Massive Dirac neutrinos, radiative decays and nucleosynthesis

We study the cosmological implications of massive Dirac neutrinos. The conditions for a fast radiative decay, ν → ν'γ, with the lifetime τ < 5 x 103 s, and the compatibility of three Dirac neutrino species with the predictions of nucleosynthesis are examined. We use a class of E6 superstring-inspired models as a suitable framework for this discussion, where we find that for lepto-quark masses in excess of about 100 GeV, the tau neutrino must weigh less than 50 eV. If ντ is heavy, the above cosmological bounds also lead to possible enhancements of rare processes, such as τ → μγ, B+ → K+νanti ν and B0 → τμ. (orig.)