Baryon Electric Charge Correlation as QCD Magnetometer

The detection of strong magnetic fields in peripheral heavy-ion collisions is crucial for observing effects such as the chiral magnetic effect but has proven exceptionally difficult. To address this, we propose the baryon electric charge correlation $\chi^{\rm BQ}_{11}$ and the chemical potential ratio $\mu_{\rm Q}/\mu_{\rm B}$ as sensitive probes of magnetic fields, based on (2+1)-flavor lattice QCD simulations at the physical pion mass. Along the transition line, $\chi^{\rm BQ}_{11}$ and $(\mu_{\rm Q}/\mu_{\rm B})_{\rm LO}$ in Pb-Pb collisions increase by factors of 2.1 and 2.4 at $eB \simeq 8M_\pi^2$, respectively. To bridge theoretical predictions with experimental observables, we implement systematic kinematic cuts that emulate detector acceptances of the STAR and ALICE experiments within the hadron resonance gas model. This allows us to construct experimentally relevant proxy observables. Furthermore, we demonstrate that $(\mu_{\rm Q}/\mu_{\rm B})_{\rm LO}$ is also sensitive to the collision system, showing a $1.5$-fold increase from Zr-Zr to Ru-Ru isobar collisions. Our findings offer new insights into thermo-magnetic effects and provide experimentally relevant guidance for the detection of magnetic fields in heavy-ion collisions.