Constraints on Lifetime and Mass of Heavy Lepton Neutrinos Imposed by Big Bang Nucleosynthesis

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Abstract
If there exist massive neutral leptons (heavy neutrinos), they would have been produced in thermal equilibrium in the early stages of the universe. The effects of their presence and decay on the big bang nucleosynthesis are investigated in detail and abundances of the products 4He, 2H and 7Li are compared with the observed cosmic abundances. We have determined a region in the lifetime-mass diagram of the heavy neutrino which should be ruled out in order for the big bang nucleosynthesis not to conflict with observed abundances of the elements. In addition, if a lower limit of the lifetime obtained from the Weinberg-Salam type theory, τ≥6 ×107 (1 MeV/mνh)5sec, is assumed, where mνh is the mass of the heavy neutrino, the mass range of 70 eV,< mνh < 10 MeV is ruled out. The other constraints on the mass and the lifetime obtained from astrophysical considerations are also discussed and summarized.

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