Cosmological bounds on heavy neutral leptons with dark decay channels

Key findings from this study

• The study found that heavy neutral leptons with dark decay modes face stronger cosmological constraints from big bang nucleosynthesis, contrary to prior expectations.
• The authors report that enhanced radiation energy density during the BBN epoch produces measurable effects on primordial helium fractions and N_eff.
• The researchers demonstrate that dark sector decay channels cannot circumvent stringent mixing angle constraints in cosmologically viable HNL models.

Overview

Heavy neutral leptons (HNLs) are theoretical particles that mix with active neutrinos. Sub-GeV HNLs face stringent cosmological constraints from big bang nucleosynthesis (BBN). Current proposals suggest dark sector decay channels could circumvent these constraints and permit terrestrial detection of HNLs in otherwise forbidden parameter spaces.

Methods and approach

The authors analyze cosmological constraints on HNL properties by examining implications for primordial nucleosynthesis and radiation density. They assess how dark decay modes affect the extra relativistic degrees of freedom during the BBN epoch. The analysis evaluates observable consequences in primordial helium abundance and effective neutrino number (N_eff).

Results

HNLs with significant dark decay modes produce stronger cosmological bounds rather than weaker ones. The counterintuitive result arises from increased radiation energy density in the early Universe around the BBN epoch. These additional relativistic contributions generate detectable alterations in primordial helium fraction and N_eff measurements, thereby constraining HNL parameters more severely than previously anticipated.

The study demonstrates that naively adding dark sector decay channels cannot evade BBN constraints on sub-GeV HNL mixing angles. The mechanism operates through enhanced radiation content during nucleosynthesis, which propagates as observable signatures in primordial abundance patterns.

Implications

The findings substantially narrow the viable parameter space for detecting HNLs in terrestrial experiments. Laboratory searches targeting mixing angles previously assumed cosmologically accessible may require fundamental theoretical revisions or alternative new physics frameworks. The work establishes that dark decay channels cannot serve as the justification for pursuing HNL searches in previously excluded regions.

These constraints inform future experimental design and target selection for HNL detection programs. Cosmological bounds now represent the dominant limitation on certain HNL properties, potentially redirecting research efforts toward alternative mass ranges or coupling regimes.

Scope and limitations

This summary is based on the study abstract and available metadata. It does not include a full analysis of the complete paper, supplementary materials, or underlying datasets unless explicitly stated. Findings should be interpreted in the context of the original publication.