Magnetic field effect on hadron yield ratios and fluctuations in a hadron resonance gas

This work studies the influence of an external magnetic field on hadron yields and fluctuations in a hadron resonance gas by performing calculations within an updated version of the open-source Thermal-FIST package. The presence of the magnetic field has a sizable influence on certain hadron yield ratios. Most notably, it leads to enhanced $p/\pi$ and suppressed $n/p$ ratios, which may serve as a magnetometer in heavy-ion collisions. By attributing the centrality dependence of the $p/\pi$ ratio in Pb-Pb collisions at 5.02~TeV measured by the ALICE Collaboration entirely to the magnetic field, its maximal strength at freeze-out is estimated to be $eB \simeq 0.12$~GeV$^2 \simeq 6.3 m_\pi^2$ in peripheral collisions. The magnetic field also enhances various conserved charge susceptibilities, which is qualitatively consistent with recent lattice QCD data and is driven in the HRG model by the increase of hadron densities. However, the variances of hadrons do not show any enhancement when normalized by the means, therefore, measurements of second-order fluctuations in heavy-ion collisions appear to offer limited additional information about the magnetic field relative to mean multiplicities.