3rd International Conference on the Initial Stages in High-Energy Nuclear Collisions (InitialStages2016)

23 May 2016, 08:30 → 27 May 2016, 18:30 Europe/Lisbon

Grande Anfiteatro (Centro de Congressos, Instituto Superior Técnico, Alameda Campus)

Guilherme Teixeira De Almeida Milhano (Instituto Superior Tecnico (PT))

The 3rd International Conference on the Initial Stages in High-Energy Nuclear Collisions (InitialStages2016), to be held at Instituto Superior Técnico, Alameda Campus, Lisbon, Portugal, on May 23rd-27th 2016. The meeting follows up on the 2014 (Napa) and 2013 (A Toxa) editions.

Practical information.pdf

Monday, 23 May

Registration

Opening session

Convener: Guilherme Teixeira De Almeida Milhano (Instituto Superior Tecnico (PT))

1 Opening by IST president (Prof. Arlindo Oliveira), CENTRA vice-president (Profª Ana Mourão) and LOC chair (Guilherme Milhano)

2 Initial Stages of a HI collision: where are we at? [TH]

Speaker: Adrian Dumitru (Baruch College (City University of New York))

talk.pdf

3 Initial Stages of a HI collision: where are we at? [EXP]

Speaker: Barbara Jacak (Lawrence Berkeley National Lab. (US))

init_state_jacak.pdf

init_state_jacak.pptx

10:30 Coffee break + poster session

Collectivity in small systems

Convener: Jean-Paul Blaizot (CEA Saclay)

4 Signatures of collectivity in small systems

Speaker: Wei Li (Rice University (US))

IS2016-WeiLi-05222016v4_noanim.pdf

5 Initial state collectivity (CGC and related approaches) including longitudinal structures

Speaker: Alexander Kovner (University of Connecticut)

Kovner-Initial Stages2016.pdf

6 Hydro in small systems

Speaker: Harri Niemi (Frankfurt Institute for Advanced Studies)

is2016_niemi.pdf

7 New flow results in ALICE from multiparticle azimuthal correlations

Multiparticle azimuthal correlations are utilized in anisotropic flow stud- ies as observables sensitive to the properties of Quark-Gluon Plasma (QGP), an extreme state of matter which can be produced in ultra-relativistic heavy- ion collisions. We will present the first results [1] for elliptic (v2), triangular (v3) and quadrangular flow (v4) of charged particles in Pb–Pb collisions at √sNN = 5.02 TeV recorded by ALICE during the LHC Run 2 operations. The comparison of experimental measurements to various theoretical calculations will be discussed. In addition, results are reported for so-called symmetric cumulants. De- veloped by ALICE, these new multiparticle observables depend only on the correlations between event-by-event amplitude fluctuations of anisotropic flow harmonics vn, and are therefore independent on the corresponding sym- metry planes. The centrality dependence of correlation between fluctuations of the elliptic, v2, and quadrangular, v4, flow harmonics, as well as of anti- correlation between v2 and triangular, v3, flow harmonics will be discussed in two different regimes for the initial stages: geometry-dominated (in mid- central heavy-ion collisions) and fluctuation-dominated (in the most central heavy-ion collisions). The measurement of individual flow harmonics together with symmetric cumulants further constrain details of initial stages and the transport prop- erties of the produced QGP. These results offer a unique opportunity to test the validity of the hydrodynamic picture and to discriminate between vari- ous possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced QGP.

Speaker: Ante Bilandzic (Technische Universitaet Muenchen (DE))

ab_InitialStages2016_2d.pdf

13:00 lunch

Collectivity in small systems

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

8 PHENIX Results on Small System Collectivity

Small collision systems have long been considered a mere testing ground to understand how the partonic structure of the nucleus affects the formation of QGP in larger A+A collisions. However, recent measurements challenge this picture, suggesting the existence of collective behavior in small systems at RHIC and LHC energies. It is not currently clear whether these observations can be understood in terms of the hydrodynamic evolution of a thermalized medium, or if they originate instead from initial state effects. A key test to elucidate the matter is to carefully control the initial geometry of the collision and correlate it with the final-state particle emission. This talk will present the latest set of PHENIX measurements of flow observables in systems with one, two, and three initial hotspots---(p,d,3He)+Au at 200 GeV, comprising azimuthal two-particle correlations and anisotropy coefficients v_n. We will present comparisons with a variety of available theory calculations, and we will discuss opportunities for the further study of small system collectivity within the context of the RHIC d+Au beam energy scan program.

Speaker: Javier Orjuela Koop (University of Colorado Boulder)

InitStages_ORJUELA-KOOP_PHENIX.pdf

InitStages_ORJUELA-KOOP_PHENIX.pdf

InitStages_ORJUELA-KOOP_PHENIX.pptx

9 Summary and news from LHCb (including First measurements of long-range near-side angular correlations in √sNN = 5TeV proton-lead collisions in the forward region with LHCb)

Proton-lead and lead-proton data taking in 2013 has allowed LHCb to expand its core physics program into the regime of heavy ion physics. The results obtained so far will be briefly summarized. They include the measurement of the nuclear modification factors and forward-backward production of prompt and displaced $J/\psi$, $\Psi(2S)$ and $\Upsilon$s, and the production of prompt $D^0$ mesons. $Z$-production in proton-lead collisions and angular particle correlations have also been measured for events of varying charged particle activity. In 2015 the LHCb experiment expand its scope further in heavy ion physics and participated for the first time in the data-taking of Pb-Pb collisions. Simulation studies showed that up to semi-central lead-lead collisions can be analysed. Furthermore, a system for the injection of small amounts of gas into the LHCb collision area has been used, which allows fixed target physics with proton and lead beams. The physics reach of the experiment as well as first results from exploratory studies with Pb-Pb and fixed target interactions will be presented.

Speaker: Johan Blouw (Max-Planck-Gesellschaft (DE))

blouw.pdf

10 Latest news on collectivity in high-multiplicity pp collisions at CMS

Observation of a long-range, near-side, two-particle correlation (known as the “Ridge”) in high-multiplicity pp and pPb collisions opened up new opportunities of exploring novel QCD dynamics in small collision systems. While extensive studies of this long-range correlation phenomenon in pPb collisions have revealed its collective properties, the nature of the ridge in pp collisions still remains unknown. Latest measurements of long-range particle correlations in high-multiplicity pp collisions are presented. Anisotropy Fourier harmonics v2 and v3 are extracted as a function of pT and multiplicity, for charged particles, and identified $K^0_s$ and $\Lambda$ hadrons. A multi-paricle cumulant technique is also used to explore the collective nature of the long-range correlations. The results are directly compared with data in pPb and PbPb collisions, which shed light on the physical origin on the novel collective correlations in small systems.

Speaker: Maxime Guilbaud (Rice University (US))

IS2016Talk.pdf

16:00 coffee break + poster session

Collectivity in small systems

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

11 Measurement of long-range azimuthal anisotropies in $pp$ and $p$+Pb collisions with the ATLAS detector.

(speaker known later) ATLAS measurements of correlations between particle pairs in relative azimuthal angle ($\Delta\phi$) and pseudorapidity separation ($\Delta\eta$), in $pp$ collisions at $\sqrt{s}$=2.76, 5.02 and 13~TeV, and in $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=5.02~TeV are presented. Prior measurements have shown that in $pp$ collisions with a large multiplicity of produced particles, a long-range structure,commonly called the ``ridge'', develops along $\Delta\eta$ at $\Delta\phi\sim$0. However, due to the presence of the large away-side jet, the full $\Delta\phi$ dependence of the long-range correlation could not be studied previously. In this analysis, a template fitting procedure is implemented to determine the contributions from dijets to the correlations, using low-multiplicity events, and extract the genuine long-range correlation. The Fourier harmonics of the genuine long-range correlation for orders $n$=2--4 are extracted, and their dependence on $p_{\mathrm{T}}$, event-multiplicity and collision energy are studied. It is shown that the second Fourier coefficient, $v_{2,2}$, dominates the long range correlation, with small but significant contributions from $v_{3,3}$ and $v_{4,4}$. The $v_{2,2}$ is shown to factorize into the product of single-particle anisotropies $v_2$. A large $v_2$ is shown to be present even in events with a small multiplicity of produced particles, implying that the long-range correlations are not unique to rare high multiplicity events, but are present even in low multiplicity $pp$ collisions. Comparisons to the $v_{n,n}$ and $v_{n}$ obtained when applying the template fitting procedure to $p$+Pb collisions are also presented. These measurements can help determine if the long range correlations in $pp$ and $p$+Pb collisions arise from similar mechanisms or not.

