COST Workshop on Interplay of hard and soft QCD probes for collectivity in heavy-ion collisions

25 Feb 2019, 09:00 → 1 Mar 2019, 12:40 Europe/Stockholm

Lundmarksalen (Lund university)

Christian Bierlich (Lunds universitet) , Roman Pasechnik (Lunds universitet)

Since the discovery of collective, heavy-ion like phenomena in small systems, like the appearance of a near-side ridge at high multiplicity, and strangeness enhancement with a smooth transition to pA and AA collision systems, a key question for both the hadron-hadron and heavy-ion communities have been if the origin of collectivity is the same across systems. A relevant point to answering this question is to address the heretofore non-observation of jet quenching in small systems. If the above mentioned collective phenomena in small systems are indeed a sign of final-state interactions, the key to understanding the differences and similarities between small and large systems, must be to thoroughly investigate the interplay between hard and soft probes on the collective phenomena.

In this workshop, we review the state of the art theoretical approaches to modelling both the soft and hard parts of collective effects in hadron-hadron and heavy-ion collisions. This will be followed up with discussions and ideas of how to tackle the challenge of unifying the descriptions. We aim for a discussion between theoreticians working on different approaches (effective models and MC generators) from several points of view. The goal is to come to a common understanding of the advantages and disadvantages of the different approaches. We plan to produce a short document summarising the present status and the output of the discussions.

The workshop would start with an overview of small systems, recent hydro and jet quenching results results, relation between hydro and jet quenching in large and small systems and what we learn about QCD. The follow up sessions would be devoted to the topics of jet fragmentation modification in high density environments, jet chemistry aspects, multiplicity and outlook on what we learn from the collider data. There will be a few experiments talks on prospects from HL-LHC, ALICE upgrade, STAR, ALICE and ALTAS results.

The first two days of the workshop will be devoted to a mini-school, with a few lectures primarily targeted at Ph.D. students and young postdocs, reviewing some of the main theoretical microscopic and macroscopic approaches to hadron-hadron and heavy-ion collisions. The school will also include a hands-on tutorial session of the Rivet framework for comparing Monte Carlo predictions to the data, with its recent updates to including heavy-ion processes.

Confirmed invited lecturers and topics:

• Ilkka Helenius - Nuclear PDFs
• Leif Lönnblad - Angantyr
• Torbjörn Sjöstrand - Pythia
• Volodymyr Vovchenko - Statistical thermal model
• Christian Bierlich - Rivet hand-on session
• Klaus Werner - EPOS
• Stefan Prestel - Jets in perturbative QCD
• Liliana Apolinario - Jet quenching
• Christopher Plumberg - Flow

Internet access is available through the eduroam wireless network.

Dates for the School: from 25 February 2019 13:00 to 26 February 2019 17:15 
Dates for the Workshop: from 27 February 2019 09:00 to 1 March 2019 12:00

Location: Department of Astronomy and Theoretical Physics, Lund University, Sölvegatan 27, Lund

Room: Lundmarksalen

Registration and workshop programme: Since the number of particpants has reached the room capacity limit, the registration is closed now (contact the organisers in case you really wish to attend but forgot to register). The agenda is full and almost finalised. Any requests for late changes should sent via email to Roman.Pasechnik@thep.lu.se.

Local oraganising committee:

• Roman Pasechnik (chair)
• Christian Bierlich
• Korinna Zapp
• Peter Christiansen
• Leif Lönnblad

 

     

Slides COST_Lecture_Plumberg.pdf

Monday, 25 February

11:00 → 12:30 Registration

The registration desk will be placed at the sound floor of Astronomy building, in front of Lundmarksalen

12:30 → 13:30 Jets in pQCD

Speaker: Stefan Prestel (Lund U.)

Slides lund20190225.pdf

13:30 → 14:30 PYTHIA and other MC generators for pp physics

Speaker: Torbjörn Sjöstrand (Lund U.)

Slides sjostrand.pdf

14:30 → 15:00 Coffee break

15:00 → 16:00 Angantyr

Speaker: Leif Lönnblad (Lund U.)

