No Hiding in the Dark: Cosmological Bounds on Heavy Neutral Leptons with Dark Decay Channels

TL;DR

This paper demonstrates that introducing dark decay channels for heavy neutral leptons does not relax cosmological constraints; instead, it strengthens them due to increased radiation energy density affecting Big Bang Nucleosynthesis and cosmic parameters.

Contribution

The study challenges the assumption that dark decay modes weaken cosmological bounds on HNLs, showing they actually lead to stricter constraints from early universe observations.

Findings

Dark decay channels increase radiation energy density during BBN.

Enhanced radiation leads to tighter bounds on HNL properties.

Laboratory search regions are more constrained than previously thought.

Abstract

Heavy neutral leptons (HNLs) are well-motivated new physics candidates. The mixing of sub-GeV HNLs with active neutrinos is severely constrained by cosmology. In particular, the success of Big Bang Nucleosynthesis (BBN) requires the HNL lifetime to be shorter than about 0.02 sec if they were in thermal equilibrium, thus excluding a wide range of mixing angles accessible to terrestrial experiments. In order to justify the laboratory searches in this cosmologically-forbidden region, it is often argued that adding new dark sector decay modes of HNLs can evade the stringent BBN constraint. Here we rule out this possibility and show that, contrary to the naive expectation, HNLs with significant dark decay modes actually lead to stronger cosmological bounds. This is mainly because of the increase in the extra radiation energy density in the Universe around the BBN epoch, which causes observable effects in the primordial helium fraction and $\Delta N_{\rm eff}$. Our result has major implications for laboratory searches of HNLs.