The Neutrino Mass Window for Baryogenesis

TL;DR

The paper investigates thermal leptogenesis as the origin of the cosmological baryon asymmetry, deriving an improved upper bound on the CP asymmetry generated by heavy Majorana neutrino decays and analyzing washout effects. By solving Boltzmann equations and leveraging neutrino oscillation data, it establishes a robust upper limit of $\bar{m} \lesssim 0.12$ eV on the sum of light-neutrino masses, almost independently of initial conditions. The analysis shows that the effective neutrino mass $\tilde{m}_1$ drives washout, with $\tilde{m}_1 \gtrsim m_*$ ($m_* \simeq 10^{-3}$ eV) erasing memory of initial asymmetries and yielding a unique leptogenesis-generated baryon asymmetry within the solar and atmospheric mass scales. Quasi-degenerate light neutrinos are strongly disfavored unless extreme degeneracy among heavy Majorana neutrinos enhances the CP asymmetry; overall, the solar-atm mass range naturally fits leptogenesis as the baryogenesis mechanism.

Abstract

Interactions of heavy Majorana neutrinos in the thermal phase of the early universe may be the origin of the cosmological matter-antimatter asymmetry. This mechanism of baryogenesis implies stringent constraints on light and heavy Majorana neutrino masses. We derive an improved upper bound on the CP asymmetry in heavy neutrino decays which, together with the kinetic equations, yields an upper bound on all light neutrino masses of 0.1 eV. Lepton number changing processes at temperatures above the temperature T_B of baryogenesis can erase other, pre-existing contributions to the baryon asymmetry. We find that these washout processes become very efficient if the effective neutrino mass \tilde{m}_1 is larger than m_* \simeq 10^{-3} eV. All memory of the initial conditions is then erased. Hence, for neutrino masses in the range from (Δm^2_sol)^{1/2} \simeq 8*10^{-3} eV to (Δm^2_atm)^{1/2} \simeq 5*10^{-2} eV, which is suggested by neutrino oscillations, leptogenesis emerges as the unique source of the cosmological matter-antimatter asymmetry.

Figures (10)

• Figure 1: Neutrino masses as functions of $\overline{m}$ for normal hierarchy (cf. (\ref{['numanor1']})-(\ref{['numanor3']})).
• Figure 3: Neutrino masses as functions of $\overline{m}$ for inverted hierarchy (cf. (\ref{['numainv1']})-(\ref{['numainv3']})).
• Figure 5: The $C\!P$ asymmetry global suppression factor $\beta_{\rm max}$ for normal and inverted hierarchy.
• Figure 6: The function $f(\widetilde{m}_1,\overline{m}=0.15\,{\rm eV})$ for normal (inverted) hierarchy. It is defined for $\widetilde{m}_1\geq m_1\simeq 0.08\,(0.07)\,{\rm eV}$.
• Figure 7: The $C\!P$ asymmetry enhancement $\xi-1$ (short dashed line) and $\langle\overline{m}_{\rm max}^0\rangle\pm \Delta\overline{m}_{\rm max}^0$ for normal hierarchy (solid and dashed lines) as functions $\Delta M_{21}/M_1$.