The collectivity of transverse momentum fluctuations

¹ Department of Physical Sciences, Indian Institute of Science Education and Research Berhampur, Transit Campus (Govt ITI), Berhampur - 760010, Odisha, India
² Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Cracow, Poland
³ Université Paris Saclay, CNRS, CEA, Institut de physique théorique, 91191 Gif-sur-Yvette, France

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
^** e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Published online: 17 April 2026

Abstract

We study the observable v₀(p_T), which quantifies the relative change of p_T spectra induced by event-by-event density fluctuations in the medium created in heavy-ion collisions. This quantity provides a direct measure of radial flow and serves as a probe of collectivity, complementing anisotropic flow coefficients. Using hydrodynamic model calculations, we predict the behavior of v₀(p_T) and show that the scaled quantity v₀(p_T)/v₀ exhibits very little dependence on centrality and transport coefficients. We further find that the apparent influence of transport coefficients-particularly bulk viscosityon v₀(p_T) largely originates from modifications of the event-averaged mean transverse momentum, 〈p_T〉. By expressing v₀(p_T)/v₀ as a function of p_T/〈p_T〉, the genuine sensitivity of v₀(p_T) to transport coefficients can be isolated. Moreover, since v₀(p_T) is the p_T-differential measure of event-by-event [p_T] fluctuations, it naturally explains the observed p_T-cut dependence of σ_pT measured by ATLAS collaboration.