XVIII Polish Workshop on Relativistic Heavy-Ion Collisions: Strange and Heavy Flavour Physics

13 Dec 2025, 08:00 → 14 Dec 2025, 17:00 Europe/Warsaw

Faculty of Physics, Astronomy and Applied Computer Science JU

Janusz Brzychczyk (Jagiellonian University), Paweł Staszel (Jagiellonian University), Roman Płaneta (Jagiellonian University)

We are pleased to announce that the XVIII Polish Workshop on Relativistic Heavy-Ion Collisions will be held at the 

Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University in Kraków

on December 13-14, 2025 (Saturday-Sunday)

 

Invited contributions are 25 min, and all others are 15 min each.

The abstract submission is now open, and we look forward to your contribution!

Poster_XVIII_Polish_Workshop_Krakow.pdf

Saturday 13 December

08:30 → 09:00 Registration

09:00 → 10:30 Session 1

09:00 Highlights in heavy-ion physics including strangeness and heavy-flavour production from ATLAS

This talk presents an overview of recent findings from the ATLAS experiment using heavy-ion collisions. These include measurements of how energetic jets are produced and modified as they travel through the hot, dense medium known as the Quark-Gluon Plasma (QGP), which is created in collisions between atomic nuclei at near light speed. The results also include studies of softer particles emerging from the collisions, which help reveal the collective behaviour and properties of the QGP. In particular, results from the recent oxygen-oxygen run at the LHC are discussed. In addition, the talk will cover the latest results from ultra-peripheral collisions (UPC), where the nuclei pass close to each other without directly colliding but still interact via their intense electromagnetic fields. These rare events allow the study of photo-nuclear and photon-photon processes. Photo-nuclear interactions provide insights into the structure of atomic nuclei, similar to deep inelastic scattering, while photon-photon interactions are used to probe the electromagnetic properties of the tau lepton, search for exotic particles like magnetic monopoles, and explore possible physics beyond the Standard Model.

Speaker: Iwona Grabowska-Bołd (AGH University of Kraków)

09:25 Measurement of coherent exclusive $J/\psi$ production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV with the ATLAS detector

The fully stripped ions used in heavy-ion collisions at the LHC are an excellent source of high-energy quasi-real photons. These can interact with photons emitted by the oncoming nucleus, or with the nucleus itself, either directly in inelastic processes or diffractively via pomeron exchange. Diffractive photonuclear processes can produce exclusive vector mesons that are uniquely sensitive to the spatial and momentum structure of the nuclear parton distribution functions, as well as spatial fluctuations (hotspots). In Run 3, the ATLAS experiment utilized a low-multiplicity track trigger in heavy-ion collisions for the first time, allowing the collection of a large sample of events with a few tracks. A substantial fraction of these events are dilepton decays of vector mesons, including the $J/\psi$. This paper presents the first results on $J/\psi$ differential cross section measurement as a function of $J/\psi$ rapidity.

Speaker: Paweł Rybczyński (AGH University of Kraków)

09:40 ALICE Experiment: Recent Highlights and Upgrade Plans

Since 2009, ALICE has successfully collected data from various collision systems, including A–A (Pb–Pb, Xe–Xe, O–O), p–A (p–Pb, p–O), and pp. This presentation will review recent highlights from ALICE and outline its ambitious short- and long-term upgrade plans.

Speaker: Jacek Otwinowski (Institute of Nuclear Physics PAS)

10:05 Low-mass Drell-Yan measurements at forward rapidity with the upgraded ALICE detector in LHC Run 3

The measurement of the Drell-Yan (DY) production at forward rapidity in proton-proton (pp) collisions at the LHC with the upgraded ALICE detector in Run 3 provides a unique tool for probing the Parton Density Functions (PDFs) and partonic structure of hadrons and nuclei. There is a lack of a hard and clear probe of nuclear matter at relatively small Bjorken-x (down to 10^–5), which could provide information about initial stages in collisions involving heavy ions. In principle, the nuclear PDFs (nPDFs) are not well known for x < 10^–4. The low-mass DY dimuon (Mμ+μ– > 4 GeV/c^2) measurements at forward rapidity with the upgraded ALICE detector will allow us to gain knowledge about small-x physics at the LHC. These measurements in pp collisions will serve as a reference for the future proton-lead (p-Pb) data. Moreover, in p-Pb collisions, at very small x, the ratio of the nuclear modification factors (R_pPb) of DY and J/ψ can provide important constraints on gluon densities. In this contribution, the initial performance of the Drell-Yan simulations with the upgraded ALICE detector will be presented.

