10th International Workshop on High-pT Physics in the RHIC/LHC era

9 Sept 2014, 08:15 → 12 Sept 2014, 13:40 Europe/Paris

Pascal Auditorium (SUBATECH Nantes)

HP_T Workshop 2014 in SUBATECH

The 10^th HP_T workshop at RHIC and LHC will be held in SUBATECH / Mines de Nantes from Sept. 9 to Sept. 12. 2014 located in the north east part of Nantes.
The main topics of the workshop are presented on the homepage.

The meeting is in a workshop format, with only plenary presentations and ample room for discussion between the presentations. The workshop will end Friday 12^th of September before lunch.

The conference will publish proceedings.

The regular registration fee is 150€. It will include:

•  Coffee breaks and 3 lunches
•  A dinner in a restaurant located downtown
•  A social event which will be defined soon

Any accompanying person willing to join the social event and/or dinner, will have to pay the corresponding fee: 25€ and 70€ respectively (included for regular participant).

We will be offering a limited number of financial supports for PhD students and young postdocs (within two years of the completion of their Ph.D). In order to apply, please send a recommendation letter to us or upload it using the registration form.

A list of hotels can be found here.

Tuesday 9 September

08:15 → 10:00 Registration

10:00 → 10:15 Welcome address

Speakers: Alexandre Shabetai (CERN - PH/UAI), Bernd Grambow, Magali Danielle Estienne (Laboratoire de Physique Subatomique et des Technologies Associe)

Slides Welcome_HPT2014_ok.pdf

10:15 → 12:35 Heavy flavor and quarkonia I

Convener: Dr Andreas Morsch (CERN)

10:15 Recent quarkonia results from the PHENIX experiment at RHIC

The measurement of quarkonia production in relativistic heavy ion collisions provides a powerful tool for studying the properties of the hot and dense matter created in these collisions. To be really useful, however, such measurements must cover a wide range of quarkonia states, collision energies and colliding species. In this talk I will present recent results from the PHENIX collaboration on quarkonia production in different nucleus-nucleus collisions at various energies in a wide rapidity range, their interpretation, and comparison to p+p collisions.

Speaker: Alexandre Lebedev (Department of Physics and Astronomy-Iowa State University-Unknow)

Slides Quarkonia_Nantes2014.pdf

10:55 Coffee break

11:25 Open heavy flavor production from STAR

Speaker: Daniel Kikola (Warsaw University of Technology)

Slides HighPtWorkshop_2014_HeavyFlavor.pdf

12:05 Constraining heavy-quark energy loss with the joint analysis of D-mesons and non-prompt J/Psi nuclear modification factors

The mass dependence of particle production at intermediate transverse momentum in ultrarelativistic heavy ion collisions provides a unique opportunity to better constrain the models of parton energy loss in the quark gluon plasma. This mass-hierarchy, first observed for most central collisions at LHC, has been recently extended to several classes of centrality, combining D-mesons nuclear modification factor ($R_{\rm AA}$) of the ALICE collaboration with the one of non-prompt $J/\psi$ stemming from B-meson decay of the CMS collaboration. In our contribution, we first analyse the respective mass dependence of collisional energy loss and of radiative energy loss. For the later case, a special care of the gluon phase-space boundary is taken. The quantitative importance of such an effect at intermediate energies has indeed recently been demonstrated in [1]. We then present theoretical calculations for the joint $R_{\rm AA}$ of D-mesons, B-mesons and non-prompt $J/\psi$ vs centrality, adopting either pure collisional energy loss scenario, pure radiative, or a cocktail of both types. For this purpose, energy loss mechanisms have been implemented in a Boltzmann transport (which does not suffer from the flaws of the Fokker-Planck evolution equation), while light partons are distributed in space and time along the hydrodynamical evolution of the hot medium initiated according to the state of the art EPOS. This approach has proven [2] to explain successfully several observables measured at RHIC and LHC, such as the $R_{\rm AA}$ of open heavy flavor mesons and of non-photonic single electrons in most central collisions as well as the elliptic flow of those particles. References ---------- [1] J. Aichelin, P.B. Gossiaux and Th. Gousset, Phys. Rev. D 89 (2014) 074018; [2] M. Nahrgang, J. Aichelin, P.B. Gossiaux and K. Werner, Phys. Rev. C 89, 014905