Speaker: Brian Cole (Columbia University (US))

IS2016ATLAS Correlations_v5.pdf

12 Discussion on collectivity in small systems

Speakers: Alexander Milov (Weizmann Institute of Science (IL)), Piotr Bozek (AGH University of Science and Technology)

IS2016_discussion_UH-1.pdf

pppred.pdf

wounded_quarks.pdf

Tuesday, 24 May

Initial stages and equilibriation

Convener: Alexander Kovner (University of Connecticut)

13 Particle production near t = zero in Proton-Nucleus and Nucleus-Nucleus collisions

Speaker: Alfred Mueller (Columbia University)

14 Particle production at tau~0

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

20160524-yenjie-InitialStages2016-v5.pdf

15 Experimental signatures of equilibration beyond the success of hydro

Speaker: Michele Floris (CERN)

mfloris-is2016.pdf

10:30 Coffee break + poster session

Initial stages and equilibriation

Convener: David d'Enterria (CERN)

16 Progress in higher-order CGC computations

Speaker: DIONYSIS TRIANTAFYLLOPOULOS (ECT*)

Triantafyllopoulos.pdf

17 Equilibration in weak coupling approaches

Speaker: Soeren Schlichting (Brookhaven National Lab)

SchlichtingIS2016.pdf

18 Strongly coupled approaches to equilibration

Speaker: David Mateos (ICREA & U. Barcelona)

Lisbon May 2016-Initial stages 3.pdf

19 Early time dynamics: pressure and 3+1D flow from classical gluon fields

Speaker: Rainer Fries (Texas A&M University)

IS_2016_Fries_nf.pdf

IS_2016_Fries_nf.pptx

13:00 Lunch

Parallel: benchmarking [TH+EXP]

Convener: Barbara Jacak (Lawrence Berkeley National Lab. (US))

20 ALICE measurements on quarkonium production in p-Pb and their impact on the understanding of Pb-Pb results

ALICE is the LHC experiment devoted to the study of heavy-ion collisions. The main purpose of ALICE is to investigate the properties of the deconfined state of nuclear matter, the Quark-Gluon Plasma (QGP). Quarkonium measurements play a crucial role in this investigation. Indeed, heavy quarks are created during the initial stages of the collision, before the QGP formation, and their number is conserved throughout the partonic and hadronic phases of the collision. The sequential suppression of the quarkonium states by colour screening has long been suggested as a signature and thermometer of the QGP. However, the first results on quarkonium suppression in Pb-Pb collisions at the LHC seem to indicate that for charmonia both regeneration and colour screening mechanisms play a role, while for bottomonia the regeneration mechanism should be small. Additionally, ALICE has recently reported an excess of J/$\psi$ at very low transverse momentum (below 300 MeV/$c$), presumably of electromagnetic origin that could become a valuable probe of the system.

We will present the latest results on quarkonium production in Pb-Pb collisions at LHC energies measured by ALICE. Quarkonia are measured at mid-rapidity in the dielectron decay channel and at forward rapidity in the dimuon one. Comparisons of measurements at different collision energies and to available theoretical calculations will be discussed.

Speaker: Dr Andry Rakotozafindrabe (CEA/IRFU,Centre d'etude de Saclay Gif-sur-Yvette (FR))

AndryR_IS2016_v5.pdf

21 Neutron skin at the LHC - the case of W production

The notion of centrality in high-energy collisions involving heavy nuclei has lately become a subject of an active discussion. Specifically, the experimental definitions of centrality that have appeared to be well in line with the theoretical expectations in heavy-ion collisions have led to rather weird behaviour when applied to the proton-lead collisions at the LHC and deuteron-gold collisions at RHIC. In this talk, I will address the prospects of calibrating the experimental centrality measures by asymmetries in inclusive W production that are caused by the neutron-skin effect - mutually different spatial distributions of protons and neutrons inside the nuclei.

Speaker: Hannu Paukkunen (University of Jyväskylä ; Helsinki Institute of Physics ; Universidade de Santiago de Compostela)

NeutronSkin_Paukkunen.pdf

22 Nuclear glue constraints from top-quark pair production in p-A and A-A collisions at the LHC and beyond

I will present phenomenological studies of top-quark pair production in p-Pb and Pb-Pb collisions at the CERN LHC and at the future circular collider (FCC), obtained with NLO pQCD calculations including nuclear PDFs. In their fully-leptonic decay mode (ttbar->W+W-bbbar->l+l- nunu bbbar), one expects 90 (Pb-Pb) and 300 (p-Pb) top-quarks cleanly tagged per nominal LHC-year, and 47,000--100,000 per FCC-year at sqrt(s_NN)= 39, 63 TeV, after typical ATLAS/CMS-like acceptance and efficiency cuts. Using a Hessian PDF reweighting technique, we demonstrate that the rapidity distributions of the t-tbar decay leptons provide quantitative constraints of the Pb gluon density at high virtualities, whose uncertainty can be reduced by up to 30% at the LHC (full heavy-ion programme), and by 70% per FCC-year [1]. [1] D. d'Enterria, K.Krajczar, H.Paukkunen, Phys. Lett. B746 (2015) 64-72; arXiv:1501.05879

Speaker: David d'Enterria (CERN)

dde_top_glue_ISlisbon_may16.pdf

23 Reconstructed jet probes of small and large systems with the PHENIX detector

Measurements of the particle jets arising from a hard scattering are valuable experimental tools for understanding the physics of small and large systems created in collisions involving nuclei. In small systems, such as in proton- or deuteron-nucleus collisions, they are useful benchmarks of the effects of the nuclear environment on the initial parton densities or the emerging jet energy. In nucleus-nucleus collision systems, they are sensitive probes of the energy loss experienced by the showers of hard-scattered partons that traverse the hot QCD medium. We present recently published PHENIX results on fully reconstructed jet production rates in 200 GeV proton-proton and deuteron-gold collisions, and a preliminary measurement in copper-gold collisions. In particular, the deuteron-gold measurement has revealed that the traditional frameworks used to understand the collision geometry of small systems break down in extreme hard-scattering kinematic regimes. These measurements represent an important step towards a robust experimental jet program at RHIC.

Speaker: Dennis Vadimovich Perepelitsa (Brookhaven National Laboratory (US))

dvp-IS16-PHENIX-5-24-16.pdf

24 Latest jet results in pp, pPb and PbPb collisions from CMS

This talk summarizes the latest results on jets in pp, pPb and PbPb collisions from the CMS collaboration

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

160524_initialStages2016_KayaTatar.pdf

25 Measurements of jet fragmentation, electro-weak boson, and charmonium production in $pp$ and $p$+Pb collisions at $\sqrt{s_{NN}}$ = 5 TeV in ATLAS

Speaker: Miguel Ignacio Arratia Munoz (University of Cambridge (GB))

ATL-COM-PHYS-2016-540.pdf

Parallel: initial state [TH]

Convener: Alfred Mueller (Columbia University)

26 Rapidity evolution of gluon TMD from low to moderate $x$

I discuss how the rapidity evolution of gluon transverse momentum dependent distribution changes from nonlinear evolution at small x ≪ 1 to linear evolution at moderate x ∼ 1.

Speaker: Prof. Ian Balitsky (JLab/ODU)

gTMD_20min.pdf

27 Gluon Production in Heavy Ion Collisions: Beyond the Leading Order

We calculate the classical single-gluon production amplitude in nucleus--nucleus collisions including the first saturation correction in one of the nuclei (the projectile) while keeping multiple-rescattering (saturation) corrections to all orders in the other nucleus (the target). In our approximation only two nucleons from the projectile nucleus interact: the single-gluon production amplitude we calculate is order-$g^3$ and is leading-order in the atomic number of the projectile, while resumming all order-one saturation corrections in the target nucleus. Our result is the first step towards obtaining an analytic expression for the first projectile saturation correction to the gluon production cross section in nucleus--nucleus collisions.

Speaker: Douglas Wertepny (The Ohio State University)

wertepny_InitialState2016_final.pdf

28 Single inclusive forward hadron production at next-to-leading order

We discuss single inclusive hadron production from a high energy quark scattering off a strong target color field in the Color Glass Condensate formalism. Recent calculations of this process at the next-to-leading order accuracy have led to negative cross sections at large transverse momenta. We identify the origin of this problem in an oversubtraction of the rapidity divergence into the Balitsky-Kovchegov evolution equation for the target. We propose a new way to implement the kinematical restriction on the emitted gluons to overcome this difficulty.

