Cosmic Microwave Background, Matter-Antimatter Asymmetry and Neutrino Masses

TL;DR

The paper analyzes thermal leptogenesis within the seesaw framework, showing that the observed baryon asymmetry can be expressed in terms of four neutrino parameters: $M_1$, $|\varepsilon_1|$, $\widetilde{m}_1$, and $\overline{m}$. By solving the Boltzmann equations for $N_1$ and $B-L$ and enforcing the CMB-measured $\eta_{B0}$, it derives parameter bounds and demonstrates that quasi-degenerate light neutrinos are incompatible with this mechanism, while hierarchical spectra remain viable with a characteristic baryogenesis temperature $T_B$ around $10^{10}$ GeV. The results organize into small and large $M_1$ regimes, showing that washout effects bound $M_1$ from below for small $M_1$ and suppress the asymmetry at large $M_1$, thereby linking cosmological data to heavy neutrino scales. Overall, the work connects cosmology with neutrino mass models, informing expectations for neutrino masses and guiding future experimental searches such as neutrinoless double beta decay.

Abstract

We study the implications of thermal leptogenesis for neutrino parameters. Assuming that decays of N_1, the lightest of the heavy Majorana neutrinos, initiate baryogenesis, we show that the final baryon asymmetry is determined by only four parameters: the CP asymmetry epsilon_1, the heavy neutrino mass M_1, the effective light neutrino mass \tilde{m}_1, and the quadratic mean \bar{m} of the light neutrino masses. Imposing the CMB measurement of the baryon asymmetry as constraint on the neutrino parameters, we show, in a model independent way, that quasi-degenerate neutrinos are incompatible with thermal leptogenesis. For maximal CP asymmetry epsilon_1, and neutrino masses in the range from (Δm^2_{sol})^{1/2} to (Δm^2_{atm})^{1/2}, the baryogenesis temperature is T_B = O(10^{10}) GeV.