Speaker: Pol Gossiaux (Subatech)

Slides high_pT_gossiaux.pdf

12:35 → 14:05 Lunch

14:05 → 15:05 Heavy flavor and quarkonia II

Convener: Dr Gergely Gabor Barnafoldi (Wigner RCP Hungarian Academy of Sciences (HU))

14:05 Measurements of heavy-flavour decay leptons with ALICE

In heavy-ion collisions at ultrarelativistic energies sufficiently high temperature and energy density can be reached to form the Quark-Gluon Plasma (QGP), a deconfined state of strongly-interacting matter. Heavy quarks, i.e. charm and beauty, are well suited to probe the interaction dynamics inside the medium, since they are mainly produced in initial hard scattering processes and experience the full evolution of the system. The elliptic azimuthal anisotropy ($v_2$) of heavy-flavour hadrons at low transverse momentum is an observable sensitive to the collective motion of heavy quarks in the medium created in ultrarelativistic heavy-ion collisions. Moreover $v_2$ of heavy-flavour hadrons at high transverse momentum is sensitive to the path-length dependence of the energy loss of heavy quarks. Further insight into the parton energy loss within the QGP can be gained from measurements of the modification of the transverse momentum distributions of heavy flavours in heavy-ion collisions with respect to binary scaled pp collisions ($R_{\rm{AA}}$). In addiction, measurements of angular correlations between heavy-flavour hadrons and other charged hadrons in Pb-Pb collisions can give more differential access to heavy-quark energy loss and to possible modifications of the parton shower and hadronisation in the presence of the medium. Measurements of heavy-flavour correlations in pp collisions, apart from being the reference for nuclear collision systems, provide a testing ground for perturbative QCD calculations. Furthermore, heavy-flavour correlations in p-Pb collisions can be used to investigate cold nuclear matter effects and a potential existence of collective phenomena in such collisions. The ALICE collaboration has measured the production of open heavy-flavour hadrons via their semi-electronic decays at mid-rapidity and via their semi-muonic decay channel at forward rapidity in pp collisions at $\sqrt{s}$ = 2.76 and 7 TeV, p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, and Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. We will present the ALICE results on the elliptic azimuthal anisotropy of heavy-flavour decay leptons in Pb-Pb collisions, the nuclear modification factor of heavy-flavour decay leptons in p-Pb and Pb-Pb collisions, and the angular correlations between heavy-flavour decay electrons and charged particles in pp, p-Pb, and Pb-Pb collisions.

Speaker: Denise Aparecida Moreira De Godoy (Laboratoire de Physique Subatomique et des Technologies Associe)

Paper proceedings_DeniseGodoy.pdf

Slides DeniseGodoy_HighPT.pdf

14:35 Charms and prompt photons production with the event generator EPOS

EPOS is an event generator using a unified formalism for the description of p-p, p-A and A-A collisions. Recently, prompt photons and charms have been implemented. One of our aims is to use this work for the study of the Quark-Gluon Plasma. After a short presentation of EPOS formalism, our results for pp collisions will be presented.

Speaker: Mr Benjamin GUIOT (Subatech)

Paper guiot-highPt.pdf

Slides High-pT-2014.pdf

15:05 → 16:35 Photons

Convener: Dr Gergely Gabor Barnafoldi (Wigner RCP Hungarian Academy of Sciences (HU))

15:05 Challenges of direct photon production at forward rapidities and large pT

Using two different models we investigate production of direct photons in proton-nucleus and nucleus-nucleus interactions at RHIC and LHC energies. Direct photons produced in interactions with nuclear targets represent a cleaner probe for investigation of nuclear effects than hadrons, since photons have no final state interaction and no energy loss or absorption is expected in the produced hot medium. Therefore, besides the Cronin enhancement at medium-high transverse momenta pT and isospin effects at larger pT, one should not expect any nuclear effects. However, this fact is in contrast to the PHENIX data providing an evidence for a significant large-pT suppression at mid rapidities in central d+Au and Au+Au collisions that cannot be induced by coherent phenomena (gluon shadowing, Color Glass Condensate). We demonstrate that such an unexpected results is subject to deficit of energy induced universally by multiple initial state interactions (ISI) towards the kinematic limits (large Feynman xF and/or large pT). To enhance the effects of coherence, one should be cautious going to forward rapidities and higher energies. In the LHC kinematic region ISI corrections are irrelevant at mid rapidities but cause rather strong suppression at forward rapidities and large pT. Contribution of coherent effects associated with gluon shadowing is effective predominantly at small and medium-high pT. We perform a comparison of numerical calculations in the color dipole approach with calculations in the QCD improved parton model and compare both models with available data from the RHIC and LHC collider experiments. We perform also predictions for expected onset of ISI effects at forward rapidities which can be verified by the future measurements at LHC.