Speaker: Sahil Upadhyaya (IFJ PAN)

10:30 → 11:00 Coffee break

11:00 → 12:30 Session 2

Convener: Piotr Salabura (Jagiellonian University)

11:00 Heavy-ion program of the HADES experiment at GSI

We report the recent results from the HADES experiment obtained in Au+Au and Ag+Ag collisions at center-of-mass energies per nucleon pair of 2.42 and 2.55 GeV, respectively. In particular, measurements of hadronic and dilepton observables are presented, with focus on particle correlations and prospects for the future experimental program are outlined.

Speaker: Hanna Zbroszczyk (Warsaw University of Technology)

11:25 Review of strangeness production in elementary collisions at HADES

The HADES (High-Acceptance Di-Electron Spectrometer) detector is a versatile device operating at SIS18 synchrotron at GSI Darmstadt with a vital list of results in the elementary and heavy ion collisions. It combines unique capability of dileptons and hadrons identification. HADES provides a unique platform for investigating elementary reactions, with a particular focus on the strangeness and dilepton production channels. These reactions offer critical insights into the dynamics of hadronic and nuclear matter, hadron structure in non-perturbative QCD regime, as well as the searches for new particles like axions in rare meson decays. The strangeness program at HADES explores the production of strange hadrons in proton-proton, proton-nucleus, and pion-nucleus collisions, serving as a sensitive probe of the underlying mechanisms of quark dynamics and structures. With the recent experiment at 4.5 GeV beam kinetic energy, we enabled more precise studies of heavier mass hyperons like Σ(1385), Λ(1405) and Λ(1520), and double-strangeness like Ξ(1320)particles, including the electromagnetic decay channels of hyperons, various mesons like omega, eta also with their electromagnetic and hadronic decay channels. Dilepton production serves as a powerful tool to investigate in-medium modifications of hadrons and provides key information on the properties of hot and dense matter, especially in the context of the early stages of hadronic reactions. Thus, the results from dilepton production in the elementary reactions are vital for the current and future heavy ion collisions at SIS100. Dilepton production with pion and proton induced reactions on proton targets in HADES provides unique possibility to study electromagnetic structure of baryons, including hyperons. This talk will present recent results from proton and pion induced elementary collisions from the HADES collaboration, emphasising the importance of these elementary reaction studies in advancing our understanding of the strong force, hadron structure, and the production mechanisms in various particle collision processes.

Speaker: Rafał Lalik (Jagiellonian University)

11:50 Exploring the QCD matter at the crossroads with CBM

Heavy-ion collisions at a few AGeV give insight into the intriguing hot and dense region of QCD diagram. In this region, the basic properties of hadrons, like mass or branching ratios may be modified with respect to the values in vacuum.

With rising beam energy, the production of many hadrons increases from rare to abundant. It gives a chance to obtain a multifaceted picture of created matter in extreme conditions. This pertains to (multi-)strange hadrons, strange resonances, hypernuclei, and possibly charm. The collisions are also a source of dileptons, whose measurement allows e.g. to probe the temperature of the system at the pre-freezeout stage. Also, with rising beam energy, the hadronic picture of the collision gives way to a description by quarks and gluons.

The Compressed Baryonic Matter (CBM) experiment is currently being installed at the FAIR facility. It is designed for high-precision measurements of a wide range of types of particles emitted from heavy-ion collisions. The planned reaction rate of up to 10 MHz is unprecedented. Thus, CBM is designed with radiation-hard detectors, fast electronics, and a free-streaming acquisition system. To test the performance of built components, the mCBM, an up-and-running demonstrator of CBM, is developed. The results of the in-beam tests at mCBM will be shown. They are a promising demonstration of capabilities of the future CBM setup.

Speaker: Krzysztof Piasecki (University of Warsaw)

12:15 $^3$H and $^3$He nuclei production in a combined thermal and coalescence framework for heavy-ion collisions in the few-GeV energy regime

A thermal model describing hadron production in heavy-ion collisions in the few-GeV energy regime is combined with the idea of nucleon coalescence to make predictions for the $^3$H and $^3$He nuclei production. A realistic parametrization of the freeze-out conditions is used, which reproduces well the spectra of protons and pions. It also correctly predicts the deuteron yield that agrees with the experimental value. The predicted yields of $^3$H and $^3$He appear to be smaller by about a factor of two compared to the experimental results. The model predictions for the spectra can be compared with future experimental data.

