Relaxing Limits from Big Bang Nucleosynthesis on Heavy Neutral Leptons with Axion-like Particles

TL;DR

This paper explores a new model where heavy neutral leptons decay into axion-like particles and neutrinos, allowing earlier decay times that bypass Big Bang Nucleosynthesis constraints, and identifies a viable parameter space for future experiments.

Contribution

It introduces a novel decay channel for HNLs involving ALPs, expanding the allowed mass and mixing parameter space compatible with cosmological constraints.

Findings

New parameter region for HNLs with 1 MeV to 1 GeV mass

Active-sterile neutrino mixing between 10^{-9} and 10^{-6}

Implications for future HNL searches at NA62 and DUNE

Abstract

Heavy neutral leptons (HNLs) are constrained by requirements of Big Bang Nucleosynthesis (BBN) as their decays significantly impact the formation of the primordial elements. We propose here a model where the primary decay channel for the HNLs is to an axion-like particle (ALP) and a neutrino. Consequently, HNLs can decay earlier and evade the BBN bound for lower masses, provided the ALPs themselves decay considerably later. Further cosmological and astrophysical constraints limit severely the range of validity of the ALP properties. We find that a new parameter region opens up for HNLs with masses between 1 MeV and 1 GeV, and active-sterile neutrino mixing strengths between $10^{-9}$ and $10^{-6}$ that is consistent with constraints and can be probed in future searches. In such a scenario, current bounds as well as sensitivities of future direct HNL searches such as at NA62 and DUNE will be affected.