Speaker: Bertrand Ducloue (University of Jyvaskyla)

Ducloue_nlosinc.pdf

29 Small-x evolution beyond the eikonal approximation

Following the recent studies in the effects of next-to-eikonal and next-to-next-to-eikonal propagation in high-energy proton-nucleus collisions, we calculate the corresponding corrections to small-x evolution. These corrections arise explicitly both from non-eikonal propagation of the softer gluon fluctuations, and from emission/absorption of gluons in the projectile wave function while inside the nuclear target. We show for what kinematical regions and which observables these corrections can be important.

Speaker: Fabio Dominguez (Universidade de Santiago de Compostela)

DominguezIS2016.pdf

30 Entanglement entropy and the Color Glass Condensate

We compute the entanglement entropy of soft gluons in the wave function of a fast moving hadron and discuss its basic properties. We also derive the expression for entropy production in a high energy hadronic collision within the Color Glass formalism. We show that long range rapidity correlations give negative contribution to the production entropy. We calculate the (naturally defined) temperature of the produced system of particles, and show that it is proportional to the average transverse momentum of the produced particles.

Speaker: Michael Lublinsky

IS2016.pdf

31 Full NLO corrections for DIS structure functions in the dipole factorization formalism

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 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, up to the formation of the Quark-Gluon Plasma. So far, phenomenological studies have been performed successfully at LO in the dipole factorization, with LL resummation, 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, I will present an update on the calculation of the (fixed order) NLO corrections to DIS structure functions on a dense target in the dipole factorization picture. In earlier studies, only one part of the NLO corrections has been calculated, the one corresponding to a quark-antiquark-gluon Fock state interacting with the target. By contrast, I will present the first direct calculation of the other part of NLO corrections, for which a quark-antiquark Fock state interacts with the target. I will also discuss issues related with the combination of the two pieces, which is complicated by the presence of UV divergences. Along the way, various techniques have been developped, which will simplify the calculation of NLO corrections to most other observables relevant for gluon saturation.

Speaker: Guillaume Beuf (ECT* Trento)

GBeuf_IS2016.pdf

16:30 Coffee break + poster session

Parallel: azimuthal asymmetries, correlations and fluctuations [TH]

Convener: Hannah Petersen

32 Principal Component Analysis of Correlation Data without Nonflow Effects

We extend the recently presented Principal Component Analysis (PCA) method to reduce the nonflow effects present in the two-particle correlation data. We illustrate this technique by applying it to simulated pseudorapidity correlation data obtained with A Multi-Phase Transport (AMPT) model for Pb-Pb collisions at the LHC energy 2.76 TeV. Measurable subleading modes are seen in the elliptic and triangular flows as a function of pseudorapidity. Although we show here only two-particle correlation results, the technique is applicable to also multi-particle correlations.

Speaker: Rajeev Bhalerao (TIFR)

RajeevBhalerao_Lisbon.pdf

33 Universal scaling of the azimuthal anisotropies due to energy loss

It is shown that the RHIC and LHC experimental data on v2(pt) and v3(pt) for all centralities satisfy an scaling law.All data lie on a curve that is obtained assuming are proportional to the energy loss of a produced parton interacting with the color field of the rest of produced partons.It is studied the extensión of the scaling to v4 and v5

Speaker: Prof. Carlos Pajares (University santiago de Compostela)

lisbon2016-2 Copy.pdf

34 Correlations and collective flow in pp and p+Pb collisions at the LHC energies

Flow measurements in p+Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV have indicated the development of collective flow in the small systems created at the LHC. Using the ultrarelativistic quantum molecular dynamics (UrQMD) model, we investigate the azimuthal correlations in p+Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV with two- and four-particle cumulants. Our calculations indicate that pure hadronic interactions can not generate the collective flow as measured in experiments, additional effects from initial state and/or from the QGP are needed to reproduce the flow data[1]. Using the same flow analysis method, we explore the collective flow in pp collisions at $\sqrt{s}$=13 TeV with two different baseline calculations[2], including: 1. VISHNU hybrid model simulations that connect viscous hydrodynamics for the QGP expansion and UrQMD for the hadron resonance gas evolution, using smooth initial conditions, 2. VISHNU simulations with fluctuating initial conditions. Our calculations of multiplicity dependent elliptic flow are comparable with the recent measurements from ATLAS. We find that triangular flow (if observed) is one of the unique observable directly associated with the domain structures in the created small pp systems. References: [1] Y. Zhou, X. Zhu, P. Li and H. Song, "Investigation of possible hadronic flow in $\sqrt{s_{\rm NN}}$=5.02TeV p+Pb collisions,'' Phys. Rev. C 91, no. 6, 064908 (2015). [2] X. Zhu, H. j. Xu, Y. Zhou, W. T. Deng and H. Song, in preparation.

Speaker: Dr Xiangrong Zhu (Peking University)

IS2016LargeSmallSystems.pdf

35 Understanding rapidity fluctuations in the initial state

Rapidity correlations are analyzed in a simple model, where in the initial stage multiple sources, extended in space-time rapidity, are formed. We show how the varying longitudinal extent of the sources generates correlations in the initial entropy deposition, which later contribute to the observed correlations in hadron production. Our analytic analysis allows to understand their structure and to identify the component due to the underlying fluctuation of the number of sources and the component from the genuine longitudinal fluctuations. Our results reproduce semiquantitatively the basic features of the recent measurements at the LHC.

Speaker: Wojciech Broniowski (IFJ PAN)

is_16.pdf

36 Mapping initial state correlations in rapidity unsing collective flow

We present results of a 3+1 dimensional hydrodynamic model for flow correlations in pseudorapidity. The method can be used to unfold the correlations in the initial fireball at different space-time rapidities.
We discuss possible mechanism of generating initial correlations in rapidity, both with nucleon and parton degrees of freedom. Particular sensitivity to the initial correlations is found in collisions of small asymmetric systems, such as p-Pb collisions.

Speaker: Piotr Bozek (AGH University of Science and Technology)

ininst.pdf

37 Thermodynamics and kinetics of Gribov-Zwanziger plasma

Dynamic features of a plasma consisting of gluons whose infrared dynamics is improved by the Gribov-Zwanziger quantization are presented. This approach includes important features of color confinement which set the plasma apart from conventional quasiparticle systems in several aspects [1]. We obtain explicit expressions for the transport coefficients $\eta$ and $\zeta$ (shear and bulk viscosity) and check that they are consistent with the numerical solutions of the kinetic equation. At high temperature, for constant both the Gribov parameter and the relaxation time used in the kinetic equation, we find linear scaling of the ratio $\zeta/\eta$ with the conformal measure 1/3 - $c_s^2$ (sound velocity squared), which is typical for a strongly coupled system. The newest results obtained for the temperature dependent Gribov parameter show also the enhancement of the bulk viscosity at the phase transition [2]. [1] W. Florkowski, R. Ryblewski, N. Su, K. Tywoniuk, arXiv:1504.03176 & arXiv:1509:01242 [2] V. Begun, W. Florkowski, R. Ryblewski, in preparation

Speaker: Wojciech Florkowski (Institute of nuclear Physics, Krakow)

IS2016-florkowski.pdf

Parallel: initial state and small systems [EXP]

Convener: Jiangyong Jia (State University of New York (US))

38 Study of prompt $D^0$ meson production and cold nuclear matter effects in proton-lead collisions at √sNN = 5TeV in the forward region with LHCb

The productions of prompt $D^0$ mesons in proton-lead collisions in the forward and backward configurations were studied. The data are collected with the LHCb detector with at a centre-of-mass energy of 5TeV. The integrated cross-section is measured to be 237±1±15mb (259±3±19mb) for the forward (backward) rapidity range 1.5 < y< 4 (5 < y < 2.5), in the pT range 0 < pT < 8 GeV/c. Nuclear modification factors and forward-backward ratios are determined, suggesting suppression in the forward direction.

Speaker: Alvaro Dosil Suarez (Universidade de Santiago de Compostela (ES))

Adosil_InitStages.pdf

39 Charged-particle production in PbPb and pp collisions at 5 TeV with CMS

During the fall of 2015, LHC provided high-luminosity PbPb and pp collision data at 5 TeV collision energy per nucleon pair, allowing for precision measurements of hot-medium effects. The measured charged particle spectra from the two collision systems are presented up to approximately 100 GeV of transverse momentum, for charged particles produced in the central rapidity region. The nuclear effects on the particle spectrum are accessed via the construction of nuclear modification factors. The centrality dependence of the nuclear modification factor is explored.