Speaker: Michal Krelina (Czech Technical University (CZ))

Paper HPT2014-Krelina_v4.pdf

Slides krelina-HPT2014_v2_nDS.pdf

15:35 Coffee break

16:05 Electromagnetic radiation as a deep probe with finite chemical potential from quark-gluon plasma

We study the electromagnetic radiation from quark-gluon plasma using a simple model incorporating the phenomenological flow parameter of quarks and gluons in thermal dependent quark mass with the variation of quark chemical potential. The production rate is observed in the relevant range of transverse momentum. The photon production increases with increases the value of quark chemical potential. The results are thus compatible with the formation of quark-gluon plasma and compare with other work.

Speaker: Dr Yogesh Kumar (University of Delhi)

Slides yogesh_photon_france.pdf

16:35 → 17:35 Initial conditions / saturation / event activity I

Convener: Dr Gergely Gabor Barnafoldi (Wigner RCP Hungarian Academy of Sciences (HU))

16:35 Multi-Reggeon exchanges at high pT from gluon saturation

To be submitted

Speaker: Jamal Jalilian-Marian

Paper jamal.pdf

Slides Jalilian-Marian-highpt-nantes-2014.pdf

17:05 Investigating the Saturated Glauber Picture in pA/dAu Collisions in the RHIC/LHC Era

Glauber model has been used successfully so far in heavy ion collisions. This was especially important for the determination on the number of collisions, $\langle N_{bin}\rangle$, which is a necessary to determine the strength of the nuclear effect measured in proton-nucleus and nucleus-nucleus collisions compared to proton-proton. Recent preliminary RHIC & LHC data have been taken in various centralities in $pA$/$dAu$ collisions, might give a new lieu of the saturated Glauber picture introduced earlier in Ref. [1,2,3]. [1] G. Papp, P. Lévai and G. Fai, Phys. Rev. **C61**, 021902 (2000); [2] Y. Zhang, G. Fai, G. Papp, G.G. Barnaföldi, and P. Lévai, Phys. Rev. **C65**, 034903 (2002); [3] G. David talk at the High pT physics at LHC Workshop – Grenoble, France "Event characterization in (very) asymmetric collisions" (Sep. 26, 2013)

Speaker: Szilveszter Miklos Harangozo (Hungarian Academy of Sciences (HU))

Slides talk_Harangozo.pdf

Wednesday 10 September

09:00 → 11:50 QCD and medium effect

Convener: Jan Rak (University of Jyvaskyla (FI))

09:00 Is $\hat{q}$ a physical quantity or just a parameter? and other unanswered questions in High-${p_T}$ Physics}

