Effective Theory for Baryogenesis with a Majorana Fermion Pair Coupled to Quarks

TL;DR

This paper proposes a novel extension of the Standard Model involving Majorana fermions coupled to quarks to explain the Universe's baryon asymmetry, incorporating baryon number violation, CP violation, and providing numerical estimates.

Contribution

It introduces a new effective operator with Majorana fermions to generate baryon asymmetry and explores its ultraviolet completions and phenomenological implications.

Findings

The model can produce the observed baryon asymmetry.

It predicts neutron-antineutron oscillation rates within experimental reach.

The interference effects can generate CP violation in baryon processes.

Abstract

With a goal toward explaining the observed baryon asymmetry of the Universe, we extend the standard model (SM) by adding a vector-vector dimension-six effective operator coupling a new Dirac fermion $\chi$, uncharged under the SM gauge symmetries but charged under baryon number, to a quark-like up-type fermion and two identical down-type fermions. We introduce baryon number violation by adding Majorana masses to $\chi$, which splits the Dirac fermion into two Majorana fermions with unequal masses. We speculate on the origin of the effective operator, the Majorana mass, and the new physics sector connection to the SM, by considering some ultraviolet completion examples. In addition to the baryon number violation, we show that $C$ and $CP$ invariances can be violated in the theory, and the interference between tree and loop amplitudes with on-shell intermediate states can lead to a baryon asymmetry in $\chi$ decay and scattering processes. We write down the Boltzmann equation for baryon number in the early Universe incorporating the decay and scattering baryon asymmetries. We provide numerical estimates for the baryon asymmetry generated, and for the neutron-antineutron oscillation rate.