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

Baty_IS2016_RAA_v10.pdf

40 Femtoscopy with identified charged pions in proton-lead collisions at $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$ with the ATLAS detector

Bose-Einstein correlations between identified charged pions are measured for $p$+Pb
collisions at $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$ with the ATLAS detector with
a total integrated luminosity of $28~\inb$. Pions are identified using ionization
energy loss measured in the pixel detector. Two-particle correlation functions and
the extracted source radii are presented as a function of average transverse pair
momentum ($k_{\mathrm{T}}$) and rapidity ($y^{*}_{k}$) as well as collision centrality.
Pairs are selected with a rapidity $-2 < y^{*}_{k} < 1$ and with an average transverse
momentum $0.1 < k_{\mathrm{T}} < 0.8 \GeV$. The effect on the two-particle correlation
function from jet fragmentation is studied, and a new method for constraining its
contributions to the measured correlations is described. The measured source sizes
are substantially larger in more central collisions and are observed to decrease with
increasing pair $k_{\mathrm{T}}$. Radii are also evaluated in intervals of $y^{*}_{k}$
and a correlation with the local multiplicity $dN/dy^{*}$ is demonstrated.
The scaling of the extracted radii with the mean number of participants is also used
to compare a selection of initial-geometry models.

Speaker: Michael Ryan Clark (Columbia University (US))

clark_is_slides.pdf

41 Overview of charged-particle multiplicities with ALICE

An overview of the measured pseudorapidity densities and multiplicity distributions at the LHC's Run 1 and 2 energies will be presented for pp, p-Pb and Pb-Pb systems. The ALICE results are shown for central pseudorapidities and, if available, over a wider pseudorapidity range (-3.4 < η < 5.0). The measurement of the inclusive charged-particle production in high-energy interactions is a fundamental observable to describe the global characteristics of the collision. Particle production at LHC energies is dominated by soft processes, but with increasing collision energy the role of perturbative hard processes, like parton scatterings, grows. The ALICE measurements are compared with results of the dierent LHC and RHIC experiments. The evolution of the multiplicity with energy is studied, utilizing different parameters depending on the colliding system considered. In addition, the results are compared to models based on various mechanisms of particle production and different initial conditions.

Speaker: Valentina Zaccolo (University of Copenhagen (DK))

IS2016-VZaccolo.pdf

42 Measurement of direct photons at forward rapidities in p-­A collisions at LHC with ALICE

on the behalf of the ALICE­‐FoCal Collaboration Direct photon production at forward rapidity is a promising probe for the gluon content of protons and nuclei at small *x*. In particular, the measurement of the nuclear modification factor for direct photons in p–A collisions at the LHC is a crucial test for gluon saturation. A new forward calorimeter (FoCal) is proposed as an upgrade to the ALICE experiment. The detector will cover the range 3.5 < η < 5 which probes the gluon distributions at *x* ~ 10^‐5 and Q ~ *p*_T > 4 GeV. Performance studies show that an extremely high‐granularity calorimeter is required. Results from R&D for this project will be discussed.

Speaker: Dieter Roehrich (University of Bergen (NO))

forward_physics_IS2016_v2.pdf

43 Multiplicity dependence of strange particle production in pp collisions with the ALICE detector

Recent measurements in high-multiplicity proton-proton (pp) and proton-lead (p-Pb)
collisions show several features that are similar to those observed in heavy-ion collisions. In this respect strangeness production may provide a valuable investigative
tool.

Baryon-to-meson ratios, such as Λ/K_S⁰, have been measured
differentially in p_T and show an evolution with increasing charged particle
multiplicity in small systems similar to the one observed with centrality in heavy-ion
collisions, where this behaviour is interpreted to be strongly related to the
hydrodynamical evolution of the system. Furthermore the production rate of strange and
multi-strange hadrons relative to pions exhibits a significant increase with multiplicity
in pp collisions, similarly to that observed in p-Pb. This increase is observed to be
more pronounced for hadrons with a larger strangeness content.

In this talk strange (K_S⁰,Λ,Λ) and multi-strange (Ξ,Ω) hadron
production measurements at mid-rapidity, in pp collisions at √s = 7 TeV, will be shown
as a function of charged-particle multiplicity. Perspectives for similar studies at √s = 13 TeV will also be discussed.

Speaker: Fiorella Fionda (Universita e INFN (IT))

StrangenessIS2016.pdf

Wednesday, 25 May

Approaches to evolution

Convener: Prof. Steffen A. Bass (Duke University)

44 Out-of-equilibrium dynamics: how to deal with it

Speaker: alex buchel (UWO/Perimeter Institute)

AlexBuchel_is2016.pdf

45 Hydro vs transport

Speaker: Hannah Petersen

Initstages2016_Petersen.pdf

46 Dissipative dynamics of the expanding anisotropic quark gluon plasma

Speaker: Mauricio Martinez Guerrero (The Ohio State University)

IS2016.pdf

10:30 Coffee break + poster session

Approaches to evolution

47 Discussion on equilibration and early time dynamics

Speakers: Jorge Casalderrey Solana (University of Barcelona (ES)), Mateusz Ploskon (Lawrence Berkeley National Lab. (US))

12:30 Lunch

Parallel: azimuthal asymmetries and UPCs [EXP]

Convener: Brian Cole (Columbia University (US))

48 Ultraperipheral collisions with the ATLAS detector

(speaker known later) The large equivalent-photon fluxes accompanying Pb ion beams at the LHC initiate photon-photon and photo-nuclear interactions which dominate when the colliding nuclei have large impact parameter (ultra-peripheral collisions). These electromagnetically-induced processes are sensitive to the nuclear wave-function and in particular the nuclear modifications of the nucleon parton distribution functions (nPDFs). As such, they are complementary to the ongoing p+A program at RHIC and the LHC, as well as the upcoming electron-ion collider in the US. The absolute rates of single and multiple neutron emission into one or both ZDCs will be presented, to test theoretical predictions for the photon flux as well as nuclear absorption. The rates for exclusive vector meson production (rho and J/psi) will be studied to probe effects related to the nuclear wave function, and the high-mass dilepton pair continuum rates will also be studied to further test expectations for two-photon interactions. Finally, first measurements of jet production in photonuclear interactions will also be presented to gain insight into the nuclear wave function in a previously unexplored range of x and Q^2.

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

Steinberg_IS2016_v2.pdf

49 Photoproduction of vector mesons in ultra-peripheral collisions with ALICE

The strong electromagnetic fields generated by ultra-relativistic heavy ions offer the possibility to study photoproduction processes at the LHC. In so called ultra-peripheral collisions (UPC), when the impact parameter of the incoming hadrons is larger than the sum of their radii, hadronic processes are strongly suppressed and only electromagnetic interactions remain. ALICE has measured the exclusive photoproduction of vector mesons -- including ρ⁰, J/ψ and ψ(2s) -- in γ-p and γ-Pb using p-Pb and Pb-Pb collisions respectively. These processes provide information on the QCD structure of the targets at higher photon-target center-of-mass energies than ever before, and contribute to our understanding of saturation and nuclear gluon shadowing. The talk will cover the results from LHC Run1 data, as well as discuss the current status and prospects for analyses with LHC Run2 data.

Speaker: Jesus Guillermo Contreras Nuno (Czech Technical University (CZ))

jgcn_IS2016.pdf

50 Results of Ultraperipheral collisions with CMS

Ultraperipheral collisions (UPCs) of heavy ions or hadrons involve long range electromagnetic interactions at impact parameters larger than sum of their radii where hadronic interaction is largely suppressed and the exchanged photon materializes into qq(bar) bound state after interacting with the gluonic field of the target proton or ion. Photoproduction of heavy vector mesons (J/psi, Upsilon) thus provide direct information on the gluon distribution functions in the nucleon/nucleus at very low values of Bjorken-x. The CMS experiment has excellent capabilities for the measurement of the heavy vector mesons in the dimuon decay channel using the tracker and the muon chambers. The measured coherent J/psi photoproduction cross section in ultraperipheral Pb-Pb collisions using 2011 PbPb data and Upsilon photoproduction in ultraperipheral pPb collisions during 2013, will be presented. The prospects for future measurements using the data collected in the 2015 PbPb run will be described.

