The shape of transverse momentum spectra in hybrid hydrodynamic models

TL;DR

This study investigates how the shape of transverse momentum spectra in hydrodynamic models depends on various parameters, revealing limited flexibility and tensions in simultaneously fitting different observables, indicating potential missing physics.

Contribution

It systematically analyzes the sensitivity of transverse momentum spectra to 17 model parameters across multiple hydrodynamic models, highlighting key sensitivities and model limitations.

Findings

Strong sensitivity to bulk viscosity, free-streaming time, and nucleon width parameter w.

Limited model flexibility in describing scaled spectra despite many parameters.

Tensions between fitting mean transverse momentum and scaled spectra.

Abstract

We study the scaled transverse momentum spectra over a wide parameter space of state-of-the-art hydrodynamic simulation models in order to learn what information can be obtained from the shape of identified-particle spectra -- previously observed to be surprisingly universal across centrality and collision systems in both experimental data and hydrodynamic simulations. We study its sensitivity to each of 17 model parameters in the context of 4 different models for particlization when switching from the hydro description to the kinetic theory afterburner. We find that the strongest sensitivity is to parameters relating to bulk viscosity, free-streaming time, and the $\texttt{T$_\mathrm{R}$ENTo}$ nucleon width parameter $w$. However, we find that the model generally has surprisingly little flexibility in describing the scaled spectrum observable, despite the large number of parameters. Within this small range of parameter dependence, we further find significant tension in a simultaneous description of momentum-integrated observables. In particular, while the mean transverse momentum prefers a large value of the nucleon width parameter $w$, a small value is required to obtain scaled spectra that are consistent with experimental measurements. We speculate on the origin of these model tensions and possible missing physics in the commonly-used $\texttt{T$_\mathrm{R}$ENTo}$+free streaming+hydro+afterburner simulation model.