Proceedings Series

Vol. 12 (2019), No. 2, pp. 157 – 482

XIII Workshop on Particle Correlations and Femtoscopy

Kraków, Poland; May 22–26, 2018

Femtoscopy of Stopped Protons

Acta Phys. Pol. B Proc. Suppl. 12, 165 (2019)

DOI: 10.5506/APhysPolBSupp.12.165

article page: 165 (contains 4 pages) •


Short account of the results obtained in the paper by A. Bialas, A. Bzdak and V. Koch [Phys. Rev. C 99, 034906 (2019) arXiv:1711.09440], is presented.

Observables from a Perturbative, Accelerating Solution of Relativistic Hydrodynamics

Acta Phys. Pol. B Proc. Suppl. 12, 169 (2019)

DOI: 10.5506/APhysPolBSupp.12.169

article page: 169 (contains 6 pages) •


The discovery of the almost perfect fluid-like nature of the strongly interacting quark–gluon plasma was one of the most important discoveries of heavy-ion physics in recent decades. The experimental results are well-described by hydrodynamical models. Most of these models are numerical simulations, however the analytic solutions are also important in understanding the time evolution of the quark–gluon plasma created in the heavy-ion collisions. Here, we present a perturbative, accelerating solution on top of a known solution, the relativistic Hubble flow. We describe the perturbative class of solutions and calculate a few observables for a selected solution.

A New and Finite Family of Solutions of Hydrodynamics: Part II: Advanced Estimate of Initial-energy Densities

Acta Phys. Pol. B Proc. Suppl. 12, 175 (2019)

DOI: 10.5506/APhysPolBSupp.12.175

article page: 175 (contains 6 pages) •


We derive a new, exact formula for the estimate of the initial-energy densities from a new family of finite and exact solution of relativistic perfect fluid hydrodynamics. The new formula depends non-trivially on the speed of sound and on the shape or width parameter of the measured (pseudo)rapidity distribution.

Interferometry in Central \(p\)+Pb Collisions

Acta Phys. Pol. B Proc. Suppl. 12, 181 (2019)

DOI: 10.5506/APhysPolBSupp.12.181

article page: 181 (contains 6 pages) •


We present the results of HBT radii calculations performed for the pion emission source created in central \(p\)+Pb collisions at \(\sqrt {s_{NN}} = 5.02\) TeV. Space-time expansion of the dense matter is simulated using hydrodynamic model with the Glauber Monte Carlo initial conditions. We present momentum and rapidity dependence of all interferometry parameters and their comparison to the experimental data reported by the ATLAS Collaboration. Overall, a fairly good agreement between the model and the data is achieved. The parameterization of the correlation function includes a cross term coupling the out and long directions, which is found to be nonzero. It is a first quantitative comparison of the model prediction of the cross term values with the experimental measurements and they appear to be well reproduced.

Femtoscopic Bose–Einstein Correlations of Charged Hadrons in \(pp\) Collisions at 13 TeV in CMS

Acta Phys. Pol. B Proc. Suppl. 12, 187 (2019)

DOI: 10.5506/APhysPolBSupp.12.187

article page: 187 (contains 6 pages) •


Femtoscopic correlations between charged hadrons are measured for minimum-bias and for high-multiplicity events in proton–proton collisions at \(\sqrt {s} = 13\!\) TeV. The results are based on data collected with the CMS experiment at the LHC in 2015. Three analysis techniques are employed with different degrees of dependence on simulated events, all of them returning consistent results within the experimental uncertainties. The measured values of the lengths of homogeneity are compared with those from lower energies and from another experiment, and are also discussed in comparison with theoretical predictions.

Lévy-stable Two-pion Bose–Einstein Correlation Functions Measured with PHENIX in \(\sqrt {s_{NN}} =\) 200 GeV Au+Au Collisions

Acta Phys. Pol. B Proc. Suppl. 12, 193 (2019)

DOI: 10.5506/APhysPolBSupp.12.193

article page: 193 (contains 6 pages) •


Measurement of quantum statistical correlation functions in high-energy nuclear physics is an important tool to investigate the QCD phase diagram. It may be used to search for the critical point, and also to understand underlying processes such as in-medium mass modifications or partially coherent particle production. Furthermore, the measurements of the femtoscopic correlation functions shed light on the space-time structure of particle production. Consequently, the precise measurements and description of the correlation functions are essential. The shape of the two-pion Bose–Einstein correlation functions were often considered to be Gaussian, but the recent precision of the experiments reveals that the statistically correct assumption is the more general Lévy distribution. In this paper, we present the recent results of the measurements of two-pion Lévy-stable Bose–Einstein correlation functions in Au+Au collisions at PHENIX.

Overview of Recent Femtoscopy Measurements with ALICE

Acta Phys. Pol. B Proc. Suppl. 12, 199 (2019)

DOI: 10.5506/APhysPolBSupp.12.199

article page: 199 (contains 6 pages) •


One of the key methods used in the study of the Quark–Gluon Plasma (QGP) is femtoscopy, the technique of measuring short-range two-particle correlations as a function of relative momentum. Traditionally, femtoscopy has been utilized to measure the size of the QGP fireball created in relativistic heavy-ion collisions. However, since it is sensitive to the correlations between the particles in the final state, it has been recently shown that the parameters of the strong interaction can be probed as well. This review includes a broad range of ALICE femtoscopy results, including both traditional and novel measurements of the interaction between particles.