Speaker: Dipanwita Dutta (Bhabha Atomic Research Centre (IN))

ddutta_initialstage_2016_v3.pdf

51 Azimuthal anisotropies for low- and high-pT particles in PbPb and pPb collisions with CMS

Previous measurements have demonstrated the collective nature of multiparticle correlations in high-multiplicity pPb collisions at the LHC. This collectivity, while consistent with a hydrodynamic flow origin, can also be interpreted in terms of initial state effects arising from gluon saturation. To investigate the detailed properties of this collectivity, differential Fourier coefficients in transverse momentum and pseudorapidity are presented based on a variety of analysis methods using data obtained with the CMS detector at the LHC. The multiparticle nature of the correlations is explored using 4-, 6- and 8-particle cumulant analyses as well as a Lee-Yang Zeros analysis that accounts for correlations among all particles. An event-plane analysis using the scalar product method is performed where the influence of recently demonstrated event-plane decorrelation on the pseudorapidity dependence is considered. The pPb collision results are compared to peripheral PbPb collision results obtained with comparable mid-rapidity charged particle densities. While a significant pseudorapidity dependence is observed in the azimuthal dependence of the particle yields, a decorrelation of the event-plane angle with pseudorapidity can account for much of the observed behavior.

Speaker: Quan Wang (University of Kansas (US))

IS2016.pdf

52 Measurement of the suppression and elliptic anisotropy of heavy-flavor muons in lead-lead collisions at $\sqrt{s_{\mathrm{NN}}}$=2.76TeV with the ATLAS detector

(speaker known later) Measurements of the nuclear modification factor ($R_{\mathrm{AA}}$) and the elliptic flow ($v_{2}$) of muons from heavy-quark decays in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$=2.76 TeV from the ATLAS experiment are presented. The measurements are done over the $p_{\mathrm{T}}$ range of 4-14~GeV and over the centrality range of (0-60)%. A significance elliptic flow is observed over the full $p_{\mathrm{T}}$ range for all centralities. The $R_{\mathrm{AA}}$ results are consistent with previous measurements but have much more statistical precision. More than a factor of two suppression of the muon yield is observed in most central collisions. These measurements give us an insight into the interaction of heavy quarks with the bulk medium produced in heavy-ion collisions.

Speaker: Soumya Mohapatra (Columbia University (US))

Conference-InitialStates.pdf

53 Charge-dependent directed flow measurements in Cu+Au collisions at \sqrt{s_{NN}} =200 GeV with the STAR experiment

At the early stages of non-central heavy-ion collisions, a strong magnetic field perpendicular to the reaction plane is created. In asymmetric Cu+Au collisions, due to the difference in the number of spectators, not only the magnetic field but also a strong electric field (E-field) would be created along the reaction plane and pointing from the Au-nucleus to Cu-nucleus. The lifetime of the E-field would be very short, of the order of a fraction of a fm/c. The quarks and antiquarks that have been already produced at this time would experience the Coulomb force, which results in charge separation of directed flow. Thus, the measurement of the charge-dependent directed flow in Cu+Au provides an opportunity to study the time dependence of light quark production in heavy-ion collisions. Understanding the time evolution of the quark densities is also very important for detailed theoretical predictions of the Chiral Magnetic Effect and Chiral Magnetic Wave, which various experiments are actively searching for. In this talk, the charge-dependent directed flow in Cu+Au collisions at \sqrt{s_{NN}} =200 GeV measured in the STAR experiment will be presented. The results are compared with existing model predictions. Implications for the dynamics of quark production and system evolution will be discussed. Higher-harmonic flow will be also presented and compared with results in A+A collisions.

Speaker: Takafumi Niida (University of Tsukuba (JP))

InitialStage2016_niida_v3.pdf

Parallel: initial state [TH]

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

54 Small-x observables at forward rapidity in pA collisions

Forward rapidity observables in high-energy proton-nucleus collisions are optimal for probing gluon saturation effects in the target nucleus. We study two small-x observables at forward rapidity: dijet azimuthal correlations in proton-nucleus collisions, and induced coherent spectrum associated with single parton scattering. Both observables are first analyzed from a CGC (color glass condensate) starting point of view, and then generally expressed in terms of TMD (transverse momentum dependent)  gluon distributions. For the dijet azimuthal correlations we derive an improved TMD factorization formula valid for an arbitrary value of the momentum imbalance of the jets. We present preliminary results for the dijet production cross section at forward rapidity for LHC energies, based on the new formula. For the second observable, we first derive general expressions at finite $N_c$ for the medium-induced soft gluon radiation for both, energetic quark and gluon, and then calculate the leading asymptotics in an explicit formula. We compare with previous calculations in different frameworks.

Speaker: Elena Petreska (Universidade de Santiago de Compostela/ Ecole Polytechnique)

ISLisbonPetreska.pdf

55 The transverse structure of the proton from ISR to LHC energies

The initial energy deposited in heavy ion collisions depends on the initial size and shape of the interaction area. Therefore, a precise picture of the proton in impact parameter space is needed in order to correctly interpret the recent and forthcoming RHIC and LHC results in pp,pA and AA collisions. In this talk, I shall present a comparative study of the impact parameter profile of the proton-proton scattering amplitude constrained by the available data on the pp elastic cross section from ISR to LHC energies. Our analysis is based on a two-scale picture in which the clouds of small gluons surrounding the valence quarks are much more smaller than the hadronic radius.

Speaker: Alba Soto Ontoso (UGR)

AlbaSoto_IS2016.pdf

56 3-parton azimuthal angular correlations as a probe of gluon saturation

we calculate the cross section for production of three partons in scattering of (real or virtual) photons on a proton or nucleus target at high energy using the Color Glass Condensate formalism. We investigate the azimuthal angular correlations among the three produced partons and show that they are a sensitive probe of saturation dynamics. We outline how this cross section can be used to calculate the Next to Leading Order corrections to di-jet angular correlations at high energy.

Speaker: Jamal Jalilian-Marian (Baruch College)

initialstages-2016.pdf

57 Photon-jet Ridge in p+A collisions at RHIC and the LHC

I will present the recent analysis of the photon-jet correlations in the Color Glass Condensate (CGC) framework in proton-proton and proton-nucleus collisions at RHIC and the LHC. I show that photon-jet correlations exhibit long-range in rapidity ridge-like structure at low transverse momenta of the produced jet and photon in high-multiplicity events. These features are strikingly similar to the observed ridge effect for di-hadron correlations at RHIC and the LHC. I show that photon-jet correlations at small-x kinematics probe the gluon saturation dynamics and such measurements can help to understand the true origin of the observed di-hadron ridge in p+A collisions, and address whether the ridge is an universal phenomenon for all two particle correlations at high energy and high multiplicity events. I will also talk if there is a ridge-like structure for photon-hadron correlation at RHIC and the LHC, and discuss how the hadronization of jet can affect the photon-jet correlations.

Speaker: Amir Rezaeian (Universidad Tecnica Federico Santa Maria)

Lisbon_Amir.pdf

Lisbon_Amir.pptx

58 Four jet production in single and double parton interaction in high energy factorization

I would like to report on first study of 4-jet production in a complete high-energy factorization (HEF) framework. The results include contributions from both single-parton scattering (SPS) and double-parton scattering (DPS). The calculations are performed for kinematical situations relevant for two experimental measurements (ATLAS and CMS) at the LHC. We compare our results to those reported by the ATLAS and CMS collaborations for different sets of kinematical cuts. The results of the HEF approach are compared with their counterparts for collinear factorization. For symmetric cuts the DPS HEF result is considerably smaller than the one obtained with collinear factorization. The mechanism leading to this difference is of kinematical nature. We conclude that an analysis of inclusive 4-jet production with asymmetric pT-cuts below 50 GeV would be useful to enhance the DPS contribution relative to the SPS contribution. In contrast to the collinear approach, the HEF approach nicely describes the distribution of the ΔS variable, which involves all four jets and their angular correlations.

