Probing High Scale Dirac Leptogenesis via Gravitational Waves from Domain Walls

Abstract

We propose a novel way of probing high scale Dirac leptogenesis, a viable alternative to canonical leptogenesis scenario where the total lepton number is conserved, keeping light standard model (SM) neutrinos purely Dirac. The simplest possible seesaw mechanism for generating light Dirac neutrinos involve heavy singlet Dirac fermions and a singlet scalar. In addition to unbroken global lepton number, a discrete $Z_2$ symmetry is imposed to forbid direct coupling between right and left chiral parts of light Dirac neutrino. Generating light Dirac neutrino mass requires the singlet scalar to acquire a vacuum expectation value (VEV) that also breaks the $Z_2$ symmetry, leading to formation of domain walls in the early universe. These walls, if made unstable by introducing a soft $Z_2$ breaking term, generate gravitational waves (GW) with a spectrum characterized by the wall tension or the singlet VEV, and the soft symmetry breaking scale. The scale of leptogenesis depends upon the $Z_2$-breaking singlet VEV which is also responsible for the tension of the domain wall, affecting the amplitude of GW produced from the collapsing walls. We find that most of the near future GW observatories will be able to probe Dirac leptogenesis scale all the way upto $10^{11}$ GeV.