Slides COSTLund19.pdf

16:00 → 18:00 Rivet hands on session

Speaker: Christian Bierlich (Lund U.)

Slides cbierlich-school.pdf

18:00 → 19:00 Pizza/working dinner

Tuesday, 26 February

08:30 → 09:30 From micro to macro QCD phenomena

Speaker: Christopher Plumberg (Lund U.)

Slides COST_Lecture_Plumberg.pdf

09:30 → 10:00 Coffee break

10:00 → 11:00 Statistical thermal model

Speaker: Volodymyr Vovchenko (FIAS)

Slides vovchenko_lect.pdf

11:00 → 12:00 EPOS

Speaker: Klaus Werner (SUBATECH)

Slides lund.20.pdf

12:00 → 13:30 Lunch

13:30 → 14:30 Nuclear Parton Distribution Functions

Speaker: Ilkka Helenius (Jyvaskyla U.)

Slides nPDF_lecture.pdf

14:30 → 15:30 Photon induced processes from semi-central to ultraperipher collisions: introduction

Speaker: Wolfgang Schafer (IFJ Krakow)

Slides WS_Lund_COST.pdf

15:30 → 16:00 Coffee break

16:00 → 17:00 Jet quenching

Speaker: Liliana Apolinário (LIP Lisbon)

Slides JQ_Liliana.pdf

17:00 → 18:00 Panel discussion

Wednesday, 27 February

09:00 → 09:35 From small to large systems - What can we learn about the QGP?

Speaker: Jurgen Schukraft (NBI)

Slides Small_Systems_Lund_2019.pptx

09:35 → 10:00 Recent flow results from LHC

Speaker: You Zhou (NBI)

Slides YouZhou_LundCOST_v1.pdf

10:00 → 10:25 Overview of jet physics results from ALICE

Collisions of ultra-relativistic heavy ions are used to create strongly interacting matter in the regime of high-energy densities and temperatures. Under these conditions color confinement of quarks and gluons in hadrons breaks down and a new state of matter called Quark-Gluon Plasma is formed. Properties of this medium can be inferred based on observed modifications of produced jets. Recently, jet analyses have developed new tools to study jet properties more differentially. These observables are based on hadron-jet correlations or jet-shape and jet-substructure measurements. The talk will review recent results from these jet analyses peformed in ALICE.

Speaker: Filip Krizek (NPI Prague)

Slides Krizek_Lund2019.pdf

10:25 → 10:45 Coffee break

10:45 → 11:10 Does dN/deta scale as Npart or Ncoll

Speaker: Christian Holm Christensen (NBI)

Slides COST_20190227.pdf

11:10 → 11:35 Quark Gluon Plasma: the fastest rotating fluid

The extreme energy densities generated in ultra-relativistic heavy ion collisions produce a state of matter that behaves surprisingly like a fluid, with exceptionally high temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong local rotational structure. Spin–orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. I will present STAR measurements (Nature 548 (2017) 62, arXiv:1701.06657) of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. I will also briefly discuss observable effects due to strong magnetic fields produced in ultra-relativistic heavy ion collisions related to the restoration of fundamental symmetries of quantum chromodynamics.

Speaker: Michal Sumbera (NPI Prague)

Slides Sumbera_Vorticity_Lund.pptx

11:35 → 12:00 Multiplicity and center-of-mass energy dependence of light-flavor hadron production in pp collisions

Measurements of particle production in proton-proton collisions as a function of multiplicity have revealed intriguing signs of collective behaviour in small systems. In this talk the recent results on light-flavour hadron production in pp collisions will be compared to larger systems, such as p-Pb, Pb-Pb and Au-Au. In addition, the importance of $\sqrt{s}$ to the observed trends will be addressed and tools to separate the soft and hard scattering regimes of the QCD will be discussed.

Speaker: Vytautas Vislavicius (NBI)

12:00 → 13:30 Lunch

13:30 → 13:55 The final state swing

Speaker: Leif Lönnblad (Lund U.)