Speaker: Nikodem Witkowski (Jagiellonian University)

12:30 → 14:00 Lunch

14:00 → 16:05 Session 3

Convener: Andrzej Rybicki (Institute of Nuclear Physics PAS)

14:00 Open charm measurements with the upgraded NA61/SHINE detector system

The NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS) investigates the properties of strongly interacting matter by studying hadron production in hadron–hadron, hadron–nucleus, and nucleus–nucleus collisions over a wide range of beam energies. One of the main goals of the program is to measure open charm production, which provides a sensitive probe of the medium produced in heavy ion collisions.

This contribution presents the current status of open charm measurements with the upgraded NA61/SHINE experimental setup. The talk will include an overview of the new high-resolution Vertex Detector and the experimental configuration used for open charm reconstruction. The reconstruction strategy for $\text{D}^{0} \rightarrow \text{K}^{-}\pi^{+}$ and $\overline{\text{D}}^{0} \rightarrow \text{K}^{+}\pi^{-}$ decays and the applied topological selection criteria will be discussed. Invariant mass distributions from Pb+Pb pilot data collected in 2022 will be presented, along with the motivation for introducing the segmented target system, which has been used since 2024, to reduce background and improve vertex separation. Finally, the prospects for upcoming Pb+Pb analyses will be outlined.

Speaker: Mateusz Bajda (Jagiellonian University)

14:25 Testing c-cbar creation properties with correlations at the CERN SPS

I will summarise the results presented in two recent papers:
https://inspirehep.net/literature/2655602
https://inspirehep.net/literature/2899809

Speaker: Marek Gazdzicki (Jan Kochanowski University in Kielce)

14:50 Measurement of charged and neutral kaons in Ar+Sc collisions at the NA61/SHINE experiment

NA61/SHINE at the CERN SPS is a multipurpose fixed-target experiment designed for measurements of charged and neutral hadrons. The main physics goals of the NA61/SHINE strong interaction program are to study the properties of the onset of deconfinement and to search for signatures of the critical point of strongly interacting matter. These objectives are pursued by performing a comprehensive scan in beam momentum (13A–150(8)A GeV/c) and system size (p+p, p+Pb, Be+Be, Ar+Sc, Xe+La, Pb+Pb).

The multiplicities of hadrons belonging to the same isospin multiplet are expected to be similar, as their mass differences are small (charged kaons are heavier by only about 1\% with respect to neutral ones). However, NA61/SHINE has reported an unexpected excess of charged over neutral kaon production in central Ar+Sc collisions at a center-of-mass energy per nucleon pair of 11.9 GeV, amounting to (18.4 ± 6.1)\% at mid-rapidity. Preliminary NA61/SHINE results at a lower collision energy (8.8 GeV) support this observation.

This contribution will present the experimental details of this intriguing result, including the particle production spectra of
mesons and the charged-to-neutral kaon production ratio in Ar+Sc collisions. It will also compare these results to the world data on charged-to-neutral kaon ratios in nucleus–nucleus collisions.

Speaker: Angelika Tefelska (Politechnika Warszawska)

15:15 Theoretical status of the kaon isospin anomaly

In this talk, I review recent theoretical efforts to understand the excess of charged over neutral kaons measured by the NA61/SHINE collaboration. This excess signals a much larger breaking of isospin symmetry than expected. In particular, the results of hadron resonance gas approach(es) and of the UrQM simulation, as well as considerations involving the electromagnetic interaction and the chiral (or axial) anomaly in QCD will be presented. In the end, the consequences of isospin breaking for other hadrons and future model improvements will be briefly summarized.