Baryon–Baryon Correlations at the STAR Experiment

Acta Phys. Pol. B Proc. Suppl. 12, 205 (2019)

DOI: 10.5506/APhysPolBSupp.12.205

article page: 205 (contains 6 pages) •


Two-particle femtoscopy allows one to study the properties of matter created during heavy-ion collisions. It makes the study of space-time evolution of the source possible and may be applied to many different combinations of hadron pairs. Two-baryon femtoscopy provides additional information about source characteristics, not accessible by the two-pion femtoscopy. In this paper, we present the correlation functions obtained for identical and non-identical pairs of protons and antiprotons for Au+Au collisions at different collision energies.

Non-identical Particle Femtoscopy at STAR

Acta Phys. Pol. B Proc. Suppl. 12, 211 (2019)

DOI: 10.5506/APhysPolBSupp.12.211

article page: 211 (contains 6 pages) •


The space-time parameters of the particle-emitting source can be examined in heavy-ion collisions at high energies via femtoscopy measurements. Two-particle correlations at small relative momentum use quantum statistics and final-state interactions. This method provide estimation of source characteristics which cannot be measured directly. Information about asymmetry in the process of emission of two types of particles can be obtained by measuring correlations of non-identical particles. In this paper, status report on femtoscopic measurements of pion–kaon, pion–proton and kaon–proton systems in Au+Au collisions at selected STAR energies is presented.

A New and Finite Family of Solutions of Hydrodynamics: Part III: Advanced Estimate of the Life-time Parameter

Acta Phys. Pol. B Proc. Suppl. 12, 217 (2019)

DOI: 10.5506/APhysPolBSupp.12.217

article page: 217 (contains 6 pages) •


We derive a new formula for the longitudinal HBT-radius of the two particle Bose–Einstein correlation function from a new family of finite and exact, accelerating solution of relativistic perfect fluid hydrodynamics for a temperature-independent speed of sound. The new result generalizes the Makhlin–Sinyukov and Herrmann–Bertsch formulae and leads to an advanced life-time estimate of high-energy heavy-ion and proton–proton collisions.

Two-particle Correlations at the LHCb

Acta Phys. Pol. B Proc. Suppl. 12, 223 (2019)

DOI: 10.5506/APhysPolBSupp.12.223

article page: 223 (contains 6 pages) •


Results from the LHCb experiment on two different types of correlations in proton–proton collisions at the centre-of-mass energies of 7 and 8 TeV are presented. Kinematic \(b\bar {b}\) correlations are studied using inclusive \(b \rightarrow J/\psi X\) decays and are observed to be in good agreement with theoretical predictions. Measurement of the Bose–Einstein correlations for same-sign charged pions is performed and dependence of the correlation parameters on the charged-particle multiplicity is investigated. The determined correlation radius is observed to increase with the charged-particle multiplicity, while the chaoticity parameter decreases.

Event-by-event Fluctuations of the Source Shape: Implications for the Lévy Shape and Event Shape Sorting

Acta Phys. Pol. B Proc. Suppl. 12, 229 (2019)

DOI: 10.5506/APhysPolBSupp.12.229

article page: 229 (contains 6 pages) •


In the first part of this contribution, we show that the Lévy-stable shape of the correlation function can be caused by averaging of the measured correlation functions over a large number of events. In the second part, it is demonstrated how a sample of events sorted by the Event Shape Sorting technique exhibits different azimuthal dependence of correlation radii in each event class.

The Femtoscopy Scales in Au+Au Collisions at the Top RHIC Energy

Acta Phys. Pol. B Proc. Suppl. 12, 235 (2019)

DOI: 10.5506/APhysPolBSupp.12.235

article page: 235 (contains 6 pages) •


The pion and kaon femtoscopy radii depending on transverse momentum are studied for Au+Au collisions at the RHIC energy \(\sqrt {s_{NN}}=200\) GeV within the integrated hydrokinetic model (iHKM). The time of the maximal emission \(\tau _{\mathrm {max}}\) for both particle species is extracted from the analytical fit to the \(R_{\mathrm {long}}\) dependence on mean transverse momentum of \(k_{\mathrm {T}}\) of the particle pair.

Cumulants of Net-proton Number Fluctuations from ALICE at the LHC

Acta Phys. Pol. B Proc. Suppl. 12, 241 (2019)

DOI: 10.5506/APhysPolBSupp.12.241

article page: 241 (contains 6 pages) •


In the lattice QCD, the ratios of the various orders of quark number susceptibilities are used to determine freeze-out parameters and to locate the phase boundary. The cumulants of conserved charges fluctuations are directly related to the respective quark number susceptibilities. Therefore, the measurements of various order cumulants of conserved charges, such as the net-baryon number, can be used to determine the freeze-out parameters and to constrain the lattice QCD predictions. In this paper, we report on the first measurements of cumulants of net-proton number distributions up to the \(4^{\mathrm {th}}\) order in Pb–Pb collisions with ALICE at the LHC. We also compare our results with RHIC measurements and hadron resonance gas model calculations.

Measurement of Long-range Azimuthal Correlations in Proton–Proton and Proton–Lead Collisions with ATLAS

Acta Phys. Pol. B Proc. Suppl. 12, 247 (2019)

DOI: 10.5506/APhysPolBSupp.12.247

article page: 247 (contains 6 pages) •


Measurement of correlations between two flow harmonics using three and four-particle cumulants with the ATLAS detector are presented in \(pp\), \(p\)+Pb, and Pb+Pb collisions. The measurements probe the long-range collective nature of particle production in the small systems. Non-flow correlations in the standard cumulants are suppressed using the subevent technique. Anti-correlation between \(v_2\) and \(v_3\) and correlation between \(v_2\) and \(v_4\) over the full multiplicity range are observed with the three-subevent method, for all collision systems. The relative correlation strengths of the cumulants are obtained by dividing them with \(\langle v_n^2\rangle \) from two-particle correlation. These normalised cumulants are found to be similar in the three-collision systems with weak dependence on the event multiplicity and transverse momentum. The results provide strong evidence for a similar long-range multi-particle collectivity in \(pp\), \(p\)+Pb and peripheral Pb+Pb collisions.

