Welcome by the Rector of the University of Wroclaw, Prof. Robert Olkiewicz & Opening Room: Witold Swida Lecture Hall

Columbia plot, critical point and bound states in dense QCD (Dense QCD)

• Christian Fischer (University of Giessen, Germany)

Room: Witold Swida Lecture Hall Converging results from different approaches locate the critical end point (CEP) of QCD at large chemical potential, i.e. in the realm of dense QCD. We summarise recent theoretical results within functional methods on the location of the CEP and its variation under systematic changes of the light and strange quark masses. We furthermore discuss the physics at large chemical potential including the properties of bound states and the possible appearance of interesting new phases.

QCD transition line up to $\mu_{B}$=400 MeV from finite volume lattice simulations (Phase diagram and critical behaviour in QCD/QFT)

• Ludovica Pirelli (Bergische Universität Wuppertal)

Room: Witold Swida Lecture Hall The QCD cross-over line in the temperature (T) - baryo-chemical potential ($\mu_{B}$) plane has been computed by several lattice groups by calculating the chiral order parameter and its susceptibility at finite values of $\mu_{B}$ . In this work we extract from the peak position of the static quark entropy ($S_{Q}(T,\mu_{B})$) in T, which is based on the renormalized Polyakov loop. This observable was proved to have a peak in the vicinity of the chiral transition temperature, as was illustrated in arXiv:1603.06637, and has smaller volume effects, as was shown in arXiv:2405.12320. The small volume is here useful to mitigate the effects of the sign problem in the cancellations inside the Taylor coefficients of the Polyakov loop. We extrapolate $S_{Q}(T,\mu_{B})$ based on high statistics finite temperature ensembles on a 16$^{3}$ $\times$ 8 lattice to finite density by means of a Taylor expansion to eighth order in $\mu_{B}$ (NNNLO) along the strangeness neutral line. For the simulations the 4HEX staggered action was used with 2+1 flavors at physical quark masses. In this setup the phase diagram can be drawn up to unprecedentedly high chemical potentials. Our results are in rough agreement with phenomenological estimates of the freeze-out curve in relativistic heavy ion collisions. In addition, we study the width of the crossover. We show that up to $\mu_{B}$ ${\approx}$ 400 MeV, the transition gets broader at higher densities, disfavoring the existence of a critical endpoint in this range. Finally, we examine the transition line without the strangeness neutrality condition and observe a hint for the narrowing of the crossover towards large $\mu_{B}$.

Chiral anomaly: from vacuum to Columbia plot (Phase diagram and critical behaviour in QCD/QFT)

• Francesco Giacosa (Kielce University)

Room: Witold Swida Lecture Hall First, the importance of the chiral anomaly in the QCD vacuum is briefly reviewed. Besides pseudoscalar mesons, it may affect other resonances. To this end, a mathematical object called `polydeterminant' is introduced. Next, the role of the chiral anomaly at nonzero temperature is investigated. In particular, the Columbia plot (the order of the phase transition in dependence of the quark masses) is strongly affected by the precise realization of the chiral anomaly.

Finite-size scaling analysis of cumulants of net proton number and the QCD critical point (Phase diagram and critical behaviour in QCD/QFT)

• Agnieszka Sorensen (Facility for Rare Isotope Beams)

Room: Witold Swida Lecture Hall We explore the validity of finite-size scaling of net-proton number cumulants as a tool to search for evidence of a critical point in the QCD phase diagram. We show that in central Au+Au collisions at $\sqrt{s_{\rm{NN}}} = 7.7$ GeV and above, the second cumulant C2 as a function of rapidity bin width follows a power-law consistent with scale invariance. We then show that for any bin width, C2 increases as a power-law with baryon chemical potential $\mu_B$. This behavior is consistent with scaling near a critical point, and our analysis identifies a range of plausible critical exponents, including mean field and 3-D Ising model critical exponents, as well as critical $\mu_{B}$ values, ranging between 500 MeV and 615 MeV which is consistent with recent theoretical expectations. To understand the origin of this apparent scale-free, power-law behavior, we perform the same analysis on simulation data from SMASH hadronic transport in the cascade mode.

Using net quark number gain to probe the phases of QCD (Phase diagram and critical behaviour in QCD/QFT)

• Tomas Mari Surkau

Room: Witold Swida Lecture Hall We propose an observable to probe the content of a bath of quarks and gluons at finite temperature and chemical potential, the net quark number gain $\Delta Q$. QCD is controlled by different degrees of freedom in the various sections of its phase diagram, notably hadrons at low temperatures (and chemical potentials) and quarks and gluons at high temperatures. However, studying the relevant content of a QCD medium across the phase diagram is challenging. To this end, we correlate the Polyakov or anti-Polyakov loops to the medium’s net quark number $Q$. This gives the net quark number gain $\Delta Q$, which provides information on how the thermal bath reacts to adding a static quark or antiquark probe. While insignificant at high temperatures, it shows the bath's tendency to form hadron-like configurations at low temperatures, which would screen the probe’s color charge. Interestingly, $\Delta Q$ allows one to distinguish meson-like and baryon-like configurations as a function of the chemical potential and for the different probes. This net quark number gain also helps explain how a single quark/antiquark can be added to a supposedly hadronic medium in the first place: the latter provides the missing quarks/antiquarks to form hadron-like states. We sketch the derivation of this general result for temperatures much smaller than the quark masses and in the limits of large temperature or chemical potential, where perturbative QCD works. We discuss possible further applications to study the various features of the QCD phase diagram, like critical points or color superconductivity. [arXiv: 2504.06459 [hep-ph] ][1] [1]: https://arxiv.org/abs/2504.06459