Speaker: Francesco Giacosa (Jan Kochanowski University)

15:40 News on charged hadron production in central nucleus-nucleus collisions from NA61/SHINE at the CERN SPS

NA61/SHINE is a multipurpose fixed-target experiment located at the CERN SPS. One of its main goals is to study the onset of deconfinement and the properties of strongly interacting matter. For this purpose, a unique two-dimensional scan in collision energy (√sNN = 5.1 − 17.3 GeV) and system size was performed. Results on charged hadron spectra produced in nucleus-nucleus collisions, including the recent results for the Xe+La system, which completes the two-dimensional scan, will be presented. In addition, new results for Pb+Pb collisions obtained by NA61/SHINE will be shown. The kinematic distributions and the measured multiplicities of identified hadrons will be compared with NA49 Pb+Pb results, as well as with available world data. The obtained results, particularly the ratio of positively charged kaons to pions, are crucial for understanding the phenomena of the onset of deconfinement and the onset of fireball, which is one of the main aims of the strong interaction program of the NA61/SHINE collaboration.

Speaker: Oleksandra Panova (IFJ PAN)

16:05 → 16:30 Coffee break

16:30 → 18:30 Session 4

16:30 Lambda hyperon production in central Ar+Sc collisions at CERN SPS energies

Measurements of strange hadron production provide information on the properties of strongly interacting matter and on the mechanisms of particle production in high-energy collisions. The NA61/SHINE experiment at the CERN SPS North Area investigates these processes through a two-dimensional scan in collision energy and system size, covering a wide range of hadronic and nuclear interactions.

This contribution presents results on the production of Lambda hyperons in the 0–10% most central Ar+Sc collisions. This includes studies of the ratios of Lambda hyperons to pions and total strangeness to pions as functions of collision energy and system size. The results will be compared with predictions from selected particle production models and with available world data from proton–proton and nucleus–nucleus collisions.

Speaker: Yuliia Balkova (National Centre for Nuclear Research)

16:55 Production of Σ⁰ hyperons and first results from NA61/SHINE

Hyperons are an important probe of strangeness production and the properties of strongly interacting matter in relativistic heavy-ion collisions. While Λ and multi-strange hyperons have been measured extensively, the Σ⁰ hyperon remains much less explored. This is particularly interesting since Σ⁰ shares the same quark content as Λ but carries different isospin, so the Σ⁰/Λ ratio can provide additional constraints on hadronization and on the role of isospin in the strange-baryon sector.

In this talk I will briefly review the current experimental status of Σ⁰ measurements and then present a new analysis in the NA61/SHINE experiment at the CERN SPS. The study is based on the decay chain Σ⁰→Λγ, where the photon converts into an e⁺e⁻ pair in the detector material, allowing for the reconstruction of Σ⁰ candidates in nucleus–nucleus collisions at SPS energies. I will show preliminary results on signal extraction.

Finally, I will outline how future Σ⁰/Λ measurements at SPS energies can be combined with NA61/SHINE kaon data to study isospin effects, in particular in the context of the recently observed, unexpectedly strong isospin-symmetry breaking in kaon production in intermediate-mass systems.

Speaker: Damian Pszczel (National Centre for Nuclear Research)

17:20 The thermal origin of strange and charm hadron production yields in heavy ion collisions

We summarise the systematics of strange and charm hadron production yields in heavy ion collisions and focus on their quantitative interpretation based on the thermal model. We emphasise the role of exact charge conservation laws and indicate the differences between strange and charm hadron productions in HIC, as well as their energy and centrality dependence.  We also address the thermal interpretation of charge fluctuations in HIC and their energy dependence.

Speaker: Krzysztof Redlich (University of Wrocław)

17:40 Interactions between heavy and light flavor hadrons in heavy-ion collisions by STAR

Heavy quarks are produced in hard partonic scatterings at the very early stage of heavy-ion collisions and they experience the whole evolution of the Quark-Gluon Plasma medium. Femtoscopic correlations, i.e. two-particle correlations at low relative momentum, are sensitive to the final-state interactions as well as to the extent of the region from which the correlated particles are emitted. A study of correlations between heavy-flavor mesons and identified charged hadrons could shed light on their interactions in the hadronic phase.

   STAR has performed the first measurement of femtoscopic correlation between $D^0$-charged hadron pairs at mid-rapidity in Au+Au collisions at ${\sqrt{s_{\mathrm{NN}}}}$ = 200 GeV. $D^0$ mesons are reconstructed via the $K^{\mp}-{\pi}^{\pm}$ decay channel using topological criteria enabled by the Heavy Flavor Tracker with excellent track pointing resolution. We will present the femtoscopic correlation functions between $D^{0}/\overline{D^0}$-$\pi^{\pm}$, $D^{0}/\overline{D^0}$-$K^{\pm}$ and $D^{0}/\overline{D^0}$-$p^{\pm}$ pairs for $D^0$/$\overline{D^0}$ with transverse momentum above 1 GeV/$c$ in the $0-80\%$ centrality range. STAR results will be compared with existing theory predictions and the physics implications will also be discussed. 
   Moreover, Lednicky-Lyuboshitz model generated correlation functions with variation of interaction parameters and emission source size will be reported.