Measurements of Fourier Harmonics of Azimuthal Anisotropy in Pb+Pb and Xe+Xe Collisions

Acta Phys. Pol. B Proc. Suppl. 12, 253 (2019)

DOI: 10.5506/APhysPolBSupp.12.253

article page: 253 (contains 7 pages) •


The high-statistics data sets collected by the LHC experiments during the heavy-ion runs allow for a detailed study of the azimuthal anisotropy of charged particles. This report presents the selected results of \(v_{n}\) measured in Pb+Pb collisions at \(\sqrt {s_{{NN}}} = 5.02\) TeV and Xe+Xe collisions at \(\sqrt {s_{NN}} = 5.44\) TeV, with emphasis on the higher order Fourier coefficients, \(v_{n}\), sensitive to initial-state fluctuations of the medium created in ultra-relativistic heavy-ion collisions.

Pseudorapidity and Initial-energy Densities in \(p\)+\(p\) and Heavy-ion Collisions at RHIC and LHC

Acta Phys. Pol. B Proc. Suppl. 12, 261 (2019)

DOI: 10.5506/APhysPolBSupp.12.261

article page: 261 (contains 7 pages) •


A known exact and accelerating solution of relativistic hydrodynamics for perfect fluids is utilized to describe pseudorapidity densities of \(\sqrt {s_{NN}} =5.02\) TeV Pb+Pb and \(\sqrt {s}=13\) TeV \(p\)+\(p\) collisions at the LHC. We evaluate a conjectured initial energy densities \(\epsilon _{\mathrm {corr}}\) in these collisions, and compare them to Bjorken’s initial-energy density estimates, and to results for Pb\(+\)Pb collisions at \(\sqrt {s_{NN}} = 2.76\) TeV and \(p\)+\(p\) collisions at \(\sqrt {s} = 7\) and \(8\) TeV.

Wounded-quark Emission Function in Asymmetric Heavy-ion Collisions

Acta Phys. Pol. B Proc. Suppl. 12, 269 (2019)

DOI: 10.5506/APhysPolBSupp.12.269

article page: 269 (contains 6 pages) •


The wounded-nucleon and -quark models are compared using \(d+\)Au collisions at \(\sqrt {s_{NN}}=200\) GeV. The shape of the wounded-quark emission function seems to be universal for different centralities, in contrast to the wounded-nucleon emission function. Predictions for \({\mathrm {d}}N_{\mathrm {ch}}/{\mathrm {d}}\eta \) distributions for various centrality classes in \(p+\)Al, \(p+\)Au, \(d+\)Au, and \(^{3}\)He\(+\)Au collisions are presented and compared to recent PHENIX results.

Forward–Backward Correlations and Multiplicity Fluctuations in Pb–Pb Collisions at \(\sqrt {s_{NN}}=\) 2.76 TeV from ALICE at the LHC

Acta Phys. Pol. B Proc. Suppl. 12, 275 (2019)

DOI: 10.5506/APhysPolBSupp.12.275

article page: 275 (contains 7 pages) •


This paper presents a comparative study of forward–backward correlations and multiplicity fluctuations in the HIJING Monte Carlo simulations of Pb–Pb collisions at \(\sqrt {s_{NN}}=2.76\) TeV. The analysis focuses on two observables which are: the forward–backward correlation coefficient \(b^{n-n}_{\mathrm {corr}}\) and the strongly intensive quantity \({\mit \Sigma }\). Results are discussed in the context of the influence of event-by-event fluctuations of the geometry of the Pb–Pb collisions on the measured quantities.

The Light Scalar \(K_{0}^{\ast }(700)\) in the Vacuum and at Nonzero Temperature

Acta Phys. Pol. B Proc. Suppl. 12, 283 (2019)

DOI: 10.5506/APhysPolBSupp.12.283

article page: 283 (contains 6 pages) •


There is mounting evidence toward the existence of a light scalar kaon \(\kappa \equiv K_{0}^{\ast }(700)\) with quantum numbers \(I(J^{P})=1/2(0^{+})\). Here, we recall the results of an effective model with both derivative and non-derivative terms in which only one scalar kaonic field is present in the Lagrangian (the standard quark–antiquark “seed” state \(K_{0}^{\ast }(1430)\)): a second “companion” pole \(K_{0}^{\ast }(700)\) emerges as a dynamically generated state. A related question is the role of \(K_{0}^{\ast }(700)\) at nonzero \(T\): since it is the lightest scalar strange state, one would naively expect that it is relevant for \(\pi \) and \(K\) multiplicities. However, a repulsion in the \(\pi K\) channel with \(I=3/2\) cancels its effect.

