CP-violation in production of heavy neutrinos from bubble collisions

TL;DR

This work analyzes CP-violation during bubble collisions in first-order phase transitions and classifies three CP-violating channels: production of heavy fermions, production of heavy states with subsequent decays, and production of light SM states. It embeds these mechanisms into a minimal BC-based cogenesis model that splits lepton number between the Standard Model and a dark sector, enabling BAU generation and asymmetric dark matter while tying neutrino masses to the phase transition via an inverse seesaw. The authors compute CP asymmetries for heavy-particle production and decay as well as light-state production (on- and off-shell N contributions), and assess wash-out and equilibration effects to map viable parameter space. They also forecast gravitational wave signatures from the phase transition, showing that part of the viable region could be probed by the Einstein Telescope, linking cosmological observables to particle physics. Overall, the paper provides a coherent framework where FOPT bubble collisions drive CP-violation, BAU, ADM, and neutrino masses, with a potentially detectable GW imprint.

Abstract

First order phase transitions (FOPT) in the early Universe can be powerful emitters of both relativistic and heavy particles, upon the collision of ultra-relativistic bubble shells. If the particles coupling to the bubble wall have CP-violating interactions, the same collision process can also create a local lepton or baryon charge. This CP-violation can originate from different channels, which have only been partially addressed in the literature. We present a systematic analysis of the different channels inducing CP-violation during bubble collisions: 1) the decay of heavy particles 2) the production of heavy particles and 3) the production of light and relativistic Standard Model (SM) particles. As an illustration of the impact that such mechanisms can have on baryon number and dark matter (DM) abundance, we then introduce a simple model of cogenesis, separating a positive and a negative lepton number in the SM and a dark sector (DS). The lepton number asymmetry in the SM can be used to explain the baryon asymmetry of the Universe (BAU), while the opposite asymmetry in the DS is responsible for determining the abundance of DM. Moreover, the masses of light neutrinos can be understood via the inverse seesaw mechanism, with the lepton-violating Majorana mass originating from the FOPT. A typical signal produced by a FOPT is the irreducible gravitational wave (GW) background. We find that a substantial portion of the parameter space can be probed at future observatories like the Einstein Telescope (ET).

Figures (8)

• Figure 1: Left Panel: Cartoon of the particle production from the BC mechanism. The bubbles in blue collide, producing boosted particles via the channels $\phi^\star \to \chi N$ and $\phi^\star \to \chi HL$, with an off-shell $N$. Right Panel: Diagrams inducing the CP-violation via one loop insertion, the analytic form of those diagrams will be computed in this paper.
• Figure 2: Feynman diagrams for the CP-violation in the production of the $N_i$ and $\chi_I$.
• Figure 3: CP-violation resulting from the process $\phi^\star \rightarrow H L_\alpha \chi_I^c$.
• Figure 4: Comparison of the off-shell production computed numerically (colored points) with analytic approximation (solid lines) using Eq.\ref{['eq:OFFshellAnalytic']}, for $y=Y= \sqrt{4\pi}$ (left-panel) and $y=Y=1$ (right panel).
• Figure 5: Scheme of the cascade of interactions leading to the production of ADM in the dark sector. The Lagrangian in Eq.\ref{['eq:model_DM']} allows $\chi$ and $\phi$ to quickly decay into $\tilde{\chi}$ and $\tilde{\phi}$. The coupling of $\tilde{\phi}$ to $H$ will allow to freeze-out the abundance of $\tilde{\phi}$ and the symmetric abundance $\tilde{\chi}$. This requires typically large enough couplings and $m_{\tilde{\phi}} \lesssim 500$ GeV.