Slides COSTLund19b.pdf

13:55 → 14:20 Influence of electromagnetic fields on small systems

The recent experimental observations at RHIC and LHC indicate the formation of quark-gluon plasma even in small systems such as proton-nucleus collisions. In the early stage of relativistic heavy ion collisions extremely intense magnetic fields, with a magnitude up to 5-50 m_π^2, are produced; in asymmetric collisions, and in proton-induced reactions in particular, not only the magnetic field but also the electric field is very high. Moreover, in asymmetric colliding systems the particle rapidity distributions are strongly asymmetric inside the overlap region. By means of microscopic calculations within the Parton-Hadron-String Dynamics (PHSD) approach we study the distributions of electromagnetic fields in central p+Au collisions and investigate the influence of these fields on final hadronic observables, such as particle distributions and flow harmonics.

Speaker: Lucia Oliva (GSI, Darmstadt)

Slides Lund2019_slides.pdf

14:20 → 14:45 Colour evolution

We consider soft gluon evolution of a system of clusters forming the initial state of the cluster hadronization model, in order to constrain colour reconnection models from a perturbative point of view. We show that this ansatz produces clusters with properties attributed to a colour pre-confined state and find strong evidence for formerly investigated colour reconnection models based on geometric properties. We also explore the possibility of colour flows giving rise to baryonic clusters and propose simple parametrizations in order to incorporate the effects of soft gluon evolution in a Monte Carlo Event Generator. In the first part of my talk I will explain the underlying theory, while in the second part I will focus on the implementation in the Monte Carlo Event Generator Herwig.

Speaker: Patrick Kirchgaeßer (KIT)

Slides ColourEvolution_Lund_2019.pdf

14:45 → 15:10 Non-flow Effects on Q-cumulants

One of the most used methods to compute flow coefficients in high-energy particle collisions is the Q-cumulant method. The method employs the mathematical concept of a cumulant which is meant to identify correlations to some order (between more particles in this case). For this reason, it is often said that the method removes lower order non-flow effects. A semi-theoretical study has been done to show that this is not (by design) true. Additionally, other effects of the particle distribution function can produce artificial flow signals. Such effects are demonstrated and discussed.

Speaker: Kristjan Gulbrandsen (NBI)

15:10 → 15:40 Coffee break

15:40 → 16:05 Multiplicity dependence of particle production at the LHC in (canonical) statistical model

System-size dependence of hadrochemistry at vanishing baryon density is studied within the statistical model in the context of p-p, p-Pb, and Pb-Pb data of the ALICE collaboration. Evaluations are done in the framework of a new Thermal-FIST package. Effects of exact conservation of charges, incomplete equilibration of strangeness, finite resonance widths, are multiplicity dependence of the chemical freeze-out temperature are covered.

Speaker: Volodymyr Vovchenko (FIAS)

Slides vovchenko_lund2019.pdf

16:05 → 16:30 In what way is a QGP interacting?

It is often thought that if the anisotropic flow in small systems is due to final state interactions then one should also observe significant jet quenching in the same systems. In this talk, I will discuss this statement based on ideas of Kinetic theory and results from the new Angantyr generator and show that one can have final state interactions that does not give significant jet quenching. Finally, I will try to relate this to perfect fluidity.

Speaker: Peter Christiansen (Lund U.)

Slides slides_peter_cost_workshop.pptx

16:30 → 16:55 Strangeness and space-time model in Herwig

Speaker: Stefan Gieseke (KIT)

16:55 → 17:20 Role of string collectivity and semihard process in multiplicity-dependent transverse momentum and the strangeness enhancement

