Hadron resonance gas is not a good model for hadronic matter in a strong magnetic field

TL;DR

This work investigates the reliability of the hadron resonance gas (HRG) model for describing hadronic matter in a strong external magnetic field $eB$. By implementing Landau quantization and exploring detailed balance, the authors examine how finite $eB$ alters particle yields and conserved-charge fluctuations, comparing HRG predictions to lattice QCD results for the net-baryon susceptibility $\chi_{BB}$. They find that large $eB$-induced changes to proton yields and $\chi_{BB}$ largely reflect the treatment of higher-spin resonances and that neutral resonances can be affected via density constraints imposed by detailed balance, challenging the HRG premise that neutral states are B-inert. The study also shows that a naive single-particle HRG description breaks down near $eB \sim m_\pi^2$ and argues for incorporating hadronic structure and interactions (e.g., via a $\pi$-$\rho$ gas with a fully in-medium spectral function) to reliably capture magnetised hadronic matter.

Abstract

We study the effect of magnetic field on particle yields and charge fluctuations in hadron resonance gas. We argue that the big changes in the proton yield and baryon number susceptibility are due to ill-defined description of higher-spin states, and that because of detailed balance, neutral resonances must be affected by the field too.

Figures (2)

• Figure 1: Particle yields normalised to the yields at vanishing magnetic field at $T=155~$MeV (Left) and net-baryon number susceptibility at $T=0.145$ (red) and $0.155~$GeV (blue) (Right).
• Figure 2: $\rho^0$ meson density at $T = 155$ MeV using the detailed balance and T-matrix approaches normalised to densities at $eB=0$.