Speaker: Priyanka Roy Chowdhury (Warsaw University of Technology)

18:00 Constrained kinetics and nonabelian symmetry driven correlations

The condition to preserve the internal symmetry of the system leads to new constraints for the kinetic equations.
Abelian symmetries lead to extensive variables that scale proportionally with the system size or number of particles. Nonabelian symmetries lead to variables that do not scale proportionally with the system size or number of particles and are realized by higher factorial moments.

Isospin symmetry will be treated as the example here.

Speaker: Ludwik Turko (University of Wrocław)

18:15 Multi-fluid description of heavy-ion collisions

Fluid-dynamical modeling of heavy-ion collisions at RHIC BES energies is complicated by primary collisions and collective expansion taking place simultaneously. In a multi-fluid model this problem is circumvented by describing the colliding nuclei and produced particles as three separate fluids. I describe the present status of our multi-fluid model MUFFIN (MUlti Fluid simulation of Fast IoN collisions), its improved friction terms and how the new friction terms leave room for finite shear viscosity which allows the reproduction of the elliptic flow data.

Speaker: Pasi Huovinen (University of Wrocław)

19:30 → 22:00 Dinner

Sunday 14 December

09:00 → 11:10 Session 5

09:00 Spin hydrodynamics

The concept of spin hydrodynamics is introduced and the newest developments in this field are reviewed.

Speaker: Wojciech Florkowski (Jagiellonian University)

09:25 From novel Wigner function for spin-1/2 particles to causal and stable spin hydrodynamics

We develop a new form of the Wigner function for a relativistic gas of spin-1/2 particles in local equilibrium [1]. This new expression resolves the previously encountered issue with the normalization of the mean spin polarization and also leads to the generalized thermodynamic relations for perfect spin hydrodynamics, agreeing therefore with kinetic theory with classical spin treatment. Moreover, we demonstrate that perfect spin hydrodynamics based on the new equilibrium Wigner function is nonlinearly causal and stable [2]. These properties are crucial for constructing a physically meaningful and numerically consistent dynamical description of strongly interacting matter.

References:
[1] arXiv:2505.02657.
[2]arXiv:2511.09580.

Speaker: Valeriya Mykhaylova (Jagiellonian University)

09:40 Exact Wigner functions for chiral spirals

At present, our understanding of the non-equilibrium aspects of a relativistic spin fluid is derived from the study of Wigner function coupled with the semiclassical expansion. In our work, we present an exact computation (up to quantum loop corrections) of the Wigner functions for quarks in a field of oscillating pion condensate. The oscillating pion condensate enters the equation of motion as a mean field, allowing for an exact solution. The spinors thus obtained are used to compute all components of the Wigner functions. Axial current obtained using these exact Wigner functions reproduce expressions in existing literature. Furthermore, we find that during an adiabatic change of the periodic potential into a uniform one, the polarization vector is twisted from its original direction. Finally, our analysis also reveals a scenario that indicates incompatibility with the conventional semi-classical expansion.

Speaker: Sudip Kumar Kar (Jagiellonian University)

09:55 Spin description in relativistic hydrodynamics: classical vs. quantum

We compare two recently developed frameworks of perfect spin hydrodynamics for spin-1/2 particles, based respectively on classical kinetic theory and the Wigner function. We show that the conserved currents in both approaches have the same form at each order of the expansion in the components of the spin polarization tensor ω. The only difference is a relative multiplicative factor, which is equal to 1 at the lowest nontrivial order and increases monotonically with the expansion order. We also discuss application range of both frameworks. The results are important for practical applications of spin hydrodynamics to model heavy-ion collisions.