The many different theoretical studies of energy loss of a quark or gluon with their color charges fully exposed passing through a medium with a large density of similarly exposed color charges (i.e. a QGP), have one thing in common: the transport coefficient of a gluon in the medium, denoted $\hat{q}$, which is defined from the mean 4-momentum transfer$^2$/collision but is expressed as the mean 4-momentum transfer$^2$ per mean free path of a gluon in the medium. Thus the mean 4-momentum transfer$^2$ for a gluon traversing length $L$ in the medium is, $\left<{q^2(L)}\right>=\hat{q}\,L=\mu^2\,L/\lambda_{\rm mfp}$, where $\lambda_{\rm mfp}$ is the mean free path for a gluon interaction in the medium, and $\mu$, the mean momentum transfer per collision, is the Debye screening mass acquired by gluons in the medium. In this, the original BDMPSZ formalism, the energy loss of an outgoing parton due to coherent gluon bremsstrahlung per unit length ($x$) of the medium, $-dE/dx$, takes the form: $${-dE \over dx }\simeq \alpha_s \left<{q^2(L)}\right>=\alpha_s\, \hat{q}\, L=\alpha_s\, \mu^2\, L/\lambda_{\rm mfp} \qquad ,$$ so that the total energy loss in the medium goes like $L^2$. Also the accumulated transverse momentum$^2$, $\left<{k_{\perp}^2}\right>$, for a gluon traversing a length $L$ in the medium is well approximated by $\left<{k_{\perp}^2}\right>\approx\left<{q^2(L)}\right>=\hat{q}\, L$. A simple estimate shows that the $\left<{k_{\perp}^2}\right>\approx\hat{q}\,L$ should be observable at RHIC via the broadening of di-hadron azimuthal correlations. For a trigger particle with $p_{T_t}$, assume that the away-parton traverses slightly more than half the diameter of the QGP\ for central collisions of Au+Au, say 8 fm. This corresponds to $\left<{k_{\perp}^2}\right>=\hat{q}\,L=8$ GeV$^2$, for $\hat{q}=1$ GeV$^2$/fm, compared to the measured $\left<{k_T^2}\right>=8.0\pm 0.2$ (GeV/c)$^2$ for di-hadrons in $p-p$ collisions at $\sqrt{s_{_{NN}}}$=200 GeV, which should be visible as an azimuthal width $\sim\sqrt{2}$ larger in Au+Au than in $p-p$ collisions . Measurements relevant to this issue will be discussed including recent STAR jet results presented at QM2014. Other topics to be discussed include the danger of using forward energy to define centrality in $p(d)+$A collisions for high $p_T$ measurements, the danger of not using comparison $p-p$ data at the same $\sqrt{s}$ in the same detector for $R_{AA}$ or lately for $R_{pA}$ measurements. Also, based on a comment at last year's 9th workshop that the parton energy loss is proportional to $dN_{\rm ch}/d\eta$, results on the depencence of the shift in the $p_T$ spectra in A+A collisions from the $T_{AA}$-scaled $p-p$ spectrum (to be presented in detail in another talk) will be further discussed.

Speaker: Michael Tannenbaum (Brookhaven National Laboratory (US))

Paper Tannenbaum-HighpTLHC-2014-proc.pdf Tannenbaum-HighpTLHC-2014-procv2.pdf

Slides MJT-HighpTLHC-2014-asgiven.pptx

09:40 Intrinsic transverse momentum distribution of jet constituents in pPb collisions at ALICE

To be submitted

Speaker: Jiri Kral (University of Jyvaskyla (FI))

Slides jk_hpt_nantes.pdf

10:10 Coffee break

10:30 coherent medium-induced parton energy loss

Recent results on the coherent energy loss of an energetic compact partonic system crossing a cold nucleus are reviewed. Coherent energy loss strongly affects hadron production rates in nuclear (p-A, A-A) collisions, such as quarkonium, but also light hadron production. It also provides an interesting template for long-distance but however process-dependent effects in QCD.

Speaker: Stephane Peigne

Slides peigne-HPT.pdf

11:10 Multiple scattering versus fully coherent scattering in pA and AA collision

Multiple scattering is the main process that is proposed for jet quenching in AA collision. Random multiple scattering leads to a very weak energy dependence of the jet energy loss and to an $L^2$ dependence. Energy dependent energy loss was attributed to a single scattering term in some formalisms and it was missing in other formalisms. Recently a model was proposed where besides multiple soft scatterings a hard scattering is added, this leads to a modified energy-dependence in medium induced jet energy loss. We give an alternative description/formalism of this model based on a path integral approach.

