Welcome Address

• Bjoern Schenke

Modeling the Little Bang: 50 years in 20 minutes

• Ulrich Heinz (The Ohio State University)

The 60th birthdays of two major contributors to our present understanding of collective dynamics of hot QCD matter in relativistic heavy-ion collisions are an opportunity for us to look back how this understanding was achieved. Appropriately, I will focus on relativistic kinetic transport theory and fluid dynamics where Dirk and Carsten left their deepest permanent footprints. The talk will build up to a recent development in my own group, Maximum Entropy Hydrodynamics, that aims to extend the applicability of both types of approaches into the far-off-equilibrium domain that is being probed in high-energy collisions between small nuclei as well as during the early evolution stages of the matter created in collisions between large nuclei. Some open questions for the second 60 years of Carsten's and Dirk's careers will be briefly addressed.

Light from the Little Big Bangs

• Charles Gale (McGill University)

Nuclear Matter Equation of State, QGP Dynamics & Jets

• John Harris (Yale University)

The quest to determine the nuclear matter equation of state (NMEOS) and its properties has been a central focus of our field for over fifty years. Numerous theoretical predictions regarding the behavior of the NMEOS have motivated major experimental efforts aimed at revealing its properties over a range of densities and temperature. Both Carsten Greiner and Dirk Rischke have made valuable contributions to this effort, which I will briefly point out. But with limited time, I’ll focus more on a few of Dirk Rischke’s contributions that overlap with experiments in which I’ve been directly involved.

Dissociation and Regeneration of Charmonia within microscopic Langevin simulations

• Naomi Oei

We present a microscopic model to study the formation, as well as dissociation and recombination processes of charmonium states in the quark gluon plasma. In this classical approach, heavy quarks are described as Brownian particles in a background medium of light constituents. The heavy-quark dynamics are modelled by a Fokker-Planck equation with constant transport coefficients, which is then implemented through relativistic Langevin simulations. The heavy quarks interact classically via a Coulomb-like screened potential, enabling the formation of charmonium states. These bound states may dissociate due to screening effects of the potential and through scatterings with plasma particles. Using box simulations at fixed temperature and volume, we demonstrate the full equilibration of the system with equilibrium charmonium yields consistent with the Statistical Hadronization Model. In order to model the phenomenology of a heavy-ion collision, we then implement a dynamical description, where the evolution of the expanding medium is parametrized by a boost invariant fireball. This allows us to study the elliptic flow and nuclear modification factor of the charm and anticharm quarks, as well as of charmonia, at RHIC and LHC energy, and compare the results to experimental data.

Shakhov collision model in relativistic kinetic theory

• Victor Ambrus (West University of Timișoara)

We present a systematic method to construct the Shakhov collision term, as an extension of the relaxation time approximation (RTA) of the Boltzmann colllision term ([1]). Our construction is based on the collision matrix in the method of moments and allows an increasing number of first- and second-order transport coefficients to be separately tuned ([2]). The talk is focussed on the ultrarelativistic gas of hard spheres, for which the transport coefficients are known ([3]). Numerical simulations show that good agreement with the first-principles BAMPS results reported by DNBMXRG ([4]) can be achieved only if (almost) all second-order transport coefficients are correctly recovered ([2]) (see Figure). ![Comparison between BAMPS data (black circles) for the Sod shock tube problem, the Anderson-Witting RTA model (AW) and various Shakhov models ($N_1$, $N_2$, $s_1$, $s_2$) with vector and tensor collision matrices of sizes $(N_1 + s_1)^2$ and $(N_2+s_2)^2$, respectively.][5] ([1]) VEA, E. Molnar, Phys. Lett. B 855 (2024) 138795. DOI: [10.1016/j.physletb.2024.138795][1]. ([2]) VEA, D. Wagner, Phys. Rev. D 110 (2024) 056002. DOI: [10.1103/PhysRevD.110.056002][2]. ([3]) D. Wagner, VEA, E. Molnar, Phys. Rev. D 109 (2024) 056018. DOI: [10.1103/PhysRevD.109.056018][3]. ([4]) G. S. Denicol et al, Phys. Rev. D 89 (2014) 074005. DOI: [10.1103/PhysRevD.89.074005][4]. [1]: https://doi.org/10.1016/j.physletb.2024.138795 [2]: https://doi.org/10.1103/PhysRevD.110.056002 [3]: https://doi.org/10.1103/PhysRevD.109.056018. [4]: https://doi.org/10.1103/PhysRevD.89.074005 [5]: http://quasar.physics.uvt.ro/~victor/KQGP/results/case1_q.pdf

