Finite temperature expansion of the dense-matter equation of state, talk by GS Débora Mroczek

Realistic simulations of neutron star mergers require an equation of state that reflects the equilibrium properties of the matter created in these systems. We presented a new framework for generating equations of state for neutron star mergers based on the equation of state of the cold, neutron-rich matter present in isolated neutron stars. We also suggested new thermodynamic quantities that can inform the equation of state in these systems. These new quantities can be calculated from theoretical models or directly inferred by experimental data from low to intermediate energy heavy-ion collisions. 

Link to Débora's talk

Dependence of the bulk viscosity of neutron star matter on the nuclear symmetry energy, talk by GS Yumu Yang

Neutron star merger/inspiral is a dynamical process that requires an understanding of the transport coefficients. For matter composed of neutrons, protons, and electrons, we presented, given the reaction rates, how the weak-interaction-driven bulk viscosity and the bulk relaxation time can be fully determined by the experimentally constrained observable called the nuclear symmetry energy and the symmetry energy slope. We showed that variations in the slope within the current experimental uncertainties can lead to orders-of-magnitude differences in the bulk viscosity, which suggests that astrophysical observations in inspirals may help constrain nuclear symmetry properties. We also showed that nuclear matter near saturation densities in neutron star inspirals may exhibit elastic behavior. 

Link to Yumu's talk

Does the coordinate system matter for the Beam Energy Scan?, poster by GS Kevin P. Pala*

Due to the high Lorentz factor, in high-energy collisions, the nuclei become approximately flat, making $2+1D$ simulations with hyperbolic coordinates well-suited. At lower energies, however, the longitudinal radius becomes comparable to the transverse radius, and the passing time increases—raising the question of whether hyperbolic coordinates remain optimal in this regime. In this study, we perform a hybrid-hydrodynamic simulation using the newly developed CCAKE hydrodynamics framework, varying the coordinate system (Cartesian vs. hyperbolic) and the initialization method. For each coordinate choice, we compare two initialization strategies: one using the geometric overlap time and another using dynamical initialization (i.e., initialization via source terms). Simulations are conducted at two beam energies, $\sqrt{s_{NN}} = 19.6$ GeV and $\sqrt{s_{NN}} = 7.7$ GeV. We find that the impact on observables is minimal—especially at mid-rapidity—but that computational performance differs: Cartesian coordinates yield faster runtimes under dynamical initialization, while hyperbolic coordinates are more efficient when using the overlap time.

(*) Kevin is a visiting Grad Student from the University of São Paulo on a fellowship for a year from FAPESP. 

Link to Kevin's  poster

Transport Coefficients from pQCD to the Hadron Resonance Gas at finite BSQ densities, talk[*] by PDRA Isabella Danhoni

The Quark Gluon Plasma is known to be the closest to a perfect fluid known in nature. This has been shown through model-to-data comparisons from heavy-ion collisions at vanishing densities. At large baryon densities, significantly less is known since this cannot be calculated using lattice QCD. Within heavy-ion collisions, there are three conserved charges: baryon number (B), strangeness (S), and electric charge (Q). Here, we presented the first calculation of shear viscosity as a function of temperature and multiple conserved charges. We perform these calculations in two limits: using leading log order perturbative QCD for the hot deconfined region of the QCD phase diagram and an excluded-volume hadron resonance gas with the state-of-the-art list of resonances for the cold confined region. We then developed a framework that interpolates these two limits to calculate shear viscosity across a wide range of finite BSQ densities, considering a crossover transition between these two regimes. We observed that the two regimes have very different BSQ densities dependence, leading to a non-trivial shear viscosity at finite densities. 

Additionally, we have also studied the convergence of the perturbative series for perturbative QCD calculations. It is known from next-to-leading order calculations of shear viscosity at vanishing baryon densities that the perturbative series has convergence problems caused by strong gluon mutual interactions. Here, we presented the extension of these next-to-leading order calculations to the high baryon density region. In this regime, the physics is dominated by quarks, and the contribution from gluons should be less relevant, improving the convergence of the perturbative series. We showed in this work that although the series does not converge at experimentally achievable T and μ, the convergence is better at μ > T than at μ = 0. 

[*] Isabella's contribution received an EPJ award to support her participation in the Conference.

Link to Isabella's talk

The ATLAS Transition Radiation Tracker and Zero Degree Calorimeter: the progress on triggering on ultraperipheral processes, talk by GS Matthew Hoppesch 

Large semi-real photon fluxes are created around the highly relativistic colliding ion species in heavy ion collisions at the LHC.  These photons can either interact directly with the ions, via photo-nuclear processes, or interact with other photons, via photon-photon fusion. The ATLAS detector can measure these so-called ultra-peripheral collisions (UPCs) and use them to study fundamental questions.  Two crucial detector upgrades improve the ability of ATLAS to identify collisions of interest and save them for further analysis. The Transition Radiation Tracker has an upgraded FastOR trigger system that allows ATLAS to trigger on collisions that produce only a few charged particles, including exclusive charmonium states like the J/Psi.  Next, the Zero-Degree Calorimeter (ZDC) has an improved ability to identify UPCs that, in addition to an interesting ultra-peripheral process, include an interaction with a very low-energy photon.  This is important for understanding the impact parameter dependence of UPCs.  Finally, plans were presented for the High-Luminosity ZDC to be used in Run 4 of the LHC.  This project is headed by UIUC and is being developed jointly by ATLAS and CMS.

