QCD in strong magnetic fields: fluctuations of conserved charges and equation of state

Abstract

We present continuum-estimated (2+1)-flavor lattice QCD results for second-order fluctuations of conserved charges and the leading-order equation of state in the presence of strong magnetic fields at nonzero baryon chemical potential, using the HISQ action at the physical pion mass. The baryon-electric charge correlation ${\chi }_{11}^{\mathrm{BQ}}$ exhibits striking sensitivity to the magnetic field: $R_{cp}$-like double ratios ${\chi }_{11}^{\mathrm{BQ}}/{\chi }_2^{\mathrm{Q}}$ and ${\chi }_{11}^{\mathrm{BQ}}/{\chi }_{11}^{\mathrm{QS}}$ reach enhancements of $\sim 2$ and $\sim 2.25$ at $eB\simeq 8M_{\pi }^2$ along the transition line, establishing ${\chi }_{11}^{\mathrm{BQ}}$ as a magnetometer of QCD. To bridge theoretical predictions and experimental observations, we construct HRG-based proxy observables and apply systematic kinematic cuts emulating STAR and ALICE detector acceptances, which retain $\sim 80\%$ of the lattice QCD magnetic sensitivity. Extending to the QCD equation of state under strangeness neutrality and isospin asymmetry, we determine the chemical potential ratio $q_1\equiv ({\mu }_{\mathrm{Q}}/{\mu }_{\mathrm{B}}{)}_{\mathrm{LO}}$ and the pressure coefficient $P_2$ for magnetic field strengths up to $eB\simeq 0.8{\mathrm{GeV}}^2\sim 45M_{\pi }^2$. The results reveal temperature-band crossings, hierarchy reversals, and non-monotonic structures driven by the nontrivial interplay between thermal and magnetic effects.

Comments: 10 pages, 3 figures, contribution to the 42nd International Symposium on Lattice Field Theory (LATTICE2025), 2-8 November 2025, Tata Institute of Fundamental Research, Mumbai, India