Speaker: haitham Zaraket (Lebanese University)

Paper zaraket-nantes.pdf

Slides Haitham_zaraket-Nantes.pdf

11:50 → 13:10 Lunch

13:10 → 18:00 Social event

Thursday 11 September

09:00 → 13:00 Jets I

Convener: Michael Tannenbaum (Brookhaven National Laboratory (US))

09:00 Jet fragmentation and energy loss in heavy ion collisions with the ATLAS detector

To be submitted

Speaker: Martin Spousta (Charles University)

Paper MartinSpousta.pdf

Slides Spousta_HighPtWorkshop_20140829_v2.pdf

09:40 Measurement of inclusive jet spectra in pp, p-Pb, and Pb-Pb collisions with the ALICE detector

10th International Workshop on High-pT Physics at RHIC/LHC, abstract draft for a talk, Rüdiger Haake (Westfälische Wilhelms-Universität Münster, Germany) on behalf of the ALICE collaboration. Title: Measurement of inclusive jet spectra in pp, p-Pb, and Pb-Pb collisions with the ALICE detector Highly energetic jets are sensitive probes for the kinematic properties and the topology of high energy hadron collisions. Jets are collimated sprays of charged and neutral particles which are produced in fragmentation of hard scattered partons in an early stage of the collision. In ALICE, jets have been measured in pp, p-Pb, and Pb-Pb collisions at several collision energies. While analyses of Pb-Pb events unveil properties of the hot and dense medium formed in heavy ion collisions, pp and p-Pb collisions can shed light on hadronization and cold nuclear matter effects in jet production. Additionally, pp and p-Pb serve as a baseline for disentangling hot and cold nuclear matter effects. p-Pb analyses can also serve as an important constraint for the nuclear parton density functions providing information about the nuclear environment. The exact evaluation of the background from the underlying event is an especially important ingredient. Due to the different nature of underlying events, each collision system requires a different analysis technique for removing the effect of the background on the jet sample. The focus of this talk is on the ALICE measurements of jet spectra and nuclear modification factors connecting p-Pb and Pb-Pb events to pp collisions. Furthermore, the radial jet structure is presented by comparing jet spectra reconstructed with different resolution parameters.

Speaker: Rudiger Haake (Westfaelische Wilhelms-Universitaet Muenster (DE))

Paper HPT2014_Proceedings_Haake.pdf

Slides 14-09-11-Nantes-Haake.pdf

10:20 Production of jets and high-pT hadrons in p-Pb collisions with the ATLAS detector

To be submitted

Speaker: Aaron Angerami (Columbia University (US))

10:50 Coffee break

11:20 Studies of high-pT jet and hadron production in pPb collisions with CMS

The suppression of high-$p_{\rm T}$ jets and hadrons is one of the important signatures of the strongly interacting medium produced in PbPb collisions; nevertheless observed as a combination of various effects from initial and final states of the interaction. The study of pPb collisions aid the dissociation of these effects and lead to a clearer understanding of jet quenching. In this talk the CMS results for nuclear modification factors of jets and hadrons in pPb collisions are presented, including the methods for obtaining pp reference spectra at $\sqrt{s_{_{NN}}} = 5.02$ TeV.

Speaker: Yetkin Yilmaz (Laboratoire Leprince-Ringuet, France)

Slides highpt10_nantes_yetkin_2014_v2.pdf

11:50 The state of jet physics - what do the data tell?

I give an assessment of the current knowledge on jet-medium interaction based on systematic studies across a large body of model combinations. Such a study allows to identify key observables which are especially sensitive to certain physics properties. For instance, utilizing observables which maximize the sensitivity to pathlength dependence, it can be shown that the data require a highly coherent jet-medium interaction. This rules out a large class of models which does not have this property and has interesting implications for the medium degrees of freedom as seen by a jet. Similarly, the P_T dependence of the subleading hadrons in a jet shows a clear breaking of the self-similarity of the fragmentation function by the medium, thus ruling out ideas of fractional energy loss. Across all available observables, the properties of the jet-medium interaction are very tightly constrained and allow to start utilizing hard processes as calibrated tomographic probes of the medium.

