G. S. Rocha | Searcharxiv

arXiv2026

Longitudinal Dynamics of Large and Small Systems from a 3D Bayesian Calibration of RHIC Top-energy Collision Data

A comprehensive Bayesian analysis of the 3D dynamics of high-energy nuclear collisions is presented. We perform a systematic model-to-data comparison using simulations of large and small collision systems, and a broad range of measurements from the PHENIX, STAR, PHOBOS, and BRAHMS collaborations spanning nearly two decades of RHIC operations. In particular, we perform fully 3D multi-stage simulations including rapidity-dependent energy deposition with global energy conservation using the 3D Glauber model, along with relativistic viscous hydrodynamics with MUSIC. We calibrate the model on rapidity- and $p_T$-differential observables and analyze the respective constraints on initial state and transport properties they provide. We emphasize the additional constraints provided by rapidity-dependent measurements, the differences in large and small system calibrations, and the tension exhibited by particular observables. We use our calibrated model to make predictions of observables in p-Au and $^3$He-Au collisions. Furthermore, we facilitate direct comparison of experimental measurements by highlighting the dependence of flow measurements on the rapidity of the regions of interest and reference, as well as the importance of the centrality selection. In particular, we examine the apparent differences between the STAR and PHENIX $v_2$ and $v_3$ measurements in small systems.

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arXiv2026

Species-dependent viscous corrections at particlization: A novel relaxation time approximation approach

We assess the effects of a recently proposed generalized relaxation time approximation (RTA) for multi-species relativistic gases within a realistic numerical hybrid framework and study its phenomenological consequences in p-Pb and Pb-Pb collisions. The novel approximation introduces counter-terms to the collision kernel, allowing for momentum-dependent relaxation times $τ_i(p)$ while preserving local energy-momentum conservation. As a consequence, the resulting first-order viscous corrections $δf_i$ to the phase-space distribution functions depend explicitly on the particle species mass $m_i$. We systematically investigate the impact of these species-dependent corrections on particle production at particlization, focusing on identified hadron yields and transverse momentum ($p_T$) spectra obtained from Cooper-Frye sampling. We find that the yields and spectra of light hadrons ($π, K, p$) are significantly affected, leading to modifications of relative particle yields such as the $K/π$ and $p/π$ ratios. We show that these effects persist, albeit with reduced magnitude, after the inclusion of the hadronic cascade stage. In contrast, the impact on inclusive charged-particle observables is strongly reduced due to compensating enhancements and suppressions among different species. This controlled deformation of identified hadron observables, which selectively modifies flavor-sensitive quantities, makes the new prescription particularly well suited for Bayesian inference, as it introduces new sensitivity directions without spoiling existing constraints. Overall, our results demonstrate that species-dependent viscous corrections arising from the generalized RTA can leave significant and observable imprints on identified hadron production and relative yields, while remaining fully consistent with the successful description of bulk collective flow observables.

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