Polarization phenomena in heavy-ion collisions: from initial to final state

• Enrico Speranza (CERN)

In recent years, there has been a growing interest in the study of spin-polarization phenomena in heavy-ion collisions. Most current approaches assume that polarization is a final-state effect that is induced by specific fluid properties such as vorticity. In this talk, I will discuss an alternative mechanism: a spin density is present in the initial state of the collisions, as it is carried by the nucleons within the colliding nuclei. This initial spin density induces a net polarization of the QCD fireball, whose event-by-event fluctuations can be probed through the spin correlation of final-state hyperons. The discovery of this phenomenon would have profound implications on our understanding of spin in relativistic hydrodynamics.

Exact solutions for the moments of the binary collision integral and its relation to the relaxation-time approximation in leading-order anisotropic fluid dynamics

• Etele Molnar (University of Wroclaw)

We compute the moments of the nonlinear binary collision integral in the ultrarelativistic hard-sphere approximation for an arbitrary anisotropic distribution function in the local rest frame. The corresponding moments of the binary collision integral are obtained in terms of quadratic products of different moments of the anisotropic distribution function and couple to a well defined set of lower-order moments. To illustrate these results we compare the moments of the binary collision integral to the moments of the widely used relaxation-time approximation of Anderson and Witting in case of a spheroidal distribution function. We found that in an expanding system the nonlinear Boltzmann collision term leads to twice slower equilibration than the relaxation-time approximation. Furthermore we also show that including two dynamical moments helps to resolve the ambiguity which additional moment of the Boltzmann equation to choose to close the conservation laws.

Analytic structure of stress-energy tensor correlation functions and new Kubo formulae

• Juhee Hong (Yonsei University)
• Alina Czajka (National Centre for Nuclear Research)
• Sangyong Jeon (McGill University)

Utilizing the gravitational Ward identities and gravity-hydrodynamics results, we clarify the analytic structure of all stress-energy correlation functions in the small frequency and the small wave-number limit. From the correlation functions, we obtain several new Kubo formulae for the viscosities. Interestingly, these new Kubo formulae are all obtained with the zero frequency limit taken first. We also clarify why in the usual ladder diagram calculation of viscosities, it does not seem to matter whether the zero frequency limit is taken first or the zero wave-number limit is taken first.

Local symmetries of the effective field theory of hydrodynamics

• Giorgio Torrieri (Unicamp)

We examine the length scales usually associated with the applicability of hydrodynamics as an effective theory in the light of the observation of hydrodynamic behavior in small systems. We argue that the explanation of such behavior might reside in the requirement of local equivalence between fluctuations and dissipative dynamics. In a way somewhat analogous to the equivalence of gravity and acceleration in general relativity, this leads to a theory which is generally covariant and non-perturbative in fluctuations. We argue that the scale separation in this theory is very different from the Knudsen number, thus explaining how strongly coupled systems with just a few degrees of freedom are characterized by an ensemble of trajectories whose average could be close to that of an ideal fluid. Based on https://arxiv.org/abs/2504.17152

The Climate Apocalypse is Coming! How can Nuclear Physicists Help?

• Wolfgang Bauer (Michigan State University)

The last couple of years were the hottest in recorded history, and all climate models under-predicted the severity this global temperature change. These data indicate that exponential feedback loops in Earth's climate are beginning to dominate, eventually causing Earth to become a second Venus. A necessary, but perhaps not sufficient, condition to avoid this catastrophe is to terminate combustion of fossil fuels. Renewable power sources alone may not be enough to substitute quickly enough. This is where new nuclear fission and fusion reactor designs need to step in.

From QGP and DCC to nanofusion - memories of collaboration with Frankfurt

• Tamas Biro (Wigner RCP)

Hydrodynamics of spin and conformal currents

• Xu-Guang Huang (Fudan University)

Recently, there have been a lot of discussions about the (pseudo-)hydrodynamic theory of spin transport, termed spin hydrodynamics. We discuss its extension to include conformal symmetries, and compare the resultant theory with the known non-relativistic microstretch fluid mechanics.