Emission Asymmetry and Time Delays in Non-identical Particle Femtoscopy

Acta Phys. Pol. B Proc. Suppl. 12, 289 (2019)

DOI: 10.5506/APhysPolBSupp.12.289

article page: 289 (contains 6 pages) •


Non-identical particle femtoscopy measures the size of the system emitting particles in heavy-ion collisions as well as the difference between mean emission space-time coordinates of two particle species (“emission asymmetry”). Hydrodynamic models predict a significant emission asymmetry between pions and kaons, coming from collective flow, enhanced by contribution from flowing resonances. We present calculations of pion–kaon, pion–proton, and kaon–proton femtoscopic correlations within the (3+1)D hydrodynamic model coupled to statistical hadronization code THERMINATOR 2, corresponding to Pb–Pb collisions at \(\sqrt {s_{NN}}=2.76\) TeV. We show the extracted system size and emission asymmetry. Recently, ALICE Collaboration presented results on kaon femtoscopy which suggest that there is a delay in emission time of kaons of the order of 2 fm/\(c\). It is interpreted as a signature for the existence of extended hadronic rescattering phase in Pb–Pb collisions at the LHC, and, in particular, the importance of the rescattering via the \(K^{*}\) resonance. We discuss the influence of emission time delays on the extracted system size and emission asymmetry. We propose a sensitive and independent experimental test to confirm the existence of kaon emission time delay (and, consequently, the existence of rescattering via the \(K^{*}\) resonance) based on the femtoscopic correlations for non-identical particles.

Results on Resonance Production in Heavy-ion Collisions from the ATLAS Experiment at the LHC

Acta Phys. Pol. B Proc. Suppl. 12, 295 (2019)

DOI: 10.5506/APhysPolBSupp.12.295

article page: 295 (contains 6 pages) •


Recent ATLAS measurements of quarkonia and open charm production are presented for proton–lead and lead–lead collisions. The studies of charmonia production in ultrarelativistic nucleus–nucleus collisions offer a handle on the properties of the hot and dense quark–gluon plasma created in these collisions. In particular, a strong suppression relative to proton–proton collisions is expected. On the other hand, measurements of quarkonia and charmed meson production in proton–nucleus collisions allow to study cold nuclear matter effects such as initial-state energy loss or nuclear modifications of parton distribution functions.

\(\Delta \eta \)–\(\Delta \phi \) Correlations of Identified Particles in the Beam Energy Scan Program at STAR

Acta Phys. Pol. B Proc. Suppl. 12, 301 (2019)

DOI: 10.5506/APhysPolBSupp.12.301

article page: 301 (contains 6 pages) •


The angular correlation function (CF) refers to the correlation of particles in the relative pseudorapidity and relative azimuthal angle. It is used to study strongly interacting matter properties at relativistic energies. Recent observations suggest that the study of CF of identified particles can provide more detailed insight into strongly interacting matter properties, in comparison with measurements of unidentified particles. In these proceedings, recent STAR experimental results of two-pion and two-proton CF are shown. The dependence of CF on collision energy and centrality is presented.

all authors

X.G. Cao, E.J. Kim, K. Schmidt, J.B. Natowitz, K. Hagel, M. Barbui, J. Gauthier, S. Wuenschel, R. Wada, N. Blando, G. Giuliani, H. Zheng, A. Bonasera, M.R.D. Rodriguez, S. Kowalski, M. Huang, G.Q. Zhang, C.Y. Wong, A. Staszczak, Z.X. Ren, Y.K. Wang, S.Q. Zhang, J. Meng

Evidence for Resonances in the 7\(\alpha \) Disassembly of \(^{28}\)Si

Acta Phys. Pol. B Proc. Suppl. 12, 307 (2019)

DOI: 10.5506/APhysPolBSupp.12.307

article page: 307 (contains 9 pages) •


Observations of resonance structures in the excitation function of the 7\(\alpha \) de-excitation of \(^{28}\)Si nuclei in the collisions of 35 MeV/nucleon \(^{28}\)Si with \(^{12}\)C may indicate that toroidal high-spin isomers such as those predicted by recent theoretical calculations are populated.

Electromagnetic Interactions of Ultrarelativistic Nuclei: A Challenge for Present and Future Heavy-ion Colliders

Acta Phys. Pol. B Proc. Suppl. 12, 317 (2019)

DOI: 10.5506/APhysPolBSupp.12.317

article page: 317 (contains 6 pages) •


One of the options within the project of the Future Circular Collider (FCC) at CERN is represented by a proton–proton collider (FCC-hh) with \(\sqrt {s_{pp}}=100\) TeV collision energy. As planned, the collisions of lead nuclei at \(\sqrt {s_{NN}}=39.4\) TeV will be also possible at the FCC-hh. In this work, the electromagnetic interactions of nuclei at the Large Hadron Collider (LHC) and at the FCC-hh are compared with respect to their impact on the operation of these accelerators and to the relation between the rates of hadronic and electromagnetic collision events. Ion species, like \(^{115}\)In, may be advantageous at the FCC-hh due to a reduced frequency of electromagnetic processes in comparison to heavy \(^{208}\)Pb presently used at the LHC. The cross sections of production of secondary nuclei in electromagnetic dissociation of \(^{115}\)In and \(^{208}\)Pb at the FCC-hh are calculated with RELDIS model. These cross sections can be used for estimating the impact of secondary nuclei on components of the FCC-hh to design its collimator system.

Recent Theoretical Results for Electromagnetically-induced Ultraperipheral Reactions of Heavy Ions

Acta Phys. Pol. B Proc. Suppl. 12, 323 (2019)

DOI: 10.5506/APhysPolBSupp.12.323

article page: 323 (contains 10 pages) •


We briefly review our works on ultraperipheral heavy-ion collisions. We discuss both \(\gamma \gamma \) and rescattering of hadronic photon fluctuation induced by one nucleus in the collision partner. Production of one and two leptonic and pionic, and \(p \bar p\) pairs is discussed as an example of photon–photon processes. The production of single vector mesons (\(\rho ^0\) or \(J/\psi \)) is an example of the second category. The double-scattering mechanisms of two \(\rho ^0\) meson production is discussed in addition.

