Bound on the neutrino magnetic moment from chirality flip in supernovae

For neutrinos with a magnetic moment, we show that the collisions in a hot and dense plasma act as an efficient mechanism for the conversion of ${\nu }_L$ into ${\nu }_R$. The production rate for right-handed neutrinos is computed in terms of a resummed photon propagator which consistently incorporates the background effects. Assuming that the entire energy in a supernova collapse is not carried away by the ${\nu }_R$, our results can be used to place an upper limit on the neutrino magnetic moment ${\mu }_{\nu }<(0.1-0.4)\times {10}^{-11}{\mu }_B$