Improved big bang nucleosynthesis constraints on heavy neutral leptons

We constrain the lifetime of thermally produced heavy neutral leptons (HNLs) from big bang nucleosynthesis. We show that even a small fraction of mesons present in the primeval plasma leads to the overproduction of primordial helium-4. This constrains the lifetime of HNLs to be ₍<0. 02 s for masses above the mass of the pion (as compared to 0. 1 s reported previously). In combination with accelerator searches, this allows us to put a new lower bound on the HNL masses and define the ``bottom line'' for HNL searches at the future Intensity Frontier experiments.