent tool to constrain initial state effects which affect hard scattering process rates in nucleon-nucleon interactions. The production yields of massive electroweak bosons, observed via their leptonic decay channels, offer a high-precision test of the binary collision scaling expected in Pb+Pb and a way to quantify nuclear modifications of the parton distribution functions. The large sample of Pb+Pb data at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV obtained by the ATLAS experiment in 2015, and the corresponding high-statistics $pp$ data at the same collision energy used as a baseline, allow for a detailed experimental study of these phenomena and comparisons to predictions from a variety of theoretical calculations. This talk presents the latest ATLAS results on electroweak boson production, including updated results on Z production and high-precision W boson results in lead-lead collisions.

Speaker: Jakub Kremer (AGH University of Science and Technology (PL))

HP2018-ATLAS-WZ.pdf

09:00 → 10:20 Parallel 2: Jets & high-pT: Jet substructure

Convener: Gunther Roland (Massachusetts Inst. of Technology (US))

09:00 Novel tools and observables for jet physics in heavy-ion collisions

Studies of fully reconstructed QCD jets in heavy-ion collisions aim to extract properties of the created hot and dense nuclear matter. Recently, jet substructure observables have extended the plethora of established observables by introducing techniques that facilitate more differential measurements of jet modifications. This talk will summarize the main lines of discussion at the 5th Heavy Ion Jet Workshop and CERN TH institute "Novel tools and observables for jet physics in heavy-ion collisions" in 2017. We present a first attempt at outlining a strategy for isolating and identifying the relevant physical processes that are responsible for the observed modifications in the kinematical Lund plane. Furthermore, we report on performed studies of jet substructure observables using two of the available Monte Carlo models of jet quenching, QPYTHIA and JEWEL, exploiting grooming techniques to elucidate differences between the models. These investigations also outline the strategy for the study of the parton shower shape dependence of the jet quenching using inclusive jet or photon-tagged jets.

Speaker: Marta Verweij (Vanderbilt University (US))

181002_MVerweij.pdf

09:20 Jet substructure and jet energy loss distributions

Recent measurements of jet substructure provide insights
into how the hard and soft parts of the jets are modified
by jet-medium interaction. One can also learn the flavor
dependence of jet quenching via comparisons between boson
tagged jet and inclusive jet. In this study, with the
Linear Boltzmann Transport (LBT) model, we investigate
the jet shape, jet splitting function and groomed jet
mass in single jet, photon-jet and heavy flavor jet
events. Our study on jet shape function shows that the
medium modifications for the core of quark jets and gluon
jets are quite different and the broadening of jet
profile at the edge of the jet cone is mainly carried by
soft particles from medium recoil. The groomed jet study
shows that jet induced medium response is responsible for
the enhancement of large groomed mass tail.We further
implement different scenarios of coherent and incoherent
energy losses in LBT simulation and find that they have a
significant impact on the pT dependence of the nuclear
modification of jet splitting function.

Speaker: Xin-Nian Wang (Central China Normal University (China)) / Lawrence Berkeley Na)

xnwang-HP2018.pdf

09:40 Jet substructure measurements in pp collisions at $\sqrt{s} = 13 $ TeV with ALICE

We present a variety of jet substructure measurements performed in pp collisions at $\sqrt{s} = 13$ TeV with the focus on grooming, among them $z_g$, $M_g$ and $R_g$, in a wide range of $p_{\rm T}$ between 20 and 200 GeV/$c$ and jet resolution $0.2 < R < 0.5$. Thanks to the capabilities of the ALICE apparatus jet substructure measurement are possible with an infrared constituent cutoff at 0.15 GeV. Furthermore, the angular resolution of the ALICE detectors allows the measurement of jet substructure observables with a high precision. Comparisons between groomed and ungroomed observables are discussed. The measurements are compared to pQCD calculations and MC generators. Furthermore, the measurement of track-based jets at the same centre-of-mass energy and its dependence on the event activity are presented.

Speaker: Markus Fasel (Oak Ridge National Laboratory - (US))

JetSubstructHardProbes18.pdf

10:00 Probing medium-induced jet splitting in heavy-ion collisions

Medium-induced jet splitting function is one of the central quantities in studying the interaction of hard jets with the dense nuclear medium. Soft-drop groomed jet measurements provide a direct tool to probe jet splitting function and its medium modification. Based on higher-twist formalism, we study the nuclear modification of groomed jet splitting in heavy-ion collisions at RHIC and the LHC energies [1]. Assuming coherent energy loss for two splitted subjets, we obtain non-monotonic jet energy dependence for the nuclear modification of jet splitting function: strongest modification at intermediate jet energies. This helps to understand CMS and STAR groomed jet data: strong modification of the momentum sharing zg distribution at the LHC whereas no obvious modification of the zg distribution at RHIC. On the other hand, the assumption of independent energy loss for two subjets cannot explain the observed nuclear modification pattern of the groomed jet zg distribution. We also study the dependence on the angular separation between two subjets and it is found that the nuclear modification of zg distribution decreases with decreasing angular separation. The nuclear modification of groomed jet mass is also investigated. Future groomed jets measurements with lower jet energies at the LHC and larger jet energies at RHIC, for different angular separations between the two subjets will provide more stringent test of our understanding of jet-medium interaction.

Reference:
[1] Ning-Bo Chang, Shanshan Cao, Guang-You Qin, Phys.Lett. B781 (2018) 423-432

Speaker: Guang-You Qin (Central China Normal University)

Qin_HP2018vq.pdf

09:00 → 10:20 Parallel 3: HF & QQbar: Quarkonia

Convener: Carlos Lourenco (CERN)

09:00 Quarkonium measurements in heavy-ion collisions at $\sqrt{s_{\text{NN}}}$ = 200 GeV with the STAR experiment

Measurements of quarkonium production are an important tool to study the properties of the Quark-Gluon Plasma (QGP) formed in relativistic heavy-ion collisions. Quarkonium suppression due to the color-screening effect was proposed as a direct evidence of the QGP formation. However, other effects, such as cold nuclear matter effects and regeneration, add additional complications to the interpretation of the observed suppression. Different bottomonium states with different binding energies are expected to dissociate at different temperatures, and therefore measurement of this ``sequential melting" can help constrain the temperature of the medium. J/$\psi$ polarization provides a valuable insights into the $J/\psi$ production mechanism, which can further deepen our understanding of quarkonium behavior in the QGP.

In this talk, we will present measurements of $\Upsilon$ and J/$\psi$ production in p+p, p+Au and Au+Au collisions at $\sqrt{s_{_\text{NN}}}$ = 200 GeV by the STAR experiment. Compared to previous results in Au+Au collisions, the latest $\Upsilon$ measurements reach higher precision by combining the data sets taken in 2011, 2014 and 2016. The nuclear modification factors for the ground and excited $\Upsilon$ states will be shown as a function of transverse momentum and centrality, and compared to those measured at the LHC as well as to theoretical calculations. Besides, measurements of the nuclear modification factor for $J/\psi$ in p+Au collisions over a broad kinematic range will be shown to quantify the cold nuclear matter effects. Measurements of the J/$\psi$ polarization parameters in p+p and for the first time in p+Au collisions will also be presented. The polarization parameters are extracted in the transverse momentum range of 0-5 GeV/c in both helicity and Collins-Soper reference frames.

Speaker: Ms Zhen Liu (University of Science and Technology of China and Brookhaven National Laboratory)

HP2018_Quarkonium_v7.pdf

09:20 Quarkonium production in Pb-Pb and Xe-Xe collisions with ALICE at the LHC

Quarkonium production is an excellent probe to investigate the properties of the hot and dense medium, which can be created in heavy-ion collisions. The production and hadronisation of heavy quarks are well separated in the space-time evolution of a collision and provide a wealth of information of the underlying QCD dynamics from the dense to the eventually diluting system.

We report on the latest results from ALICE at the LHC on quarkonium production in heavy-ion collisions at mid- and forward rapidity. The nuclear modification factor as a function of centrality, transverse momentum, and rapidity is presented for charmonium and bottomonium states together with the elliptic flow of J/$\psi$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. The status of the J/$\psi$ polarization measurement in Pb-Pb collisions is addressed. In addition, results on the nuclear modification factor for Xe-Xe collisions at $\sqrt{s_{\rm NN}}=5.44 $ TeV are presented. Experimental results are compared with available phenomenological calculations and results obtained at lower collision energies.

Speaker: Markus Kohler (Ruprecht-Karls-Universitaet Heidelberg (DE))

mkk_HP2018.pdf

09:40 Quarkonium production in heavy ion collisions: coupled Boltzmann transport equations

Heavy quarkonia can be used as probes of the quark-gluon plasma (QGP), a hot nuclear environment produced in heavy ion collisions. Quarkonia become unbound or ``melt" at sufficiently high temperature due to the screening of the color attraction between quark and antiquark. In this sense, quarkonia can be thought of as thermometers for the QGP. But extracting the melting temperature from experimental measurements is complicated. One needs to take into account cold nuclear matter (initial state) effects, static and dynamic screening, in-medium dissociation and recombination and feed-down processes in a consistent way. To this end, we develop a set of coupled Boltzmann transport equations of heavy quarks and quarkonia. It includes elastic and inelastic scattering of heavy quarks with the medium, color screening, as well as dissociation and recombination inside the medium. The dissociation and recombination processes are calculated in potential non-relativistic QCD (pNRQCD). The effective field theory pNRQCD is valid when the heavy quark mass is large and the distance between the heavy quark antiquark pair inside the quarkonium bound state is small compared with the thermal scales. We solve the Boltzmann equations by Monte Carlo simulations with given initial conditions and time dependent medium background.

We will discuss how the coupled system of heavy quarks and quarkonium approaches equilibrium in a static QGP box, which indicates that the dissociation and recombination are implemented in a consistent way. We then will present calculations with realistic initial conditions and hydrodynamical background. The initial momenta of heavy quarks and quarkonia are generated from event generators such as PYTHIA with nuclear parton distribution functions while the initial positions are sampled from binary collisions. We will also compare the calculations with experimental measurements of $R_{AA}$ and $v_2$ of quarkonia at both RHIC and LHC. Finally, we will present similar studies of the production of doubly heavy baryons and discuss future plans towards a more complete understanding of quarkonium production mechanisms in heavy ion collisions.

Speaker: Xiaojun Yao (Duke University)

2018hardprobe_XiaojunYao.pdf

10:00 Quarkonia production in pPb collisions with LHCb

We present new results on quarkonia production in proton-lead
collisions, using the data collected in 2016 at 8.16 TeV nucleon-nucleon
center-of-mass energy, in the unique forward region (pseudorapidity
between 2 and 5) covered by the LHCb detector. Both forward and backward
rapidities are covered thanks to the possibility of beam reversal.
Measurements include J/psi and psi', where the prompt and from-b-decay
components can be disentangled, and the 1-- bottomonia states. The large
increase in size of the heavy flavour sample, compared to 5 TeV sample
collected in 2013, allows a remarkable improvement in the accuracy of
the determination of nuclear modification factors.

Speaker: Giulia Manca (Universita` degli studi di Cagliari and INFN, Cagliari, IT)

Manca_HP2018_v4.pdf

09:00 → 10:20 Parallel 4: Electroweak: Photons

Convener: Mr Jean-Philippe Guillet (LAPTh CNRS)

09:00 Probing heavy ion collisions with photons at low and high center-of-mass energies

Photons are penetrating probes of the strongly-coupled nuclear plasma created in relativistic heavy-ion collisions. The entire spacetime evolution of the Quark-Gluon Plasma (QGP) is imprinted in their spectrum and momentum anisotropies, making them effective probes the earlier stage of heavy ion collisions. In this talk we use photons to study the complex initial dynamics of lower energy collisions across RHIC's beam energy scan. We show that photons can distinguish different scenarios of energy deposition in lower energy collisions. We present results for both the momentum and rapidity dependence of photon production.

