Improved BBN 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 $m_{N}>m_{\pi}$ (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 "bottom line" for HNL searches at the
future Intensity Frontier experiments.
