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Improved big bang nucleosynthesis constraints on heavy neutral leptons
We constrain the lifetime of thermally produced Heavy Neutral Leptons (HNLs) from primordial nucleosynthesis. We show that even a small fraction of mesons present in the primordial plasma leads to the over-production of the primordial helium. This puts an upper bound on the lifetime of HNLs $\tau_{N}<0.02$ s for masses …