We combine these study of thermal and prompt photon production in low energy collisions with calculations at LHC energies to clarify the dependence of photon production on the center-of-mass energy of collisions. We discuss our finding in light of the recent scaling in direct photons identified by the PHENIX collaboration [1].

[1] PHENIX Collaboration (2018) arXiv:1805.04084

Speaker: Jean-Francois Paquet (Duke University)

jfpaquet_HP2018.pdf

09:20 PHENIX measurement of direct photons radiation from p+p and p+A collisions

In recent years data from small collision systems at LHC and RHIC have revealed evidence for collective behavior of the produced hadrons, and indications that a hot strongly-coupled system is being formed in these collisions. If so, the matter formed must also radiate thermal or low momentum direct photons. The versatility of RHIC puts PHENIX in an ideal position to search for any indications for thermal photon emission from recent large data sets collected with high multiplicity triggers in p+p, p+A, d+A and 3He+Au collisions at 200 GeV center-of-mass energy. In this talk we will present new measurements of direct photons from p+p and p+Au collisions reaching down to 1 GeV/c. We find that in high multiplicity p+Au events we observe a factor of 2-3 enhancement of direct photons below 4 GeV/c compared to p+p collisions, consistent with thermal photon emission. The status of the measurements in d+Au will also be presented.

Speaker: Norbert Novitzky (University of Tsukuba (JP))

HardProbes2018Novitzkyv2.pdf

09:40 Possible thermal photons in pp collisions and their effects in AA analyses

Direct photons are a powerful tool for elucidating the properties of the hot QCD matter in heavy-ion collisions. They are estimated by assuming that the scaled baseline of proton-proton collision result, which exhibits fair agreement with perturbative QCD calculations, represents prompt photons. On the other hand, recent analyses on azimuthal momentum anisotropy show that a primordial fluid could be formed in small systems at top collision energies. There could also be other source of photons such as glasma photons. We investigate possible non-prompt photon contributions in proton-proton collisions and the effects of modified baseline on the interpretation of direct photon spectra and elliptic flow in heavy-ion collisions at CERN Large Hadron Collider.

Speaker: Akihiko Monnai (High Energy Accelerator Research Organization (KEK))

hp18_monnai.pdf

10:00 Direct photon production and flow at low transverse momenta in pp, p-Pb and Pb-Pb collisions

Direct photons are produced by various processes in pp, p–A and A–A collisions and are sensitive to details of the space-time evolution the medium produced in heavy-ion collisions. In A-A collisions the low $p_{\rm T}$ part of the direct photon spectrum is expected to be dominated by thermal direct photons - thermal radiation of hot matter, i.e., of the quark-gluon plasma and the hadron gas. At RHIC energies it is a challenge for hydrodynamical models to simultaneously describe the yield and the elliptic flow of direct photons in A-A collisions, which is often referred to as the direct-photon puzzle.

Within ALICE photons can be detected with either of the two electromagnetic calorimeters, EMCal and PHOS, and via reconstruction of $e^+e^-$ pairs from conversions in the ALICE detector material using the central tracking system. An additional hybrid method, combining the conversion information with that of the calorimeters has been developed. Whereever possible the results were combined to reflect our best estimate of the inclusive photon spectra and flow, $R_\gamma$, as well as the extraction of direct photon spectra or their upper limits and the
direct photon flow.

In this talk the first measurements on the direct photon production in pp collisions at $\sqrt{s}$ = 2.76 and 8 TeV, as well as in p–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV will be presented. Below 3 GeV/$c$, $R_\gamma$ was found to be consistent with unity in these collision systems. Furthermore, for $p_{\rm T} > 5$ GeV/$c$ the direct photon spectra are in agreement with pQCD next-to-leading order calculations within the uncertainties. In addition the final results on the direct photon elliptic flow ($v_{2}^{\gamma,dir}\{SP\}$) at $\sqrt{s_{\rm NN}}$ = 2.76 TeV from the Pb-Pb run in 2010 in the $p_{\rm T}$ range of $0.9 < p_{\rm T} < 6.2$ GeV/$c$ will be presented in this talk. Comparisons to PHENIX results and to predictions of several theoretical models will be presented in order to shed light on the status of the direct photon puzzle at LHC energies.

Speaker: Nicolas Schmidt (ORNL)

nschmidt_dirgammaflow_ALICE.pdf

10:20 → 10:45 Coffee break

10:45 → 12:25 Parallel 1: Initial State: Nuclear PDFs

Convener: Yen-Jie Lee (Massachusetts Inst. of Technology (US))

10:45 Probing nuclear PDFs with dijets in ultra-peripheral Pb+Pb collisions

Precision of the current nuclear PDF (nPDF) analyses is limited due to the lack of data constraints especially at small-x region. Ultimately the best way to pin down the nPDF uncertainties would be the clean photon-induced processes in a high-energy lepton-ion collider. To some extend this can also be accomplished with ultra-peripheral heavy-ion collisions at the LHC where a quasi-real photon from one nucleus interacts with a nucleus from the other beam in an event where a large impact parameter prevents hadronic interactions.

In this talk we apply the photoproduction framework recently implemented into the Pythia 8 event generator to study the potential of photo-nuclear dijets in ultra-peripheral collisions to further constrain the nPDFs. The focus is in the x-region below the reach of the LHC data currently used in global analyses. In particular, we quantify the small-x reach with different jet kinematics and show how well the values of x derived from reconstructed jet momenta correlate with the actual values of x probed in these measurements. To demonstrate the constraining potential, we compare the expected experimental uncertainties to the current nPDF errors and discuss about other theoretical uncertainties including the uncertainty arising from poorly-constrained photon PDFs. Also the role of diffractive processes in jet production is briefly discussed.

Speaker: Ilkka Helenius (Tübingen University)

HeleniusHP2018.pdf

11:05 Constraints on nuclear parton distribution functions with dijets and isolated photons in pp, pPb and PbPb collisions at 5.02 TeV with the CMS detector

Dijet measurements in pPb collisions have been shown to be one of the most important tools for constraining the gluon nuclear parton distribution functions (nPDFs) at large Bjorken-x. The dijet pseudorapidity distributions are measured as a function of dijet average transverse momentum in order to study the nuclear modifications of PDFs at various factorization scales. The final results from pp and pPb data samples are compared with next-to-leading-order perturbative QCD predictions obtained from both nucleon and nuclear PDFs. Likewise, high transverse momentum prompt photons produced directly from the hard scattering of two partons provide a direct test of perturbative QCD (pQCD) and the nPDFs, as colorless objects that do not interact with the medium. The fragmentation and decay contribution to the photon signal can be suppressed by imposing an isolation requirement. Isolated photon spectra are compared in pp and PbPb to extract the nuclear modification factor. In this talk, we show the latest constraints on nPDFs from studies of dijet pseudo-rapidity distributions in pp and pPb, and isolated photon spectra in pp and PbPb with the CMS detector.

Speaker: Yeonju Go (Korea University (KR))

HP2018_nPDF_dijet_eta_cms_yeonju.pdf

11:25 Impact of CMS dijets in 5.02 TeV pPb and pp collisions on EPPS16 nuclear PDFs

The CMS measurement of dijet pseudorapidity distributions in pPb versus pp collisions at 5.02 TeV [1] provides a direct constraint on nuclear gluon PDFs. We show that while the pPb pseudorapidity distributions have sizable free-proton PDF uncertainties, the ratios of the pPb and pp distributions at high average transverse momentum are practically insensitive to scale and free-proton PDF choices. We find the CMS data on pPb to pp ratios to be in good agreement with the EPPS16 nuclear modifications [2]. Using a non-quadratic extension of the Hessian PDF reweighting method [3], we study the impact of the high average transverse momentum data on the EPPS16 nuclear PDFs. Relative to EPPS16, we find stronger evidence for mid-x gluon antishadowing as well as indication for larger gluon shadowing at small x. The data are also able to further constrain the gluon PDF in the EMC region. Furthermore, we discuss complications in interpreting the low average transverse momentum data, arising from a relatively high transverse momentum cut of the leading jet.

[1] A. M. Sirunyan et al. [CMS Collaboration], arXiv:1805.04736 [hep-ex].
[2] K. J. Eskola, P. Paakkinen, H. Paukkunen and C. A. Salgado, Eur. Phys. J. C 77 (2017) no.3, 163.
[3] H. Paukkunen and P. Zurita, JHEP 1412 (2014) 100.

Speaker: Petja Paakkinen (University of Jyväskylä)

HP2018_Paakkinen.pdf

11:45 Probing the high-x content of the nuclei in the fixed-target mode at the LHC

Using the LHCb and ALICE detectors in the fixed-target mode at the LHC offers unprecedented possibilities to study the quark, gluon and heavy-quark content of the nuclei in the poorly known region of the high-momentum fractions. We will review our projections for studies of Drell-Yan, W, charm, beauty and quarkonium production with both detector set-ups used with various nuclear targets and the LHC proton beams. Based on this, we will show the expected improvement in the determination of the quark, charm and gluon nuclear PDFs as well as discuss the implication for a better understanding of the cold-nuclear-matter effects in hard-probe production in proton-nucleus collisions.

Speaker: Aleksander Kusina (IFJ PAN)

kusina_HardProbes2018.pdf

12:05 Identified particle production in p-Pb collisions at 8.16 TeV with ALICE at the LHC

Measurement of identified charged particle production as a function of multiplicity in pp and p-Pb collisions are important tools for understanding the similarities and differences between small and large interacting systems. Due to their short lifetime, resonances are useful tools to understand the mechanism of particle production and properties of the hadronic phase. The yield of resonances might be modified due to in-medium effects such as re-scattering and regeneration. In particularly, studying the resonance production in p-Pb collisions along with pp provides a baseline measurement for comparison with Pb-Pb results and helps in understanding the cold nuclear matter contributions such as shadowing and Cronin effects.

The excellent tracking and particle identification capabilities of the ALICE detector allow us to study identified particle production from very low to high transverse momentum. We will present the measurement of $K^{∗}(892)^{0}$ and $\phi(1020)$ resonances as well as pions, kaons and protons performed in p-Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV in the rapidity range $−0.5 < y <0$. The results include the transverse momentum spectra ($p_{\rm T}$), integrated yields, mean transverse momenta and $p_{\rm T}$ integrated yields ratio for various centrality classes. The results will be compared with p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV.

Speaker: Sandeep Dudi (Panjab University (IN))

sandeep_hard_probe2018.pdf

10:45 → 12:25 Parallel 2: Jets & high-pT: Jet substructure

Convener: Jorn Henning Putschke (Wayne State University (US))

10:45 Confronting jet quenching with jet grooming: groomed jet mass distributions in heavy ion collisions

I will present a calculation of jet mass distributions for small-radius jets in proton-proton and heavy ion collisions using soft-collinear effective theory (SCET). A process-independent groomed jet mass function is defined which captures the soft-collinear radiation inside jets. The factorization expression simplifies significantly, allowing one to calculate jet mass distributions for jets in any hard process with the corresponding jet cross section. Additional contributions from initial and final state radiation as well as underlying events are discussed. With the medium-induced splitting functions calculated using SCET with Glauber gluon interactions, the medium correction to jet mass distributions is incorporated consistently within the resummation framework. We provide calculations of jet mass distributions for inclusive jets and jets recoiling against a prompt photon or a $Z$ boson, and we compare with Pythia simulations of pp collisions and currently available measurements in pp and AA collisions at the LHC. In the end I will discuss the calculation of a new, collinear-drop jet observable and its use for systematically probing the inner working of the quark gluon plasma.

Speaker: Dr Yang-Ting Chien (Massachusetts Institute of Technology)

HP2018_Chien_mass.pdf

11:05 Inclusive jet measurements in pp and Pb-Pb collisions with ALICE

Measurements of jet yields in heavy-ion collisions can be used to constrain jet energy loss models, and in turn provide information about the physical properties of deconfined QCD matter. ALICE reconstructs charged particle jets (charged jets) with high-precision tracking of charged particles down to $p_{\rm T}^{track} = 150$ MeV/$c$, and jets (full jets) with the addition of particle information from the electromagnetic calorimeter down to $p_{\rm T}^{cluster} = 300$ MeV/$c$. By including low momentum jet constituents, ALICE is uniquely situated at the LHC to measure jets down to low jet momentum, to determine the modification to the soft components of jets, and to measure medium recoil particles.

