Fluid velocity from transverse momentum spectra

TL;DR

This paper introduces a generalized blast-wave model to analyze transverse momentum spectra in heavy-ion collisions, comparing it with hydrodynamics and applying it to experimental data, revealing limitations of ideal hydrodynamics.

Contribution

The paper develops a generalized blast-wave parametrization based on fluid velocity distribution, extending the conventional model and analyzing its differences with hydrodynamic calculations.

Findings

The fit describes data reasonably up to 6 GeV/c but is not perfect.

Ideal hydrodynamics cannot fit all particle spectra simultaneously.

Data show a broadening of fluid velocity distribution with decreasing collision centrality.

Abstract

We parametrize the transverse momentum distribution of outgoing hadrons in ultrarelativistic nucleus-nucleus collisions as a superposition of boosted thermal distributions. In this approach, which generalizes the conventional blast wave, the momentum distribution is determined by the distribution of the fluid velocity. We analyze the difference between this generalized blast-wave parametrization and a full hydrodynamic calculation. We then apply the generalized blast-wave fit to experimental data on Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76\ \mathrm{TeV}$. The fit is reasonable up to $p_t\sim 6\ \mathrm{GeV}$, much beyond the range where hydrodynamics is usually applied, but not perfect. Based on the differences between the fit and the data, we argue that an ideal hydrodynamic calculation cannot fit simultaneously all identified particle spectra, irrespective of the specific implementation. In particular, data display a significant excess of pions at low $p_t$, whose physical interpretation is discussed. Data also show that the distribution of the fluid velocity becomes broader as the collision becomes less central. This broadening is explained by event-by-event hydrodynamic calculations, where it results from the centrality dependence of initial-state fluctuations.