Neutrino masses and the baryon asymmetry

TL;DR

The paper tackles the origin of the cosmological baryon asymmetry by connecting it to neutrino properties through leptogenesis: sphalerons couple $B$ and $L$ in the early universe, so a nonzero $B-L$ generated by out-of-equilibrium decays of heavy Majorana neutrinos can yield the observed asymmetry via $B\!\text{−}\!L$ processing. It develops a Boltzmann-equation framework to compute the CP-violating decays and washout effects, and applies it to two explicit mass-matrix realizations based on $SU(5)\times U(1)_F$ and $SU(3)_c\times SU(3)_L\times SU(3)_R\times U(1)_F$, showing that hierarchical heavy neutrinos with $B-L$ broken at the GUT scale can naturally reproduce $n_B/s\sim 10^{-10}$ with a baryogenesis temperature around $T_B\sim 10^{10}$ GeV. The analysis yields concrete predictions for the CP asymmetry $\varepsilon_1$, the washout parameter $\kappa$, and the scale of $B-L$ breaking, while also outlining supersymmetric variants and the resulting shifts in yields and viable parameter ranges. Collectively, the work links neutrino mass scales and mixings to cosmological observables, constraining high-scale physics and informing searches for lepton-number-violating processes and neutrino oscillation patterns. It further highlights the need for beyond-Boltzmann, quantum-mechanical treatments to fully capture the non-equilibrium dynamics of leptogenesis.

Abstract

Due to sphaleron processes in the high-temperature symmetric phase of the standard model the cosmological baryon asymmetry is related to neutrino properties. For hierarchical neutrino masses, with $B-L$ broken at the unification scale $Λ_{GUT}\sim 10^{16} $GeV, the observed baryon asymmetry $n_B/s \sim 10^{-10}$ can be naturally explained by the decay of heavy Majorana neutrinos. We illustrate this mechanism with two models of neutrino masses, consistent with the solar and atmospheric neutrino anomalies, which are based on the two symmetry groups $SU(5)\times U(1)_F$ and $SU(3)_c\times SU(3)_L\times SU(3)_R\times U(1)_F$. We also review related cosmological bounds on Majorana neutrino masses and the use of Boltzmann equations.

Figures (12)

• Figure 1: One of the 12-fermion processes which are in thermal equilibrium in the high-temperature phase of the standard model.
• Figure 2: Jump of the Higgs vacuum expectation value at the critical temperature. Comparison of 4d simulations (triangles, squares) with 3d simulations (stars) and perturbation theory.
• Figure 3: Critical line of first-order phase transitions.
• Figure 4: Effective lepton number violating interaction.
• Figure 5: Tree level and one-loop diagrams contributing to heavy neutrino decays.