Neutrino mass generation and leptogenesis via pseudo-Nambu-Goldstone Higgs portal

We consider an extension of the Standard Model with the global symmetry-breaking pattern $\mathrm{SO}(5)/\mathrm{SO}(4)$, where the Higgs boson arises as a pseudo-Nambu-Goldstone boson. The scalar content of the theory consists of a Standard-Model-like Higgs field and an extra real scalar field. The flavor sector of the model is extended by two right-handed neutrinos compatible with the observed light-neutrino phenomenology, and we find that the correct vacuum alignment determines the mass of the heavier neutrino eigenstate to be around 80 TeV. The new singlet-scalar state generates dynamically a Majorana mass term for the heavy-neutrino states. We show how the model leads to the correct baryon asymmetry of the Universe via leptogenesis in the case of two degenerate or hierarchical heavy neutrinos.

Figure 1

The values of the vacuum expectation value and the (absolute value of the) Majorana Yukawa coupling, $|{\alpha }_0|$, as a function of $M_0$, when the correct Higgs mass and the vacuum solution are imposed. Solid (dashed) lines correspond to the democratic (diagonal) $\alpha$-matrices in Eq. (10).

Figure 2

The minimum allowed physical Higgs mass value as a function of $M_0$ when the vacuum solution is imposed. The figure corresponds to the democratic texture in Eq. (10). The result of the diagonal case is practically indistinguishable.

Figure 3

The values for the symmetry-breaking scale as a function of the Majorana neutrino Yukawa coupling for one and two heavy RH neutrinos with identical couplings. The points correspond to values at $M_0=0$ in Fig. 1.

Figure 4

The benchmark scenario (case 1) in Table 1 with mass hierarchy $M_2=79.5\mathrm{TeV}$ and $M_1=8.6\mathrm{TeV}$. Shown in the figure are the abundances of the two neutrinos with solid blue and red lines. The dashed lines show the corresponding equilibrium densities. The purple curve shows the resulting net lepton abundance normalized by a factor ${10}^{-8}$ so that the value corresponding to the observed BAU is $Y_{L-\overline{L}}=0.11\times {10}^{-8}$ shown by the horizontal dotted-dashed line.

Figure 5

We show in these plots a comparison of the main results presented in Fig. 4 (dashed purple curve) and cases 2 (upper panel) and 3 (lower panel) presented in Table 1. See the text for more details.