  The measurement of the magnetic field created in high-energy heavy-ion
collisions is challenging, due the the fact that the magnetic field decays so
drastically that in a thermalized quark-gluon plasma the field strength becomes
rather weak. By incorporating the weak magnetic effect into the medium, and
especially into the production formalism of dileptons from the quark-gluon
plasma, the effect of dilepton polarization is studied through the dilepton
angular distribution. We find that the anisotropic coefficients in the dilepton
spectrum are quite sensitive to the orientation and strength of the weak field.
Accordingly, these coefficients provide ideal probes for the magnetic field in
realistic experiments.
