Limiting temperature of pion gas with the van der Waals equation of state

TL;DR

This paper investigates the thermodynamics of an interacting pion gas using the van der Waals equation of state with Bose statistics, revealing a limiting temperature where equilibrium ceases and critical behavior emerges.

Contribution

It introduces a model incorporating Bose statistics into the van der Waals framework, identifying a limiting temperature and analyzing critical phenomena in pion gases.

Findings

Existence of a limiting temperature $T_0$ for the pion gas.

Divergence of specific heat and fluctuations at $T_0$.

Vanishing speed of sound at the limiting temperature.

Abstract

The grand canonical ensemble formulation of the van der Waals equation of state that includes the effects of Bose statistics is applied to an equilibrium system of interacting pions. If the attractive interaction between pions is large enough, a limiting temperature $T_0$ emerges, i.e., no thermodynamical equilibrium is possible at $T>T_0$. The system pressure $p$, particle number density $n$, and energy density $\varepsilon$ remain finite at $T=T_0$, whereas for $T$ near $T_0$ both the specific heat $C=d\varepsilon/dT$ and the scaled variance of particle number fluctuations $\omega[N]$ are proportional to $(T_0-T)^{-1/2}$ and, thus, go to infinity at $T\rightarrow T_0$. The limiting temperature corresponds also to the softest point of the equation of state, i.e., the speed of sound squared $c_s^2=dp/d\varepsilon$ goes to zero as $(T_0-T)^{1/2}$. Very similar thermodynamical behavior takes place in the Hagedorn model for the special choice of a power, namely $m^{-4}$, in the pre-exponential factor of the mass spectrum $\rho(m)$.