Photon–Photon Scattering at the LHC in Collisions of Nuclei

Acta Phys. Pol. B Proc. Suppl. 12, 333 (2019)

DOI: 10.5506/APhysPolBSupp.12.333

article page: 333 (contains 7 pages) •


This paper is focused on an analysis of the light-by-light scattering in ultraperipheral heavy-ion collisions at the energy available at the CERN Large Hadron Collider. Here, contribution from fermionic boxes, resonance scattering, VDM-Regge model, two-gluon exchange as well as pionic background will be compared. Each of these processes dominates at different ranges of two-photon invariant masses. The usage of the equivalent photon approximation in the impact parameter space gives results that are in good agreement with recently measured ATLAS and CMS data. Predictions including ALICE and LHCb experimental cuts for the next run at the LHC are shown.

Production of \(p\bar {p}\) Pairs in UPC at the LHC

Acta Phys. Pol. B Proc. Suppl. 12, 341 (2019)

DOI: 10.5506/APhysPolBSupp.12.341

article page: 341 (contains 6 pages) •


We discuss production of \(p\bar {p}\) pairs in two-photon interactions in heavy-ion collisions. We present predictions for the ultraperipheral, ultrarelativistic, heavy-ion collisions (UPC) \(^{208}\)Pb\(^{208}\)Pb \(\to ^{208}\)Pb\(^{208}\)Pb\(\,p\bar {p}\). The parameters of vertex form factors are adjusted to the Belle data for the \(\gamma \gamma \to p \bar {p}\) reaction. To the described Belle data, we include the proton-exchange, the \(f_2(1270)\) and \(f_2(1950)\) \(s\)-channel exchanges, as well as the hand-bag mechanism. Then, the total cross section and several differential distributions for experimental cuts corresponding to the LHC experiments are presented. The distribution in \({y}_{\mathrm {diff}}\), the rapidity distance between the proton and antiproton, is particularly interesting. We find the total cross sections: 100 \(\mu \)b for the ALICE cuts, 160 \(\mu \)b for the ATLAS cuts, 500 \(\mu \)b for the CMS cuts, and 104 \(\mu \)b taking into account the LHCb cuts. This opens a possibility to study the \(\gamma \gamma \to p \bar {p}\) process in UPC at the LHC.

Spectator-induced EM Effects on Charged Meson Ratios in Heavy-ion Collisions

Acta Phys. Pol. B Proc. Suppl. 12, 347 (2019)

DOI: 10.5506/APhysPolBSupp.12.347

article page: 347 (contains 6 pages) •


This paper gives an overview of studies of spectator-induced electromagnetic (EM) effects on charged pion emission in ultrarelativistic heavy-ion collisions. These effects, caused directly by the electromagnetic field generated by positively charged spectators, are found to induce large distortions in spectra and ratios of produced charged \(\pi ^{+}\), \(\pi ^{-}\) mesons as well as induce a charge splitting of measured pion directed flow. Recent studies demonstrate that the analysis of spectator-induced EM effects shows sensitivity to the actual distance \(d_{\mathrm {E}}\) between the pion formation zone at freeze-out and the spectator matter. This gives a new possibility of studying the space-time evolution of dense and hot matter created in the course of the collision. The analysis of the dependence of the distance \(d_{\mathrm {E}}\) as a function of pion rapidity gives a first estimate of the pion decoupling time from EM effects which can be directly compared to existing HBT data. Consequently, spectator-induced EM interactions appear as an alternative tool for studying the space-time characteristics and longitudinal evolution of the system.

Electromagnetic Effects on Charged Pion Spectra at SPS Energies

Acta Phys. Pol. B Proc. Suppl. 12, 353 (2019)

DOI: 10.5506/APhysPolBSupp.12.353

article page: 353 (contains 7 pages) •


One of the main goals of the NA61/SHINE experiment at the CERN SPS is to study the properties of strongly interacting matter by a two-dimensional scan of elementary and nuclear reactions as a function of system size and collision energy. This paper presents new results on a new observable relevant to this part of the NA61/SHINE programme, which is the modification of positively (negatively) charged particle spectra by the electromagnetic repulsion (attraction) of final-state particles by the charged nuclear remnant, the spectator system. Preliminary measurements of \(\pi ^+/\pi ^-\) ratios in central and intermediate Ar+Sc collisions at beam momentum of 150 \(A\) GeV/\(c\) are shown, and compared to NA49 data in peripheral Pb+Pb collisions at 158 \(A\) GeV/\(c\) as a function of longitudinal and transverse pion momentum. In spite of a dramatic decrease in the magnitude of spectator charge, spectator-induced electromagnetic effects remain clearly visible in Ar+Sc reactions. This is the first measurement of these effects in the Ar+Sc system at SPS energies. Moreover, a comparison with electromagnetic Monte Carlo simulations is made, and conclusions on the space-time evolution of pion production in Ar+Sc and Pb+Pb collisions are presented.

all authors

K. Mazurek, A. Rybicki, A. Szczurek, V. Ozvenchuk, P.N. Nadtochy, A. Kelic, A. Marcinek

Role of the Spectator System in Electromagnetic Effects

Acta Phys. Pol. B Proc. Suppl. 12, 361 (2019)

DOI: 10.5506/APhysPolBSupp.12.361

article page: 361 (contains 7 pages) •


The electromagnetic effects on charged pion (\(\pi ^+,\pi ^-\)) spectra provide new, independent information on the space-time evolution of the ultrarelativistic heavy-ion collision. The spectator lifetime and its excitation energy may also be of importance for the understanding of the space-time evolution of the participant zone. This paper gives an overview of our coordinated effort to understand the interplay between electromagnetic phenomena and processes related to the fragmentation of the spectator system at forward rapidity in peripheral Pb+Pb collisions at top CERN SPS energies. Our study includes, on the one hand, the experimental analysis of electromagnetic effects and corresponding phenomenological Monte Carlo simulations and, on the other hand, dedicated theoretical calculations based on the abrasion–ablation model ABRABLA and the 4DLangevin approach.

