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 𝒵8 discrete symmetry which is broken down to a 𝒵2, 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 θ23, the Dirac CP-violating phase δ and the absolute neutrino mass scale mlightest. In particular, in most cases, the model selects one θ23 octant with δ ≃ 3π/2. Moreover, the obtained lower bounds on mlightest 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 GeV ≲ mDM ≲ 90 GeV is viable and will be probed by future DD searches. In the fermion DM case, correct relic density is always obtained for mDM ≳ 45 GeV thanks to dark fermion-scalar coannihilation channels.

Discrete dark matter mechanism as the source of neutrino mass scales

“…In this model, the dominance of the tree-level seesaw mechanism over the scotogenic model is a prediction. This is not a generic feature from scoto-seesaw models [7,8,[16][17][18][19][20], it is a result from the fact that the mass mediators for both mechanisms in this model belong to the same…”

Neutrino mass hierarchy from the discrete dark matter model

“…As stated in the next section (for a detailed account, see [1]), neutrino masses are generated by a type-I seesaw and a scotogenic mechanism [5][6][7]. The key feature of this model, in contrast to previous works, is that it naturally predicts the dominance of the seesaw over the scotogenic mechanism [8][9][10][11][12][13][14].…”

Hybrid scoto/seesaw: flavour and dark matter

“…-LHC Higgs data: The odd-neutral scalars 𝑆 𝑖 contribute to the Higgs invisible decay through 3For a detailed analysis on the fermion DM scenario see ref. [6]. 4The computation of Ωℎ 2 and 𝜎 SI , is performed with MicrOmegas [21].…”

“…It can be shown that these LHC Higgs data constraints also exclude the low-mass region 45.6 GeV ≲ 𝑚 𝑆 1 ≲ 60 GeV (see ref. [6] for details). Thus, we conclude that the intermediate mass region 68 GeV ≲ 𝑚 𝑆 1 ≲ 90 GeV for scalar DM is viable since it leads to a correct relic density while evading current collider and DD experimental constraints.…”