Based on:
[1] Phys.Rev.D 112 (2025) 5, L051901
[2] arXiv:2509.06014

Speaker: Zbigniew Drogosz (Jagiellonian University)

10:10 Boost-invariant perfect Fermi–Dirac spin hydrodynamics

We analyze the effect of using the Fermi–Dirac statistics, rather than its Boltzmann approxima-
tion, in numerical simulations of perfect spin hydrodynamics of particles with spin 1/2. The system
considered is a one-dimensional boost-invariant expansion with corrections to the energy–momentum
tensor and the baryon current that are second order in the spin polarization tensor ω. The study
shows the feasibility of this approach, as the special functions defined by integrals that appear
in the coefficients in the Fermi–Dirac case can be pre-tabulated and interpolated.
For sample sets of initial conditions representing hydrodynamic simulations of heavy-ion collisions
producing Λ hyperons, the differences in parameter evolution between the two underlying particle
statistics are about one order of magnitude smaller than corrections coming from spin feedback.

Speaker: Natalia Łygan (Jagiellonian University)

10:25 Neural network enhanced Bayesian global analysis of EKRT+viscous hydrodynamics model of relativistic heavy ion collisions

In this talk I present a Bayesian global analysis of a (2+1)-d viscous hydrodynamics model with event-by-event EKRT initial state describing simultaneously four collision systems: Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, Pb+Pb collisions at 2.76 TeV and 5.02 TeV, and Xe+Xe collisions at 5.44 TeV. To make the analysis computationally feasible, two layers of model emulation are utilized: First, neural networks are used to estimate model output for a single event, which allows us to evaluate hundreds of thousands of events with reasonable computational resources. Based on these single-event neural network computations, Gaussian process emulators are then trained to produce estimates for the event-averaged output which can be compared with the experimental data. This procedure allows us to obtain constraints on the temperature dependence of shear and bulk viscosities for the QCD medium formed in relativistic heavy ion collisions.

Speaker: Jussi Auvinen (University of Wrocław)

10:40 Magnetic susceptibility of hot hadronic medium and quark degrees of freedom near the QCD cross-over point

Lattice QCD results for the magnetic susceptibility of the medium just below the cross-over temperature are difficult to be reproduced by the available hadronic models. In particular, the widely used hadron resonance gas, remarkably successful to describe numerous other lattice results, is substantially too strong diamagnetic as compared to the data, at all temperatures available in the lattice simulations, starting at low temperature $T\sim130$ MeV. We propose a possible approach with quark-meson model where the quark masses are fixed from baryon-baryon and baryon-strangeness susceptibility data in order to be consistent. We show that the quark-meson model can reproduce the lattice data for magnetic susceptibility, generating the lacking paramagnetism in HRG, provided its vacuum contribution is treated carefully. The fitted quark masses are consistent with other model results e.g quasi-particle picture. We also compute the contribution of the pion-vector meson loops in the evaluation of the magnetic susceptibility via the photon polarization, showing it is small.

Speaker: Rupam Samanta (Institute of Nuclear Physics PAS)

10:55 Baryon Dynamics as a Background to Electromagnetic Signatures in Directed Flow Observables at RHIC

In non-central heavy-ion collisions, the presence of strong electromagnetic (EM) fields causes charged hadrons to experience different deflections along and opposite to the impact parameter direction. This results in a splitting of the directed flow between oppositely charged hadrons. Since the strength of the EM field varies with both centrality and system size, studying the directed flow splitting across different collision systems and centralities can provide valuable insights into the signal and magnitude of the EM field. Recent measurements by the STAR Collaboration (PRX 14, 011028, 2024) at RHIC have reported such directed flow splitting, which has been interpreted as evidence of EM field effects. In this talk, I will discuss the observed notable splitting in the directed flow between positively charged protons and negatively charged anti-protons, along with its interesting centrality dependence. I will argue that this effect cannot be solely attributed to the EM field but also receives significant contributions from the background physics of baryon stopping and baryon dynamics (arXiv:2305.08806 & 2503.04660). In this context, I will present the role of baryon diffusion within the Beam Energy Scan (BES) phenomenology. Furthermore, I will emphasize the importance of studying conserved charge dynamics in low-energy heavy-ion collisions to better understand the interplay between electromagnetic and baryonic effects.

