Leptogenesis constraints on the mass of right-handed gauge bosons

We discuss leptogenesis constraints on the mass of the right-handed $W$ boson ($W_R$) in a TeV-scale left-right seesaw model for neutrino masses. For generic Dirac mass of the neutrinos, i.e. with all Yukawa couplings $\lesssim 10^{-5.5}$, it has been pointed out that successful leptogenesis requires a lower bound of 18 TeV on the $W_R$ mass, pushing it beyond the reach of the LHC. Such TeV-scale left-right seesaw model must, however, be parity asymmetric for type-I seesaw to give the observed neutrino masses. This class of models can accommodate larger Yukawa couplings, which give simultaneous fits to charged-lepton and neutrino masses, by invoking either cancellations or specific symmetry textures for Dirac ($M_D$) and Majorana ($M_N$) masses in the seesaw formula. We show that in this case the leptogenesis bound on $M_{W_R}$ can be substantially weaker, i.e. $M_{W_R}\gtrsim 3\mathrm{TeV}$ for $M_N\lesssim M_{W_R}$. This happens due to considerable reduction of the dilution effects from $W_R$-mediated decays and scatterings, while the washout effects due to inverse decays are under control for certain parameter ranges of the Yukawa couplings. We also show that this model is consistent with all other low-energy constraints, such as lepton flavor violation and neutrinoless double beta decay. Thus, a discovery of the right-handed gauge bosons alone at the LHC will not falsify leptogenesis as the mechanism behind the matter-antimatter asymmetry in our Universe.

Figure 1

The dimensionless collision rate parameters defined in Eqs. (50)–(53), compared with the rescaled Hubble rate $H=zH(z)/H_N$. We have shown the results for $\alpha =1$ and $l=1$, and for the fit given in Sec. 2a. The vertical dashed line shows the critical temperature $z_c=m_{N_1}/T_c$, beyond which the sphaleron transitions freeze out.

Figure 2

The final lepton asymmetry as a function of the scale of the Yukawa coupling matrix for various $W_R$ masses. The dashed horizontal line shows the value required to satisfy the observed baryon asymmetry.

Figure 3

Realistic fits shown by the dots, compared with the value obtained by rescaling the Yukawa couplings (solid line) for the $M_{W_R}=3\mathrm{TeV}$.

Figure 4

The allowed parameter space in our TeV-scale L-R seesaw model, satisfying all the constraints in the lepton sector, including the neutrino oscillation data, LFV and leptogenesis constraints.

Figure 5

The final lepton asymmetry as a function of the $W_R$ mass for various values of the degeneracy parameter.