COSMOLOGICAL EVOLUTION OF AN EARLY-DECAYING PARTICLE

The cosmological evolution of an early-decaying particle is investigat ed by solving a set of the Boltzmann equations which describe the time evolution of the momentum distribution functions of the decaying part icle and its decay products. The inverse decay process plays an import ant role when the decay occurs at a temperature greater than the rest mass of the particle. As a result, the abundance of the decaying parti cle does not decrease much before it becomes non-relativistic. We appl y these generic results to the specific case of the decay of a heavy M ajorana tau-neutrino with mass between 10 keV and 40 MeV and lifetime between 10(-4) s and 100 s. We assume that the tau-neutrino decays int o a mu-neutrino and an invisible particle (e.g. majoron), and explore the nucleosynthesis constraint on mass and lifetime of the tau-neutrin o.