Speaker: Tribhuban Parida (AGH University of Kraków)

11:10 → 11:35 Coffee break

11:35 → 13:35 Session 6

11:35 New vistas in $\gamma \gamma \to \gamma \gamma$ scattering in ultraperipheral collisions of heavy ions

The current theoretical estimations lead to cross-sections for $AA \to \gamma \gamma AA$ which are somewhat smaller than the measured ones by the ATLAS and CMS Collaborations, In our recent paper, we estimated the contribution of inelastic channels to the Light-by-Light (LbL) scattering in ultraperipheral collisions (UPC) of heavy ions, in which one or both of the incident nuclei dissociate ($A A \to \gamma \gamma X Y$ where $X, Y = A, A'$) due to the photon emission. These new mechanisms are related to extra emissions that are difficult to identify and may be misinterpreted as enhanced $\gamma \gamma \to \gamma \gamma$ scattering.

We include processes of coupling of photons to individual nucleons in addition to coherent coupling to the whole nuclei. Both elastic (nucleon in the ground state) and inelastic (nucleon in an excited state) are taken into account. The inelastic nucleon fluxes are calculated using CT18qed photon distribution in nucleon. The inelastic photon fluxes are shown and compared to standard photon fluxes in the nucleus.

We present the ratio of the inelastic corrections to the standard contribution to the nuclear cross section. We find that for the ATLAS kinematics the inelastic corrections grow with $M_{\gamma \gamma}$ and rapidity difference. Our results indicate that the inelastic contributions can be of the order of 20-40 \% of the traditional (no nuclear excitation) predictions. We discuss also uncertainties due to the choice of factorization scale.

We calculate cross sections for the production of different neutron classes 0n0n, 0nXn+Xn0n and XnXn, possible to measure in Zero Degree Calorimeters (ZDCs), associated with photon-photon scattering in UPC of lead on lead. The calculations are performed for the ATLAS kinematics. Our calculations for neutron classes are tested against existing data for $\rho^0$ production in UPC. We get a good agreement for absolute cross section as well as very good results for fractional cross section which gives confidence to the analogous calculation for diphoton production, corresponding to $\gamma \gamma \to \gamma \gamma$ scattering. We present both absolute as well as fractional cross sections for the different neutron classes. We also show different distributions in impact parameter, photon rapidity, transverse momentum, diphoton invariant mass and photon rapidity difference. The shapes of the photon distributions depend on the neutron classes which we quantify. It would be valuable to test our predictions using the ATLAS main detector and the ATLAS ZDCs.

Speaker: Antoni Szczurek (Intitute of Nuclear Physics PAS and University of Rzeszów)

11:50 Glueballs in the Hopfion approach

We work out the Hopfion description of glueballs by inclusively comparing the energy spectra obtained by quantizing Hopfions with experimental data and lattice QCD. Identifying a Hopfion carrying a unit topological charge as f0(1500), the Hopfions with the topological charge two are classified as glueballonia, i.e., two glueballs are bound together. We find a tightly and a loosely bound glueballonia complying with f0(2470) and a novel scalar particle carrying the mass around 2814 MeV, respectively, and calculate their binding energies. By the rigid body quantization of Hopfions, we predict a characteristic multiplet structure of tensor glueball states. Some of them are missing in the current experimental data and can be verified in future measurements.

Speaker: Chihiro Sasaki (University of Wrocław & SKCM2 at Hiroshima University)

12:05 The Wróblewski factor revisited for proton-proton at $\sqrt{s}\simeq$10 GeV

The Wróblewski factor, proposed 40 years ago [1] and still widely used, is a measure of strangeness production compared to the light quark production. The method is based on the direct comparison of the number of quarks in produced particles. Several assumptions were needed as not all particles have been measured.

Numerous multiplicities of particles (charged and several neutrals) produced in inelastic proton-proton interactions have been precisely measured at $\sqrt{s}\simeq$ 10 GeV available at the CERN SPS. The experimental multiplicities are reasonably well (<20%) described [2] within hadron-resonance gas model, which estimates multiplicities of several unmeasured particles, in particular $\pi^0$ mesons and $\eta$ mesons (containing hidden strangeness). Calculations of the Wróblewski factor will be presented.

[1] A. Wróblewski, Acta. Phys. Pol. B16, 379 (1985).
[2] T. Matulewicz, K. Piasecki, Acta Phys. Pol. B54, 12-A1 (2023).

Speaker: Tomasz Matulewicz (University of Warsaw)

12:20 Quantum mechanics in heavy-ion collisions

In the standard picture of heavy-ion collisions, it seems that a large part of the evolution of the produced fireball can be described in terms of classcal theories: relativistic hydrodynaics, the relativistic Boltzmann equation. The quantum effects seem to be restrivcted to the very initial stages, and the computation of some material properties, for instance the transport coefficients.
Recent results highlight that the quantum corrections are, in fact, large. The succes of such classical models can be explained within quantum field theory.