The multiplicity dependence of the mean transverse momentum as well as strangeness and multi-strangeness production is studied in the framework of two models: dipole-based Monte Carlo model with string fusion [1-2], and extended multi-pomeron exchange model [3-4]. The first model allows the unifying description of pp, pA and AA collisions [5]. The partonic interactions are described as collisions of dipoles. The hardness of the collision is determined by transverse size of the colliding dipoles. The production of the observed particles is implemented in the string mechanism, taking into account string fusion [6-7]. The second model [3-4] is based on Regge-Gribov approach. The model includes string collectivity, parametrized by one parameter, beta, which is determined by data on pt-multiplicity correlation at wide energy range. For the particles species discrimination, both models use the modified Schwinger mechanism [8], where the effective string tension, according to the string fusion prescription, depends on the string density. We show that two effects contribute to the transverse momentum: the hardness of the partonic collision, and the modification of the string tension. Only second effect leads to the modification of strangeness and multi-strangeness production. Taking it into account allows one to describe the experimental data [9] at LHC energy. [1] V. N. Kovalenko, Phys. Atom. Nucl. 76, 1189 (2013), arXiv:1211.6209 [hep-ph]. [2] V. Kovalenko, V. Vechernin, PoS (Baldin ISHEPP XXI) 077, arXiv:1212.2590 [nucl-th], (2012). [3] E. Bodnia, D. Derkach, G. Feofilov, V. Kovalenko, A. Puchkov, PoS (QFTHEP 2013) 060 (2013), arXiv:1310.1627 [hep-ph]. [4] E. O. Bodnia, V. N. Kovalenko, A. M. Puchkov, G. A. Feofilov, AIP Conf. Proc. 1606, 273-282 (2014), arXiv:1401.7534 [hep-ph]. [5] V. N. Kovalenko, Phys. Part. Nucl, 48, 945 (2017). [6] M. A. Braun, C. Pajares, Nucl. Phys. B 390, 542 (1993). [7] N. S. Amelin, N. Armesto, M. A. Braun, et al., Phys. Rev. Lett. 73, 2813 (1994). [8] J. Schwinger, Phys. Rev. 82, 664 (1951); T. S. Biro, et al, Nucl. Phys. B. 245, 449 (1984). [9] J. Adam, et al (ALICE Collaboration), Nature Physics 13, 535-539 (2017).

Speaker: Vladimir Kovalenko (St.Petersburg State U.)

Slides LundPresKovalenko.pdf

19:00 → 21:00 Workshop dinner

Thursday, 28 February

09:00 → 09:25 Generating functional for quenched observables

Hard probes, and in particular jets, are at the same time both promising and challenging tools for probing the hot and dense nuclear matter created in high-energy heavy-ion collisions at the RHIC and LHC. Radiative processes in the medium lead to a redistribution of energy within the jet cone and an enhanced energy leakage out of it. These modifications are governed by the interplay of the relevant momentum- and time-scales, and are sensitive to the way partonic fluctuations within the jet are resolved by the medium. Mapping out all relevant regimes of jet-medium interactions onto the Lund kinematical plane, we establish a probabilistic picture of the fragmentation in the form of a generating functional (GF). In contrast to the GF in vacuum, in medium it contains terms describing fragmentation at short and long time-scales, compared to the medium scales, and the transition between them. Due to the mismatch between real and virtual emissions, the GF is normalized to the collimator function that describes how jet substructures are quenched and can be measured from the inclusive jet spectrum in heavy-ion collisions. We put a special emphasis on the role of color coherence for hard, small-angle fluctuations.

Speaker: Konrad Tywoniuk (Bergen U.)

Slides LundCOST2019_Konrad.pdf

09:25 → 09:50 Soft modifications to pp fragmentation: The shoving model

In this talk the recently developed shoving model, based on interacting Lund strings, is discussed. The model is developed with soft collective effects in pp in mind, most prominently a description of the ridge in pp collisions. Since Lund strings also connect the hardest fragment of a jet to beam remnants, shoving the connecting strings will affect jet observables as well. Similarities to jet quenching in AA collisions are highlighted in this connection.

Speaker: Christian Bierlich (Lund U.)

