Radiative neutrino decay in hot media

We calculate the rate for the radiative neutrino decay in a thermal background of electrons and photons, taking into account the effect of the stimulated emission of photons in the thermal bath. We show that the rate is enhanced by a large factor relative to the rate for the corresponding process in the vacuum. The notion that the presence of an ambient medium modifies the properties of elementary particles is now well known. Sometimes the effects are dramatic, as is the case of the MSW mechanism which has been invoked to explain the solar neutrino puzzle [1]. There, under favorable conditions, the conversion of one neutrino flavor into another is greatly enhanced by the presence of the background. Some time ago it was observed [2] that the electromagnetic properties of neutrinos are also drastically modified within a medium when compared to the properties in vacuum. This fact can lead to such interesting phenomena as the Cherenkov radiation by massless neutrinos [3] and the possible explanation of pulsar velocities by neutrino oscillations biased by a magnetic field [4]. It has also been shown previously [5, 6] that the rate for the radiative decay of a massive neutrino that propagates through a background of particles, ν(k) → ν(k) + γ(q) , (1) can be much larger than the rate for the corresponding process in the vacuum. In the calculation of Ref. [5] the background was taken to be a thermal bath of electrons, nucleons and photons, as is the case of a plasma made of ordinary matter, as well as possibly their antiparticles. Because the medium contains electrons but not muons or tauons, the GIM mechanism that inhibits the radiative neutrino decay in a vacuum is not operative anymore, and as a result