Spin alignment from gluon polarization

• Mei Huang (University of Chinese Academy of Sciences(UCAS))

The spin-1 component of massless gluon will form condensation under rotation thus induce gluon polarization, which can explain the puzzle of enhancement of the phase transition temperature under rotation, and the induced spin alignment of vector mesons is also in agreement with experiment data.

Spin physics in heavy ion collisions

• Qun Wang (University of Science and Technology of China)

Polarization has opened a new physics chapter in relativistic heavy-ion collisions. Since the first prediction and experimental observation of global spin polarization, a lot of progress has been made in understanding its features, both at experimental and theoretical level. In this talk, I will give a brief overview on the recent advances in this field.

Advancements in relativistic spin hydrodynamics

• David Wagner (Florence University)

For the past years, different observables related to the spin polarization of hadrons such as $\Lambda$-Baryons have continued to attract attention and pose theoretical challenges. While some measurements (such as the global polarization) can be reproduced by assuming the spins of the particles to be equilibrated, more differential observables (such as the local polarization) remain a subject of active discussion. In order to account for the finite duration of the process of spin relaxation, i.e., the approach of the spin potential to the thermal vorticity, a theory of relativistic spin hydrodynamics can be used. Similar to the Bloch equations that describe the (nonrelativistic) dynamics of magnetization, such a theory has to provide evolution equations for the spin potential, which remains a dynamical quantity. I will showcase the construction of this theoretical framework over the past years. Starting from the equations of quantum field theory, an intermediate quantum-kinetic framework is developed, which subsequently produces the equations of dissipative spin hydrodynamics in the appropriate limit. Finally, the equations are solved numerically and are shown to be able to reproduce the experimental data.

Hybrid stars: equilibrium structure and bulk viscosity

• Armen Sedrakian (University of Wrocław and FIAS)

I will discuss the recent work on the construction of equilibrium hybrid stars, the emergence of twins and triplets, and their signatures, such as mass, radius, and tidal deformability. I will further discuss the bulk viscosity of hybrid star matter, which is relevant for the studies of binary neutron star mergers and their gravitational wave signals.

Neutrino emission from magnetized quark matter in compact stars

• Igor Shovkovy (Arizona State University)

We present a field-theoretic analysis of neutrino emission from dense quark matter in strong magnetic fields, focusing on conditions relevant to magnetar interiors. In the unpaired quark phase, we study both direct Urca and synchrotron processes. For the Urca process, we develop an approximation that includes Landau quantization for electrons but not quarks, which is appropriate for high-density matter. This reveals anisotropic and field-dependent emission rates with potential consequences for magnetar cooling and pulsar kicks. Although subdominant, synchrotron emission offers insight into the magnetic-field-driven dynamics of quark matter. Our results underscore the complex role of magnetic fields in neutrino production from magnetized quark cores.

NJL-model studies of color superconductivity revisited: removing cutoff artifacts using an RG consistent approach

• Michael Buballa (TU Darmstadt)

The Nambu–Jona-Lasinio (NJL) model and particularly its extension to color superconductivity is a powerful framework for explorative studies of dense, but not asymptotically dense quark matter. However, its reliability is limited by regularization artifacts that emerge near the cutoff energy scale. Unfortunately, this already affects the phenomenologically most interesting density regime, relevant for compact-star cores and mergers. In this talk, I show how these artifacts can effectively be removed by employing an approach that is based on the principle of renormalization-group (RG) consistency. Our study reveals qualitative modifications of previous NJL-model results for the phase diagram of color superconducting matter. Notably, this also resolves an old puzzle about the melting pattern of the Color-Flavor Locked phase, which is now in agreement with earlier Ginzburg-Landau predictions.

From gluon radiation to gravitational radiation: A classical double copy

• Raju Venugopalan (Brookhaven National Laboratory and Stony Brook University)

I discuss a remarkable mathematical correspondence between QCD and gravity in Regge asymptotics. We discuss the implications for gravitational radiation, on Black-Hole formation, and in turn back on QCD.

