Near-Resonant Thermal Leptogenesis

TL;DR

This paper demonstrates that successful thermal leptogenesis can occur at electroweak scale energies without resonance, by analyzing near-resonant conditions and flavor effects, thus broadening the viable parameter space.

Contribution

It provides a non-resonant, controlled framework for near-resonant leptogenesis, extending the viable mass scale down to the electroweak scale and incorporating flavor effects during reheating.

Findings

Successful leptogenesis for M > 100 GeV independent of washout regime.

Reheating temperatures as low as 10 GeV can produce baryon asymmetry.

Universal upper bound on CP asymmetry parameter, ta  1/200.

Abstract

We study leptogenesis in the quasi-degenerate but non-resonant regime. Expanding the CP asymmetry parameter near degeneracy and imposing the conservative non-resonance condition that the mass splitting must be much greater than the right-handed neutrino decay rates $\Delta M > 100\Gamma_i$, yields the universal upper bound $\epsilon \leq 1/200$, independent of both the effective neutrino masses and the right-handed neutrino mass. We investigate vanilla and flavoured near-resonant leptogenesis and find that successful leptogenesis by right-handed neutrino decays can occur for $M \gtrsim 100~\mathrm{GeV}$ independent of washout regime, extending the viable parameter space of thermal leptogenesis down to the electroweak scale without invoking resonance. We also analyse near-resonant thermal leptogenesis during reheating and show that successful baryon asymmetry generation is compatible with reheating temperatures as low as $T_{RH}\simeq 10\rm GeV$ without relying on non-thermal production. Finally, we present a consistent framework for incorporating flavour effects in near-resonant leptogenesis during reheating. Overall, near-resonant thermal leptogenesis offers a controlled alternative regime to resonant leptogenesis, lowering the leptogenesis scale to the electroweak scale, without reliance on a disputed regulator used in resonant leptogenesis.