Analytic calculation of quark-number susceptibilities in heavy-quark QCD (Lattice QCD)

• Kei Tohme (Kyoto University)

Room: Witold Swida Lecture Hall We investigate the thermodynamics of heavy-quark QCD in the hopping-parameter expansion (HPE) in an analytic manner. We develop a procedure to calculate the grand potential of heavy-quark QCD in the HPE using a technique based on the cumulant expansion. Applying this method to the analysis of thermodynamics, we derive the formula for the grand potential expressed in terms of the cumulants of the Polyakov loop and loop operators with windings along the temporal direction. These results immediately lead to the fact that the ratio of quark-number cumulants at even orders is unity in the deconfined phase and nine in the confined phase at leading order in the HPE. We also explore properties of excitations carrying quark number in both phases, such as their excitation energies.

Towards an optimally sensitive temperature probe in heavy-ion collisions (Correlations, bound states and hadron spectroscopy)

• Víctor López Pardo

Room: Witold Swida Lecture Hall The high-precision heavy quarkonium data from LHC Run 2 and the ongoing Run 3 provide a unique window into the properties of hot nuclear matter and the Quark-Gluon Plasma (QGP). To make full use of this data, it is crucial to go beyond traditional observables such as the nuclear modification factor $R_{AA}$ and elliptic flow $v_2$, and instead develop new probes that are more directly sensitive to the characteristics of the medium. We adopt the open quantum systems perspective for in-medium quarkonium and take inspiration from cold atom metrology techniques to construct observables with optimal sensitivity to specific QGP parameters. Focusing on the bulk temperature, we develop such optimal observables based the Caldeira-Leggett master equation as a simplified setup, with the goal of extending it to a full Quantum Brownian Motion Lindblad equation.

The finite-temperature QCD static potential: a higher order calculation (Correlations, bound states and hadron spectroscopy)

• ME Carrington (Brandon University)

Room: Witold Swida Lecture Hall We calculate the leading and subleading corrections to the real-time static potential in a high-temperature quark-gluon plasma for distances smaller than the screening length. The calculation involves one-loop 2- and 4- point functions in the hard-thermal-loop effective theory. We use our results to estimate the binding energy, the decay width, and the dissociation temperature of the bottomonium ground state. We compare with recent lattice results in the literature.

Nuclear chiral density wave in neutron stars? (Dense QCD)

• Andreas Schmitt (University of Southampton)

Room: Witold Swida Lecture Hall Cold and dense QCD matter may break rotational invariance spontaneously. One candidate phase is the chiral density wave, which exhibits an anisotropic chiral condensate, and which is conjectured to occur in the vicinity of the quark-hadron transition. I will employ a nucleon-meson model that allows for a chiral phase transition to explore the possibility of a chiral density wave in nuclear matter and throughout the transition. Besides the simplest case of isospin-symmetric matter, I will discuss the fate of the chiral density wave under neutron star conditions. I will show that the chiral density wave is only favored for model parameters that result in very soft matter, too soft to allow for realistic neutron stars.

3D Pion crystal from the chiral anomaly (Dense QCD)

• Geraint Evans (Academia Sinica)

Room: Witold Swida Lecture Hall Including the effects of the chiral anomaly in Chiral Perturbation Theory at finite baryon chemical potential, it has been shown that neutral pions form an inhomogeneous phase dubbed the "Chiral Soliton Lattice" (CSL) above a certain critical magnetic field. Above a second, even higher critical field, the CSL becomes unstable to fluctuations of charged pions, implying they condense. I will point out the similarity of this second critical field to the upper critical magnetic field in conventional type-II superconductors, suggesting that an inhomogeneous phase of superconducting charged pion exists beyond this point. Applying similar methods originally used by Abrikosov, I will present results where we've constructed such a phase and show the region where it is preferred in the baryon chemical potential-magnetic field phase diagram at zero temperature. This new phase has a non-zero baryon number density which is periodic in all three spatial dimensions.

Spin density and local equilibrium Wigner function for spin-half particles (Other)

• Samapan Bhadury (Jagiellonian University)

Room: Witold Swida Lecture Hall Analyzing the connections between the spin density matrix and the Wigner function for spin-half particles, we propose a new form of the local equilibrium Wigner function that solves the long-standing problem related to normalization of mean spin polarization. Interestingly, this new proposal for the local equilibrium Wigner function coincides with the recently developed generalized thermodynamic relations for perfect spin hydrodynamics using classical spin. The expressions of the conserved currents of the theory, obtained from the new Wigner function and the classical spin, match up to the second order in the spin polarization tensor. However, as expected, the two approaches fail to coincide at large values of the mean polarization.

Cold quark matter: Renormalization group improvement (Dense QCD)

• Loïc Fernandez (Helsinki Institute of Physics, University of Helsinki)

Room: Witold Swida Lecture Hall We will discuss the recent developments in renormalization group improvements of the cold and dense QCD pressure (Phys. Rev. D 111, 034020 and Phys. Rev. Lett. 129, 212001) at next-to-next-to leading order (NNLO) through the renormalization group optimized perturbation theory (RGOPT) and at all-order resummation of the soft modes. RGOPT applied for the very first time at NNLO displayed a significant reduction in sensitivity to variations of the arbitrary renormalization scale as compared to the state-of-the-art NNLO results. This confirms previous NLO investigations that the RGOPT resummation scheme provides improved convergence properties and reduced renormalization scale uncertainties, thus being a promising prescription to improve perturbative QCD at high and mid range baryonic densities.