09:50 → 10:15 Probing the QGP time structure from large to small(er) systems with top quarks

Top quarks have been recently proposed as a unique probe to determine the space-time evolution of the medium that is created in heavy-ion collisions [1]. In particular, they allow an approximate determination of the time at which the top’s decay products start interacting with the QGP, making this channel most sensitive to the late time dynamics of the produced medium. This is achieved by using the hadronic decay chain of the top quark: in addition to the finite lifetimes of the top and W-boson, the time-delay in the interaction of the (colour-singlet) W-boson’s decay products with the medium adds to a finite total time-delay that can be applied to probe different QGP timescales. Top quarks have already been measured in nuclear collisions. With the forthcoming HL-LHC and HE-LHC, the increase in luminosity and energy can be further explored to use top quark observables as a unique probe of the QGP. In this talk, we show the latest projections for the use of this particular channel in both PbPb and lighter ions collisions in these two setups. [1] L. Apolinário, J. G. Milhano, G. Salam, C. Salgado, Phys.Rev.Lett. 120 (2018) no.23, 232301

Speaker: Liliana Apolinário (LIP Lisbon)

Slides TopsTalk_Liliana.pdf

10:15 → 10:45 Coffee break

10:45 → 11:10 Suppression of anisotropic flow without viscosity

Relativistic viscous hydrodynamics calculations can describe a wide range of observables in relativistic heavy ion collisions. However, many studies suggest that the hot and dense system created is not in local thermal equilibrium, resulting in a contradiction for the applicability of hydrodynamics. In hydrodynamic calculations, a crucial step is to convert the fluid fields into observable particles. The most common approach is to use the Cooper-Frye formula, which assumes thermal (Boltzmann/Bose/Fermi) distributions at the conversion. In this talk, I show how local equilibrium distributions of the Tsallis form can affect observables, resulting in non-exponential particle distributions and a viscous-like suppression of the anisotropic flow. By comparing the calculation to experimental data, I will estimate the degree of non- Boltzmann effects on the freeze-out surface.

Speaker: Adam Takacs (Bergen U.)

Slides Adam_Takacs_COST_Workshop_Lund_2019.pdf

11:10 → 11:35 A Monte Carlo study of in-medium quark and gluon jet colour scaling

Most jet quenching models predict that, at the parton level, the quark to gluon energy loss due to in-medium interactions scales with the ratio of the corresponding Casimirs $C_F/C_A$. However, in the jet framework, one expects a violation of this scaling due to the finite extension of the jet. In this talk, I will present results from a Monte Carlo study of calibrated Z+jets events which allow addressing this problem. The results presented here show an interesting scaling violation which indicates that gluon and quark jets are much more similar in medium than the corresponding partons. In addition, a parametrization of the average energy loss for quarks and gluons and for its variance is provided. Such a parametrization can prove useful to computing observables for in-medium events without needing a full Monte Carlo simulation.

Speaker: Joao Barata (Santiago de Compostela)

Slides Lund.pdf

11:35 → 12:00 The future of JEWEL

Speaker: Korinna Zapp (Lund U.)

Slides lund-COSTwshop_1902.pdf

12:00 → 13:30 Lunch

13:30 → 14:05 Dynamical description of heavy-ion collisions

We present the basic ideas of the Parton-Hadron-String Dynamics (PHSD) transport approach which is a microscopic covariant dynamical model for strongly interacting systems formulated on the basis of Kadanoff-Baym equations for Green's functions in phase-space representation (in 1st order gradient expansion beyond the quasi-particle approximation). The approach consistently describes the full evolution of a relativistic heavy-ion collision from the initial hard scatterings and string formation (based on LUND string model) through the dynamical deconfinement phase transition to the strongly-interacting quark-gluon plasma (sQGP) as well as hadronization and the subsequent interactions in the expanding hadronic phase. The PHSD approach has been applied to p+p, p+A and A+A collisions from lower SIS to LHC energies and been successful in describing a large number of experimental data including single-particle spectra, collective flow and electromagnetic probes. The highlights of the PHSD results will be presented with the focus on charm production.

