  We studied effect of finite quark mass on the dynamics of axial charge using
the D3/D7 model in holography. The mass term in axial anomaly equation affects
both the fluctuation (generation) and dissipation of axial charge. We studied
the dependence of the effect on quark mass and external magnetic field. For
axial charge generation, we calculated the mass diffusion rate, which
characterizes the helicity flipping rate. The rate is a non-monotonous function
of mass and can be significantly enhanced by the magnetic field. The diffusive
behavior is also related to a divergent susceptibility of axial charge. For
axial charge dissipation, we found that in the long time limit, the mass term
dissipates all the charge effectively generated by parallel electric and
magnetic fields. The result is consistent with a relaxation time approximation.
The rate of dissipation through mass term is a monotonous increasing function
of both quark mass and magnetic field.