QCD at finite isospin density and pion condensation (Dense QCD)

• Jens Oluf Andersen

Room: Witold Swida Lecture Hall In this talk I will discuss QCD at finite isospin density at zero temperature and zero baryon density. This is system is of particular interest since there is no sign problem and one can perform lattice simulations. I will present results for the speed of sound from NLO chiral perturbation theory (chipt) and the quark-meson model in the Bose-condensed phase. The latter is able is in good agreement with chipt for small values of mu_I and with lattice data over a large range iospin densities. I also present results for the pion condensate in chipt and it compares favorably with lattice simulations for small mu_I.

Finite spin density effects on the QCD phase transition in the linear sigma model (Phase diagram and critical behaviour in QCD/QFT)

• Pracheta Singha (west university of Timisoara)

Room: Witold Swida Lecture Hall The experimental observation of the spin polarisation of the outgoing hadrons in non-central heavy ion collisions implies the existence of a strongly vortical QGP medium with finite spin density. We consider the effect of finite spin density on the QCD phase diagram using the Linear Sigma model coupled to quarks (LSMq). In our approach, we introduce the finite spin density via a quark spin potential ($\mu_s$) in the canonical formulation of the spin operator, leading to a nonlinear modification of the energy dispersion relation. Besides an expected effect on the thermal fermion loop, the spin potential also enters non-trivially in the zero-point, temperature-independent part of the fermion loop. Employing renormalization techniques [1], we observe a substantial influence of this term on the phase structure of the system at nonnegligible $\mu_s/T$. Taking both the thermal and the zero-point terms into account, we find the expectation value of the sigma condensate by minimizing the thermodynamic potential of the system. Our findings indicate that, starting from a chiral symmetry broken phase at small $T$ and $\mu_s$, the minima of the thermodynamic potential moves towards lower values of $\sigma$ with increasing $\mu_s$, indicating the restoration of chiral symmetry. This behaviour is consistent with the results from the first principle lattice simulations [2]. Keywords: Quark-Gluon-Plasma, chiral phase transition, spin potential. References: [1] A. Ayala, L.A. Hernandez, M. Loewe, J.C. Rojas, R. Zamora, Eur. Phys. J. A 56 (2020)71. [2] V.V. Braguta, M.N. Chernodub, A.A. Roenko, arXiv:2503.18636

Berry curvature and spin-one color superconductivity (Dense QCD)

• Noriyuki Sogabe (University of Illinois Chicago)

Room: Witold Swida Lecture Hall We explore the interplay between Berry curvature and topological properties in single-flavor color superconductors, where quarks form spin-one Cooper pairs. By deriving a new relation, we connect the topological nodal structure of the gap function in momentum space to the (non-Abelian) Berry flux associated with paired quarks. This generalizes the early work by Li and Haldane [1] to systems with additional internal quantum numbers, such as color. In the ultra-relativistic limit, we uncover rich topological structures driven by the interplay of spin, chirality, and color. Specifically, we identify chirality-induced topological nodes in the transverse (opposite chirality pairing) polar and A-phases. In contrast, the color-spin-locking phase lacks these nodes due to a non-trivial color Berry flux, which in turn induces gapless excitations with total Berry monopole charges of ±3/2–differing from conventional Weyl fermions. Our findings can be potentially extended to other fermionic systems carrying additional internal degrees of freedom. This work has been published in Physical Review Letters [2]. [1] Y. Li and F. Haldane, [Phys. Rev. Lett. **120**, 067003 (2018)][a]. [2] N. Sogabe and Y. Yin, [Phys. Rev. Lett. **134**, 171903 (2025)][b]. [a]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.067003 [b]: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.134.171903

Nontrivial topology in gauge theories, and time dependent Dark Energy of the Universe. (Nuclear astrophysics)

• Ariel Zhitnitsky (UBC)

Room: Witold Swida Lecture Hall We discuss the dynamics of the topologically nontrivial sectors with non-trivial holonomy in strongly coupled QCD in the background of the expanding universe characterized by the Hubble scale $H\ll \Lambda_{QCD}$. We argue that the vacuum energy and the de Sitter phase emerge dynamically with the scale $\rho_{DE}\approx H\Lambda_{QCD}^3 \approx (10^{-3} eV)^4$, which is amazingly close to the observed value. We argue that the key element for this idea to work is the presence of nontrivial holonomy in strongly coupled gauge theories. The effect is global in nature and cannot be formulated in terms of a gradient expansion in an effective local field theory. We explain the idea in exactly solvable 2d Schwinger model and we also test these ideas with solvable models for QCD being formulated on Hyperbolic space. We also comment on some lattice QCD results when the system is formulated on a curved background modelling the expanding Universe with nonzero $H$. We also comment on recent DESI (Dark Energy Spectroscopic Instrument) results which are consistent with our findings.

Pi in the sky: Axion condensation in neutron stars (Nuclear astrophysics)

• Sanjay Reddy (University of Washington)

Room: Witold Swida Lecture Hall The QCD axion, originally proposed nearly five decades ago as a solution to the strong CP problem, remains one of the most compelling dark matter candidates. In this talk, I will explore the possibility that axions may undergo Bose-Einstein condensation in the dense cores of neutron stars, where extreme baryon densities could facilitate such a phase. Employing Chiral Perturbation Theory in conjunction with nuclear forces derived from Chiral Effective Field Theory, I will present evidence that axion condensation is not only theoretically plausible but may have astrophysically relevant consequences. Detecting - or definitively excluding - axion condensation in neutron star interiors would allow us to probe previously inaccessible regions of axion parameter space, with potentially far-reaching implications for both axion phenomenology and the microphysics of dense nuclear matter.

