Eur. Phys. J. A (2021) 57: 294
https://doi.org/10.1140/epja/s10050-021-00599-1

Regular Article - Theoretical Physics

Hybrid model with viscous relativistic hydrodynamics: a role of constraints on the shear-stress tensor

¹ Joint Institute for Nuclear Research, 141980, Dubna, Russia
² Institute of Applied Physics, Moldova Academy of Science, 2028, Kishineu, Moldova
³ Matej Bel University, 97401, Banska Bystrica, Slovakia

^b kolomei@theor.jinr.ru

Received: 20 May 2021
Accepted: 1 October 2021
Published online: 20 October 2021

Abstract

We present the hybrid model connecting the parton–hadron–string dynamic model (PHSD) and a hydrodynamic model taking into account shear viscosity within the Israel–Stewart approach. The numerical scheme, initialization, and particlization procedure are discussed in detail. The performance of the code is tested on the pion and proton rapidity and transverse mass distributions calculated for Au + Au and Pb+Pb collisions at AGS–SPS energies. The influence of the switch time from transport to hydro models, the viscous parameter, and freeze-out time are discussed. Since the applicability of the Israel–Stewart hydrodynamics assumes the perturbative character of the viscous stress tensor, , which should not exceed the ideal energy–momentum tensor, , hydrodynamical codes usually rescale the shear stress tensor if the inequality is not fulfilled in some sense. There are several conditions used in the literature and we analyze in detail the influence of different conditions and values of the cut-off parameter on observables. We show that the form of the corresponding condition plays an important role in the sensitivity of hydrodynamic calculations to the viscous parameter – a ratio of the shear viscosity to the entropy density, . It is shown that the constraints used in the vHLLE and MUSIC models give the same results for the observables. With these constraints, the rapidity distributions and transverse momentum spectra are most sensitive to a change of the ratio. We demonstrate that these constraints do not guarantee that each element of the tensor is smaller than the corresponding element . As an alternative, a strict condition is used. When applied it reduces the sensitivity of the proton and pion momentum distributions to the viscosity parameter. We performed global fits of the rapidity and transverse mass distributions of pion and protons. It was also found that as a function of the collision energy monotonically increases from up to and saturates for higher SPS energies. We observe that it is difficult to reproduce simultaneously pion and proton rapidity distribution within our model with the present choice of the equation of state without a phase transition.