Two complementary sets of measurements with ALICE over $R = 0.2 - 0.4$ and extending to low jet $p_{\rm T}$ will be presented:
(1) New results of inclusive full jet measurements in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV will be shown, and compared to theoretical predictions. These will include the jet $R_{\rm AA}$ and the jet cross-section ratio for different jet $R$, and will constitute the first such full jet measurements at low transverse jet momentum at this collision energy.
(2) The first measurements of charged jet production using a mixed-event technique to correct for combinatorial background in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV will be shown, over a variety of collision centralities. The mixed-event technique allows for extending jet spectra to lower transverse momenta and larger jet radii without imposing a bias on the jet fragmentation. Details of the mixed-event method will be provided, as well as a discussion on conceptual differences between the mixed-event approach and other background correction methods.

Speaker: James Mulligan (Yale University (US))

HardProbes2018_Mulligan.pdf

11:25 Probing heavy ion collisions using quark and gluon jet substructure

Uncovering the inner working of the quark-gluon plasma requires comprehensive jet substructure information. I will discuss the study of jet quenching utilizing quark and gluon jet substructures as independent probes of heavy ion collisions. Jet and subjet features are exploited to highlight differences between quark and gluon jets in vacuum and in a medium. I will first discuss a physics-motivated, multivariate analysis of jet substructure observables including the jet mass, the radial moments, the $p_T^D$ and the pixel multiplicity. In comparison, I will use state-of-the-art image-recognition techniques by training a deep convolutional neutral network on jet images. To systematically extract jet substructure information, I will introduce the telescoping deconstruction framework exploiting subjet kinematics at multiple angular scales. I will draw connections to the soft-drop splitting function and illuminate medium-induced jet modifications using Lund diagrams. The quark gluon discrimination performance worsens in heavy ion jets simulated in JEWEL due to significant soft event activities affecting the soft jet substructure. This work suggests a systematically improvable framework for studying modifications to quark and gluon jet substructures and facilitating direct comparisons between theoretical calculations, simulations and measurements in heavy ion collisions.

Speaker: Dr Yang-Ting Chien (Massachusetts Institute of Technology)

HP2018_Chien_qg.pdf

11:45 A step towards tagging of quenched jets

Measurements of jet substructure have augmented substantially the scope
of jet quenching studies by arguably giving experimental access to
single splitting jet properties and possibly allowing for the
identification of QGP response contributions to a jet. We show that jet
substructure observables provide a path to measure how quenched a jet is
without the need for comparison with other states in the event (eg, an
electroweak boson). By exploring the flexibility of the Soft Drop
procedure, we identify cuts (ie, Soft Drop parameters) that act as a,
background insensitive, jet quenching classifier. This, so far elusive,
classification allows for the robust identification of strongly quenched
jets from which stringent constrains on jet quenching mechanisms can be
extracted.

Speaker: Yi Chen (CERN)

20181003_AStepTowardsTaggingOfQuenchedJets_HardProbes_YiChen_v6.pdf

12:05 Jet substructure modifications in a QGP from multi-scale description of jet evolution with JETSCAPE

The JETSCAPE Collaboration (Jet Energy-loss Tomography with a Statistically and Computationally Advanced Program Envelope) has developed and released an innovative, modular and flexible event generator to be used by the heavy-ion community. The modifications of jet substructure in relativistic heavy-ion collisions represent a new class of quenching observables, which are sensitive to the redistribution of energy within the jet and the medium excitation that is triggered by the passage of the shower. In this talk we present calculations of jet substructure observables (jet shape, fragmentation function, nuclear modification factor of jet spectra with different cone radii) obtained from numerical calculations using JETSCAPE 1.0, and compare them to experimental data from both A-A and p-p collisions.

The JETSCAPE 1.0 package is a Monte-Carlo simulations of multi-stage jet evolution. The code package invokes four different modules for energy loss which have different effects on intra-jet observables. As a feature of JETSCAPE 1.0, in multi-stage jet evolution the energy loss description for a jet constituent can be switched from one model to another, depending on the scale or virtuality, or momentum during the propagation. The simulations of multi-stage jet evolution make it possible to comprehensively explore the detailed mechanisms of jet quenching.

We study the effect of introducing a space-time extent to the radiation-dominant (MATTER) portion of the shower; the effect of broadening of shower partons due to scattering; the effect of the recoil partons which propagate as a wake of the jet; the effect of the scattering dominated (MARTINI/LBT) portion of the shower; and the effect of AdS/CFT drag on a vacuum and medium modified shower. We also explore the effect of hadronization on these observables.

Speaker: Yasuki Tachibana (Wayne State University)

tachibana_hp_18_v4.pdf

10:45 → 12:25 Parallel 3: HF & QQbar: Heavy flavour

Convener: Jean-Philippe Lansberg (IPN Orsay, Paris Sud U. / IN2P3-CNRS)

10:45 Detailed studies of prompt J/$\psi$ and $\psi$(2S) production in pp, pPb and PbPb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The understanding of charmonium production in PbPb collisions requires the inclusion of many phenomena such as dissociation in the QGP, partonic energy loss, statistical recombination, on top of cold nuclear matter effects (modifications of nPDFs, initial-state energy loss, nuclear break-up). Final results on prompt J/$\psi$ and $\psi(2S)$ production, based on the pp and PbPb data collected at 5.02 TeV by CMS in 2015, are reported, showing a stronger suppression of the excited state as compared to the ground state in all measured bins, as well as a hint for a weaker J/$\psi$ suppression at low pt (possibly due to regeneration) and at high pt (possibly due to partonic energy loss). Final results on prompt J/$\psi$ and $\psi(2S)$ production in pPb data at 5.02 TeV are also reported, showing a different modification of the ground and excited state already in this smaller system, especially in the Pb-going direction, hinting to a possible breakup of the weaker bound excited states from interactions with final state particles. Finally, fully unfolded results of J/$\psi$ production inside jets in pp collisions show that production models are unable to properly model this topology.

Speaker: Batoul Diab (Centre National de la Recherche Scientifique (FR))

HP18_BatoulDiab.pdf

11:05 Spectroscopy in the quark-gluon plasma with bottomonia

The suppression of Upsilon-mesons in the hot quark-gluon medium (QGP) versus reduced feed-down is investigated in heavy-ion collisions at energies reached at RHIC and at LHC. Our centrality- and p_T-dependent model encompasses screening, collisional damping and gluodissociation in the QGP. For Y(1S) it is in agreement with both STAR and CMS data provided the relativistic Doppler effect and the reduced feed-down from the Y(nS) and chi_b(nP) states are properly considered.

At both energies, most of the Y(1S)-suppression is found to be due to reduced feed-down, whereas the main Y(2S) suppression is caused by hot-medium effects in the collectively expanding QGP. The importance of reduced feed-down increases with energy. The p_T-dependence is flat due to the relativistic Doppler effect. Possible suppression effects due to the transient electromagnetic fields in more peripheral collisions are shown to be negligible. The predicted Y(1S)-suppression in Pb-Pb at sqrt(s_NN) = 5.02 TeV is compared with CMS data. Cold nuclear matter effects are discussed for p-Pb at the same energy.

[1] J. Hoelck, F. Nendzig, and G. Wolschin, Phys. Rev. C 95, 024905 (2017).
[2] J. Hoelck and G. Wolschin, Eur. Phys. J. A, 53 (2017).

Speaker: Georg Wolschin (Heidelberg University)

HP18_wolschin.pdf

11:25 Understanding sequential quarkonium suppression with $\Upsilon(nS)$ measurements in pp, pPb and PbPb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The production cross sections of the $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$ states were measured separately using the CMS experimental apparatus, in pp, pPb, and PbPb collisions at 5.02 TeV. New results on the production of the three upsilon states in pPb are reported, including cross sections as a function of transverse momentum (p$_T$) and rapidity ($y$). The data show a stronger suppression of the excited states (2S and 3S) as compared to the ground state (1S). The event activity dependence of the forward-backward ratio of all three upsilon states is also reported. Final results on the differential production cross section and nuclear modification factor of upsilon mesons in PbPb collisions at 5.02 TeV, as a function of centrality, p$_T$ and $y$, show similar suppression pattern, more pronounced than in pPb data. A strong suppression is observed in PbPb collisions, by up to a factor of 2 and 10 for the $\Upsilon(1S)$ and $\Upsilon(2S)$ respectively. The $\Upsilon(3S)$ was not observed in PbPb collisions, being suppressed by more than a factor 14 at the 95% confidence level.

Speaker: Jaebeom Park (Korea University (KR))

HP_1801003v4.pdf

11:45 Heavy-flavour production in the SACOT-mT scheme

The hadroproduction of heavy-flavoured mesons has recently attracted a growing interest e.g. within the groups involved in global analysis of proton/nuclear parton distribution functions (PDFs), saturation physics, and physics of cosmic rays. In particular, the D- and B-meson measurements of LHCb at forward direction are sensitive to gluon dynamics at small x and are one of the few perturbative small-x probes before the next generation deep-inelastic-scattering experiments.

In this talk, we will concentrate on the collinear-factorization approach to inclusive D-meson production and describe a novel implementation (called SACOT-$m_{\rm T}$) of the general-mass variable flavour number scheme (GM-VFNS). In the GM-VFNS framework the cross sections retain the full heavy-quark mass dependence at $p_{\rm T}=0$, but gradually reduce to the ordinary zero-mass results towards asymptotically high $p_{\rm T}$. However, the region of small (but non-zero) $p_{\rm T}$ has been somewhat problematic in the previous implementations of GM-VFNS, leading to divergent cross sections towards $p_{\rm T} \rightarrow 0$, unless the QCD scales are set in a particular way. Here, we provide a solution to this problem. In essence, the idea is to consistently account for the underlying energy-momentum conservation in the presence of a final-state heavy quark-antiquark pair. This automatically leads to a well-behaved GM-VFNS descrpition of the cross sections across all $p_{\rm T}$ without a need to fine tune the QCD scales. The results are compared with the LHCb, ALICE and CMS data and a very good agreement is found. We also compare to fixed-order calculations and explain why they lead to approximately a factor of two lower D-meson production cross sections than the GM-VFNS approach.

Speaker: Dr Hannu Paukkunen (University of Jyväskylä)

GMVFNS_Paukkunen.pdf

12:05 Charm, bottom, and quarkonia cross sections for double and triple-parton scatterings in high-energy proton-nucleus and nucleus-nucleus collisions

The framework to compute the cross sections for the production of
particles with high mass and/or large transverse momentum in double- (DPS),
triple- (TPS), and in general n-parton scatterings, from the corresponding
single-parton (sigma_SPS) values in high-energy proton and nucleus will be
reviewed. The basic parameter of the factorized n-parton scattering ansatz
is an effective cross section sigma_eff encoding all unknowns about the
underlying generalized n-parton distribution in the proton (nucleon). In its
simplest and most economical form, the sigma_eff parameter can be derived
from the transverse parton profile of the colliding protons and/or nucleus,
using a Glauber approach. Numerical examples for the cross sections and
yields expected for the concurrent DPS or TPS production of heavy-quarks,
quarkonia, and/or gauge bosons in proton and nuclear collisions at LHC and
Future Circular Collider (FCC) energies will be provided. The obtained cross
sections are based on perturbative QCD predictions for sigma_SPS at
next-to-leading-order (NLO) or next-to-NLO (NNLO) accuracy including, when
needed, nuclear modifications of the corresponding parton densities.

