Decoupled Heavy Neutrino Temporarily Dominated Post-Big Bang Energy Density, Shifting Matter-Antimatter Conditions

The team modeled a decoupled heavy neutrino with extremely weak Standard Model couplings that decays far more slowly than ordinary neutrinos. Their Boltzmann-equation solutions show this particle's energy density briefly exceeded radiation before decaying, creating a short matter-dominated era at temperatures around the electroweak scale. This window shifts the freeze-out temperature of lepton-number-violating processes, allowing a larger CP-violating asymmetry to survive washout than in standard radiation-dominated scenarios.

Previous leptogenesis models assumed radiation domination throughout the relevant epoch and therefore required larger CP phases or additional resonances. The new calculation demonstrates that even a particle with interaction strength suppressed by 10^-10 can still imprint on the asymmetry through its gravitational influence on the expansion rate. This indirect route revives interest in decoupled states that had been dismissed as cosmologically inert.

The mechanism connects to ongoing searches at future colliders and neutrinoless double-beta decay experiments, where a detectable signature would require the heavy neutrino mass to lie between 10^7 and 10^9 GeV. Confirmation would also tighten constraints on the reheating temperature after inflation.

Next steps include embedding the scenario in a full UV-complete model and running lattice simulations of the phase transition to quantify the exact asymmetry yield. Without direct production of the particle or a measurable shift in primordial gravitational-wave spectra, the claim remains a plausible but untested extension of leptogenesis.