Testing the seesaw mechanism and leptogenesis with gravitational waves: Majorana versus Dirac cases

We investigate the B − L gauge extension of the Standard Model that the Dirac seesaw mechanism with thermal leptogenesis leaves imprints in the early Universe via the stochastic gravitational background emanating from a network of cosmic strings when B − L symmetry is broken. With right-handed neutrino mass lighter than the typical scale of grand unification, the B − L symmetry protecting the right-handed neutrinos leads to constraints on the Yukawa couplings for both Dirac and Majorana scenarios. Estimating the predicted gravitational wave background we find that future space-borne missions could probe the range concerning thermal Dirac leptogenesis. In a comparative analysis between such probes of gravitational waves sourced from cosmic strings in Dirac and Majorana leptogenesis in the B − L extension, based on the energy scales of the leptogenesis, for instance, gravitational waves detectors will be able to probe the scale of Dirac leptogenesis up to 10 9 GeV , while for Majorana leptogenesis it would be upto 10 12 GeV .