The collectivity of transverse momentum fluctuations

We study the observable v0(pT), which quantifies the relative change of pT 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(pT) and show that the scaled quantity v0(pT)/v0 exhibits very little dependence on centrality and transport coefficients. We further find that the apparent influence of transport coefficients-particularly bulk viscosityon v0(pT) largely originates from modifications of the event-averaged mean transverse momentum, 〈pT〉. By expressing v0(pT)/v0 as a function of pT/〈pT〉, the genuine sensitivity of v0(pT) to transport coefficients can be isolated. Moreover, since v0(pT) is the pT-differential measure of event-by-event [pT] fluctuations, it naturally explains the observed pT-cut dependence of σpT measured by ATLAS collaboration.