Flavour and dark matter in a scoto/type-II seesaw model

Abstract

The neutrino mass and dark matter (DM) problems are addressed in a Standard Model extension where the type-II seesaw and scotogenic mechanisms coexist. The model features a flavour $\mathcal{Z}_8$ discrete symmetry which is broken down to a $\mathcal{Z}_2$, stabilising the (scalar or fermion) DM particle. Spontaneous CP violation is implemented through the complex vacuum expectation value of a singlet scalar field, inducing observable CP-violating effects in the lepton sector. The structure of the effective neutrino mass matrix leads to constraints on the low-energy neutrino observables, namely the atmospheric neutrino mixing angle $θ_{23}$, the Dirac CP-violating phase $δ$ and the absolute neutrino mass scale $m_{\rm lightest}$. In particular, in most cases, the model selects one $θ_{23}$ octant with $δ\simeq 3π/2$. Moreover, the obtained lower bounds on $m_{\rm lightest}$ are typically in the range probed by cosmology. We also analyse the constraints imposed on the model by current experimental limits on charged lepton flavour violating (cLFV) processes, as well as future projected sensitivities. It is shown that the Higgs triplet and scotogenic contributions to cLFV never overlap and that the interplay among Yukawa couplings, dark charged scalar masses and mixing leads to a wide parameter-space region compatible with current experimental bounds. We investigate the scalar and fermion DM parameter space of our model by considering relic density, direct-detection (DD) and collider constraints. For scalar DM the mass interval $68 \ \text{GeV} \lesssim m_{\text{DM}} \lesssim 90 \ \text{GeV}$ is viable and will be probed by future DD searches. In the fermion DM case, correct relic density is always obtained for $m_{\text{DM}} \gtrsim 45$ GeV thanks to dark fermion-scalar coannihilation channels.