Chiral Cherenkov radiation in the presence of a time-dependent chiral chemical potential

Abstract

The photon production process $f\to f+\gamma$ in the presence of a time-dependent chiral chemical potential $\mu_5$ is studied. The validity of the adiabatic approximation is demonstrated in the ultra-relativistic limit. Analytical expressions for the photon spectrum are derived for two models of the chiral magnetic conductivity $b_0\propto \mu_5$: (i) $b_0(t)= A_1+B_1\tanh(t/\tau)$ and (ii) $b_0(t)= A_2(t/\tau)/(1+t^2/\tau^2)$. It is known that for constant $b_0$, photon emission may exhibit a resonance for one photon polarization depending on the sign of $b_0$. It is shown that in model (i), up to two resonances may occur, depending on the sign of the asymptotic values $b_0(\infty)$ and $b_0(-\infty)$. No resonances are observed in model (ii). Numerical calculations are performed using parameters relevant to quark-gluon plasma, and the universality of the results is discussed.