Speaker: Dr Alexander Snigirev (SINP MSU)

snig-hp18.pdf

10:45 → 12:25 Parallel 4: Electroweak: Photons & dileptons

Convener: Itzhak Tserruya (Weizmann Institute of Science (IL))

10:45 PHENIX measurements of beam energy and centrality dependence of direct photon emission in heavy ion collisions

PHENIX has observed that a large number of direct photons are radiated during the evolution of the strongly coupled QGP produced in Au+Au collision at 200 GeV to hadron resonance matter. The direct photons are emitted with a large azimuthal anisotropy. PHENIX has extended this study using large data samples from Au+Au collisions at 39, 62.4, and 200 GeV, and Cu+Cu collisions at 200 GeV. For a given beam energy the direct photon yield scales with the number of binary collisions Ncoll for all centrality selections at all pT. At high pT the Ncoll scaled direct photon yield from p+p collisions agrees with the heavy ion data as expected. However, at low pT the yield from heavy ion collisions exceeds the yield from p+p significantly. Comparing the low pT direct photon yield across beam energies as a function of the produced charged particle density dNch/d reveals that the yield is proportional to (dNch/d)1.25. In this talk we will discuss these new results and their implications. We will also show updated results on direct photon yields and azimuthal anisotropies v2 and v3 from high statistics Au+Au data at 200 GeV.

Speaker: Axel Drees

Drees_HP2018.pdf

11:05 Jet and photon probes of small collision systems with ATLAS

Measurements of photon and jet production in small collision systems are of great interest to understanding the partonic structure of heavy nuclei, and serve as a constraint on the initial state in large collision systems. These channels are sensitive to a broad set of physics effects such as the modification of the parton densities in nuclei, including the onset of non-linear QCD or saturation effects at low-$x$, and the energy loss of partons in the nucleus before the hard scattering. This talk presents new results from the ATLAS experiment on photon and dijet production in $p$+Pb collision data recorded in 2016. Photon yields are reported in 8.16 TeV $p$+Pb data over a large kinematic range, $p_T$ = 25-500 GeV and $|\eta| <$ 2.37, and the production rates are compared to an extrapolated $pp$ reference based on existing 8 TeV collision data. The measured spectra are used to construct nuclear modification factors and forward/backward ratios. These are compared to theoretical calculations of initial state energy loss and to the expectations from the modifications of parton distribution functions in nuclei. Additionally, new results on forward-forward and forward-central di-jet production are reported in 5.02 TeV $p$+Pb and $pp$ collisions, including jets up to $y = 4$ in the center of mass frame. The measured azimuthal angular correlations and conditional yields are compared to theoretical calculations of the decorrelation and suppression expected from saturation effects.

Speaker: Martin Spousta (Charles University)

Spousta_HP2018_20181001.pdf

11:25 Synchrotron radiation as a probe of confinement and QGP

The intensive synchrotron radiation resulting from the quarks interacting with a collective confining color field in the heavy ion collisions is discussed. The spectrum of hard photons is presented and is compared with experimental data showing a feasibility of this magnetic bremsstrahlung-like radiation. The study of the predicted early specific anisotropy in the angle distribution of leptons with respect to the three-momentum of pair is suggested as a way to select this mechanism among other sources of direct photons and dileptons and to probe the confinement properties.

Speaker: Alexey Nikolskii (Bogoliubov Laboratory of Theoretical Physics, JINR, Dubna, Russia)

Synch_Nikolskii.pdf

11:45 Measurement of the $\mu^+\mu^-$ Invariant Mass Spectra in p+p and p+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV with the Muon Telescope Detector at STAR

Dileptons (l$^+$l$^-$) are produced during all stages of heavy-ion collisions (HIC) via various production mechanisms. Since leptons interact through the electromagnetic force, not via the strong force, they carry pristine information about the produced medium. For this reason, dileptons provide an essential tool for studying the properties of the strongly interacting medium produced in HICs. The installation of the Muon Telescope Detector (MTD) allows a measurement of the dimuon ($\mu^+\mu^-$) production over a large invariant mass range (0 $< M_{\mu^+\mu^-} <$ 4 GeV/c$^{2}$) for the first time at STAR. Data have been collected with the fully installed MTD from p+p and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. These two data sets allow for new opportunities to measure the dimuon invariant mass spectra at STAR. In this talk, we will present new measurements of the $\mu^+\mu^-$ invariant mass spectra in p+p and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. The muon identification techniques employing deep neural networks and the novel background estimation methodology will be discussed. Finally, the invariant mass spectra will be compared with the hadronic cocktail and the physics will be discussed.

Speaker: James Brandenburg (Rice University)

jdb_HP18_final.pdf

12:05 Low-mass di-lepton measurements in Pb--Pb collisions with ALICE at the LHC

The production of low-mass dielectrons is one of the most promising tools for the understanding of the chiral symmetry restoration and of the thermodynamical properties of the Quark-Gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. At low invariant mass ($m_{\rm ee} <$ 1.1 GeV/$c^{2}$), the dielectron invariant-mass spectrum is sensitive to the properties of short-lived vector mesons in the medium and modifications related to the chiral symmetry restoration. Thermal radiation emitted by the system, both during the partonic and hadronic phase, contributes to the dielectron yield over a broad mass range and gives insight into the temperature of the medium. In the intermediate-mass region (1.1 $< m_{\rm ee} <$ 2.8 GeV/$c^{2}$), the measurement of thermal dielectrons from the QGP is very challenging at the LHC due to the dominant contribution of correlated e$^{+}$e$^{-}$ pairs from semileptonic decays of charm and beauty hadrons.

In this talk, we will present the final results of e$^{+}$e$^{-}$ pair production in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV and the status of the analysis at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. We will discuss the modifications of dielectron yields in Pb--Pb collisions compared to known hadronic sources and the production of virtual photons. Furthermore, runs with a reduced magnetic field of the ALICE solenoid are planned for the Pb--Pb data taking during the LHC Run 3, to improve the signal-to-background ratio, a key aspect of this analysis. It simultaneously gives the opportunity to study very soft dielectron production. We will show results obtained with pilot runs in minimum-bias pp collisions at 13 TeV.

Speaker: Alberto Caliva (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))

HardProbes_AlbertoCaliva.pdf

12:25 → 13:45 Lunch

13:45 → 17:30 Free time - excursion

Thursday, 4 October

09:00 → 10:20 Parallel 1: Initial State

Convener: Eero Aleksi Kurkela (CERN)

09:00 Toward a unified picture of particle production at all $p_{\rm T}$

We describe a new unified formalism which contains both large and small x gluons in a target proton or nucleus wave function. It generalizes the Color Glass Condensate formalism (appropriate for low pt only) and therefore can be applied to particle production in high energy hadronic/nuclear collisions in both small and large transverse momentum regions and all rapidities.

Speaker: Jamal Jalilian-Marian (Baruch College)

09:20 Multi gluon correlations in the Color Glass Condensate: quantum interference in proton-nucleus collisions

We compute multi gluon production at mid rapidity in proton-nucleus collisions within the Color Glass Condensate framework. We show that, in the dilute-dense limit valid for such collisions, the terms responsible for the multi gluon correlation have two origins. On the one hand, the Hanbury-Brown-Twiss interference in the final state. On the other hand, the Bose enhancement of gluons in the projectile and target wave functions, with the latter being suppressed by the number of colors with respect to the former. We also demonstrate that such correlations come from the highest order relevant correlator of Wilson lines in the target wave function, i.e., the quadrupole and sextupole for two and three gluon correlations respectively. We develop a general method for the computation of such high order correlators that captures the bulk of their contribution to the multi gluon production cross section but does not employ the approximation of a large number of colors.

Ref.: Tolga Altinoluk, Nestor Armesto, Alex Kovner and Michael Lublinsky, arXiv:1805.07739 [hep-ph].

Speaker: Nestor Armesto Perez (Universidade de Santiago de Compostela (ES))

armesto_HP2018_gcorr.pdf

09:40 Multi-particle correlations and collectivity in small systems from the initial state

We report on recent progress in understanding multi-particle correlations in small systems from the initial state using the Color Glass Condensate Effective Field Theory (CGC EFT). Working in the dilute-dense limit, applicable for asymmetric collision systems, we first study the Fourier harmonics $v_2$ and $v_3$ for the the systems p/d/$^3$He+Au. This semi-analytic framework, when paired with realistic modeling of nuclei, allows for a first quantitive calculation of these quantities from the initial state. We will then compare our results to recent PHENIX measurements and discuss possible sources of systematic uncertainties. Novel features of our calculation will be further elucidated by comparison to a simple parton model for multi-particle correlations. Additionally, we will discuss how this framework permits a simple power-counting argument for the multiplicity scaling of integrated Fourier harmonics $v_n$, which we will then compare to recent ATLAS data. Lastly, we give an outlook to future studies of correlations from the initial-state using our framework.

[1] M. Mace, V. V. Skokov, P. Tribedy, R. Venugopalan. Hierarchy of azimuthal anisotropy harmonics in collisions of small systems from the Color Glass Condensate. arXiv:1805.09342 [hep-ph]
[2] M. Mace, V. V. Skokov, P. Tribedy, R. Venugopalan. In preparation.
[3] K. Dusling, M. Mace, R. Venugopalan. Multiparticle collectivity from initial state correlations in high energy proton-nucleus collisions. Phys. Rev. Lett. 120, 042002 (2018). arXiv:1705.00745 [hep-ph]
[4] K. Dusling, M. Mace, R. Venugopalan. Parton model description of multiparticle azimuthal correlations in pA collisions. Phys. Rev. D 97, 016014 (2018). arXiv:1706.06260 [hep-ph]

Speaker: Mark Mace (Stony Brook University)

HP_2018_MACE_2_2.pdf

10:00 Resolving the Partonic Structure of Nuclei through Energy-Frontier eA Scattering

The Large Hadron-electron Collider LHeC is a proposed upgrade of the LHC. An energy recovery linac would provide 60 GeV electrons to collide with the proton and nuclear beams from the HL-LHC and, eventually, with those from the HE-LHC and the FCC-hh. Working concurrently with the pPb or PbPb LHC modes, it will provide ePb collisions in the TeV regime with luminosities $\sim 10^{33}$ cm$^{-2}$s$^{-1}$ to achieve integrated luminosities $\sim 10$ fb$^{-1}$ in ten years. It will offer a huge extension of the kinematical coverage with respect to the one presently covered by data used in nuclear PDF fits, down to $x \sim 10^{-6}$ in the perturbative region for the LHeC. In this talk we will show the possibilities in such machine for determining the nPDFs with unprecedented precision, in a region of interest for both HL-LHC and future AA colliders, with the possibility of extracting the complete set of PDFs of a single nucleus in a single experiment. Confronting such precision nPDFs with data from nuclear colliders will then set the eventual factorisation breaking effects and allow a more precise characterisation of the properties of the QCD matter produced in such collisions.

Speaker: Nestor Armesto Perez (Universidade de Santiago de Compostela (ES))

armesto_HP2018_LHeC.pdf

09:00 → 10:20 Parallel 2: Jets & high-pT

Convener: Constantinos Loizides (ORNL)

09:00 Inclusive jet and dijet suppression in Pb+Pb and Xe+Xe collisions with ATLAS

Jets represent an important tool allowing to access the information about the hot and dense medium created in heavy ion collisions. Both jet yields and jet internal structure are observed to be modified in heavy-ion collisions compared to proton-proton reference. This talk presents latest measurements of the nuclear modification factor, $R_{AA}$, for $R=0.4$ jets in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with the ATLAS detector at the LHC. The analysis is performed over a large range of transverse momentum, up to $p_{\mathrm{T}}=1$ TeV, and differentially in jet $p_{\mathrm{T}}$, rapidity, and collision centrality. The jet $R_{AA}$ is measured also differentially in the jet mass which provides a new information on the dependence of the energy loss on the jet's substructure. Measurements of dijets are expected to provide information about the path-length dependence of the energy loss or about the role of fluctuations in the energy loss. Latest results on the dijet momentum balance in Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV are presented and compared to the same quantity measured in Pb+Pb collisions $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV.

