Kinematic and dynamical origins of mean-$p_T$ fluctuations in heavy-ion collisions

Abstract

Event-by-event fluctuations of the mean transverse momentum (mean-$p_T$) provide a sensitive probe of collective dynamics beyond single-particle spectra and anisotropic flow. We present a systematic study of mean-$p_T$ fluctuation observables using a Bayesian-calibrated multistage hydrodynamic framework, including quantitative comparisons to RHIC measurements and model-based investigations of beam-energy and kinematic-acceptance effects. The experimental definitions employed by the STAR and ALICE Collaborations are implemented explicitly and found to yield consistent results within controlled limits. We study the centrality and beam-energy dependence of the observable, its sensitivity to key soft-sector ingredients, and the impact of the kinematic $p_T$ acceptance. By introducing scaled-$p_T$ cuts, we demonstrate that a part of the apparent energy dependence arises from kinematic projection effects, while the remaining trends reflect genuine collective dynamics. Our results establish mean-$p_T$ fluctuations as a nontrivial and independent validation of calibrated hydrodynamic descriptions of the quark--gluon plasma.

Comments: v1: 10 pages, 4 figures; v2: minor revision, published on Phys. Lett. B