 Nonrelativistic entropy at zero chemical potential

For a dilute nonrelativistic gas with $x=m/T\gg 1$ and zero chemical potential, the Bose-Einstein distribution approaches the Maxwell-Boltzmann distribution. Expanding $E=m+p^2/(2m)+\cdots$ in the number density integral gives $n\sim g(mT/(2\pi ){)}^{3/2}{e}^{-x}$. Since $\rho =mn+3nT/2+\cdots$ and $P=nT$, the entropy density at zero chemical potential gives $s=n(x+5/2+\cdots )$ and hence the displayed leading expression. This includes the internal multiplicity $g$; a single real scalar field has $g=1$.

The displayed asymptotic uses natural units $k_B=c=\hslash =1$. Thus $m$ and $T$ are both energy scales, and $x=m/T$ is dimensionless. Restoring SI units replaces this ratio by $m{c}^2/(k_{B}T)$ and includes $k_B$ in the physical entropy per particle.