Speaker: Krzysztof Kutak (Instytut Fizyki Jadrowej Polskiej Akademii Nauk)

IS-Kutak-2016.pdf

59 Incoherent diffraction and proton structure fluctuations

Exclusive vector meson production can be used to directly probe the gluon density of a hadron. Measuring the cross section differentially in momentum transfer t makes it possible to determine the transverse density profile (via coherent diffraction) and density fluctuations (incoherent diffraction) of the target hadron. This knowledge on the geometric fluctuations of the proton is particularly important for understanding collective phenomena observed in proton-nucleus collisions. We calculate coherent and incoherent diffractive vector meson production in photon-proton and photon-nucleus scattering in the Color Glass Condensate framework. The dipole model used in the calculation is constrained by the proton structure function data. By implementing sub-nucleon scale fluctuations using the constituent quark model or the IP-glasma framework we demonstrate that incoherent gamma-p scattering is sensitive to small-scale fluctuations. We also show that the effect of geometric fluctuations can be disentangled from saturation scale fluctuations. We discuss to what extent the current HERA diffractive deep inelastic scattering data can be used to constrain the proton structure fluctuations and how an Electron Ion Collider (EIC) can improve the picture.

Speaker: Dr Heikki Mäntysaari (Brookhaven National Laboratory)

is2016_mantysaari.pdf

16:00 Coffee break + poster session

Parallel: particle production, hydrodynamics and transport [TH]

Convener: Carlos Pajares Vales (Universidade de Santiago de Compostela (ES))

60 EKRT-model predictions for 5.02 TeV Pb+Pb collisions at the LHC

We present the recent results from the event-by-event next-to-leading order perturbative-QCD + saturation + viscous hydrodynamics (EKRT) model [1,2]. In particular, we show the computed predictions for the centrality dependence of charged hadron multiplicity, flow coefficients of the azimuthal-angle asymmetries and correlations of event-plane angles in 5.02 TeV Pb+Pb collisions at the LHC. The centrality dependencies of the studied observables, and the magnitude of the flow coefficients and event-plane angle correlations are predicted to be similar to those at 2.76 TeV. Overall, our predictions match very well with the new LHC measurements [3,4]. In addition, we show that the computed flow coefficients provide slightly more discriminating power on the temperature dependence of QCD matter viscosity than the 2.76 TeV measurements. [1] H. Niemi, K. J. Eskola, R. Paatelainen and K. Tuominen, Phys.Rev. C93 (2016) 1, 014912, arXiv:1511.04296 [hep-ph]. [2] H. Niemi, K. J. Eskola and R. Paatelainen, Phys. Rev. C93 (2016) 2, 024907, arXiv:1505.02677 [hep-ph]. [3] J. Adam et al. [ALICE Collaboration], arXiv:1602.01119 [nucl-ex]. [4] J. Adam et al. [ALICE Collaboration], arXiv:1512.06104 [nucl-ex].

Speaker: Dr Risto Paatelainen (University of Santiago de Compostela)

IS2016paatelainen.pdf

61 Fluid dynamic propagation of initial baryon number perturbations

Baryon number density perturbations offer a possible route to experimentally measure baryon number susceptibilities and heat conductivity of the quark gluon plasma. We study the fluid dynamical evolution of local and event-by-event fluctuations of baryon number density, flow velocity, and energy density on top of a (generalized) Bjorken expansion. To that end we use a background-fluctuation splitting and a Bessel-Fourier decomposition for the fluctuating part of the fluid dynamical fields with respect to the azimuthal angle, the radius in the transverse plane, and rapidity. We examine how the time evolution of linear perturbations depends on the equation of state as well as on shear viscosity, bulk viscosity, and heat conductivity for modes with different azimuthal, radial, and rapidity wave numbers. Finally we discuss how this information is accessible to experiments in terms of the transverse and rapidity dependence of correlation functions for baryonic particles in high energy nuclear collisions.

Speaker: Stefan Floerchinger (CERN)

FloerchingerInitialStages2016.pdf

62 Colored particle-in-cell simulations for heavy-ion collisions

We present our work on the simulation of the quark-gluon-plasma using the colored particle-in-cell (CPIC) method. CPIC is a generalization of particle-in-cell simulations - commonly used in plasma physics - to real-time non-abelian lattice gauge theories. Using this method we are able to model the early time dynamics of heavy-ion collisions in the laboratory frame in 3+1 dimensions. In particular we study the effects of finite pancake thickness in the McLerran-Venugopalan model. Our approach requires a consistent treatment of gauge fields and color currents, which is achieved using charge-conserving CPIC methods. We show that the description in the laboratory frame agrees with boost-invariant approaches as a limiting case and investigate collisions beyond boost-invariance.

Speaker: David Mueller (Vienna University of Technology)

dmueller_initstages2016.pdf

63 Bulk observables in small colliding systems using Yang-Mills dynamics and Lund string fragmentation

We compute gluon production from the Color Glass Condensate based IP-Glasma model and perform hadronization with PYTHIA's Lund string fragmentation to study bulk observables in small colliding systems at the LHC. In particular we study high multiplicity events in which observations are often attributed to final state collective effects. We find that our alternative description, based largely on initial state dynamics, can describe characteristic features of experimental observables, including the particle mass dependence of the mean transverse momentum.

Speaker: Prithwish Tribedy (Brookhaven National Lab)

tribedy_talk_IS2016.pdf

64 Anatomy of particle production and azimuthal anisotropy in small and large systems

The question of whether there are fundamental differences between soft particle production in small systems (p+p, p+A, etc) and large systems (A+A(B)) is deemed by many to be an unsettled issue. The explicit role of the initial state for these systems is similarly predisposed. I will demonstrate that the scaling properties of the pseudorapidity density (dN/deta) for p+p, p+A and A+A(B) collisions, and the mean multiplicity $\langle N_{ch}\rangle$ for ee, ep, and p+p collisions, are fully compatible with a thermal particle production mechanism for the bulk of the soft particles produced in all of these collision systems. Further, I will demonstrate, via recent azimuthal anisotropy and HBT measurements, that the expansion dynamics for both small and large systems are unquestionably similar. The implications/opportunities associated with these finding will be discussed as well.

Speaker: Roy Lacey (Stony Brook University)

5_Lacey_IS2015_talk.pdf

5_Lacey_IS2015_talk.pptx

65 Forward-backward correlations between intensive observables

Demonstrated that the investigations of the forward-backward correlations between intensive observables enable to obtain more clear signal about the initial stage of hadronic interaction, e.g. about the process of string fusion, compared to usual forward-backward multiplicity correlations. As an example the correlation between mean-event transverse momenta of charged particles in separated rapidity intervals is considered. The calculations are fulfilled in the model with string fusion by introducing a lattice in transverse plane. The dependence of the correlation strength on the collision centrality is obtained for different initial energies. It is shown that above RHIC energy the dependence reveals the decline of the correlation coefficient for most central collisions, reflecting the attenuation of color field fluctuations due to the string fusion at large string density. It is also found that contrary to the correlation between transverse momenta of single particles the strength of the correlation between mean-event transverse momenta of particles in two separated rapidity intervals is not decreasing with the total number of produced strings, remaining significant even in the case of Pb-Pb collisions, in which the total number of strings can reach several thousand. All this makes this type of correlation promising for the observation of the signatures of string fusion at the initial stage of hadronic interaction in relativistic heavy ion collisions at LНС energy.

The work was supported by the RFBR grant 15-02-02097 and the Saint-Petersburg State University grant 11.38.197.2014.

Speaker: Vladimir Kovalenko (St. Petersburg State University (RU))

Lisbon5.pdf

66 The Initial Conditions in High-Energy pp Collisions

The initial conditions in high-energy pp collisions can be well examined from the momentum distributions of both jets and hadrons in these collisions, as they provide relevant information on the collision mechanism and the initial properties of the produced medium. It was found recently that in pp collisions at the LHC, the hadron transverse momentum distributions at central rapidity spanning 14 decades of magnitude over the pT domain from the lowest of ~0.5 GeV/c to the highest of ~200 GeV/c can be adequately described by a single Tsallis distribution with only three apparent degrees of freedom [1]. The simplicity of the pT spectrum over such a large pT domain, and the subsequent hard-scattering model analysis of the jet and hadron momentum distributions suggest indirectly that a single mechanism, the hard-scattering between partons, dominates over almost the whole pT domain in these high-energy collisions [2]. Direct evidence of such a dominance is further provided by the experimental observation of the signature for the hard-scattering process, as indicated by the two-particle angular correlations with minimum-pT-biased hadrons [3]. One is led naturally to the initial conditions that in high-multiplicity events in high-energy pp collisions, parallel hard-scattering collisions of partons generate both the jet and the bulk medium whose subsequent collisions lead to the angular correlations of the near-side ridge, as described in the momentum kick model for such correlations [4-7]. [1] C.Y.Wong , G.Wilk, {Tsallis fits to pT spectra for pp collisions at LHC}, Acta.Phys.Pol. B43,247(2012). [2] C.Y.Wong , G.Wilk, {Tsallis fits to pT spectra and multiple hard scattering in pp Collisions at the LHC}, Phys.Rev.D87,114007(2013). [3] C.Y.Wong,G.Wilk,L.Cirto,C.Tsallis, {From QCD-based hard-scattering to nonextensive statistical mechanical descriptions of transverse momentum spectra in high-energy pp and p-pbar collisions}, Phys.Rev.D91,114027(2015). [4] C.Y.Wong, {The Momentum Kick Model Description of the Near-Side Ridge and Jet Quenching}, Phys.Rev.C78:064905,(2008). [5] C.Y.Wong, {Exploring Early Parton Momentum Distribution with the Ridge from the Near-Side Jet}, J.Phys.G35:104085,(2008). [6] C.Y.Wong, {Parton Momentum Distribution at the Moment of Jet-Parton Collision}, Chin. Phys. Lett. 25, 3936 (2008). [7] C.Y.Wong, {Momentum-Kick model description of the ridge in (Delta-phi)-(Delta eta) correlation in pp collisions at 7 TeV}, Phys.Rev.C84:024901,(2011).

