A Model of Annihilogenesis

TL;DR

This paper introduces a novel leptogenesis model via annihilogenesis involving Majorana neutrinos during a phase transition, producing a baryon asymmetry consistent with observations and relaxing previous neutrino mass constraints.

Contribution

It presents an explicit annihilogenesis model with CP violation from interference effects, expanding the understanding of baryon asymmetry generation during phase transitions.

Findings

CP asymmetry $ imes 10^{-9}$ to $10^{-7}$ for order-one Yukawas

Baryon asymmetry matches observed values across broad parameters

Model relaxes constraints on neutrino masses and reheating temperature

Abstract

We present an explicit model of leptogenesis via annihilogenesis in which two right-handed Majorana neutrinos couple to the Standard Model lepton doublets and Higgs, and acquire a large mass shift during a strong first-order phase transition of an additional scalar singlet. As bubbles of true vacuum expand, the $\chi_a$ are reflected off the walls and confined to shrinking pockets of false vacuum, where the density grows and the dominant CP-violating process is the $2 \to 4$ annihilation $\chi_1 \chi_1 \to L_1 L_1 \Phi^* \Phi^*$. Interference between tree-level $W$ and $B$ exchange and one-loop diagrams containing the heavier $\chi_2$ produces a CP asymmetry $\epsilon$, which we evaluate numerically and find to lie in the range $|\epsilon| \sim 10^{-9}$--$10^{-7}$ for $\mathcal{O}(1)$ Yukawa couplings. Electroweak sphalerons convert the resulting lepton asymmetry into a baryon asymmetry $Y_{\Delta B}$ that reproduces the observed value across a broad region of parameter space, with little sensitivity to the bubble-wall velocity or initial pocket size. The Majorana mass that controls $\epsilon$ is the residual mass of $\chi_1$ inside the collapsing pockets rather than its post-transition value, so the usual relation between the singlet mass and the Standard Model active neutrino masses is relaxed. As a result, the upper bound on $|\epsilon|$ from the largest light-neutrino mass that constrains standard thermal leptogenesis does not apply, and the lower limits on the right-handed-neutrino scale and the reheating temperature are relaxed.