Effective theory for baryogenesis with a Majorana fermion pair coupled to quarks

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 x, uncharged under the SM gauge symmetries but charged under baryon number, to a quarklike up-type fermion and two identical down-type fermions. We introduce baryon number violation by adding Majorana masses tox, 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 x decay and scattering processes. We provide numerical estimates for the baryon asymmetry generated, and for the neutronantineutron oscillation rate.

Automated summary

In order to explain the observed asymmetry of baryon in the Universe, the standard model (SM) was extended by introducing a vector-vector dimension-six effective operator. This operator couples a new Dirac fermion x, which is uncharged under SM gauge symmetries but charged under baryon number, to a quarklike up-type fermion and two identical down-type fermions. Majorana masses were added to x, resulting in the split of the Dirac fermion into two Majorana fermions with unequal masses. The theory predicts violation of C and CP invariances, leading to a baryon asymmetry in x decay and scattering processes through interference between tree and loop amplitudes with on-shell intermediate states. Numerical estimates for the generated baryon asymmetry and neutron-antineutron oscillation rate are provided.