Improved BBN constraints on Heavy Neutral Leptons

Understand

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 $m_{N}>m_{\pi}$ (as compared to 0.1 s reported previously).
• In combination with accelerator searches, this allows us to put a new lower bound on the HNLs masses and defining the &#34;bottom line&#34; for HNL searches at the future Intensity Frontier experiments.