Speaker: Radim Slovak (Charles University (CZ))

HardProbes2018_FINAL.pdf

09:20 Parton Energy Loss in Generalized High Twist Approach

High-Twist approach has been used to calculate parton energy loss and medium modified fragmentation function due to multiple parton scattering. We revisit this problem in deeply inelastic scattering (DIS) process, with a generalized high-twist approach. In this approach, the differential radiated gluon number distribution can be expressed in terms of unintegrated gluon distribution function, or transverse momentum dependent jet transport coefficient. The radiated gluon spectrum can be calculated without soft gluon and static medium approximation. In these limits, one can recover the GLV result in first order opacity approximation.

Speaker: Ms Yuanyuan Zhang (Central China Normal University)

HP2018_YYZ.pdf

HP2018_YYZ.pdf

09:40 Event-by-event jet suppression, anisotropy and hard-soft tomography

Event-by-event jet suppression, anisotropy and hard-soft tomography

Xin-Nian Wang$^{1,2,3}$ , Shanshan Cao$^4$, Yayun He$^1$ and Tan Luo$^1$

A consistent description of high $p_T$ particle suppression ($R_{AA}$) and azimuthal anisotropy $v_2$ has been a puzzle in the study of jet quenching, pointing to some non-perturbative native of jet transport. Event-by-event single inclusive jet suppression and azimuthal anisotropy are studied within the Linear Boltzmann Transport (LBT) model for jet propagation in QGP medium from 3+1D hydrodynamic evolution with fluctuating initial conditions. We demonstrate that LBT can describe both the single inclusive jet suppression and azimuthal anisotropy with a single adjustable parameter- effective strong coupling constant $\alpha_s$. This indicate observed jet quenching puzzle might be caused by non-perturbative phenomenon in hadronization at intermediate $p_T$. We also studied the energy and centrality dependence and the effect of fluctuation as compared to a smooth hydro evolution. A linear relationship is found between high $p_T$ jet anisotropy due to jet quenching and soft hadron anisotropy from hydrodynamic expansion

$^1$ Central China Normal University, $^2$ Lawrence Berkeley National Laboratory,
$^3$ University of California, Berkeley, $^4$ Wayne State University.

Speaker: Mr Yayun He (CCNU)

YayunHe_HP2018.pdf

09:00 → 10:20 Parallel 3: HF & QQbar: Charm

Convener: Enrico Scomparin (Universita e INFN Torino (IT))

09:00 Production of open charm and beauty states in pPb collisions with LHCb

A rich set of open heavy flavour states is observed by LHCb in pPb
collisions data collected at 5 and 8.16 TeV nucleon-nucleon
center-of-mass energy. Results include the new measurements of
production of beauty hadrons in pA collisions through cleanly
reconstructed exclusive decays. Open charm states include the Lambda_c
baryon, that was also observed in pA collisions for the first time by LHCb.

Speaker: Yanxi Zhang (Centre National de la Recherche Scientifique (FR))

HeavyIonLHCb_HardProbes.pdf

09:20 Measurements of strange and non strange charm production in PbPb collisions at 5.02 TeV with the CMS detector

The heavy-flavor particles are produced in the earlier stage in heavy-ion collision and experience the full evolution of the QGP medium. The measurement of D mesons could provide us important inputs for flavor and charge dependent transport properties. On the other hand, with abundant strange quarks presented in heavy-ion collision, the $D_{S}^{+}$ production is expected to be enhanced hadronization via recombination. Large statistics proton-proton and PbPb samples collected at 5.02 TeV with CMS detector are used for the measurement of $D^{0}$ and $D_{S}^{+}$ production over a wide transverse momentum range. Result of D-meson $p_T$-differential cross section, nuclear modification factor $R_{AA}$, and the ratio of $D_{S}^{+}$ over $D^{0}$ for both pp and PbPb collisions are presented.

Speaker: Cheng-Chieh Peng (Purdue University (US))

Hard_Probes2018_Dmeson_CMS_ChengChieh_v6.pdf

09:40 Charm production in fixed-target mode at LHCb

LHCb has the unique capability to study collisions of the LHC beams on
fixed targets. Internal gas targets of helium, neon and argon have been
used so far. First final results and prospects on open and hidden charm
productions will be presented, which can provide crucial constraints on
cold nuclear matter effects and nPDF at large x. These measurements,
together with production of antiprotons and other light hadrons, are of
great interest to cosmic ray physics as well.

Speaker: Frederic Fleuret (Laboratoire Leprince-Ringuet - Ecole polytechnique)

fleuret_HardProbes2018-v2.pdf

10:00 Charm physics in NA61/SHINE

NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) is a fixed-target experiment operating at the CERN SPS accelerator. The main goal of the Collaboration is to study the properties of the phase transition between confined matter and quark-gluon plasma by performing a two-dimensional scan of the phase diagram of strongly interacting matter. Within this program, collisions of different systems (p+p, Be+Be, Ar+Sc, Xe+La, Pb+Pb) over a wide range of beam momenta (13A-158A GeV/c) have been recorded.

Recently, the physics program of NA61/SHINE was extended by measurements of open charm production in A+A collisions which is the main goal of NA61/SHINE beyond 2020. In order to meet the challenges of the required spatial resolution of primary and secondary vertex reconstruction, the detector was upgraded by a micro vertex detector. A Small-Acceptance version of the Vertex Detector (SAVD) was successfully commissioned in December 2016 and first pilot data were collected for Pb+Pb collisions at a beam momentum of 150A GeV/c. These data allowed to validate the measurement concept and to perform the first direct measurement of open charm hadron production in collisions of nuclei at the SPS energies. In October and November 2017, the large statistics datasets for Xe+La collisions at 75A and 150A GeV/c were registered and are under analysis now. This contribution will present the motivation of open charm studies as well as the current status and details of the analysis of the collected Pb+Pb and Xe+La data.

Speaker: Wojciech Brylinski (Warsaw University of Technology (PL))

charm_na61_WB.pdf

09:00 → 10:20 Parallel 4: Novel experimental developments

Convener: Thomas Peitzmann (Nikhef National institute for subatomic physics (NL))

09:00 Enhanced hard-probe measurements in the 2020s with the ALICE Upgrade

The ALICE Collaboration is preparing a major upgrade of the detector apparatus during the second LHC long shutdown (LS2, 2019-20) in view of the LHC Runs 3 and 4 (2021 to 2029).

The objective of the ALICE upgrade for LS2 is two-fold: i) an improvement of the tracking precision and efficiency, in particular in the low-momentum range; ii) an improvement of the readout capabilities of the experiment, to fully exploit the LHC luminosity for heavy ions envisaged after LS2.

The first goal will be achieved by replacing the Inner Tracking System with a new tracker, composed of seven cylindrical layers of monolithic silicon pixel detectors (MAPS) with fast readout, high granularity and low material thickness, and by introducing a new telescope tracker, also composed of MAPS, in front of the muon spectrometer. As an example the resolution of the track spatial position will improve by about a factor of three in the direction transverse to beam line. The second goal will be achieved by replacing the readout chambers of the Time Projection Chamber with Gas Electron Multiplier (GEM) detectors, by upgrading or replacing the readout electronics of several of the other detector systems, by adding a new fast trigger detector and by implementing a new integrated online-offline architecture. The upgraded ALICE detector will be capable of reading out Pb-Pb interactions in minimum-bias trigger mode at a rate of 50 kHz, corresponding to 50 times the current rate.

The prime physics goals of the ALICE upgrade are high-precision measurements of heavy-flavour, charmonium and low-mass dilepton production, with particular emphasis on the low-$p_{\rm T}$ region. For example, in the sector of open heavy flavour, the new silicon trackers, in conjuction with the high rate capabilites, will allow us to extend the measurements of nuclear modification factor and flow coefficients of charm and beauty mesons down to or close to zero in $p_{\rm T}$. The reconstruction of heavy-flavour baryon decays will be possible down to about 2 GeV/$c$. In the quarkonium sector, the production and elliptic flow of J/ψ and ψ(2S) states will be measured with unprecedented precision down to zero $p_{\rm T}$, with the separation of prompt and B-decay contributions at both central and forward rapidity. The large samples and reduced material thickness of the new inner tracker will allow us to accurately study the ρ-meson spectral function with low-mass dileptons and to charcterize the spectrum and flow of the thermal dilepton radiation expected in Pb-Pb collisions.

Speaker: Cristina Bedda (Utrecht University (NL))

TalkHP_Bedda.pdf

09:20 Complete heavy flavor physics at RHIC with the sPHENIX MAPS vertex detector

sPHENIX is the next generation heavy ion physics experiment designed to collect a suite of unique jet and Upsilon observables with unprecedented statistics and kinematic reach at RHIC. The sPHENIX inner most tracking detector, MVTX (MAPS-based vertex detector), will provide a precise determination of the impact parameter of tracks to the primary vertex in high multiplicity heavy ion collisions. The MVTX utilizes the next generation fast Monolithic Active Pixel Sensor technology to provide precision tracking with high tracking efficiency over a broad momentum range at high luminosity environment envisioned at RHIC. These new capabilities enable precision measurements of open heavy flavor observables, covering an unexplored kinematic regime at RHIC, in particular the nuclear modification and flow of $b$-jets and $B$-mesons, which collectively cover a wide transverse momentum range of 2--35~GeV$/c$. Together with the light-quark observables, these $b$-quark probes will provide essential information to complete the microscopic description of QGP at RHIC energies. The physics program and detector development of MVTX will be discussed in this talk.

Speaker: Dr Jin Huang (Brookhaven National Lab)

sPHENIX_HF.pdf

09:40 High-luminosity fixed-target experiments at the LHC

By extracting the beam with a bent crystal or by using an internal gas target, the multi-TeV proton and lead LHC beams allow one to perform the most energetic fixed-target experiments ever and to study $pp$, $pd$ and $pA$ collisions at $\sqrt{s_{NN}}=115$ GeV and Pb$p$ and PbA collisions at $\sqrt{s_{NN}}=72$ GeV with high precision and modern detection techniques. In this talk, we will review the technical solutions to obtain a high-luminosity fixed-target experiment at the LHC and will discuss their possible implementations with the ALICE and LHCb detectors.

Speaker: Cynthia Hadjidakis (Centre National de la Recherche Scientifique (FR))

HP2018-AFTER-CynthiaHadjidakis.pdf

10:00 Next generation jet measurements with sPHENIX

The proposed sPHENIX detector at the Relativistic Heavy Ion Collider (RHIC), designed to take advantage of technological advances from the Large Hadron Collider (LHC), will allow measurements of jets and jet correlations with a kinematic reach that will overlap with measurements made at the LHC. This is made possible by taking advantage of the increased luminosity due to accelerator upgrades and the ability of sPHENIX to sample the entire luminosity. Particle jets, formed when a hard scattered parton fragments and then hadronizes into a spray of particles, are an excellent as a probe of the Quark Gluon Plasma (QGP) formed in heavy-ion collisions. As these partons traverse the QGP formed in heavy ion collisions, they lose energy to the medium, in a process called jet quenching. The physics of this process can be explored by comparing jet-based observables in heavy-ion collisions to those in proton-proton collisions. This is necessary in order to answer the fundamental questions of how and why partons lose energy in the QGP, which will require that we characterize both the medium induced modification of the jet fragmentation pattern, and the correlation of the lost energy with the jet axis. Observables which use the correlation between a high energy photon and a jet are especially useful as the photon kinematics are more closely correlated with the parton kinematics, as well as preferentially selecting quark jets over gluon jets. In addition to inclusive or semi-inclusive jet observables, advances in both theory and measurement at the LHC have provided a suite of new observables which are related to the fragmentation patter of the jet and how it is modified in the QGP medium. These observables require the precise tracking and calorimetry that the sPHENIX design has. We will show the performance of jet and gamma-jet observables within the sPHENIX detector, including the jet energy scale and resolution in both proton-proton and heavy-ion collisions. These results are simulated with the improved framework developed for understanding the performance of the new detector for measuring jets and photons in a heavy ion environment.