Bulk viscous dissipation in strange quark stars and mergers (Nuclear astrophysics)

• José Luis Hernández (Institut de Ciències de l'Espai, Institut d'Estudis Espacials de Catalunya (IEEC))

Room: Witold Swida Lecture Hall In the coalescence of neutron stars the bulk viscous dissipation offers a potential opportunity to infer properties of neutron star matter by estimating its effect on the gravitational wave emission. To this end, we compute the bulk viscosity in unpaired quark matter considering the electroweak processes allowed in the neutrino-transparent regime at low temperatures. Using the equations of state of bag models and perturbative QCD, we analyze the dependence of our results on the density, temperature and strange quark mass. Finally, we study relevant phenomena in the merger of binary compact stars where the bulk viscosity plays a significant role. First, the damping of baryon density oscillations in quark matter and its importance in the postmerger phase after the collision of two neutron stars if deconfined matter is achieved in the process. Further, the tidal heating induced by the companion in the binary inspiral of strange quark stars and its effect on the gravitational wave form. References 1. Damping of density oscillations from bulk viscosity in quark matter, José Luis Hernández; Cristina Manuel; Laura Tolos, Phys. Rev. D 109, 123022 (2024) 2. Damping of density oscillations from bulk viscosity in quark matter, José Luis Hernández; Cristina Manuel; Laura Tolos, PoS QNP2024, 175 (2025) 3. Tidal heating in binary inspiral of strange quark stars, Suprovo Ghosh; José Luis Hernández; Bikram Keshari Pradhan; Cristina Manuel; Debarati Chatterjee, Laura Tolos, arXiv:2504.07659, (2025)

Temperature and Magnetic Field effects on Isospin equilibration via Urca processes in Neutron Star Mergers. (Nuclear astrophysics)

• Pranjal Tambe (Inter-University Centre for Astronomy and Astrophysics)

Room: Witold Swida Lecture Hall Isospin-equilibrating weak processes, called ``Urca" processes, are of fundamental importance in astrophysical environments like proto-neutron stars, neutron star mergers, and supernovae. In these environments, matter can reach high temperatures of tens of MeVs and be subject to large magnetic fields. We thus investigate Urca processes at different temperatures and field strengths by performing the full temperature and magnetic-field dependent integrals of rates of Urca processes for different equations of state. I will present the finite temperature and magnetic field effects on the Isospin equilibrium condition. We find that the magnetic fields play an important role at temperatures of a few MeV, especially close to or below the direct Urca threshold, which is softened by the magnetic field. At higher temperatures, the effect of the magnetic fields can be overshadowed by the thermal effects. We observe that the magnetic field more strongly influences the neutron decay rates than the electron capture rates near the direct-Urca threshold density, leading to a shift in the isospin equilibrium condition.

Are there indications for quark deconfinement in fastly spinning massive neutron stars? (Nuclear astrophysics)

• Christoph Gärtlein (IST, University of Lisbon / University of Coimbra / University of Wroclaw)
• Violetta Sagun (University of Southampton)

Room: Witold Swida Lecture Hall We study rotating hybrid stars, with a particular emphasis on the effect of a deconfinement phase transition on their properties at high spin. Our analysis is based on a hybrid equation of state with a phase transition from hypernuclear matter to color-superconducting quark matter, where both phases are described within a relativistic density functional approach. By varying the vector meson and diquark couplings in the quark matter phase, we obtain different hybrid star sequences with varying extensions of the quark matter core, ensuring consistency with astrophysical constraints from mass, radius, and tidal deformability measurements. We test whether the early deconfinement phase transition is consistent with the present observational data. We show how the fastest spinning pulsars and the appearance of the quasi-radial oscillations and non-axisymmetric instabilities constrain the strongly interacting matter equation of state at zero temperature. Our findings reveal that incorporating the hybrid equation of state into the analysis of pulsars has significant implications for the constraints on the properties of strongly interacting matter and neutron stars.

Exploring Color-Superconducting Phases in Hybrid Star Models Using an RG-Consistent NJL Approach (Nuclear astrophysics)

• Ishfaq Ahmad Rather (Institute for Theoretical Physics, Goethe University, 60438 Frankfurt am Main, Germany)

Room: Witold Swida Lecture Hall We present a comprehensive investigation of color-superconducting phases in compact stars using the renormalization group-consistent Nambu–Jona-Lasinio (RG-NJL) model [1]. By systematically varying vector and diquark coupling constants, we analyze their effects on the equation of state, speed of sound, and resulting mass-radius relations. Our models demonstrate that stable color-flavor-locked (CFL) phases can exist in maximum-mass star configurations, typically extending several kilometers in radius. At neutron-star densities, we observe the speed of sound squared reaching values up to $c_s^2 \approx 0.6$ and CFL gaps up to $\Delta \approx 250$ MeV [2]. The parameter constraints are derived from the $2.0\,M_{\odot}$ observational limit. We further explore the viability of hybrid-star configurations by combining our quark-matter model with a hadronic equation of state through Maxwell construction. Our results indicate several possible compositional scenarios for hybrid stars that remain consistent with current astrophysical constraints while demonstrating that early hadron-quark transitions can significantly affect tidal deformability measurements at $1.4\,M_{\odot}$ [3]. References [1] H. Gholami, M. Hofmann, and M. Buballa, Phys. Rev. D 111, 014006 (2025). [2] H. Gholami, I. A. Rather, M. Hofmann, M. Buballa, and J. Schaffner-Bielich, (2024), arXiv:2411.04064 [hep-ph]. [3] J. E. Christian, I. A. Rather, H. Gholami, & M. Hofmann, (2025), arXiv:2503.13626 [astro-ph.HE]

