Heavy-quark spin precession as a magnetic field chronometer

The lifetime of the strong magnetic field generated in relativistic heavy-ion collisions remains one of the major open questions in the study of the quark–gluon plasma. We propose heavy-quark spin precession as a probe to quantify the magnetic field and its profile in such collisions. We investigate the Larmor evolution of polarized charm and bottom quarks propagating through an expanding quark–gluon plasma in the presence of the magnetic field. We show that realistic magnetic-field strengths and lifetimes generate measurable accumulated precession phases prior to hadronization, leading to characteristic charge-dependent polarization splittings between ${\mathrm{\Lambda }}_c^{+}$ and ${\bar{\mathrm{\Lambda }}}_c^{-}$, as well as between ${\mathrm{\Lambda }}_b^0$ and ${\bar{\mathrm{\Lambda }}}_b^0$, through the mixing of transverse and longitudinal polarization components. The predicted signals exhibit distinctive transverse-momentum and quark-mass dependence, providing complementary sensitivity to the magnetic-field lifetime in the charm and bottom sectors. Our results suggest heavy-flavor polarization, especially in the charm sector, as a quantitative probe for the space–time evolution of magnetic fields in relativistic heavy-ion collisions.