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 tau(N) < 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.

Automated summary

The study demonstrates that even a small fraction of mesons present in the primeval plasma can lead to the overproduction of primordial helium-4, limiting the lifetime of thermally produced heavy neutral leptons. Combining this with accelerator searches allows for a new lower bound to be set on HNL masses, defining the bottom line for HNL searches at future Intensity Frontier experiments.