  A strong magnetic field influences significantly the masses of the charged
light mesons. For example, the mass of charged pion increases with the magnetic
field increasing. We propose this mechanism as a possible way to extract the
magnitude of magnetic field at freeze-out in heavy ion collisions and thus help
constrain its lifetime which is currently a major open question to resolve.
Specifically we show that the ratio between the yield of charged pions and that
of charged rhos is very sensitive to the magnetic field value at freeze-out. By
using a viscous-hydrodynamic framework (iEBE-VISHNU) to simulate heavy ion
collisions and implementing magnetic-field-dependent meson masses, we compute
their yields and predict the dependence of such ratio on the magnetic field. We
suggest to use this ratio of charged rho yield over charged pion yield as an
experimental observable to extract the possible magnetic field at freeze-out in
heavy ion collisions.