Speaker: Elena Bratkovskaya (GSI, Darmstadt)

Slides Bratkovskaya_Lund2019.pdf

14:05 → 14:30 Particle Interferometry from Hydrodynamics and Event Generators

Particle interferometry - also known as Hanbury Brown-Twiss (HBT) interferometry - is a measurement technique based on momentum-space correlations between identical particles which has proven to be an indispensable tool in studying the space-time evolution of femtoscopic collision systems. In particular, HBT interferometry allows one to classify these collision systems on the basis of their collision geometry. This, in turn, provides a useful aid to calibrating things such as jet-quenching analyses in both large and small collision systems, where the precise geometry of the system plays a crucial role in the interpretation of experimental results. In this talk, I describe some recent and ongoing efforts to connect the techniques of HBT interferometry with the question of QGP medium formation in heavy-ion collisions and high multiplicity hadron-hadron collisions, some of which are directly relevant to the goal of understanding jet-quenching in collision systems of various sizes. Specifically, I will show how hydrodynamic predictions for the space-time geometry can be tested using HBT, and I will discuss some current work to enable similar predictions to be made using the MC event generator formalism of Pythia/Angantyr for the modeling of high-energy collision systems.

Speaker: Christopher Plumberg (Lund U.)

Slides COST_talk_Plumberg.pdf

14:30 → 14:55 New exact solutions of relativistic hydrodynamics and their applications

A recently found new family of exact solutions of 1+1 dimensional relativistic hydrodynamics will be presented, and its applications to describe the pseudorapidity density, the HBT radii and the estimation of the initial energy density in proton-proton and heavy ion collisions at LHC, RHIC and SPS energies will be reviewed. The talk will focus on a recently discovered signal of a non-monotonic behaviour of the initial energy densities in heavy ion collisions at RHIC energies. References: 1) Lifetime estimations from RHIC Au+Au data. G. Kasza and T. Csörgő, [arXiv:1811.09990]. 2) A new and finite family of solutions of hydrodynamics: Part III: Advanced estimate of the life-time parameter. T. Csörgő and G. Kasza, [arXiv:1810.00154]. 3) A new and finite family of solutions of hydrodynamics: Part II: Advanced estimate of initial energy densities. G. Kasza and T. Csörgő, [arXiv:1806.11309 [nucl-th]]. 4) A new and finite family of solutions of hydrodynamics. Part I: Fits to pseudorapidity distributions. T. Csörgő, G. Kasza, M. Csanád and Z.F. Jiang, [arXiv:1806.06794]. 5) New exact solutions of relativistic hydrodynamics for longitudinally expanding fireballs. T. Csörgö, G. Kasza, M. Csanád, Z.F. Jiang, [arXiv:1805.01427]. 10.3390/universe4060069, Universe 4 (2018) 69.

Speaker: Tamás Csörgő (Wigner RCP)

Slides 2019-02-28-Csorgo-Kasza-New-exact-rel-hydro-sol-Lund-COST.pdf 2019-02-28-Csorgo-Kasza-New-exact-rel-hydro-sol-Lund-COST.pptx

14:55 → 15:20 QGP from the quantum ground-state of QCD?

In this talk, I will elaborate on a semi-classical Hamiltonian approach to describe the QGP production mechanism in real time based upon the existence of a homogeneous initial state being a non-trivial QCD ground-state. An effect ala parametric resonance leading to a decay of the homogeneous gluon condensate into inhomogeneous gluon plasma is thought to be a possible driver of QGP production in QCD.

Speaker: Roman Pasechnik (Lund U.)

Slides COST-Pasechnik.pdf

15:20 → 15:50 Coffee break

15:50 → 16:15 Creation of quark–gluon plasma droplets with three distinct geometries

Experimental studies of the collisions of heavy nuclei at relativistic energies have established the properties of the quark–gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks and gluons are not bound into hadrons. In this state, matter behaves as a nearly inviscid fluid that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the particles produced, creating a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton–proton (p+p) and proton–nucleus (p+A) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton–gold (p+Au), deuteron–gold (d+Au) and helium–gold (3He+Au) collisions at a nucleon–nucleon centre-of-mass energy sqrt(s_NN)=200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide the best simultaneous description of these measurements.