Speaker: Cheuk-Yin Wong (Oak Ridge National Laboratory)

7_Wong_IS2016_talk.pdf

Parallel: thermalisation and equilibration [TH]

Convener: Urs Wiedemann (CERN)

67 Determination of QGP initial conditions and medium properties via a Bayesian model to data analysis

Initial state models translate the spacetime overlap of nuclear density between projectile and target nuclei to generate profiles of entropy (or energy) at the QGP thermalization time which are subsequently evolved by relativistic viscous fluid dynamics. Historically, these initial conditions were generated by one of two means: either from a two-component Glauber ansatz which asserts an admixture of soft and hard collision processes or from specific calculations in Color-Glass Condensate (CGC) effective field theory.

In contrast to the phenomenological approach of the two-component Glauber model where the binary collision fraction is tuned to fit experiment, CGC initial conditions represent predictions calculated from an underlying effective field theory. While modern CGC initial condition models have been highly effective in describing the wealth of bulk observables at RHIC and the LHC, multiple models exist in the literature which predict different results and describe the data with varying degrees of success.

In this talk, we use a parametric initial condition ansatz which mimics the behavior of CGC calculations and extends the coverage of existing two-component parametric mappings to span the full space of reasonable initial condition models. We show that the parameterized initial conditions are highly constrained by bulk observables utilizing Bayesian statistics and a multi-parameter model to data comparison that relies on a state-of-the-art hybrid model for calculating the time-evolution of the QGP phase and its subsequent decay into the hadronic final state. We compare these results to specific calculations in CGC effective field theory and discuss implications for the initial stages of QGP formation.

Speaker: Scott Moreland (Duke University)

is16.pdf

68 Thermalization and hydrodynamization in the color-flux-tube model

Detailed study of thermalization of the momentum spectra of partons produced via decays of the color flux tubes due to the Schwinger tunneling mechanism is presented. The collisions between particles are included in the relaxation time approximation specified by different values of the shear viscosity to entropy density ratio. At first we show that, to a good approximation, the transverse-momentum spectra of the produced patrons are exponential, irrespectively from the assumed value of the viscosity of the system and the freeze-out time. This thermal-like behaviour may be attributed to specific properties of the Schwinger tunneling process. In the next step, in order to check the approach of the system towards genuine local equilibrium, we compare the local slope of the model transverse-momentum spectra with the local slope of the fully equilibrated reference spectra characterised by the effective temperature that reproduces the energy density of the system. We find that the viscosity corresponding to the AdS/CFT lower bound is necessary for thermalization of the system within about two fermis. References: 1) R. Ryblewski, W. Florkowski, Phys.Rev. D88 (2013) 034028 2) R. Ryblewski, arXiv:1512.04117

Speaker: Dr Radoslaw Ryblewski (Institute of Nuclear Physics PAN)

talk.pdf

69 Initial stages in heavy-ion collisions: Isotropization and hydrodynamization

Early stages before hydrodynamization in heavy ion collision is studied by numerically solving a 2+1D effective kinetic theory of weak coupling QCD under longitudinal expansion. We find agreement with viscous hydrodynamics and classical Yang-Mills simulations in the regimes where they are applicable. By choosing initial conditions that are motivated by color-glass-condensate framework we find that for Qs = 2GeV and αs = 0.3, the system is approximately described by viscous hydrodynamics well before τ ~ 1.0 fm/c.

Speaker: Yan Zhu (University of Jyvaskyla)

zhu.pdf

70 Initial conditions for hydrodynamics from weakly coupled pre-equilibrium evolution

We use effective kinetic theory, accurate at weak coupling, to simulate the pre-equilibrium evolution of transverse energy and flow perturbations in heavy-ion collisions. We provide a Green function which propagates the initial perturbations to the energy-momentum tensor at a time when hydrodynamics becomes applicable. With this map, the complete pre-thermal evolution from saturated nuclei to hydrodynamics can be modelled in a perturbatively controlled way.

Speaker: Aleksas Mazeliauskas (Stony Brook University)

initial_stages_talk.pdf

71 Initial stage from holography

In the past years much progress has been made to understand the initial stage of heavy ion collisions at very strong coupling. I will briefly review this approach, which in particular led to a full 3+1D simulation of off-central collisions using the MUSIC hydrodynamic code. I will report on progress on collisions with a conserved charge [1], which will be relevant for the beam energy scan. New results will also include event-by-event fluctuations, which may in part explain why previous strongly coupled studies overestimated the entropy production. [1] To appear, with Jorge Casalderrey-Solana, David Mateos and Miquel Triana

Speaker: Wilke van der Schee (MIT)

Talk_Lisbon16_Wilke.pdf

Talk_Lisbon16_Wilke.pptx

72 From high-energy collisions to hydrodynamics in strongly coupled non-conformal theories

Ever since fast hydrodynamization has been observed in heavy ion collisions at RHIC, attempts to understand this feature of the very early non-equilbrium stage have been made. We use the gauge/string duality to model the evolution of strongly coupled Quark-Gluon plasma in a non-conformal gauge theory. The non-trivial equation of state and the presence of a time dependent bulk viscosity change the evolution of planar shockwaves. The effect of this non-conformality shows an increase of the relaxation times of the resulting plasma.

Speaker: maximilian attems (University of Barcelona)

IS2016_Attems.pdf

73 On chiral instability in quark-gluon plasma

We study the chiral-imbalance and the Weibel instabilities in presence of the quantum anomaly using the Berry-curvature modified kinetic equation. We argue that in many realistic situations, e.g. relativistic heavy-ion collisions, both the instabilities can occur simultaneously. The Weibel instability depends on the momentum anisotropy parameter ξ and the angle (θn) between the propagation vector and the anisotropy direction. It has maximum growth rate at θn=0 while θn=π/2 corresponds to a damping. On the other hand the pure chiral-imbalance instability occurs in an isotropic plasma and depends on difference between the chiral chemical potentials of right and left-handed particles. It is shown that when θn=0, only for a very small values of the anisotropic parameter ξ∼ξc, growth rates of the both instabilities are comparable. For the cases ξc<ξ≪1, ξ≈1 or ξ≥1 at θn=0, the Weibel modes dominate over the chiral-imbalance instability if μ5/T≤1. However, when μ5/T≥1, it is possible to have dominance of the chiral-imbalance modes at certain values of θn for an arbitrary ξ. We also calculate the coefficient of shear viscosity generated by the instability. Further we discuss consequences of our results for heavy-ion collisions.