Speaker: Songkyo Lee (Iowa State University (US))

20181004_songkyo_HP2018_v3.pdf

10:20 → 10:45 Coffee break

10:45 → 12:25 Parallel 1: Jets & high-pT

Convener: William Horowitz (University of Cape Town)

10:45 Moliere scattering in QGP: finding scatterers within the liquid

By looking for rare (but not exponentially rare) large-angle deflections of a jet or of partons within a jet, experimentalists can find the weakly coupled short-distance quark and gluon quasiparticles (scatterers) within the strongly coupled liquid quark-gluon plasma produced in heavy ion collisions, as was proposed in Ref.~1. In this previous work, the probability for picking up a given transverse momentum via a Moliere scattering was calculated, but in the limit of infinite parton energy which means zero angle scattering. Here we calculate the Moliere scattering probability for incident partons with finite energy, meaning that we have now calculated the probability for a parton showing up with a nonzero angle with respect to its initial direction due to scattering with the constituents of QGP, using leading order perturbative QCD. We include all relevant channels, including those in which the parton that shows up at a large angle was kicked out of the medium as well as the Rutherford-like channel in which what is seen is the scattered incident parton. These results will serve as valuable inputs to jet Monte Carlo calculations, allowing hard scattering to be added to Monte Carlos that do not include it or allowing these effects to be identified in Monte Carlo calculations in which they are already incorporated. Our results provide key theoretical input toward finding the scatterers within the QGP liquid, which in turn is the necessary first step toward using precise, high statistics, suitably differential measurements of jet modification in heavy ion collisions to discern the microscopic structure of QGP.

Ref. 1: F. D’Eramo, M. Lekaveckas, H. Liu and K. Rajagopal, Momentum Broadening in Weakly Coupled Quark-Gluon Plasma (with a view to finding the quasiparticles within liquid quark-gluon plasma), JHEP 05 (2013) 031, [1211.1922].

Speaker: Dr Yi Yin (MIT)

Moliere_HP_final_YiYin.pdf

11:05 Medium modified jets, soft drop and leading hadrons in a single formalism

We study the modification of reconstructed jets and high transverse momentum (high-$p_T$) hadrons, produced in the fragmentation of these jets, as they propagate through a dense medium. Vacuum and medium modified fragmentation functions are used to define the properties of hadrons produced in hard interactions, in $p$-$p$ and $A$-$A$ collisions respectively. Jets and jet modification are studied using a semi-analytical approach, based on defining a factorized jet function. The vacuum jet function is then measured in tuned PYTHIA simulations. Both the modification of the fragmentation function and that of the jet are carried out using an identical energy loss kernel (based on the higher-twist approach), with modifications introduced to make it compatible with jet algorithms. This approach treats jet and leading particle observables on an equal footing. This approach is then extended to di-hadron production within a jet and its modification within a medium (near side associated yield). Extensions of this formalism to a ``di-sub-jet'' function and its application to the soft-drop measurements are discussed. The methodology of extending these calculations to Monte-Carlo simulations is outlined. All calculations are carried out within a 2+1D viscous hydrodynamic simulation, and compared with experimental data from a variety of collision energies.

Speaker: Chathuranga Sirimanna (Wayne State University)

HardProbes2018_Chathuranga.pdf

11:25 Probing spectral properties of the QGP with real-time lattice simulations

We present a novel method to obtain spectral properties of a non-Abelian gauge theory in the region where occupation numbers are high. The method to measure the (single-particle) spectral function is based on linear response theory and classical-statistical lattice simulations. Although we use a system close to its nonthermal fixed point, a situation that typically occurs in the weak-coupling picture during the initial stages of a heavy-ion collision, its extracted spectral function can be understood within the hard-thermal loop (HTL) formalism and thus resembles thermal equilibrium at high temperatures. This allows us to obtain quantities like the life time of quasiparticles that are beyond the leading order and difficult to compute within HTL. Moreover, the approach has the potential to measure transport coefficients and can be employed beyond the range of validity of HTL.

Speaker: Kirill Boguslavski (University of Jyvaskyla (FI))

HardProbes_2018_10_Boguslavski.pdf

11:45 First calculation of $\hat{q}$ on a quenched SU(3) lattice and its scale dependence

The jet transport coefficient $\hat{q}$ is the leading parameter that controls the modification of hard jets in a dense extended medium. In this talk, we outline an ab-initio framework to compute $\hat{q}$ on a quenched SU(3) lattice. We consider a leading order diagram where a hard parton probes the thermal medium by exchanging a Glauber gluon. The non-perturbative part of this process is expressed in terms of a non-local (two-point) Field-Strength-Field-Strength operator product which can be Taylor expanded after analytic continuation to the Euclidean region. Such an expansion allows us to write $\hat{q}$ in terms of the expectation of local operators. In this talk, we demonstrate that $\hat{q}$ like other non-perturbative quantities such as parton distribution functions (PDFs) and fragmentation functions (FFs) depends on the energy of the probe and the interaction scale between the hard parton and the medium. To explore the scale dependence of the jet transport coefficient, we carry out a perturbative analysis both on the lattice and in continuum field theory. We discuss the matching of these two approaches and the effect of higher order terms from the perturbative expansion on the extracted value of $\hat{q}$.

Speaker: Amit Kumar (Wayne State University)

AmitKumar_HardProbes2018.pdf

12:05 Understanding wide jet suppression in data through the hybrid strong/weak coupling model

By means of a global fit to all available central hadron and jet RAA measurements in heavy ion collisions at the LHC, we constrain the hybrid model free parameter while obtaining an execellent description of those data. With the narrow range of parameters obtained by this procedure, we perform a detailed investigation of the mechanism of jet suppression in heavy ions as a function of the jet properties. A salient example is the dependence of jet RAA on the reconstruction radius R, for which our model yields a non-monotonic behaviour and a significant suppression even for large jet radius. We show that this interesting pattern is a consequence of the observation that wider jets lose more energy in combination with the effect of medium response. We further explore the consequences of this characteristic feature of our model on the new generation of groomed jet substructure observables, obtaining in particular good qualitative agreement with the measured difference between medium-modified and vacuum Lund map distribution.

Speaker: Dr Daniel Pablos (McGill)

pablos_hp18.pdf

10:45 → 12:25 Parallel 2: Jets & high-pT

Convener: Rainer Fries (Texas A&M University)

10:45 Quark and Gluon jet Energy Loss

Although quark/gluon single parton energy loss is theoretically well established and under control by the corresponding Casimir color factor (CF/CA), quark/gluon initiated jets will wash out the corresponding color factor ratio due to the parton shower evolution. As such, for phenomenological studies in heavy-ions, it is not only mandatory to take into account the corresponding deviation from the expected vacuum probabilistic shower, but also from the enhanced splitting rate due to the interactions with the medium. In this work, we use a calibrated energy loss channel (photon-jet events) to study, at Monte Carlo level, the expected energy loss dependence on light-quark/gluon initiated jets. Our results show that it is not possible to use such simple ansatz to quantify the sensitivity of observables to the role of the color charge as previously assumed.Further we provide robust parametrizations for both average energy loss and its variance for quark and gluon initiated jets.

Speaker: Joao Barata (LIP Laboratorio de Instrumentacao e Fisica Experimental de Part)

Quark_Gluon_Joao_Barata.pdf

11:05 Universal quark/gluon ratio in medium-induced parton cascade

We investigate the radiative break-up of a highly energetic quark or gluon in a high-temperature QCD plasma. We find that, as a result of the turbulent nature of the underlying parton cascade the quark to gluon ratio of the soft fragments tends to a universal constant value that is independent of the initial conditions, within an inertial range of momenta T ≪ ω ≪ E, where E denotes the energy of the original hard parton (jet) and T the temperature of of the medium. We discuss implications of this result to jet quenching physics and the problem of thermalization of the quark-gluon plasma in heavy ion collisions.

Speaker: Soeren Schlichting (University of Washington)

SSHardProbes18-parallel.pdf

11:25 Measurements of the jet internal sub-structure and its relevance to parton evolution in p+p and Au+Au collisions at STAR

Recent measurements of jet structure modifications at RHIC and LHC highlight the importance of differential measurements to study the nature of jet quenching. Since these jet structure observables are intimately dependent on parton evolution in both the angular and energy scale, measurements are needed to disentangle these two scales in order to probe the medium at difference length scales to study its characteristic properties such as the coherence length. To that effect, the STAR collaboration presents fully unfolded results of a jet's internal sub-structure via the Softdrop groomed jet radius ($R_{g}$) and the shared momentum fraction ($z_{g}$) in p+p collisions at 200 GeV as a function of jet momenta and for a variety of jet radii. In applying the Softdrop criterion recursively on a given jet in p+p collisions, we also showcase the first measurement of the jet virtuality evolution in both the angular and momentum scales in data. The recursive measurements allows us to test the self-similarity of the Altarelli-Parisi splitting function measured as a function of $z_{g}$. We compare our measurements to current Monte Carlo models on the market leading to stringent constraints on model parameters.

Having established the p+p baseline, we present the first measurement of the jet`s inherent angular structure in Au+Au collisions at 200 GeV via an experimentally robust observable related to the SoftDrop $R_{g}$ such as the opening angle between the two leading sub-jets ($\theta_{sj}$). We utilize the 2014 dataset with a significant increase in statistics offering extended kinematic reach and the ability to perform more differential analyses. In Au+Au collisions, we use the specific di-jet selection introduced in our previous momentum imbalance measurement. By studying traditional jet quenching observables such as $A_{J}$ and recoil coincidence spectrum as a function of jets belonging to a particular angular class based on the $\theta_{sj}$ observable, we directly probe the medium response to jets at a particular resolution scale to look for signatures of coherent or de-coherent energy loss.

Speaker: Dr Raghav Kunnawalkam Elayavalli (Wayne State University)

jetsubstructure_STAR_raghav_hp2018-1.pdf

11:45 Jet structure in integrated EPOS3-HQ approach

A consistent modelling of back reaction of the medium on the jet evolution is important for understanding the jet structure. Majority of existing models implement only one-way jet-medium interaction by coupling jets to a fixed hydrodynamic expansion and not including the energy deposition in the medium itself.
In this talk we show the first results of jet observables from a parton cascade integrated with hydrodynamic evolution within the EPOS3-HQ model. The hard (jet) partons are produced along with soft partons in the initial state EPOS approach. The soft partons, represented by strings, melt into thermalized medium which is described with a 3 dimensional event-by-event viscous hydrodynamic approach. The jet partons then propagate in the hydrodynamically expanding medium. The total jet energy gets progressively “degraded” as the partons reaching a certain lower cut off are melted into the hydrodynamic medium via the source terms. The full evolution proceeds in a concurrent mode, without separate thermalized and jet parts.
We demonstrate how the medium modification effects on the jets, and also how the medium recoil alters the jet structure. Also we discuss the dependence of this effect on the energy loss scenario.

Speaker: Dr Iurii Karpenko (SUBATECH Nantes)

HP2018-Karpenko.pdf

12:05 Multi-stage jet evolution through QGP using the JETSCAPE framework: inclusive jets, correlations and leading hadrons.

The JETSCAPE Collaboration (Jet Energy-loss Tomography with a Statistically and Computationally Advanced Program Envelope) has developed and released an innovative, modular and flexible event generator to be used by the heavy-ion community. High-energy jet evolution through deconfined QCD matter is a multi-scale problem that involves different stages, whose physics should be addressed by different effective approaches: a few-scattering vacuum-like shower phase; a multiple scattering induced rare emission phase; and a strongly coupled phase. The goal of the present work is to put all of these limited descriptions together in a single combined effective evolution, and to confront its predictions with data.

In this talk we will present a selection of first results obtained from this approach made possible by the extensive flexibility of the JETSCAPE framework, including event-by-event fluctuating hydro and jet modification simulations. We will focus on several jet and hadron observables (inclusive and correlations), measured both at RHIC and LHC. While a full accounting of the jet/plasma interplay has not yet been implemented, the observables presented in this work (involving single high pt hadrons and jets) are expected to be rather insensitive to such effects, allowing us to interpret the data in a meaningful way while providing a reliable and comprehensive framework with which to extract transport properties of the QGP.

