Dissipative properties of hot and dense hadronic matter in an excluded-volume hadron resonance gas model

Abstract

We estimate dissipative properties viz: shear and bulk viscosities of hadronic matter using relativistic Boltzmann equation in relaxation time approximation within ambit of excluded volume hadron resonance gas (EHRG) model. We find that at zero baryon chemical potential the shear viscosity to entropy ratio (η/s) decreases with temperature while at finite baryon chemical potential this ratio shows same behavior as a function of temperature but reaches close to Kovtun-Son-Starinets (KSS) bound. Further along chemical freezout curve, ratio η/s is almost constant apart from small initial monotonic rise. This observation may have some relevance to the experimental finding that the differential elliptic flow of charged hadrons does not change considerably at lower center of mass energy. We further find that bulk viscosity to entropy density (ζ/s) decreases with temperature while this ratio has higher value at finite baryon chemical potential at higher temperature. Along freezout curve ζ/s decreases monotonically at lower center of mass energy and then saturates.

Bulk viscosity for pion and nucleon thermal fluctuation in the hadron resonance gas model

“…The ζ(T ) of Ref. [34] decreases up to T ∼ 0.150 GeV after which a mild increment is observed. Most of the earlier works [20,21,25,27,28,32,[34][35][36][37] based on effective QCD model calculations [20,21,25,27,28] as well as effective hadronic model calculations [32,[34][35][36][37] predicted a decreasing function of ζ/s(T ) in the hadronic temperature domain, which is qualitatively similar with our results (dashed lines).…”

“…From the Relaxation Time Approximation (RTA) of kinetic theory approach [28,31] or from the one-loop expression of diagrammatic approach based on Kubo formula [37], we can get standard expressions of bulk viscosity coefficient for pion and nucleon components [28,31,34,37] :…”

“…Owing to that motivation, we have gone through a microscopic calculations of ζ and ζ/s, where equilibrium situations of hadronic matter are controlled by the standard HRG model and non-equilibrium picture of medium constituents is introduced via quantum fluctuation of pion and nucleon in medium. With respect to the earlier HRG calculations of ζ [33][34][35][36], the main distinguishable contribution is in the non-equilibrium properties of medium constituents, quantified by their thermal width. Assuming pions and nucleons as most abundant constituents of medium, we have calculated their thermal width, which are coming from their in-medium scattering with different possible mesonic and baryonic resonances.…”

“…These effective QCD model calculations [20][21][22][23][24][25][26][27][28] cover both QGP and hadronic phases while hadronic-model calculations of Refs. [32][33][34][35][36][37] are restricted within hadronic phase only. The present work is also addressing the estimation of ζ in the hadronic phase only.…”

“…A list of references are [2,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], where Ref. [19] addressed high temperature perturbative QCD calculations of ζ, Refs.…”