Link to Matthew's talk

Nonlinear Causality and Strong Hyperbolicity of baryon-rich Israel-Stewart
Hydrodynamics, talk by GS Ian Cordeiro

Relativistic theories of viscous hydrodynamics continue to provide successful descriptions of a diverse range of phenomena, including the quark gluon plasma (QGP) phase of heavy-ions, neutron star mergers and black hole accretion disks. One of the most historically successful theories of relativistic hydrodynamics known as Israel-Stewart theory enforces the second law of thermodynamics on an entropy current that is expanded to second order in dissipative currents. In addition to benefiting from an immediate analogy to thermodynamics, upon expanding the dynamic variables to linear fluctuations from global equilibrium, one can show that they are causal, in the sense that information is not allowed to propagate faster than the speed of light, and locally well-posed, in the sense that unique solutions exist continuously with the prescribed initial data for some finite timescale within some set of bounds. However, very little has been known about the fully-nonlinear regime due in part to its 24 independent coupled partial differential equations including various conservation laws and Einstein’s equations. Nonlinear effects are expected to be significant in heavy-ion collisions, particularly at early times, as well as in black hole accretion disks, as these systems are often far from equilibrium. 

Here, we present the first set of simultaneously necessary and sufficient conditions that define the exact region of nonlinear causality for Israel-Stewart-like theories with a bulk and shear viscous contribution. These bounds therefore constrain a significantly broad class of hydrodynamic theories whose transport coefficients are allowed to depend on dissipation, and are allowed to include a baryon current, which is especially relevant towards astrophysical phenomena. Furthermore, we provide sufficient conditions for strong hyperbolicity for the first time, which guarantees local well-posedness of solutions for a very general class of solutions. These conditions are algebraic inequalities that explicitly limit all dynamics of the system, including dissipation, and make no assumption on the spacetime metric or equation of state.

Link to Ian's talk

Probing initial state effects in p+Pb collisions via forward transverse energy and spectator neutron measurements with the ATLAS detector, Poster by Matthew Hoppesch 

Proton-nucleus collisions at LHC energies represent an incredible source of information to further understand the building blocks of matter. The proton internal structure fluctuates, but when it collides with the nucleus, it does it in a practically frozen configuration because of the very short transit time. By measuring dijet events using the full acceptance of the ATLAS calorimeter, we have explored how the proton configuration impacts both forward transverse energy and forward neutron measurements. Our results, included in a recent ATLAS paper just submitted for publication (see arXiv 2504.02638), represent an unprecedented input towards the understanding of the proton’s size fluctuations and the associated changes in its interaction strength. 

Link to Matthew's poster

Measurement of R=0.4 inclusive jet cross-section in p+p collisions at √s = 200 GeV with the sPHENIX Detector,  Poster by Apurva Narde 

The sPHENIX experiment is a next-generation collider detector at the Relativistic Heavy Ion Collider (RHIC), specifically designed to study rare jet and heavy flavor probes of the Quark-Gluon Plasma (QGP). It features large-acceptance electromagnetic and hadronic calorimeter systems, including the first hadronic calorimeter (HCal) at mid-rapidity at RHIC, as well as a high-rate data acquisition and trigger system optimized for capturing high transverse momentum (high-$p_T$) jets. During RHIC Run-24, sPHENIX collected 107 pb⁻¹ of proton-proton collision data at $\sqrt{s} = 200$ GeV using an efficient high-$p_T$ jet trigger, representing a substantial increase in the product of luminosity and acceptance compared to prior measurements at this energy. This poster presents a measurement of the inclusive jet cross-section using the full sPHENIX calorimeter system. Jets, which are collimated sprays of particles originating from high-energy quarks and gluons, are reconstructed using the anti-$k_t$ algorithm with a radius parameter R=0.4. Measuring the jet cross-section in proton-proton collisions at RHIC, especially in comparison to similar measurements at the LHC, provides valuable insight into the proton's parton distribution function and the relative contributions of quark and gluon-initiated jets. Furthermore, this proton-proton measurement serves as an essential perturbative Quantum Chromodynamics baseline for interpreting future measurements in heavy-ion collisions, where parton energy loss in the QGP medium can be studied via jet suppression. The results demonstrate both the precision and capability of the sPHENIX detector to carry out its jet physics program.

Link to Apurva's poster