Improved BBN constraints on Heavy Neutral Leptons

TL;DR

This paper uses primordial nucleosynthesis constraints to significantly tighten the bounds on the lifetime and mass of Heavy Neutral Leptons, informing future experimental searches.

Contribution

It provides the most stringent lifetime upper bound for HNLs with masses above the pion mass, refining the parameter space for future experiments.

Findings

Upper bound on HNL lifetime: τ_N < 0.02 s for m_N > m_π

Over-production of helium constrains HNL properties

New lower bounds on HNL masses for future searches

Abstract

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.