Speaker: Chanwook Park (McGill University)

Chanwook_Park_HP2018.pdf

10:45 → 12:25 Parallel 3: HF & QQbar

Convener: Cynthia Hadjidakis (Centre National de la Recherche Scientifique (FR))

10:45 PHENIX measurements of charm, bottom, and Drell-Yan via dimuons in p+p and p+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV

Dilepton spectra are a classic probe to study ultra-relativistic heavy ion collisions. At RHIC energies, the dimuon continuum is dominated by correlated pairs from charm and bottom semi-leptonic decays and the Drell-Yan process. No Drell-Yan measurement had been made at sqrt(sNN)= 200 GeV to date. A precise measurement of the Drell-Yan cross-section can provide constraints to PDFs. The dimuon spectra also contain information on heavy flavor angular correlations, which can constrain the relative contributions from different heavy flavor production mechanisms. Studying heavy flavor correlations in p+Au collisions may provide further insight to understanding cold nuclear matter effects. In this talk, we report measurements of µµ pairs from charm, bottom, and Drell-Yan in p+p and p+Au collisions at sqrt(sNN)= 200 GeV. A further shape analysis is applied to the heavy flavor pair correlations to extract the ratios of leading and next-to-leading order contributions.

Speaker: Yue Hang Leung (Stony Brook University)

hp2018vf.pdf

11:05 Charm baryon production in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Charm quarks are a powerful probe of the Quark-Gluon Plasma (QGP) created in high-energy heavy-ion collisions. They are produced in hard scattering processes on a timescale shorter than the QGP formation time and experience the whole system evolution. There have been extensive researches regarding the production of charm mesons, such as ${\rm D}^0$, ${\rm D}^+$D+, ${\rm D}^{*+}$, in heavy-ion collisions to investigate the interactions of charm quarks with the QGP constituents and the transport properties of the medium. The measurement of charm-baryon production, and in particular the baryon-to-meson ratios, provide unique information on hadronisation mechanisms, constraining the role of coalescence and testing the predicted presence of diquark states in the QGP.

Measurements of charm-baryon production in pp and p–Pb collisions are also essential to establish a baseline for Pb–Pb collisions. The measurements in pp collisions provide critical tests of pQCD calculations and models of charm hadronisation in vacuum; the measurements in p–Pb collisions are useful to study cold nuclear matter effects and the possible evolution with charged-particle multiplicity of the modification of charm hadronisation.

In this talk, ALICE results of charm-baryon measurements in pp, p-Pb, and Pb-Pb collisions are presented. The $p_{\rm T}$-differential cross sections of $\Lambda^+_{\rm c}$ -baryon production in pp collisions at $\sqrt{s}$ = 7 TeV and, for the first time, at $\sqrt{s}$ = 5.02 TeV are reported, along with the results in p–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The $p_{\rm T}$-differential cross section times branching ratio of the $\Xi^0_{\rm c}$ baryon measured in the decay channel $\Xi^0_{\rm c} \rightarrow e^+ \Xi^- \nu_e$ in pp collisions is also discussed. Finally, the first measurement of the $\Lambda^+_{\rm c}$ production in Pb–Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV is shown.

Speaker: Elisa Meninno (Universita e INFN, Salerno (IT))

Talk_ElisaMeninno_HardProbes2018_v8.pdf

11:25 Measurements of $\Lambda_c^{\pm}$, $D_s^{\pm}$, $D^{*\pm}$ and $D^{0}(\overline{D^{0}})$ Production in Au+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV at STAR

Because of their large mass, charm quarks are suggested to be an important tool for studying the properties of the Quark Gluon Plasma (QGP) produced in heavy-ion collisions. Recently, measurements at RHIC and LHC have indicated strong energy loss and large elliptic flow for open charm hadrons, similar in magnitude to those of light hadrons. The observed enhancements of $\Lambda_{c}^{\pm}$ and $D_{s}^{\pm}$ in Au+Au collisions suggest that the coalescence mechanism plays an important role also for charm quark hadronization.

In this presentation, we will report on the measurements of production of various charmed hadrons (including $D^{0}(\overline{D^{0}})$, $D_{s}^{\pm}$, $D^{*\pm}$ and $\Lambda_{c}^{\pm}$) obtained via topological reconstruction in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, utilizing the Heavy Flavor Tracker at STAR. Precise results on the $D^{0}$ yields from the 2014 data are reported for a wide transverse momentum range down to 0 GeV/c in various centrality bins. The $D_{s}^{\pm}$ and $D^{*\pm}$ spectra in different collision centralities will also be presented. With the high-statistics data collected in 2014 and 2016, and the usage of a supervised machine learning algorithm for cut optimization, the first measurement of the centrality and transverse momentum dependences of $\Lambda_{c}^{\pm}$ production will be shown. Finally, the total charm quark cross section extracted from these measurements in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV will be presented.

Speaker: Guannan Xie (University of Illinois at Chicago)

HardProbes18_OpenCharm_GXie.pdf

11:45 $\Lambda_C^{+}$ production in pp and PbPb collisions at 5.02 TeV with the CMS experiment

Due to their large mass, the interactions of heavy quarks with the quark-gluon plasma (QGP) may be different from those of light quarks. The lightest charm baryon is the $\Lambda_C^{+}$, composed of a charm quark and two light quarks. Measurements of $\Lambda_C^{+}$ production in both pp and PbPb collisions can provide important inputs to the understanding of heavy quark transport in the QGP and the creation of heavy quark mesons and baryons via coalescence. Models involving quark coalescence predict a large enhancement of $\Lambda_C^{+}$ production in PbPb collisions compared to pp collisions. The high luminosity datasets collected at a nucleon-nucleon center-of-mass energy of 5.02 TeV using the CMS detector have been used to measure $\Lambda_C^{+}$ production in both pp and PbPb collisions via the $\Lambda_C^{+} \to P^{+} K^{-} \pi^{+}$ decay channel. Results for differential cross sections for $\Lambda_C^{+}$ and ratios of $\Lambda_C^{+}$ over $D^0$ yields in pp and PbPb collisions, as well as the nuclear modification factors for $\Lambda_C^{+}$, are presented.

Speaker: Rui Xiao (Purdue University (US))

CMS_Lc_production.pdf

12:05 Heavy-flavour-production studies in a new energy and rapidity domain with the nuclear LHC beams in the fixed-target mode

The use of the lead LHC beam in the fixed-target mode allows one to study heavy-flavour production in a new energy domain, half way between SPS and RHIC. We will report on the opportunities for quarkonium and open charm/beauty production in Pb$A$ collisions including the prospects to study the relative suppression of $\Upsilon(nS)$ and new charmonium observables such as the $\chi_c$ and associated production. Both set-up using the ALICE and LHCb detectors will be discussed.

Speaker: Antonio Uras (Centre National de la Recherche Scientifique (FR))

HP2018_Uras.pdf

10:45 → 12:25 Parallel 4: HF & QQbar

Convener: Peter Alan Steinberg (Brookhaven National Laboratory (US))

10:45 Measurements of inclusive, boson-tagged, and heavy quark flavor jet energy loss in PbPb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the CMS detector

To quantify parton-medium interactions in the presence of a hot-and-dense medium, jets are studied as a function of parton flavor. In particular, gluon initiated jets are expected to lose more energy than corresponding quark jets. Inclusive jets are produced with a high gluon fraction at LHC energies, while boson-tagged jets have a strong enhancement of light-quark initiated jets. Additionally, heavy flavor quark jets can be identified with b-tagging techniques, providing unique information on the impact of the quark mass in energy loss processes. In this talk, we present measurements of jet energy loss in jets back-to-back with a Z or photon tag. With a clean tag of initial parton momentum quantifying the magnitude of energy loss, we then study the angular redistribution of energy with inclusive jet shapes, heavy-flavor tagged jet shapes, and radial scan of nuclear inclusive jet modification factor. The implications of these measurements are discussed through comparisons of jet energy loss, redistribution, and the medium response as a function of the parton flavor.

Speaker: Xiao Wang (University of Illinois at Chicago (US))

Xiao_for_HP2018.pdf

11:05 DREENA framework: predictions, comparison with experimental data, and proposal of a new observable

We will present our newly developed DREENA framework, which allows predicting energy loss of high $p_\perp$ partons traversing quark gluon plasma (QGP). The framework is based on dynamical energy loss formalism, and is applied to both the medium with constant temperature (DREENA-C) [1] and to evolving medium modeled by Bjorken 1+1D expansion (DREENA-B) [2]. The formalism allows making numerical predictions for a wide number of observables, centralities and collision energies, and for different experiments and collision systems. Accordingly, we will first show that our postdictions agree well with a wide range of data at different centralities. Furthermore, we will show that the predictions, which were published well before the data became available, agree very well with these data, again explaining some of the experimentally observed, but intuitively unexpected, suppression patterns. We will also propose a new observable [3], which allows clearly distinguishing between different energy loss mechanisms, as well as numerical predictions and simple scaling arguments that support this proposal. The first steps in our work towards the application of this model as a novel high-precision tomographic tool of QGP medium, will also be discussed.

[1] D. Zigic, I. Salom, J. Auvinen, M. Djordjevic and M. Djordjevic, arXiv:1805.03494 [nucl-th]
[2] D. Zigic, I. Salom, M. Djordjevic and M. Djordjevic, arXiv:1805.04786 [nucl-th].
[3] M. Djordjevic, D. Zigic, M. Djordjevic and J. Auvinen, arXiv:1805.04030 [nucl-th].

Speaker: Magdalena Djordjevic (Institute of Physics Belgrade)

HP18_Djordjevic.pdf

11:25 First results of EPOSHQ model for open heavy flavor production in AA at RHIC and LHC

The production of open heavy flavor in ultrarelativistic heavy ions collisions is known to be one of the best probes of the QGP phase before freeze out as well as of its coupling strength. One of the most topical issues is whether non perturbative effects could increase this strength around the critical temperature and hence communicate a stronger elliptical flow to heavy quarks. In [1], it was however shown that extra ingredients such as the bulk evolution or the hadronization scheme have drastic impact on these physical analysis, pleading for using state of the art approaches to describe all these "extra ingredients". In our contribution, we plan to provide and discuss first systematic results from the Nantes EPOS-HQ model combining the EPOS3 event generator - which has been proved to be able to reproduce a lot of observables in the soft sector for pp, pA and AA -- and the MC$@_s$HQ scheme to describe HQ evolution in QGP. One of the unique feature of EPOSHQ is to model HQ production consistently with the other particles, leading to initial state fluctuations affecting both heavy and light sector. This allows us to proceed to sophisticated analysis mixing light and heavy hadrons. In our contribution, we will take benefit from this newly developped approach to analyse the influence of the energy loss models on both event shape engineering observables such as $v_n(D)$ as a function of the corresponding $Q_n$ and on HQ correlations such as momentum imbalance which can help in constraining such models.

[1] R. Rapp et al, arXiv: 1803.03824

Speaker: Pol Gossiaux (Subatech)

EPOSHQ gossiaux.pdf

11:45 Measurements of heavy-flavour correlations and jets with ALICE at the LHC

Heavy quarks (charm and beauty) are produced in hard parton scatterings in the early stages of hadronic collisions. Therefore, they are ideal probes to investigate the properties of the Quark-Gluon Plasma (QGP) produced in ultra-relativistic heavy-ion collisions. The study of angular correlations between heavy-flavour particles and charged particles allows us to characterize the heavy-quark fragmentation process and its possible modification in a hot nuclear matter environment. The measurement of heavy-flavour jets gives more direct access to the initial parton kinematics and can provide further constraints for heavy-quark energy loss models, in particular adding information on how the radiated energy is dissipated in the medium.