Color-superconducting strange quark matter formation in compact stars (Nuclear astrophysics)

• Mirco Guerrini (University of Ferrara and INFN Ferrara)

Room: Witold Swida Lecture Hall An intriguing hypothesis states that ordinary hadronic matter in bulk is a metastable state (i.e., a local minimum) of strongly interacting matter, while strange quark matter (SQM) is absolutely stable (i.e., the global minimum) in bulk. A potential barrier that prevents a spontaneous deconfinement transition of ordinary hadronic matter into SQM would separate these two phases.  Our work aims to determine the conditions under which this barrier can be overcome and to assess whether such conditions may occur in high-energy astrophysical phenomena, such as core-collapse supernovae and binary compact star mergers. In these scenarios, the formation of a critical SQM seed could ultimately lead to the conversion of a hadronic star into a strange quark star. It is usually assumed that the local flavor composition remains fixed during the initial formation of the SQM seed, given that the weak interactions are too slow to change it significantly. However, the composition fluctuates around its average equilibrium values at the typical temperatures of high-energy astrophysical processes. I will address this effect by considering the local thermal fluctuations of the hadronic composition, showing that they make the formation of SQM much easier. This result is also relevant in the standard one-family and twin-star scenarios, leading to a much smaller finite-size effect-mediated delay in the deconfinement than the ones estimated in previous works. Moreover, I will discuss the role of color-superconductivity in such a phenomenon. Finally, some possible associated phenomenology, such as the explosion of massive progenitor stars, will be presented.

Color Superconductivity in two-flavor QCD and its role in neutron stars (Dense QCD)

• Bernd-Jochen Schaefer

Room: Witold Swida Lecture Hall We investigate color superconductivity in two-flavor QCD and its impact on the equation of state for dense matter in neutron stars. Employing a quark-meson-diquark model truncation within the functional renormalization group framework, we analyze the formation of diquark and chiral condensates and characterize the resulting superconducting phase at high baryon densities. Our results provide insights into the phase structure of QCD under extreme conditions and suggest that color-superconducting quark matter may play a significant role in the composition of neutron star cores and other astrophysical phenomena.

Particle-theory input for neutron-star physics (Dense QCD)

• Aleksi Vuorinen (University of Helsinki)

Room: Witold Swida Lecture Hall Different parts of neutron stars (NSs) probe all fundemantal interactions of particle physics: the strong nuclear force is behind the bulk thermodynamic properties of their constituent matter, electroweak interactions are needed to describe equilibration and transport phenomena during binary NS mergers, and information on QED scattering processes at ultrastrong magnetic fields is required to properly understand the NS magnetosphere. Corresponding to these three topics, I will discuss three topical problems within NS physics: inferring a model-independent equation of state for NS matter, determining the bulk viscosity of quark matter, and accounting for the full effects of background magnetic fields in QED scattering processes using Schwinger's proper time formalism. A common denominator in all cases is the need to perform high-precision calculations using resummed (thermal) field theory, the recent results of which I will showcase within each of the three categories.

Nonequilibrium dynamics in QCD and Holography (Non-equilibrium dynamics/non-equilibrium aspects of QCD)

• Matthias Kaminski

Room: Witold Swida Lecture Hall The plasma generated in heavy ion collisions goes through different phases in its time evolution. While early times right after the collision are governed by far-from equilibrium dynamics, later times are believed to be well described by near-equilibrium dynamics. While these regimes of nonequilibrium are complicated to describe within QCD, effective descriptions such as hydrodynamics provide a viable approach. In addition, holographic descriptions allow access to the full nonequilibrium dynamics. In this presentation, we review three examples of such hydrodynamic approaches and corresponding holographic descriptions: 1) nonequilibrium shear viscosity, 2) propagation of nonequilibrium sound waves, and 3) the nonequilibrium chiral magnetic effect.

Out-of-equilibrium CME conductivity from lattice QCD (Lattice QCD)

• Eduardo Garnacho Velasco

Room: Witold Swida Lecture Hall In this talk, we present a study of the out-of-equilibrium CME conductivity via Kubo formulas using lattice QCD simulations. We present an analytic study of the CME Kubo formula in 1-loop perturbation theory, which sheds light on the nature of the CME in a system of non-interacting fermions. We then determine the Euclidean correlator as a function of the (imaginary) time using full dynamical simulations at the physical point, analyzing some of its characteristics and how they can be directly related to the conductivity. Finally, we extract the spectral function from the Euclidean correlator using well-known spectral reconstruction methods, discussing the temperature dependence of the determined conductivity in QCD.

Isotropization in heavy ion collisions in the 2PI formalism (Non-equilibrium dynamics/non-equilibrium aspects of QCD)

• Francois Gelis

Room: Witold Swida Lecture Hall An important question in heavy-ion collisions is how the initial far-from-equilibrium medium evolves and thermalizes while it undergoes a rapid longitudinal expansion. In this presentation, I will show how to use the two-particle irreducible (2PI) effective action to address this question, focusing on phi^4 scalar theory truncated at three loops. I will present numerical results for quantities such as the occupation number, the thermal mass, and the number density.