What neutron stars say about the properties of strong interaction

• György Wolf (Wigner RCP)

The existence of neutron stars provide us with a challenge and a possibility to study strong interaction, too. At the center of neutron stars the densities can reach 6-8 times the normal nuclear densities, and these densities cannot be studied in terrestrial experiments. Therefore, it provides us with constraints for the properties of the cold, dense strongly interacting matter. The existence of quark matter inside the heaviest neutron stars has been the topic of numerous recent studies, many of them suggesting that a phase transition to strongly interacting conformal matter inside neutron stars is feasible. Here we examine this hybrid star scenario using a soft and a stiff hadronic model, a constituent quark model with three quark flavours, and applying a smooth crossover transition between the two. Within a Bayesian framework, we study the effect of up-to-date constraints from neutron star observations on the equation-of-state parameters and various neutron star observables. We find, consistently with other studies, that a peak in the speed of sound, exceeding 1/3, is highly favoured by astrophysical measurements.

Chromo-dynamics of Jet-Fluid Interaction

• Xin-Nian Wang (Lawrence Berkeley National Lab. (US))

I will discuss how the quark-gluon plasma responds to jet propagation in high-energy heavy-ion collisions and what properties of QGP one can learn from these responses. I will also talk about how to measure these responses which amount to very small perturbation of sounds in a hot quark-gluon plasma.

Captain Dirk, I, and color superconductivity

• Robert Pisarski

I review work which Captain Dirk and I did on color superconductivity in the last millennium.

Femtoscopy of kaon-deuteron pairs

• Juan Torres-Rincon (Universitat de Barcelona)

Pseudogauge-invariant local equilibrium distribution

• Francesco Becattini (Università di Firenze)

The ambiguity in the choice of the spin tensor has an impact on the description of local thermodynamic equilibrium and, as a consequence, on the theoretical formulae for the spin polarization in relativistic nuclear collisions. In this talk, I will propose a new definition of the local equilibrium density operator which is free of this ambiguity, that is it is pseudo-gauge invariant. Phenomenological consequences are addressed.

Heavy Quarks as Messengers of the Little Bang

• Steffen Bass (Duke University)

I will discuss research efforts by the Duke QCD group to utilize heavy quarks as probes of the Quark-Gluon-Plasma and extract the heavy-quark transport coefficient of the QGP.

The Extended Linear Sigma Model: History, Ideas, and Lessons Learned

• Francesco Giacosa (Jan Kochanowski University)

In this talk, I recall the key moments, fundamental steps, and main considerations that led to the construction of the extended Linear Sigma Model. I compare the initial plans for this chiral model of QCD with what has been achieved, and highlight the main contributions to the development of the approach. Selected results for hadrons, both in vacuum and in medium, will also be presented.

Chiral symmetry restoration in hot and dense baryon-rich QCD matter

• Jochen Wambach (TU Darmstadt)

Rare Connections: Physics and Apfelwine

• Agnes Mocsy

The human element in science is often overlooked. But physics is more than equations and experiments, it is also apfelwine shared among colleagues, and the quiet rituals that shape community. It’s the collective dynamics of little bangs and little moments, that shape how science is lived and shared. It’s collaboration, creativity, friendships, vulnerability, insecurity. Above all it’s full of stereotype-challenging stories. We may go through the same field, but don’t experience the same field. From thermal field theory to cinema, my journey has taken me across disciplinary boundaries, while the passion for physics and for exploring new territories remains unchanged. In this talk, I’ll share how physics and artistic expression converge into powerful storytelling, and how art can open up new ways of connecting with each other and with the world. I’ll reflect on how scientific communities, including those I share with Frankfurters, laid the foundation for this work. And I’ll share excerpts from Rare Connections, a feature documentary that explores the human side of nuclear science at the Facility for Rare Isotope Beams in Michigan.

Two Conjectures about Late-Time Dynamics in Diffusive Systems

• Navid Abbasi

We now understand, at least in certain microscopic models like holography, how rapidly two-point functions hydrodynamize following a quench. Motivated by the need to go beyond Gaussian observables in the search for the QCD critical endpoint, a natural question arises: What about higher-point functions? For instance, does a three-point function hydrodynamize earlier or later than a two-point function? We address this question by proposing two conjectures concerning the hierarchical structure of hydrodynamization times in large-N, strongly coupled quantum field theories. We show that these conjectures hold in a specific class of microscopic models described by a Schwinger-Keldysh effective field theory associated with such theories.