Speaker: Thorsten Renk (University of Jyväskylä)

Slides trenk-nantes.pdf

12:30 Production of strange particles in charged jets in p–Pb and Pb–Pb collisions measured with ALICE at the LHC

Studies of jet production can provide information about the properties of the hot and dense strongly interacting matter created in ultra-relativistic heavy-ion collisions. Specifically, measurement of strange particles in jets may clarify the role of fragmentation processes in the anomalous baryon to meson ratio at intermediate particle $p_\textrm{T} $ that was observed in Pb--Pb and, to a lesser extent, in p--Pb collisions. In this contribution, measurements of the $p_\textrm{T}$ spectra of $\Lambda$ and $\overline{\Lambda}$ baryons and $\textrm{K}^0_\textrm{S}$ mesons produced in association with charged jets in Pb--Pb collisions at $\sqrt{s_\textrm{NN}}=2.76\:\textrm{TeV}$ and p--Pb collisions at $\sqrt{s_\textrm{NN}}=5.02\:\textrm{TeV}$ are presented. The analysis is based on data which was recorded by ALICE at the LHC, exploiting its excellent particle identification capabilities. The baryon/meson ratios of the spectra of strange particles associated with jets are studied for different event activities in p-Pb and are restricted to central events in Pb-Pb. A comparison to the ratios obtained for inclusive particles and for particles stemming from the underlying event as well as to PYTHIA simulations is shown.

Speaker: Alice Zimmermann (Ruprecht-Karls-Universitaet Heidelberg (DE))

Paper ProceedingsHighPtWorkshop_AliceZimmermann.pdf

Slides HighPtNantes2014_final.pdf

13:00 → 14:40 Lunch

14:40 → 16:20 Jets II

Convener: Thorsten Renk (University of Jyväskylä)

14:40 Statistical Jet Fragmentation at LHC

Fragmentation functions measured in e+e- and pp collisions are well reproduced via a fragmentation model based on microcanonical statistics and superimposed Euler--Gamma-type multiplicity fluctuations [1,2]. The power of the obtained analytic fragmentation function (which is a cut-power function in x) develops a double-logarithmic dependence on the QCD scale Q^2 [3]. Besides, this function also describes transverse hadron spectra measured in pp and AA collisions at RHIC and LHC energies [4,5]. Interestingly, the power of the spectra of pions, kaons and protons stemming from pp collisions exhibits a similar double-logarithmic dependence on the collision energy sqrt{s} and on the hadron multiplicity N (measured in the |eta|<1 region) [6]. [1] K. Urmossy etal, Phys. Lett. B, 718 (2012) 125-129, arXiv:1204.1508 [2] K. Urmossy etal, Phys. Lett. B, 701 (2011) 111-116, arXiv:1101.3023 [3] G. G. Barnafoldi etal, Gribov 80 Conference: C10-05-26.1, p.357-363 [4] K. Urmossy etal, Phys. Lett. B, 689 (2010) 14-17, arXiv:0911.1411 [5] G. G. Barnafoldi etal, 'Hot Quarks 2010', J. Phys. Conf. Ser. 270 (2011) 012008 [6] K. Urmossy, Submitted to EPJC, arXiv:1212.0260

Speaker: Karoly Uermoessy (Hungarian Academy of Sciences (HU))

Paper Urmossy_HighpT_2014.pdf

Slides Urmossy_HighpT2014.pdf

15:20 sPHENIX Jet Physics

First thought to be a gas of nearly non-interacting particles, the Quark-Gluon Plasma (QGP) was unexpectedly found to behave as a near perfect fluid. How the small viscosity of the QGP arises from the microscopic system of quarks and gluons is not yet known and requires a greater understanding of the state's microscopic description. Fast partons are useful probes of that structure and are best studied via full reconstruction of jet showers. The PHENIX collaboration is pursuing a series of aggressive upgrades aimed at enhanced jet reconstruction capabilities to make use of the full enhanced luminosity at RHIC and complement those being made at the LHC. With increased coverage and the addition of hadronic calorimetry, we will demonstrate that the sPHENIX upgrade will be well positioned to provide a broad and exciting program of jet probe measurements and will be combined with a flexible accelerator facility capable of providing a wide range of collision systems and beam energies.

Speaker: Dr Michael McCumber (Los Alamos National Laboratory)

Slides mccumber_slides_nantes_2014.pdf

15:50 Momentum balance of dijets from in-medium hard scattering partons

In a recent line of work, we have been studing the energy-momentum deposited by fast moving partons within a medium using linearized viscous hydrodynamics. We have shown that for the conditions arising in heavy-ion collisions, energy momentum is preferentially deposited along the head shock of the fast moving partons. We now focus on the analysis of the momentum balance of dijets produced by partons that deposit energy and momentum in the medium and hadronize via Cooper-Frye. We take into account the collision geometry and explore power-like energy loss schemes. Our preliminary results show that we can account for the jet momentum imbalance in central collisions, with both the Cooper-Frye profile and the contribution from three in-medium parton production.