Speaker: Leonardo Tinti (Jan Kochanowski University)

12:35 Abrupt switching from hadron gas to quark-gluon plasma at smooth chiral crossover

Based on a generalized Beth-Uhlenbeck approach to thermodynamics of QCD motivated by cluster decomposition we present a unified equation of state of hot strongly interacting matter and analyze its properties in a wide range of temperatures. The hadrons are treated as color singlet multiquark clusters in medium with a background gluon field in the Polyakov gauge. The confining aspect of QCD is accounted for by the Polyakov loop mechanism and by a large vacuum quark mass motivated by a confining density functional approach. We demonstrate that an abrupt switching between hadronic and partonic degrees of freedom, which is one of striking manifestations of dynamical restoration of chiral symmetry, is accompanied by a smooth behavior of entropy density at chiral crossover. By applying a reaction-kinetic criterion for the chemical freeze-out of multi-quark
clusters in heavy-ion collisions, we demonstrate that the chemical freeze-out coincides with their Mott transition. Individual contributions of different components of strongly interacting matter to its speed of sound are analyzed for the first time. It is shown that restoration of chiral symmetry drives speed of sound of hadron gas to negative values, manifesting its mechanical instability and being in a strike disagreements with the lattice QCD data. Accounting for the partonic excitations naturally resolves this contradiction.

Speaker: Oleksii Ivanytskyi (University of Wrocław)

12:50 Eta condensates on the extended Columbia plot with axial anomaly

CP violation is known to emerge at finite topological angle, including $\theta=\pi$, which can be related to a negative quark mass parameter. Such a phase with nonstrange and strange eta condensates was indeed identified also in the Columbia plot in the presence of the axial anomaly [2410.08185]. However, anomaly terms corresponding to different topological charge give rise to different behavior in this region. We discuss the effect of higher-order anomalous terms in an effective model framework, focusing on the emergence of CP-violating solutions and their stability. We show that, while eta condensates emerge in several scenarios, such a solution might be associated only with local minima.

Speaker: Győző Kovács (University of Wrocław & Wigner RCP)

13:05 Photon-photon femtoscopy in Ag+Ag collisions at $ \sqrt{s_{NN}}=2.55$ GeV

Exploring correlations between photon pairs is a challenging frontier in the field of femtoscopic measurements. Unlike hadrons, direct photons escape the hot and dense medium without final-state interactions, carrying pristine information from the earliest moments of the collision. Therefore, they offer a unique possibility to probe the initial space-time dynamics of the system. Additionally, due to the nature of photon correlations, it can be used to estimate the average number of direct photons in high-multiplicity events at the low $p_{T}$ region.

The HADES experiment, part of the FAIR/GSI facility, specialises in dielectron ($e^\pm$) measurements in collisions at beam energies of 1–2 A GeV, allowing for the detection of photons via the conversion method. The experiment’s electromagnetic calorimeter extends its capabilities to real photon detection, paving the way for photon-photon femtoscopy within this energy regime.

Preliminary results for Ag+Ag collision data at $\sqrt{s_{NN}} = 2.55$ GeV will be presented.

Speaker: Mateusz Grunwald (Warsaw University of Technology)

13:20 Overview of femtoscopic analyses at HADES

HADES is an experiment located at the SIS-18 accelerator in GSI, Darmstadt,
Germany. It is a detector setup primarily developed for precise measurements of
dielectrons, but it soon found other uses, including femtoscopy. High angular
acceptance and interaction rates enable the collection of large quantities of
high-quality data, making them useful for any type of analysis. HADES is focused
on probing the high-density parts of the phase diagram, making it an interesting
choice for femtoscopic analyses.

Femtoscopic correlations are a method for probing the size and the dynamics
created in proton-proton or heavy-ion collisions. A multitude of beamtimes allows
for analyses of different systems (p+p at √SNN = 4.5 GeV, Ag+Ag at
√sNN =2.55 GeV, Au+Au at √ SNN =2.4 GeV).
Results for different femtoscopic analyses (e.g, pion, NN, and NY correlations)
performed with the HADES experiment will be presented during the talk.
Keywords: HADES, femtoscopy

Speaker: Michał Prędota (Warsaw University of Technology)