Speaker: Prof. Jitesh Bhatt (Physical Research Laboratory)

6_ISlisboa16.pdf

Thursday, 26 May

Benchmarking

Convener: Kari J. Eskola (University of Jyvaskyla)

74 Benchmarking in pA: is it still possible?

Speaker: Maria Pia Zurita

IS2016-Pia.pdf

75 pA/eA/ultraperipheral AA complementarity: what, where and how can we learn about the AA initial state

Speaker: Tuomas Lappi (University of Jyvaskyla)

is16.pdf

76 EIC

Speaker: J.H. Lee (Brookhave National Laboratory)

is2016.pdf

10:30 Coffee break

Benchmarking

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

77 LHeC

Speaker: Bruce Mellado Garcia (University of the Witwatersrand)

LISBON260516.pdf

LISBON260516.ppt

78 Heavy flavour production in pp and PbPb collisions with the CMS detector

The measurement of heavy flavour production is a powerful tool to study the properties of the high-density QCD medium created in heavy-ion collisions as heavy quarks are sensitive to the transport properties of the medium and may interact with the QCD matter differently from light quarks. In particular, the comparison between the nuclear modification factors of light and heavy flavoured particles provides insights into the expected flavour dependence of in-medium parton energy loss. Using the large statistics proton-proton and PbPb samples collected at 5.02 TeV during the 2015 LHC runs, high precision open charm and beauty measurements are performed with the CMS detector in a wide transverse momentum range, from few GeV up to approximately 100 GeV. This allows us to set an important milestone in our understanding of energy loss phenomena. In this talk, the most recent results of nuclear modification factor of $D^{0}$ mesons in PbPb collisions at 5.02 TeV are presented and compared to the charged hadron nuclear modification factor at the same energy.

Speaker: Gian Michele Innocenti (Massachusetts Inst. of Technology (US))

IS2016talk_Innocenti.pdf

79 Discussion on benchmarking in pA and complementarity with other approaches

Speakers: Carlos Albert Salgado Lopez (Universidade de Santiago de Compostela (ES)), Yen-Jie Lee (Massachusetts Inst. of Technology (US))

13:00 Lunch

Extraction of medium properties

Convener: Eero Aleksi Kurkela (CERN)

80 Heavy-flavour production measurements in small systems with ALICE

Hadrons carrying heavy flavour, i.e. charm or beauty quarks, are unique probes in hadronic collisions at high energies. Due to their large masses, heavy quarks are predominantly produced in hard parton scattering processes in the initial phase of the collisions. Therefore, measurements of heavy-flavour hadron production in pp collisions provide tests of perturbative QCD and constitute a crucial baseline for the study of heavy-flavour production in heavy-ion collisions. In particular, the more differential measurements of heavy-flavour particle multiplicity dependence or angular correlations with charged hadrons provide insight into their production mechanisms, as well as into the interplay between the hard and soft processes or fragmentation. At the LHC energies, multiple parton interactions may also influence heavy-flavour production. In p-Pb collisions heavy-flavour production is affected by cold nuclear matter effects, such as the modification of the parton density distribution of nucleons bound in nuclei with respect to those of free nucleons, multiple soft scatterings of partons in the initial state ($k_{\rm T}$ broadening) or gluon radiation (energy loss). The modelisation of those effects usually considers a dependence on the collision geometry. Heavy- flavour measurements in p-Pb collisions help to characterise the initial state of heavy-ion collisions and are the mandatory reference for the interpretation of the corresponding studies in Pb-Pb collisions, where heavy quarks interact with the medium constituents providing information on the transport properties of the medium. ALICE capabilities allow for full reconstruction of the hadronic decays of open- charmed D mesons at central rapidity, as well as to study leptons from charm and beauty decays at central and forward rapidity. An overview of the heavy-flavour measurements in pp and p-Pb collisions with ALICE will be given. Special attention will be put on the production cross sections, nuclear modification factors, angular correlations between D mesons and charged hadrons, and the studies of the multiplicity dependence of D-meson yields, which will be discussed in terms of collision centrality (centrality-dependent nuclear modification factor) or as a function of charged-particle multiplicity (per-event yield as a function of charged-particle multiplicity). Results will be compared with model calculations when possible.

Speaker: Elena Bruna (Universita e INFN Torino (IT))

ebruna_IS2016.pdf

81 The role of longitudinal correlations and fluctuations [TH]

Speaker: Dr Long-Gang Pang (Frankfurt Institute for Advanced Studies, Goethe University)

LGPang_InitialStage_201605.pdf

82 The role of longitudinal correlations and fluctuations [EXP]

Speaker: Jiangyong Jia (State University of New York (US))

Jiangyong_is2016v5.pdf

16:00 Coffee break

Extraction of medium properties

Convener: Rainer Fries (Texas A&M University)

83 Interplay of different parameters in hydro: role of preequilibrium dynamics and initialization time

Speaker: Ulrich Heinz (The Ohio State University)

IS2016_Heinz.pdf

84 Measurement of forward-backward multiplicity and flow correlations in $pp$, $p$+Pb and Pb+Pb collisions with the ATLAS detector

(speaker known later) Correlations between particle multiplicities at different pseudorapidities are studied in pp, p+Pb and Pb+Pb collisions using the ATLAS detector at the LHC. A data driven procedure is used to separate the short-range and long-range contributions to the correlation function. The short-range component differs considerably between opposite and same charged pairs and shows a strong collision system dependence. The long-range component is expanded in an orthogonal basis of Legendre polynomials, and is found to be dominated by the first order (linear) modulation. The coefficient of the first order modulation, $a_1$, is studied as a function of the event-multiplicity. It is found to be independent of the charge combination of the pair, and similar between the three collision systems at a given multiplicity. The $a_1$ is known to be strongly correlated with the forward-backward asymmetry in the initial state. The multiplicity and collision system dependence of $a_1$ can provide important insights into the early time entropy production in high energy collisions. Measurements of the azimuthal anisotropy of charged particles in Pb+Pb, p+Pb and pp collisions over broad transverse momentum and pseudorapidity ranges, using the ATLAS detector are presented. The measurements are done via multi-particle cumulants, event-plane and scalar-product methods. The analysis uses high-statistics $p$+Pb data at $\sqrt{s_{\mathrm{NN}}}$=5.02~TeV, $pp$ data at $\sqrt{s}$=13~TeV and Pb+Pb data at $\sqrt{s_{\mathrm{NN}}}$=2.76~TeV. The measurements for different systems are compared as a function of charge particle multiplicity. In addition, the measurements of flow harmonics, $v_n$, in the new 5.02~TeV Pb+Pb data collected during the 2015 LHC heavy-ion run, will be reported. This study provides important insight into the potential collective behavior in small collision systems.

Speaker: Mingliang Zhou (State University of New York (US))

ATLAS_LongCorr_Mingliang.pdf

85 Beam Energy Dependence of Azimuthal Correlations in Au+Au Collisions at mid and forward rapidity

The STAR collaboration has made an extensive set of flow measurements for Au+Au collisions spanning the collision energy range of 7.7 - 200 GeV in beam energy scan I (BES - I). Differential results for the n$^{th}$ harmonic coefficients $v_{n}\{2\}$, used to characterize the flow magnitudes, will be presented for charged hadrons measured at mid and forward rapidity ($\eta < 1$ and $ 2.5 < \eta < 4$) with a pseudorapidity separation $\Delta\eta>0.7$. The use of these measurements to understand aspects of the reaction dynamics and the QCD phase diagram will also be discussed.

Speaker: Niseem Abdelwahab (Stony Brook University)

3_Abdelwahab_IS2016_talk.pdf

3_Abdelwahab_IS2016_talk.pptx

19:30 Conference Dinner

Friday, 27 May

Multi-particle interactions

Convener: Cyrille Marquet (CPHT - Ecole Polytechnique)

86 Multi-parton and multi-nucleon correlations: theory

Speaker: Christian Bierlich (Lund University (SE))

bierlich_mpi.pdf

87 Multi-parton and multi-nucleon correlations: experiment

Speaker: Andreas Morsch (CERN)

IS2016_morsch-Final.pdf

10:30 Coffee break + poster session

Multi-particle interactions

88 Discussion on MPIs in pp/pA/AA

Speakers: Cyrille Marquet (CPHT - Ecole Polytechnique), Peter Alan Steinberg (Brookhaven National Laboratory (US))

mpi-discussion-slides.pdf

Outlook

Convener: Ulrich Heinz (The Ohio State University)

89 What features of the initial stages will not be tractable (or will, but without sufficient precision) with current or foreseable theoretical tools?

Speaker: Derek Teaney (Stony Brook University)

is_teaney.pdf

90 What aspects of the initial stages will not be feasibly tested in the near future?

Speaker: Mateusz Ploskon (Lawrence Berkeley National Lab. (US))

ploskon_init_stages2016.pdf

13:00 Lunch

Outlook

91 New experimental conditions in the near future at RHIC and LHC

Speaker: David Morrison (BNL)

morrison_is2016.key

morrison_is2016.pdf

92 Expected accelerator conditions for the near future

Speaker: Dr John Jowett (CERN)

InitialStage2016 Jowett 27May2016v2.pdf

InitialStage2016 Jowett 27May2016v2.pptx

93 Closing