Speaker: Tamás Csörgő (Wigner RCP)

Slides 2019-02-28-Csorgo-Csanad-4-PHENIX-COST-Workshop-Interplay-Hard-and-Soft-v2.pdf

16:15 → 16:40 Effect of Rope hadronisation on Strangness enhancement in pp collisions at the LHC

The p-p collisions at high multiplicity at LHC show small scale collective effects similar to that observed in heavy ion collisions such as enhanced production of strange and multi-strange hadrons, long range azimuthal correlations, etc. The observation of strangeness enhancement in p-p collisions at 7 TeV and 13 TeV as measured by ALICE experiment is explored using Pythia8 event generator within the framework of microscopic rope hadronization model which assumes the formation of ropes due to overlapping of strings in high multiplicity environment. The spectral shape and its hardening with multiplicity is well described by the model. The mechanism of formation of ropes also described the observed experimental strangeness enhancement for higher multiplicity classes in p-p collisions at 7 TeV and 13 TeV. The enhancement with multiplicity is further investigated by studying the mean pT (< pT >) and the integrated yields (< dN/dy >) of strange and multi-strange hadrons and comparing the predictions to the mea- sured data at LHC for 7 TeV and 13 TeV.

Speaker: Ranjit Nayak (IIT Bombay)

16:40 → 17:05 Jet production in ultra-peripheral collisions with Pythia 8

Traditionally jet production in different collision systems has been used to study different physics: jet quenching in AA, initial-state nuclear modifications in pA and pQCD baseline in pp. There are, however, recent observations of similar collective effects in all these systems. This challenges the picture where the measurements in these different systems would provide information only on well-separated physics phenomena. In addition to these widely-studied collision systems, it is also possible to study photon-proton and photon-nucleus interactions in ultra-peripheral collisions (UPCs) at the LHC. These photon-induced processes could offer new insights on the collective phenomena and provide a clean environment to study purely initial-state effects. In this talk we discuss how these processes can be simulated with Pythia 8 general-purpose Monte Carlo event generator by applying the recent photoproduction framework. As an application, we study the potential of photo-nuclear dijets in PbPb collisions at the LHC to further constrain nuclear PDFs. In addition, we introduce our dynamical rapidity gap survival model for hard diffraction which is now implemented also for photoproduction in Pythia 8, and present predictions for diffractive dijet production in UPCs at the LHC.

Speaker: Ilkka Helenius

Slides Lund_Helenius.pdf

17:05 → 17:30 Hadron yields and fluctuations at the CERN Super Proton Synchrotron: system size dependence from Pb+Pb to p+p collisions

The kaon to pion ratio K^+/\pi^+ and the scaled variance \omega^- for fluctuations of negatively charged particles are studied within the statistical hadron resonance gas (HRG) model and the Ultra relativistic Quantum Molecular Dynamics (UrQMD) transport model. The calculations are done for p+p, Be+Be, Ar+Sc, and Pb+Pb collisions at the CERN Super Proton Synchrotron energy range to reveal the system size dependence of hadron production. For the HRG calculations the canonical ensemble is imposed for all conserved charges. In the UrQMD simulations the centrality selection in nucleus-nucleus collisions is done by calculating the forward energy E_{\rm F} deposited in the Projectile Spectator Detector, and the acceptance maps of the NA61/SHINE detectors are used. The role of centrality selection on fluctuation measures is studied in detail. A comparison of the HRG and UrQMD results with the data of the NA61/SHINE Collaboration is done. To understand a difference of the event-by-event fluctuations in p+p and heavy ion collisions the centrality selection procedure in the sample of all inelastic p+p events is proposed and analyzed within the UrQMD simulations. References: [1] A. Motornenko, K. Grebieszkow, E. Bratkovskaya, M.I.Gorenstein, M.Bleicher and K.Werner, J. Phys. G 45, no. 11, 115104 (2018), arXiv:1711.07789 [nucl-th] [2] A. Motornenko, V.V. Begun, V. Vovchenko, M.I. Gorenstein and H. Stoecker, arXiv:1811.10645 [nucl-th].