Glasma as a fluid (Fluid dynamics and collectivity)

• Stanislaw Mrowczynski

Room: Witold Swida Lecture Hall At the earliest stage of ultrarelativistic heavy-ion collisions the produced matter is a highly populated system of gluons called glasma which can be approximately described in terms of classical chromodynamic fields. Although the system's dynamics is governed by Yang-Mills equations, glasma evolution is shown to strongly resemble hydrodynamic behavior. We discuss glasma collective flow and we check how well the glasma energy-momentum tensor can be represented by a hydrodynamic energy-momentum tensor. Keeping in mind the recently discovered polarization phenomena of hyperons and vector mesons, we study the global angular momentum of the glasma, which is perpendicular to the reaction plane, and the local angular momentum along the beam direction. The vorticity of the glasma fluid is discussed as well.

Exploring QCD phase diagram at finite temperature and density via lattice calculations (Lattice QCD)

• Sipaz Sharma (Technische Universitat Munchen)

Room: Witold Swida Lecture Hall In recent years, lattice QCD calculations have significantly advanced our understanding of the QCD phase diagram at finite temperature and baryon density. In this talk, I will discuss the QCD equation of state at finite baryon chemical potential, with particular emphasis on the Taylor expansion method used to extend lattice results to nonzero chemical potentials. I will highlight the role of cumulants of conserved charge fluctuations as essential probes of the properties of hot and dense QCD matter. Additionally, I will review the current understanding of the curvature of the chiral phase transition lines as functions of the baryon chemical potential, both for physical light and strange quark masses and in the (2+1)-flavor chiral limit. Throughout the talk, I will continuously draw connections between these lattice results and the experimental efforts to locate the QCD critical point.

Nonlocal Operators on the Lattice for the Higgs-Confinement Phase Transition (Phase diagram and critical behaviour in QCD/QFT)

• Yusuke Shimada (U Tokyo)

Room: Witold Swida Lecture Hall In this talk, I will report our recent achievements based on ref. [1]. **Quark-Hadron Continuity on QCD and Gauge-Higgs model** A big puzzle of QCD and its phase diagram is whether quark matter at high density and hadronic matter are smoothly connected or separated by the phase transition. On the one hand, since both phases have the same symmetry, the continuity between hadrons and quark matter (**quark-hadron continuity**) seems to be realized. On the other hand, intuitively, the two phases are distinguished by the presence or absence of the color charge, and the phase transition seems to exist. To settle this issue, the color charge needs to be calculated numerically from **nonlocal operators** such as the Wilson loop and **Aharonov--Bohm phase**. We have examined the usefulness of nonlocal operators in analyzing the behavior of the phase transition by using an Abelian Higgs model as a testing ground. This model does not have the sign problem and has the same continuity problem as QCD if the gauge group is extended. **Phase Diagram of Abelian Higgs model** We calculated the **Polyakov loop** and **'t Hooft loop** of the charge-2 Abelian Higgs model on the lattice. These nonlocal operators are order parameters of confinement. We confirmed the phase transition lines of the theory and obtained the phase diagram. Furthermore, the Higgs and Coulomb deconfined phases were distinguished. **Aharonov--Bohm Phase on Lattice** We also calculated the Wilson loop and the **Aharonov--Bohm phase** on the lattice. A vortex of the Higgs field is generated, and several size loops are considered. We found the first-order phase transition on the Wilson loop, but phase values become difficult to read at the confined phase. [1] Yusuke Shimada and Arata Yamamoto, "Analyzing the Higgs–Confinement Transition with Nonlocal Operators on the Lattice", *Progress of Theoretical and Experimental Physics*, Volume 2025, Issue 4, April 2025.

Approaching the continuum with anisotropic lattice thermodynamics (Lattice QCD)

• Jon-Ivar Skullerud

Room: Witold Swida Lecture Hall The FASTSUM collaboration has a long-standing programme of using anisotropic lattice QCD to investigate strong interaction thermodynamics, and in particular spectral quantities. Here we present first results from our new ensemble which has a temporal lattice spacing $a_\tau=15$ am and anisotropy $a_s/a_\tau=7$, giving unprecedented resolution in the temporal direction. We show results for the chiral transition, parity doubling of baryons, and heavy quarkonium, and compare them with earlier results on coarser lattices.

Equation of state of finite-density 2color QCD at lower temperature from lattice simulations (Lattice QCD)

• Etsuko Itou

Room: Witold Swida Lecture Hall We investigate the equation of state (EoS) of two-color QCD at low temperature and high density using lattice Monte Carlo simulations. Our results show that the speed of sound exceeds the relativistic limit (c^2_s/c^2= 1/3) after the BEC–BCS crossover in the superfluid phase. We also discuss the temperature dependence of the EoS within the superfluid phase. This talk is based on arXiv:2207.01253, 2405.20566, and work in progress.

Flux tube structure in finite temperature QCD (Lattice QCD)

• Volodymyr Chelnokov (Institut für Theoretische Physik, Goethe Universität)

Room: Witold Swida Lecture Hall We present a study of the structure of the chromoelectrical field created by a static quark-antiquark pair in lattice QCD with 2+1 flavours of dynamical quarks, where the quark masses are set to their physical values. The analysis covers a wide range of temperatures both above and below the chiral crossover, and explores varying quark-antiquark distances, with the aim of identifying signals of deconfinement and string breaking in the field structure. To this end we apply the zero-curl perturbative field subtraction method, developed in our earlier studies of pure gauge SU(3) theory and of full QCD at zero temperature.