Speaker: Maria Elena Tejeda-Yeomans (DF-USON)

Slides Tejeda-HighPtWkshp2014.pdf

16:20 → 16:50 Coffee break

16:50 → 17:50 Initial conditions / saturation / event activity II

Convener: Thorsten Renk (University of Jyväskylä)

16:50 Particle rapidity distribution in proton-nucleus collisions using the proton-contributor reference frame

I define the proton-contributor reference frame in proton nucleus (p--A) collisions as the center of mass of the system formed by the proton and the participant nucleons of the nucleus. Assuming that the rapidity distribution of produced particles is symmetric in the proton-contributor reference frame, several measurements in p-Pb collisions at sqrt(sNN)=5.02TeV can be described qualitatively. These include rapidity distributions of charged particles, J/ψ and Z bosons.

Speaker: Gines Martinez-Garcia (Laboratoire de Physique Subatomique et des Technologies Associe)

Slides GinesMartinez_HighpTWorkshop.pdf GinesMartinez_HighpTWorkshop.pptx

17:20 Geometry and event activity in very asymmetric collisions

Recent results at RHIC and LHC on centrality dependence of various high pT observables in very asymmetric collisions (p/d+A) raised a lively discussion on the ways event-by-event geometry is determined experimentally. In other words: are the methods based on average soft production still applicable once a truly hard scattering occurs in a very asymmetric system? In the kinematic limit the answer is obviously no; at some point the naive factorization of soft and hard processes should break down. We will compare strength, weaknesses and limitations of the original Glauber MC and some alternative procedures suggested so far, and explore whether at least part of the model assumptions can be verified or falsified, which would lead to a reduction of the ambiguities in basic quantities like impact parameter, number of participants and collisions.

Speaker: Gabor David (Brookhaven National Laboratory)

Slides gdavid_hptlhc_nantes2014.pdf

19:30 → 23:00 Social dinner

Friday 12 September

09:00 → 13:00 Jets, high pT hadrons and correlations

Convener: Stephane Peigne

09:00 Overview of ALICE results on azimuthal correlations using neutral- and heavy-meson triggers

The ALICE detector is dedicated to studying the properties of hot and dense matter created in heavy-ion collissions. Amongst the probes used to investigate these properties are high-momentum particles, which originate in hard-scatterings occuring before the fireball creation. The traversing fragments of hard scatterings interact with the hot and dense matter and via this interaction their spectra and azimuthal distribution are modified. This is probed by the measurement of nuclear modification factor, where the p_{T} spectra obtained in Pb--Pb collisions are compared to a pp baseline. A strong suppresion of charged hadrons as well as neutral- and heavy-mesons was observed at p_{T} > 4 GeV/c. Azimuthal correlations, using the high-momentum hadrons as triggers, can provide further insight into how the presence of medium modifies the final particle production and from the comparison with theoretical models can test their predictions of its properties. We give an overview of ALICE azimuthal-correlation measurements of neutral- and heavy-meson triggers with charged hadrons in pp at \sqrt_{s_{NN}}=7 TeV and Pb--Pb at \sqrt_{s_{NN}}=2.76 TeV. We also present a measurement of \pi_{0} correlation with jets in pp at \sqrt_{s_{NN}}=7 TeV.

Speaker: Sona Pochybova (Hungarian Academy of Sciences (HU))

Slides HPT2014_SonaPochybova_vf.pdf

09:40 Jet and high pt hadron production overview from STAR

To be submitted

Speaker: Sonja Kabana (Laboratoire de Physique Subatomique et des Technologies Associe)

10:20 Detailed study of parton energy loss via measurement of fractional momentum loss of high $p_T$ hadrons in heavy ion collisions

High $p_T$ hadrons as fragments of hard scattered patrons are a powerful tool to probe the opacity of the system produced in nucleus collisions, through the loss of their momenta in the medium created. Since the last decade, the momentum loss has been quantified in terms of suppression of the yield at given $p_T$. We recently measured the fractional momentum loss ($\delta p_T/p_T$, where $p_T$ being original momentum of a hadron, and $\delta p_T$ being the loss of its momentum in the medium) of the hadrons statistically, and found that the partons lose 1.5 times larger $\delta p_T/p_T$ in central 2.76TeV Pb+Pb collisions at the LHC than that in 200GeV Au+Au collisions at RHIC. We are extending the measurement of $\delta p_T/p_T$ over to different collision species and cms energies and investigating a systematic trend. We recently found that the $\delta p_T/p_T$ nicely scales with $dN/d\eta$. We will report the latest investigation of parton energy loss in A+A collisions using high $p_T$ hadrons.