Speaker: Anton Motornenko (Frankfurt U.)

Slides Motornenko_Lund_2019.pdf

Friday, 1 March

09:00 → 09:35 Parton tomography: Wigner distributions in nucleon and nuclear targets

Speaker: Emmanuel de Oliveira (UFSC Florianopolis)

Slides wigner.pdf

09:35 → 10:00 Vector meson production by photon - induced interactions in heavy ion collisions: Recent results and prospects

In this contribution we will present a brief review of the recent results that demonstrate that the single and double vector meson production by photon - induced interactions in heavy ion collisions can be used to constrain the QCD dynamics at high energies and improve our understanding about the gluon Sivers function.

Speaker: Victor Goncalves (Universidade Federal de Pelotas)

Slides Talk_UPHIC_Lund19.pdf

10:00 → 10:25 Photon induced production of J/psi and dilepton pairs in semi-central nucleus-nucleus collisions

We calculate total and differential cross sections for J/\psi photoproduction in ultrarelativistic lead-lead collisions at the LHC energy \sqrt{s_{NN}}=2.76 TeV. In the present approach we use a simple model based on vector dominance picture and multiple scattering of the hadronic (c \bar c) state in a cold nucleus as an example. In our analysis we use both the classical mechanics and quantum (Glauber) formulae for calculating \sigma_{tot, J/psi Pb} which is a building block of our model. We compare our UPC results with ALICE and CMS data. For semi-central collisions (b < R_A+R_B) a modification of the photon flux is necessary. We discuss different motivated by physics approximations. We try to estimate the cross sections for different centrality bins and for J/psi mesons emitted in forward rapidity range (2.5 < y < 4) corresponding to recent ALICE experimental results. Reasonable results are obtained and open questions are discussed. We study the invariant-mass distributions of dileptons produced in ultrarelativistic heavy-ion collisions at very low pair transverse momenta, P_T\leq 0.15 GeV. Specifically, we investigate the interplay of thermal radiation with initial photon annihilation processes, \gamma \gamma \to l^+ l^-, triggered by the coherent electromagnetic fields of the incoming nuclei. For the thermal radiation, we employ the emission from the QGP and hadronic phases with in-medium vector spectral functions which describes the inclusive excess radiation observed over a wide range of collision energies. For the coherent photon fusion processes, whose spectrum is much softer than for thermal radiation, we employ initial fluxes from the Fourier transform of charge distributions of the colliding nuclei in the equivalent-photon approximation. We first verify that the combination of photon fusion, thermal radiation and final-state hadron decays gives a fair description of the low-P_T invariant-mass as well as P_T spectra as recently measured by the STAR collaboration in \sqrt{s_{NN}}=200 GeV Au+Au collisions for different centrality classes, including experimental acceptance cuts. The coherent contribution dominates in peripheral collisions, while thermal radiation shows a markedly stronger increase with centrality. We extend the calculations to lower collision energies (\sqrt{s_{NN}}=17.3 GeV) and compare to the acceptance-corrected dimuon excess spectra measured by the NA60 experiment at the CERN SPS; the contribution from photoproduction turns out to be subleading. We also provide predictions for the ALICE experiment at the LHC. The resulting excitation function from SPS to LHC energies reveals a nontrivial interplay of photoproduction and thermal radiation.

Speaker: Antoni Szczurek (IFJ Krakow)

10:25 → 10:55 Coffee break

10:55 → 11:30 What must we learn about QGP-like effects in small systems from Run 3+

Speaker: Naghmeh Mohammadi (CERN)

Slides LundCostWorkshop.pdf

11:30 → 11:55 ALICE upgrade and prospects

Speaker: David Silvermyr (Lund U.)

11:55 → 12:20 The CLASH project

In this talk, the KAW sponsored CLASH project will be described. CLASH is a 5 year project started summer 2018 where experimentalists and theorists in Lund aims to pin down the origin of collective effects in small systems.

Speakers: Leif Lönnblad (Lunds universitet) , Peter Christiansen (Lunds universitet)

Slides slides_why_we_clash.pptx