Topological observables and $\theta$ dependence in high temperature QCD from lattice simulations (Lattice QCD)

• Andrey Kotov

Room: Witold Swida Lecture Hall We discuss topology in Quantum Chromodynamics at high temperatures $T~\gtrsim~180$ MeV obtained from lattice simulations. Our setup consists of $N_f=2+1+1$ Wilson twisted mass fermions with physical quark masses and results are extrapolated to the continuum limit. We compare the results for the topological susceptibility obtained with the field-theoretic definition with those obtained from an observable constructed with the disconnected part of the chiral susceptibility, and we confirm their agreement on our range of temperatures. We also study the topological charge distribution, the next order cumulant $b_2$ and, for the first time, the Free Energy as a function of the $\theta$ angle. We find a rapid crossover to the Dilute Instanton Gas behaviour above $ T \simeq 300$ MeV for all the observables we have considered.

Equation of State of QCD with $N_f=3$ flavours up to the electroweak scale (Lattice QCD)

• Matteo Bresciani

Room: Witold Swida Lecture Hall The Equation of State of Quantum Chromodynamics with $N_f=3$ flavours is determined non-perturbatively with a precision of about 0.5%-1.0% in the range of temperatures between 3 GeV and 165 GeV. The computation is carried out by means of numerical simulations of the gauge theory discretized on the lattice. At each given temperature the entropy density is computed at several lattice spacings in order to extrapolate the results to the continuum limit. The pressure and energy density are then determined by integrating the entropy density with respect to the temperature. The numerical data show a linear behaviour in the strong coupling constant squared, which points to the Stefan-Boltzmann limit at infinite temperature. They are also compatible with the known perturbative formula supplemented by higher order terms in the coupling constant, containing non-perturbative contributions. This parametrization describes well our data together with those present in the literature down to 500 MeV.

QCD in strong magnetic fields: fluctuations of conserved charges and EoS (Lattice QCD)

• Arpith Kumar (Central China Normal University)

Room: Witold Swida Lecture Hall Strong magnetic fields, attaining magnitudes comparable to the QCD scale, can significantly affect the equilibrium properties and bulk thermodynamics of strongly interacting matter. While such strong fields are expected in off-central heavy-ion collisions, directly measuring their imprints, such as chiral magnetic effects, remains challenging. In this talk, we propose the baryon-electric charge correlations $\chi^{\rm BQ}_{11}$ and the chemical potential ratio $\mu_{\rm Q}/\mu_{\rm B}$ as effective probes for magnetic fields in heavy-ion collisions [1, 2]. This is based on our (2+1)-flavor QCD lattice simulations at physical pion masses, focusing on the pseudocritical temperature regime. To bridge theoretical predictions with experiment, we construct proxy observables and implement systematic kinematic cuts within the hadron resonance gas (HRG) framework to emulate detector acceptances of the STAR and ALICE experiments. Extending our investigation into the QCD equation of state (EoS), we examine the behavior of bulk thermodynamic expansion coefficients under extremely strong magnetic fields [3]. Our current $eB$ window spans up to $0.8~{\rm GeV}^2 \sim 45 m_{\pi}^2$, revealing intriguing non-monotonic behaviors in the $T$-$eB$ dependence of pressure and number density, which are absent at vanishing and relatively weak magnetic fields. The HRG model and the magnetized ideal gas serve as useful references for the low-$T$, weak-$eB$ and high-$T$, strong-$eB$ regimes, respectively. Furthermore, we also discuss higher-order thermodynamic observables, energy and entropy density, highlighting the significant impact of magnetic fields. [1] Heng-Tong Ding, Jin-Biao Gu, Arpith Kumar, Sheng-Tai Li, Jun-Hong Liu, Phys. Rev. Lett. 132, 201903 (2024) [2] Heng-Tong Ding, Jin-Biao Gu, Arpith Kumar, Sheng-Tai Li, arXiv:2503.18467 [hep-lat] [3] Heng-Tong Ding, Jin-Biao Gu, Arpith Kumar, Sheng-Tai Li, PoS LATTICE2024 (2025) 175, and work in progress

Announcement of the XXIVth Conference on Quark Confinement and the Hadron Spectrum

• David Blaschke

Room: Witold Swida Lecture Hall

How to explain the unreasonable success of hydrodynamics in heavy-ion collisions (Fluid dynamics and collectivity)

• Leonardo Tinti (Uniwersytet Jana Kochanowskiego w Kielcach)

Room: Witold Swida Lecture Hall Relativistic hydrodynamics is the covariant generalization of a classical (non-relativistic, non-quantum) model. Recent developments in the field can explain how it is possible for relativistic hydrodynamics to be predictive in a very small system, in which quantum effects like diffraction and discrete (rather than continuous) spin states are not suppressed. This can be particularly surprising, if one considers that the gradients, as well as the non-ideal corrections are large, especially during the initial stages. This is in contrast to the classical requirements for the applicability of viscous hydrodynamics.

First Demonstration that Quark-Gluon Plasma has a Nonzero Resolution Length (Phenomenology & Experiment)

• Arjun Srinivasan Kudinoor (Massachusetts Institute of Technology)

Room: Witold Swida Lecture Hall We begin by using Hybrid Model calculations to reproduce experimental results published by ATLAS in 2023 on $R_{AA}$ for $R=1$ jets in Pb+Pb collisions. These jets are identified by first reconstructing anti-$k_t$ $R=0.2$ subjets and then re-clustering them. Following ATLAS, we investigate how $R_{AA}$ for these jets depends on the angle between the two subjets involved in the final clustering step of the $R=1$ jet. We also study the dependence of $R_{AA}$ for these jets on the resolution length of quark-gluon plasma (QGP), defined such that the QGP-medium can only resolve partons in a jet shower that are separated by more than this length. We demonstrate that this measurement pioneered by ATLAS rules out any picture in which an entire parton shower loses energy coherently as a single entity. We further use this setup of $R=1$ jets reclustered from $R=0.2$ skinny subjets to evaluate the soft drop angle, notated as $dR_{12}$, using all charged-particle tracks that are associated with each $R=1$ jet. Following another ATLAS measurement published in 2025, we use Hybrid Model calculations to investigate the dependence of $R=1$ jet $R_{AA}$ on the soft drop angle $dR_{12}$. We demonstrate that this new measurement from ATLAS rules out any picture in which all partons in a shower lose energy fully incoherently. Therefore, our analysis demonstrates, for the first time, that the QGP has a finite nonzero resolution length.