Speaker: Takao Sakaguchi (BNL)

Paper SakaguchiHPT2014Proc.pdf

Slides Sakaguchi_HighpT2014_ver2.pdf

11:00 Coffee break

11:30 Study of high-p_{T} hadron-jet correlations in ALICE

Jets provide unique probes of the medium created in ultra-relativistic heavy-ion collisions. Here, the observed jet quenching phenomena in central collisions prove that jets are sensitive to interesting properties of strongly-coupled matter. In addition, jet production in elementary processes, such as pp collisions, is well understood within the framework of perturbative QCD, providing a rigorous theoretical basis for jet quenching calculations. In the talk we will report the measurement of semi-inclusive p_{T} spectra of charged particle jets that recoil from a high-p_{T} hadron trigger in Pb--Pb and pp collisions at \sqrt{s_{NN}} = 2.76 TeV and \sqrt{s} = 7 TeV, respectively. In this analysis, the copious yield of uncorrelated trigger hadron-jet matchings in central Pb+Pb collisions is removed by calculating the difference between two spectra corresponding to disjoint trigger hadron p_{T} ranges. This procedure does not impose any fragmentation bias on the recoil jet population, which is therefore collinear and infrared safe. The resulting distributions obtained for different values of jet resolution parameter are used to study the modification of jet structure in the medium. The results will be compared with calculations based on PYTHIA and JEWEL.

Speaker: Filip Krizek (Acad. of Sciences of the Czech Rep. (CZ))

Paper krizek_v4.pdf

Slides KrizekNantes_v3.pdf

12:00 Jet-track correlations in PbPb collisions with CMS

The measurements of fragmentation functions, jet shapes and missing transverse momenta by the CMS collaboration show that jet properties in central PbPb collisions are modified compared to pp collisions. Modification of jets at small angles are investigated by the first two measurements and jets are observed to get softer and broader. Missing transverse momenta measurement provides detailed characterisation of angular distribution of momentum flow with respect to jets at large angles. Focussing on dijet events with large dijet transverse momentum asymmetry makes it possible to investigate momentum flow around jets which undergo a significant energy loss as they transverse the medium.

Speaker: Doga Can Gulhan (Massachusetts Inst. of Technology (US))

Slides doga_10thhighptnantes_v1.pdf

12:30 Investigations on identified two-particle correlations and quantum number conservation in p-p, p-Pb and Pb-Pb collisions at LHC energies

Unidentified two-particle correlations are widely pursued at RHIC and LHC to investigate particle production and hadronization mechanisms as well as collective effects. Two-particle correlations with identified trigger and identified associated particles give information on quantum number conservation and the flavour balance during parton fragmentation and hadronization, however their detailed measurement at LHC is just started. We present a systematic study of two-particle correlations with identified trigger and identified associated particles at LHC energies using several state of art LHC era Monte Carlo generators. The identified two-particle correlation studies of bulk particles: charged pions, kaons and protons reveal interesting trigger-hadron dependent splitting of the associated identified hadron spectra in all collision systems. Furthermore, the identified correlation functions show change in the identified trigger-hadron dependent evolution with transverse momenta compared to unidentified correlation functions. The splitting effect and their evolution from p-p and p-Pb to Pb-Pb collisions will be discussed for PYTHIA, EPOS and HIJING generated events.

Speaker: Gyula Bencedi (Hungarian Academy of Sciences (HU))

Paper GBencedi_Nantes_HPT4LHC_2014_Proceedings.pdf

Slides GBencedi_highpT_Nantes_Sep2014_v1.pdf

13:00 → 13:20 Adjourn