Dániel Kincses

Event-by-event transverse momentum correlations are sensitive to the equation of state of strongly interacting matter and are expected to exhibit anomalous fluctuations in the vicinity of the QCD critical point. We report the first measurements of two-particle transverse momentum ( p T ) correlations for mid-rapidity charged particles in fixed-target Au + Au collisions at nucleon-nucleon center-of-mass energies s NN = 3.0 – 7.7 GeV , measured by the STAR experiment during the Beam Energy Scan (BES) Phase II program. The dependence of the scaled correlator on the number of participating nucleons ( N part ) is studied to test expectations from an independent-source scenario, where the correlations are expected to scale as 1 / N part . We observe a clear breakdown of the expected scaling behavior in central collisions and identify a statistically significant nonmonotonic dependence of the p T correlations on collision energy, with a significance of approximately 5 σ . In contrast, transport-model calculations and data from mid-central collisions yield significances of only 2 σ and 1.4 σ , respectively—insufficient to support a claim of nonmonotonicity. These observations provide new constraints on the equation of state at high baryon density and may be sensitive to the presence of a QCD critical point.

Particle correlations and femtoscopy are a rich subfield of high-energy physics. As the experimental data become more precise, there is an increasing need for the theoretical calculations to provide better and more general descriptions of the measurements. One of the important new directions is the investigation of the precise shape of the Bose–Einstein correlation functions utilizing Lévy-stable distributions. This work is a direct follow-up to our previous study, in which we developed a novel method for calculating Bose–Einstein correlation functions including the Coulomb final-state interaction. In this paper, we present a self-consistent generalization of the previous approach to non-spherical source functions and investigate the validity of the previously applied approximations assuming spherical symmetry. We present a software package that includes the calculation of a fully three-dimensional correlation function including the Coulomb interaction.

One of the most vital topics of today’s high-energy nuclear physics is the investigation of the nuclear structure of the collided nuclei. Recent studies at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have shown that several observables, such as the collective flow and transverse-momentum correlations of the produced particles, can be sensitive to various nuclear structure and deformation parameters. Femtoscopy, another essential tool for investigating the space-time geometry of the matter created in nuclear collisions, has not yet been widely applied to such studies. Using a multiphase transport (AMPT) model, in this Letter, it is demonstrated that the femtoscopic source parameters of pion pairs can also serve as a robust signal of unique nuclear structure. Through an analysis of Pb 208 + Ne 20 and Pb 208 + O 16 collisions at s N N = 68.5 GeV , two collision systems especially relevant to the SMOG2 program of the LHCb experiment, it is shown that a deformed initial shape can significantly affect femtoscopic source parameters. This study highlights the importance of expanding the nuclear structure investigations to femtoscopic observables and serves as a baseline for numerous possible future studies in this new direction.

The process of Lévy walk, i.e., movement patterns described by heavy-tailed random walks, plays a role in various phenomena, from chemical and microbiological systems through marine predators to climate change. Recent experiments have suggested that this phenomenon also appears in heavy-ion collisions. However, the theoretical interpretation supporting such findings is still debated. In high-energy collisions of heavy nuclei, the strongly interacting Quark Gluon Plasma is created, which, similarly to the early Universe, undergoes a rapid expansion and transition back to hadronic matter. In the subsequent expanding hadron gas, particles interact until kinetic freeze-out, when their momenta stop changing, and they freely transition toward the detectors. Measuring spatial freeze-out distributions is a crucial tool in understanding the dynamics of the created matter and the interactions among its constituents. In this paper, we introduce a three-dimensional analysis of the spatial freeze-out distribution of pions (the most abundant particles in such collisions). Utilising Monte-Carlo simulations of high-energy collisions, we show that the chain of processes ending in a final state pion has a step length distribution leading to Lévy-stable distributions. Subsequently, we show that simulated pion freeze-out distributions indeed exhibit heavy tails and can be described by a three-dimensional elliptically contoured symmetric Lévy-stable distribution.