Signatures of local acceleration of quark-gluon plasma in the dilepton production (QCD in external fields)

• Aritra Bandyopadhyay

Room: Witold Swida Lecture Hall Dilepton emissions represent a key probe for characterising the Quark-Gluon Plasma (QGP). A central role in computing dilepton yields is played by the imaginary part of the electromagnetic current-current correlation, or equivalently, of the photon polarisation tensor [1]. In this work, we investigate the influence of local acceleration on dilepton production. We compute this quantity in a thermal medium subject to acceleration. We assume the acceleration is sufficiently small so that it can be treated as a perturbation. We employ the thermal Dirac propagator in an accelerated frame, recently formulated within the imaginary-time formalism in [2]. Using a small acceleration expansion, we evaluate the imaginary part of the photon polarization tensor. Our perturbative results are then compared with the case of vanishing acceleration, allowing us to clearly isolate and identify the effects introduced by local acceleration. Keywords: Dilepton production rate, Accelerated medium, Quark gluon plasma. References: [1] L. D. McLerran and T. Toimela, Phys. Rev. D 31, 545 (1985). [2] V. E. Ambruș and M. N. Chernodub, Phys. Lett. B 855, 138757 (2024).

Guiding center hydrodynamics with spin (Dense QCD)

• Rajeev Singh (West University of Timisoara, Romania)

Room: Witold Swida Lecture Hall We build upon the recently formulated guiding-center kinetic theory for guiding-center plasma by incorporating spin degrees of freedom in the presence of electromagnetic fields. This approach yields a streamlined set of equations for guiding-center ideal hydrodynamics with spin--fewer than those in traditional spin hydrodynamics--owing to a restriction on motion perpendicular to the magnetic field. We propose that this framework offers a promising tool to comprehending how matter behaves under the influence of intense magnetic fields, such as spin polarization effects observed in experimental measurements.

Results on magnetohydrodynamics simulations with BHAC-QGP (Fluid dynamics and collectivity)

• Ashutosh Dash

Room: Witold Swida Lecture Hall We present BHAC-QGP, a numerical framework for simulating the evolution of quark-gluon plasma (QGP) in relativistic heavy-ion collisions under strong magnetic fields. Based on the general-relativistic magnetohydrodynamics code BHAC, originally developed for astrophysical applications, BHAC-QGP incorporates finite electrical conductivity and uses Adaptive Mesh Refinement (AMR) to capture QGP dynamics across multiple scales with high precision. Simulations of Au+Au collisions at RHIC energies demonstrate excellent agreement with established results and improved resolution of electromagnetic effects. Furthermore, we show that causality and a consistent treatment of Ohm’s law are essential for modeling early-time electromagnetic fields in such collisions. In particular, finite electrical conductivity plays a critical role in describing the directed flow recently measured by the STAR experiment. These features make BHAC-QGP a valuable tool for studying magneto-hydrodynamic effects in heavy-ion collisions.

Universal Real-Time Dynamics in Diffusive Systems (Fluid dynamics and collectivity)

• Navid Abbasi

Room: Witold Swida Lecture Hall The search for the QCD critical point in heavy-ion collisions requires the inclusion of fluctuations in hydrodynamic simulations of Quark-Gluon Plasma (QGP). Schwinger-Keldysh Effective Field Theory (SK-EFT) systematically incorporates these fluctuations, extending beyond classical hydrodynamics, which accounts only for dissipation. After a brief review of SK-EFT, we discuss how linear response results obtained from experimental data can be incorporated into this framework to predict nonlinear response in diffusive systems. This is closely related to the emergence of a universal scaling function in the structure of real-time correlation functions. Furthermore, we argue that this universality persists even in a causal diffusive system. Finally, we comment on the relevance of these results to QGP dynamics near the QCD critical point.

Novel early time hydrodynamic attractor in ultracold quantum gases (Fluid dynamics and collectivity)

• Clemens Werthmann (Ghent University)

Room: Witold Swida Lecture Hall Hydrodynamic attractors were first studied in conformal Bjorken flow in the context of heavy ion collisions to discuss how the fireball of strongly interacting matter approaches hydrodynamic behaviour. Recently, a hydrodynamic attractor has also been constructed for the case of ultracold atom gases, meaning it now has a second real world application and may soon be measured directly in experiment. Interestingly, the newly discovered instance of this behaviour has some significant differences to Bjorken flow: it appears in a deeply nonrelativistic, nonconformal and isotropically contracting system. This suggests that the phenomenon is universal to all dynamical systems where equilibration competes with expansion. Through more general studies of how the behaviour plays out in different situations, one might eventually be able to construct an effective dynamical description that would supercede hydrodynamics. We take a first step in this direction by examining continuous modifications of conformal Bjorken flow. In some scenarios, the competition between equilibration and expansion can play out very differently, even leading to long term stable states of coexistence. We discuss how concepts from the known case transfer to the new case, for example the scaling variable and the distinction between the early time and late time attractors, and propose a modification of how to identify the behaviour that we believe to be a more accurate representation.