\(XYZ\) States at the LHCb

Acta Phys. Pol. B Proc. Suppl. 12, 369 (2019)

DOI: 10.5506/APhysPolBSupp.12.369

article page: 369 (contains 6 pages) •


The recent results obtained in the sector of heavy flavour spectroscopy of exotic states are presented. The observation of hidden-charm pentaquarks is reviewed, followed by the presentation of new evidences for \(X\) and \(Z\) states.

Baryonic and Mesonic Resonances at the LHCb

Acta Phys. Pol. B Proc. Suppl. 12, 375 (2019)

DOI: 10.5506/APhysPolBSupp.12.375

article page: 375 (contains 6 pages) •


The LHCb experiment is designed to study properties and decays of heavy-flavoured hadrons produced from \(pp\) collisions at the LHC. LHCb has recorded the world’s largest data sample of beauty and charm hadrons, enabling precise spectroscopy studies of such particles. The valuable results obtained by LHCb include observation of doubly charmed baryons \({\mit \Xi }_{cc}\) and five new narrow \({\mit \Omega }_c\) states.

Multiplicity Dependence of Resonance Production with ALICE at the LHC

Acta Phys. Pol. B Proc. Suppl. 12, 381 (2019)

DOI: 10.5506/APhysPolBSupp.12.381

article page: 381 (contains 6 pages) •


Particle production in ultra-relativistic heavy-ion collisions at the LHC energies is discussed in the light of the results obtained with ALICE, with special emphasis on the light-flavour hadronic resonances. The most recent measurements from the LHC Run 2, including the Xe–Xe run, are discussed and shown to be in agreement with previous findings.

The Two Particle Correlation in \(Z\)-boson Tagged Events

Acta Phys. Pol. B Proc. Suppl. 12, 387 (2019)

DOI: 10.5506/APhysPolBSupp.12.387

article page: 387 (contains 6 pages) •


These proceedings present the result of the first study of the long-range hadron correlations in \(pp\) collisions with a constraint on collision geometry. The constraint is implemented by requiring events in which a \(Z\) boson is produced. Selecting collisions with hard scattering causes the impact parameter distribution to be different from the inclusive \(pp\) collision. The analysis uses 19.4 fb\(^{-1}\) of \(\sqrt {s}=8\) TeV \(pp\) data obtained by the ATLAS experiment at the LHC. The correlations between the charged-particle pairs in relative azimuthal angle over the transverse momentum range from 0.5 GeV to 5 GeV are studied as a function of the charged particle multiplicity of the event. The number of charged particle tracks and the correlation functions are corrected to account for the significant pileup contribution present in the events. The correlation in the \(Z\)-tagged events is found to be independent of multiplicity, and its magnitude is \(8\pm 6\)% larger than that in inclusive \(pp\) events.

Kinetic Approach to Polarization–Vorticity Coupling and Hydrodynamics with Spin

Acta Phys. Pol. B Proc. Suppl. 12, 393 (2019)

DOI: 10.5506/APhysPolBSupp.12.393

article page: 393 (contains 6 pages) •


Recently introduced equilibrium Wigner functions for spin-\(1/2\) particles are used in the semiclassical kinetic equations to study the relation between spin polarization and vorticity. It is found, in particular, that such a framework does not necessarily imply that the thermal-vorticity and spin polarization tensors are equal. Subsequently, a procedure to formulate the hydrodynamic framework for particles with spin-\(1/2\), based on the semiclassical expansion, is outlined.

Dynamics of Relativistic Spin-polarized Fluids

Acta Phys. Pol. B Proc. Suppl. 12, 399 (2019)

DOI: 10.5506/APhysPolBSupp.12.399

article page: 399 (contains 6 pages) •


We briefly review the foundations of a new relativistic fluid dynamics framework for polarized systems of particles with the spin one half. Using this approach, we numerically study the dynamics of the spin polarization of a rotating medium resembling the ones created in high-energy heavy-ion collisions.

Partial Correlation Analysis in Ultra-relativistic Nuclear Collisions

Acta Phys. Pol. B Proc. Suppl. 12, 405 (2019)

DOI: 10.5506/APhysPolBSupp.12.405

article page: 405 (contains 6 pages) •


We show that the method of partial covariance is a very efficient way to introduce constraints (such as the centrality selection) in data analysis in ultra-relativistic nuclear collisions. The technique eliminates spurious event-by-event fluctuations of physical quantities due to fluctuations of control variables. Moreover, in the commonly used superposition approach to particle production the method can be used to impose constraints on the initial sources rather than on the finally produced particles, thus separating out the trivial fluctuations from statistical hadronization or emission from sources and focusing strictly on the initial-state physics. As illustration, we use simulated data from hydrodynamics started on the wounded-quark event-by-event initial conditions, followed with statistical hadronization, to show the practicality of the approach in analyzing the forward–backward multiplicity fluctuations. We mention generalizations to the case with several constraints and other observables, such as the transverse momentum or eccentricity correlations.

