Dark decay channels strengthen cosmological bounds on heavy neutral leptons

What the study found: The study finds that heavy neutral leptons (HNLs), which are hypothetical particles beyond the Standard Model, are not rescued by adding dark sector decay channels. Instead, the authors say that significant dark decay modes can strengthen cosmological bounds.

Why the authors say this matters: The authors conclude that this has major implications for laboratory searches of HNLs, because the cosmologically allowed region is not enlarged by dark decay channels as sometimes expected.

What the researchers tested: The paper examines sub-GeV HNLs that mix with active neutrinos and considers the effect of adding new decay modes into a dark sector. It focuses on cosmological constraints from big bang nucleosynthesis (BBN, the early-universe formation of light elements), the primordial helium fraction, and ΔN_eff, a measure of extra radiation energy density.

What worked and what didn't: The authors report that the usual idea of using dark decay modes to avoid the BBN lifetime constraint does not work. They say the added dark decays can increase the extra radiation energy density around the BBN epoch, which leads to stronger bounds rather than weaker ones.

What to keep in mind: The abstract does not provide detailed model assumptions, parameter values, or the full derivation of the bounds. It also does not describe specific laboratory experiments, only that the result affects their interpretation.

Key points

• The study says dark sector decay channels do not evade cosmological limits on heavy neutral leptons.
• The authors report that significant dark decays can make the cosmological bounds stronger.
• The effect is linked to increased extra radiation energy density around the big bang nucleosynthesis epoch.
• The paper notes observable consequences for the primordial helium fraction and ΔN_eff.
• The abstract says the result has major implications for laboratory searches of heavy neutral leptons.