Studies in pp collisions are mandatory to characterise heavy-quark production and fragmentation in vacuum, constituting the necessary reference for interpreting heavy-ion collision results. Differences between results from pp and p-Pb collisions can give insight on how the heavy-quark production and hadronisation into jets is affected by cold nuclear matter effects.

This contribution will include the latest heavy-flavour correlation and jet measurements with the ALICE detector in pp, p-Pb and Pb-Pb collisions from the LHC run-2 data. In particular, the results of azimuthal correlations of D mesons with charged particles in pp collisions at $\sqrt{s}$ = 5.02, 7 and 13 TeV and in p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV will be presented. Measurements of multiplicity and centrality dependent azimuthal correlations of heavy-flavour hadron decay electrons with charged particles in p-Pb and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV will be shown. In addition, the new measurement of D-meson tagged jet production in pp collisions at $\sqrt{s}$ = 5 TeV will be presented, along with the final results at $\sqrt{s}$ = 7 TeV, that include the study of the jet- momentum fraction carried by the D meson. Recent results in p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV will be reported. The first measurement of the nuclear modification factor of D-tagged jet in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV will also be discussed.

Speaker: Salvatore Aiola (Yale University (US))

SAiola_ALICE_HF_Corr_Jets.pdf

12:05 PHENIX results on system size dependence of J/$\psi$nuclear modification in p(d)+A collisions at $\sqrt{s_{\rm NN}}$=200 GeV

The dissociation of quarkonia in the medium created in heavy ion collisions is still one of the most debated topics in the heavy ion community. Progress in understanding the effect relies on a broad range of measurement in collisions with different nuclear sizes, covering a broad rapidity coverage. This presentation will report the J/Psi measurements performed by the PHENIX collaboration at forward and backward rapidities of 1.2<|y|<2.2 in p+Au, p+Al and 3He+Au collisions at sqrt(sNN) =200 GeV and the corresponding nuclear modification factors. The results will be compared to current quarkonia breaking models.

Speaker: John Matthew Durham (Los Alamos National Laboratory)

Durham_HardProbes2018.pdf

12:25 → 14:00 lunch

14:00 → 15:20 PLENARY: Jets & high-pT: Radiation and fragmentation

Convener: Raju Venugopalan (Brookhaven National Laboratory)

14:00 Vacuum-like jet fragmentation in a dense QCD medium

A main difficulty in understanding the dynamics of jets produced in the high-density environment of ultrarelativistic heavy ion collision, is to provide a unified description for the two sources of radiation that are a priori expected: the "vacuum-like" emissions responsible for the parton shower from large virtualities (of the order of the hard scale) down to the hadronisation scale and the "medium-induced" emissions responsible for the energy loss by the jet.

Within this talk, which is based on the recent paper [1], we demonstrate that these two mechanisms can be factorized from each other within a controlled, "double-logarithmic", approximation in perturbative QCD. We show that, due to the scatterings off the plasma, the in-medium parton showers differ from the vacuum ones in two crucial aspects: their phase-space is reduced and the first emission outside the medium can violate angular ordering. We compute the jet fragmentation function and find results in qualitative agreement with measurements at the LHC.

We also present our first results going beyond the double logarithmic approximation, which include both vacuum-like and medium-induced emissions and allow us to estimate the energy loss by the jet.

[1] P. Caucal, E. Iancu, A.H. Mueller, G. Soyez, e-Print: arXiv:1801.09703 [hep-ph], Phys.Rev.Lett. 120 (2018) 232001

Speaker: Mr Paul Caucal (IPhT)

presentation.pdf

14:20 Jet fragmentation and charged particle angular distribution within and around jets in Pb+Pb collisions with ATLAS

Abstract: In this talk, we present the latest measurements of the jet fragmentation and the angular distributions of charged particles within and around jets, as performed with the ATLAS detector in Pb+Pb collisions at the LHC. Jets are direct probes of the QCD medium created in these collisions, and studying jet fragmentation provides insight into the strength and mechanism of jet quenching. Fragmentation functions in Pb+Pb collisions and distributions of the transverse momentum of charged particles are compared to the same quantities measured in $pp$ collisions at the same collision energy. Measurements are presented as a function of jet transverse momentum and jet rapidity at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. The charged-particle angular distributions are also measured at distances extending outside the jet radius of $R = 0.4$.

Speaker: Akshat Puri (Univ. Illinois at Urbana Champaign (US))

HP2018_Puri.pdf

14:40 Exploring the phase space of jet splittings at ALICE in pp and Pb-Pb collisions using jet shapes and grooming techniques

Recent results in jet substructure measurements in pp and Pb--Pb collisions are presented at $\sqrt{s} = 7$ TeV and $\sqrt{s_{\rm NN}}= 2.76$ TeV respectively. The jet shapes discussed focus on the splitting process of jets either by quantifying the 2-prongness of the jet (τ2/τ1) or by parameterising the splitting process itself (zg and Rg). The Nsubjettiness of a jet could be sensitive to a semi-hard medium induced splitting occurring in-cone that takes a jet from being characteristically n-pronged to (n+1)-pronged. In a similar way such a process is expected to modify the momentum fraction (zg) and angular separation (Rg) of splittings identified using Soft Drop jet grooming. In addition to the kinematics of the splittings, the total number identified in the jet evolution (nSD) could indicate the amount of additional semi-hard in cone radiation occurring due to the presence of the medium. Jets are studied differentially across these observables using inclusive and semi-inclusive samples and results are compared to a variety of predictions from models and Monte Carlo generators modelling a range of different medium induced processes.

Speaker: Nima Zardoshti (University of Birmingham (GB))

Hard_Probes_Nima_Zardoshti.pdf

15:00 Probing properties of the QCD Medium using jet substructure techniques in pp and PbPb collisions at 5.02 TeV at CMS

We present recent results on measurements of inclusive jet substructures using grooming techniques and constituent correlations with pp and PbPb data collected with the CMS detector at a center-of-mass energy of 5.02 TeV per nucleon pair. Jet grooming techniques are used to focus on the hard structure of the jet by extracting the two subjets corresponding to the hardest parton splitting. The hard jet structure is also sensitive to the role of (de)coherent gluon emitters. Measurement of constituent jet charge and multiplicities are used in conjunction with parton flavor template fits to further map the interior structure of jets. Modifications relative to Monte Carlo baseline are interpreted as modification from Quark Gluon Plasma on a propagating parton and aid in understanding of mass and splitting function measurement previously performed by the CMS collaboration. Results and prospects of future jet substructure measurements of identified partons are discussed.

Speaker: Dhanush Anil Hangal (University of Illinois at Chicago (US))

HP18_JetSubstructure_DhanushHangal.pdf

15:20 → 15:45 Coffee break

15:45 → 18:35 PLENARY: Futures of Heavy-Ion Physics (Hard Probes)

Convener: David d'Enterria (CERN)

15:50 Intro: Futures of Heavy-Ion Physics (Hard Probes)

Speaker: David d'Enterria (CERN)

HP2018_future.pdf

15:55 Future heavy-ion facilities: sPHENIX experiment

Speaker: Gunther Roland (Massachusetts Inst. of Technology (US))

sPHENIX_HP2018_v2.pdf

16:15 Future heavy-ion facilities: Electron-Ion Collider (EIC)

Speaker: Abhay Deshpande (Stony Brook University)

EIC-Discussion-Abhay-R.pdf

US-EIC@hardprobes.pdf

US-EIC@hardprobes.pptx

16:35 Future heavy-ion facilities: Fixed-target LHC (AFTER)

Speaker: Jean-Philippe Lansberg (Centre National de la Recherche Scientifique (FR))

AFTER-HP2018_041018.pdf

16:55 Future heavy-ion facilities: LHeC/FCC-eh

Speaker: Max Klein (University of Liverpool (GB))

17:15 Future heavy-ion facilities: FCC-AA

Speaker: Andrea Dainese (INFN - Padova (IT))

Dainese_FCC_HP18_v1.pdf

17:35 Round-table discussion: Future of hard probes & heavy-ion collisions

Speakers: Abhay Deshpande (Stony Brook University), Andrea Dainese (INFN - Padova (IT)), Gunther Roland (Massachusetts Inst. of Technology (US)), Jean-Philippe Lansberg (Centre National de la Recherche Scientifique (FR)), Nestor Armesto Perez (Universidade de Santiago de Compostela (ES))

EIC Dicussion Abhay.pdf

19:25 → 19:30 Map Casino

casino.001.jpeg

19:35 → 21:35 Social Dinner

Friday, 5 October

09:00 → 10:40 PLENARY

Convener: Marco Van Leeuwen (Nikhef National institute for subatomic physics (NL))

09:00 Direct-photon and heavy-flavour production in proton-proton collisions at $\sqrt{s}$ = 7 TeV with ALICE

Speaker: Horst Sebastian Scheid (Johann-Wolfgang-Goethe Univ. (DE))

Scheid_Flash_Talk.pdf

09:05 The problem of overlapping formation times

Speaker: Shahin Iqbal (National Centre for Physics)

HardProbes2018_Shahin.pdf

09:10 Heavy flavour dynamics in event-by-event viscous hydrodynamic backgrounds

Speaker: Roland Katz (University of São Paulo)

Katz_Hard_Probes_2018_HF.pdf

09:15 Measurement of the azimuthal anisotropy of charged particles in 5.02 TeV Pb+Pb and 5.44 TeV Xe+Xe collisions with ATLAS

Speaker: Klaudia Burka (Polish Academy of Sciences (PL))

HP18_kburka_flashtalk.pdf

09:20 Measurements of Bottomonium production in pp, pPb and PbPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV

Speaker: Geonhee Oh (Chonnam National University (KR))

Geonhee_HP2018_Flash_Talk_vFinal.pdf

09:25 Initial-state in heavy-ion collisions at colliders: Theory summary

Speaker: Vladimir Skokov (Brookhaven national laboratory)

Skokov_HP.pdf

09:55 Initial-state in heavy-ion collisions at colliders : Experimental summary

Speaker: Émilien Chapon (CERN)

chapon_IS_exp_HP18.pdf

10:20 Coffee break

10:40 → 12:20 PLENARY

Convener: Marta Verweij (Vanderbilt University (US))

10:40 Ultraperipheral collisions: Summary

Speaker: Janet Elizabeth Seger (Creighton University (US))

UPCSummary_Seger_V1.pdf

11:05 Jets in QCD matter: Experimental summary

Speaker: Michael Linus Knichel (CERN)

MKNICHEL_HP_2018-10-05.pdf

11:30 Jets in QCD matter: Theory summary

Speaker: Korinna Christine Zapp (LIP Laboratorio de Instrumentacao e Fisica Experimental de Part)

HP18_KZapp.pdf

11:55 Open heavy flavour: Experimental summary

Speaker: Deepa Thomas (University of Texas at Austin (US))

OpenHFSummary_Deepa-10-05.pdf

12:20 → 14:00 Lunch break

14:00 → 15:15 PLENARY

Convener: Christina Markert (University of Texas at Austin (US))

14:00 Quarkonia: Experimental summary

Speaker: Hugo Denis Antonio Pereira Da Costa (Université Paris-Saclay (FR))

Hugo_Pereira_HardProbes2018.pdf

14:25 Open heavy flavour and quarkonia: Theoretical summary

Speaker: Ralf Rapp (Texas A&M University)

hp18.pdf

hp18.pptx

14:50 Electroweak observables: Summary

Speaker: Friederike Bock (CERN)

HP2018_ElectroweakProbes.pdf

15:15 → 15:45 Coffee break

15:45 → 16:10 PLENARY

Convener: Christina Markert (University of Texas at Austin (US))

15:45 HP'20 Austin presentation

Speaker: Christina Markert (University of Texas at Austin (US))

HP2020_final_HP2018.pdf

16:00 Talk (closing)

Speakers: Andreas Morsch (CERN), David d'Enterria (CERN), Philippe Crochet (Univ. Blaise Pascal Clermont-Fe. II (FR))

HardProbes2018_closing.pdf