Recent Results on Flow and Correlations from the ATLAS Experiment

Acta Phys. Pol. B Proc. Suppl. 12, 411 (2019)

DOI: 10.5506/APhysPolBSupp.12.411

article page: 411 (contains 7 pages) •


The azimuthal anisotropy in Xe+Xe collisions at the energy of \(\sqrt {s_{NN}}=5.44\) TeV was recently investigated in the ATLAS experiment at the LHC. Preliminary results on the \(p_{\mathrm {T}}\) and centrality dependence of \(v_2\) and \(v_3\) harmonics obtained with multi-particle cumulants are presented. The measurements indicate a presence of nearly Gaussian \(v_n\) harmonic fluctuations in the Xe+Xe system. For Pb+Pb collisions at the energy of \(\sqrt {s_{NN}}=2.76\) and 5.02 TeV, the longitudinal flow harmonic correlations are presented, providing evidence that the correlations between \(v_n\) (\(n=2\)–4) separated in pseudorapidity do not factorise into the product of single-particle \(v_n\). Additionally, for Pb+Pb collisions at \(\sqrt {s_{NN}}=5.02\) TeV, a preliminary determination of the modified Pearson’s correlation coefficient for the event-wise mean transverse momentum and \(v_n^2\) (\(n=2\)–4) is shown. Significant, non-zero correlation coefficients are measured for all \(v_n\) harmonics.

New Measurement of Pion Directed Flow Relative to the Spectator Plane by the NA49 Experiment at CERN

Acta Phys. Pol. B Proc. Suppl. 12, 419 (2019)

DOI: 10.5506/APhysPolBSupp.12.419

article page: 419 (contains 6 pages) •


This article reports on a new measurement of negatively charged pion directed flow \(v_1\) relative to the spectator plane for Pb+Pb collisions at the beam energy of 40\(A\) GeV/\(c\) recorded by the NA49 experiment at CERN. \(v_1\) is shown as a function of rapidity and transverse momentum in different classes of collision centrality. The projectile spectator plane is estimated using the transverse segmentation of the NA49 forward hadron calorimeter. The new results extend the NA49 data for \(v_1\), which was previously measured only relative to the participant plane, and complement recent preliminary data by the NA61/SHINE Collaboration and published results of STAR from the RHIC beam energy scan program.

Onset of Deconfinement and the Critical Point at SPS Energies

Acta Phys. Pol. B Proc. Suppl. 12, 425 (2019)

DOI: 10.5506/APhysPolBSupp.12.425

article page: 425 (contains 8 pages) •


The NA61/SHINE experiment at the CERN SPS pursues a rich program on strong interactions, which covers the study of the onset of deconfinement and the search for the critical point. A broad region of the phase diagram of strongly interacting matter is scanned by varying the beam momentum (13 \(A\)–158 \(A\) GeV/\(c\)) and the size of the colliding system (\(p\)+\(p\), \(p\)+Pb, Be+Be, Ar+Sc, Xe+La, and Pb+Pb reactions). This paper presents a selection of NA61/SHINE results on particle production and intermittent behavior, discussed together with existing data from the NA49 Collaboration. The evolution of non-monotonic structures in pion and strangeness production as a function of system size and energy is addressed in detail. The change of hadron production properties from \(p\)+\(p\) up to Pb+Pb collisions can be interpreted as the beginning of the formation of large clusters of strongly interacting matter (the onset of fireball). The intermittency signal gives an indication for a possible critical behavior in the Si+Si system at top SPS energy but none in Be+Be reactions. Additionally, the potential of large acceptance NA61/SHINE measurements for investigating the longitudinal evolution of the system is discussed.

Deconfinement in \(pp\) Collisions at LHC Energies

Acta Phys. Pol. B Proc. Suppl. 12, 433 (2019)

DOI: 10.5506/APhysPolBSupp.12.433

article page: 433 (contains 6 pages) •


Recently, the CMS Collaboration has published identified particle transverse momentum spectra in high multiplicity events at LHC energies \(\sqrt s =0.9\)–13 TeV. In the present work, the transverse momentum spectra have been analyzed in the framework of the color fields inside the clusters of overlapping strings, which are produced in high-energy hadronic collisions. The initial temperature and shear viscosity to entropy density ratio \(\eta /s\) are obtained. For the higher multiplicity events at \(\sqrt s=7\) and 13 TeV, the initial temperature is above the universal hadronization temperature and is consistent with the creation of deconfined matter. In these small systems, it can be argued that the thermalization is a consequence of the quantum tunneling through the event horizon introduced by the confining color fields, in analogy to the Hawking–Unruh effect.

all authors

J.N. Guenther, Sz. Borsányi, A. Pásztor, Z. Fodor, M. Giordano, K. Kapás, S.D. Katz, I. Portillo, C. Ratti, K.K. Szabó

Lattice Investigations of the QCD Phase Diagram from Analytical Continuation

Acta Phys. Pol. B Proc. Suppl. 12, 439 (2019)

DOI: 10.5506/APhysPolBSupp.12.439

article page: 439 (contains 5 pages) •


At zero baryon density, lattice QCD is an established tool that provides precise theoretical results. Calculations at non-zero densities, however, require new techniques to deal with the sign problem. In this work, we will review our recent effort to investigate QCD at non-vanishing baryon chemical potential.

