Scale of Dirac leptogenesis and left-right symmetry in the light of recent PTA results

Abstract

Motivated by the recent release of new results from five different pulsar timing array (PTA) experiments claiming to have found compelling evidence for primordial gravitational waves (GW) at nano-Hz frequencies, we study the consequences for two popular beyond the Standard Model (SM) frameworks, where such nano-Hz GW can arise due to annihilating domain walls (DW). Minimal framework of Dirac leptogenesis, as well as left-right symmetric model (LRSM) can lead to formation of DW due to spontaneous breaking of $Z_2$ symmetry. Considering the NANOGrav 15 yr data, we show that the scale of Dirac leptogenesis should be above $10^7$ GeV for conservative choices of Dirac Yukawa couplings with fine-tuning at the level of the SM. The scale of {\it minimal} LRSM is found to be more constrained $M_{\rm LR} \sim 10^6$ GeV in order to fit the NANOGrav 15 yr data. On the other hand, the {\it non-minimal} LRSM can be compatible with the NANOGrav data for $10^2 \, {\rm TeV} \lesssim M_{\rm LR} \lesssim 10^3$ TeV but with the corresponding $B-L$ breaking scale violating collider bounds.