PHENIX Results on Collision Energy-dependent Lévy HBT Correlations from \(\sqrt {s_{NN}} = 15\) to 200 GeV

Acta Phys. Pol. B Proc. Suppl. 12, 445 (2019)

DOI: 10.5506/APhysPolBSupp.12.445

article page: 445 (contains 6 pages) •


Different regions on the QCD phase diagram can be investigated by varying the collision energy and the centrality in heavy-ion collisions. In our latest measurements at the PHENIX experiment at RHIC, we utilize Lévy-type sources to describe the measured HBT correlation functions. In this paper, we report on the current status of the analysis of the centrality and beam energy dependence of the Lévy source parameters in Au+Au collisions from \(\sqrt {s_{NN}} = 15\) GeV to \(\sqrt {s_{NN}} = 200\) GeV.

NA61/SHINE Lévy HBT Measurements in Be+Be Collisions at 150\(A\) GeV/\(c\)

Acta Phys. Pol. B Proc. Suppl. 12, 451 (2019)

DOI: 10.5506/APhysPolBSupp.12.451

article page: 451 (contains 6 pages) •


The program of NA61/SHINE allows for the investigation of the phase diagram of strongly interacting matter. The nature of the quark–hadron transition can be studied through analyzing the space-time structure of the hadron emission source via the measurement of Bose–Einstein momentum correlations. These can be described by correlation functions based on Lévy-distributed sources. This report presents the performance plots of Bose–Einstein correlation analysis in Be+Be collisions at 150\(A\) GeV\(/c\) beam momentum. The transverse mass dependence of the Lévy source parameters and their possible interpretations are discussed.

PHENIX Measurements of Low Momentum Direct Photon Radiation from Large and Small Systems in (Ultra)Relativistic Heavy-ion Collisions: Direct Photon Scaling

Acta Phys. Pol. B Proc. Suppl. 12, 457 (2019)

DOI: 10.5506/APhysPolBSupp.12.457

article page: 457 (contains 7 pages) •


The PHENIX Collaboration has measured low momentum direct photon radiation in Au+Au collisions at 200 GeV, 62.4 GeV and 39 GeV, in Cu+Cu at 200 GeV as well as in \(p\)+\(p\), \(p\)+Au and \(d\)+Au at \(\sqrt {s_{NN}} = 200\) GeV. In these measurements, PHENIX has discovered a large excess over the scaled \(p\)+\(p\) yield of direct photons in \(A\)+\(A\) collisions, and a non-zero excess, observed within systematic uncertainties, over the scaled \(p\)+\(p\) yield in central \(p\)+\(A\) collisions. Another finding is that at low-\(p_{\mathrm {T}}\), the integrated yield of direct photons, \({\mathrm {d}}N_{\gamma }/{\mathrm {d}}y\), from large systems shows a behavior of universal scaling as a function of the charged-particle multiplicity, \(({\mathrm {d}}N_{\mathrm {ch}}/{\mathrm {d}}\eta )^{\alpha }\), with \(\alpha = 1.25\), which means that the photon production yield increases faster than the charged-particle multiplicity.

NA61/SHINE: Detector Upgrades and Physics Plans Beyond 2020

Acta Phys. Pol. B Proc. Suppl. 12, 465 (2019)

DOI: 10.5506/APhysPolBSupp.12.465

article page: 465 (contains 6 pages) •


The measurements of open charm meson production was proposed as an important tool to investigate the properties of hot and dense matter formed in nucleus–nucleus collisions. Recently, the experimental setup of the NA61/SHINE experiment was supplemented with a Vertex Detector which was motivated by the importance and the possibility of the first direct measurements of open charm meson produced in heavy-ion collisions at SPS energies. First test data taken in December 2016 on Pb+Pb collisions at 150\(A\) GeV/\(c\) allowed to validate the general concept of \(D^0\) meson detection via its \(D^0~\rightarrow ~\pi ^{+} + K^{-}\) decay channel and delivered a first indication of open charm production. The physics motivation of open charm measurements at SPS energies, pilot results on open charm production and, finally, the future plans of measurements in the NA61/SHINE experiment after LS2 are presented.

Double-parton Scattering Effects in Heavy Meson Production

Acta Phys. Pol. B Proc. Suppl. 12, 471 (2019)

DOI: 10.5506/APhysPolBSupp.12.471

article page: 471 (contains 6 pages) •


We present results of our theoretical investigation of double-parton scattering (DPS) effects in production of heavy flavour mesons (charm and bottom). We discuss production of charm–bottom and bottom–bottom meson–meson pairs in proton–proton collisions at the LHC. The calculation of DPS mechanism is performed within factorized Ansatz where each parton scattering is calculated in the framework of the \(k_{\mathrm {T}}\)-factorization. The hadronization is done with the help of independent parton fragmentation picture. For completeness, we compare results for double- and single-parton scattering (SPS). As in the case of double charm production, also here the DPS dominates over the SPS, especially for small transverse momenta. We present several distributions and integrated cross sections with realistic cuts for simultaneous production of \(D^0 B^+\) and \(B^+B^+\), suggesting future experimental studies at the LHCb detector.

Three-dimensional Lévy HBT Results from PHENIX

Acta Phys. Pol. B Proc. Suppl. 12, 477 (2019)

DOI: 10.5506/APhysPolBSupp.12.477

article page: 477 (contains 6 pages) •


In heavy-ion collisions, we measure the Bose–Einstein correlation functions to map out the femtoscopic homogeneity region of the particle emitting source. Previously, a one-dimensional measurement with a Lévy-shaped source was done at the PHENIX experiment. In this paper, we present the three-dimensional Lévy HBT results of PHENIX and their comparison with the one-dimensional results.


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