arXiv subjects
Publications and source records attributed to Erik Schumacher.
The same leptoquarks that explain the recently observed anomaly in $R_K$ can generate naturally small Majorana neutrino masses at one-loop level through mixing with the standard model Higgs boson. This is particularly relevant in models with at least two leptoquarks contributing to $b \rightarrow s ll$ transitions.
Lepton flavor violating Higgs decays can arise in flavor symmetry models where the Higgs sector is responsible for both the electroweak and the flavor symmetry breaking. Here we advocate an $S_4$ three-Higgs-doublet model where tightly constrained flavor changing neutral currents are suppressed by a remnant $Z_3$ symmetry. A small breaking of this $Z_3$ symmetry can explain the $2.4\,σ$ excess of Higgs decay final states with a $μτ$ topology reported recently by CMS if the new neutral scalars are light. The model also predicts sizable rates for lepton flavor violating Higgs decays in the $eτ$ and $e μ$ channels because of the unifying $S_4$ flavor symmetry.
We propose a model based on the $SU(5)$ grand unification with an extra $A_{4}\otimes Z_{2}\otimes Z_{2}^{\prime }\otimes Z_{2}^{\prime \prime}\otimes U\left( 1\right) _{f}$ flavor symmetry, which accounts for the pattern of the SM fermion masses and mixings. The observed hierarchy of charged fermion masses and quark mixing matrix elements arises from a generalized Froggatt-Nielsen mechanism triggered by a scalar $\mathbf{24}$ representation of $SU(5)$ charged under the global $U(1)_{f}$ and acquiring a VEV at the GUT scale. The light neutrino masses are generated via a radiative seesaw mechanism with a single heavy Majorana neutrino and neutral scalars running in the loops. The model predictions for both quark and lepton sectors are in good agreement with the experimental data. The model predicts an effective Majorana neutrino mass, relevant for neutrinoless double beta decay, with values $m_{ββ}=$ 4 and 50 meV for the normal and the inverted neutrino spectrum, respectively. The model also features a suppression of CP violation in neutrino oscillations, a low scale for the heavy Majorana neutrino (few TeV) and, due to the unbroken $Z_{2}$ symmetry, a natural dark matter candidate.
We discuss neutrino masses and mixing in the context of seesaw type I models with three right-handed Majorana neutrinos and an approximately diagonal Dirac sector. This ansatz is motivated by the idea that the flavor structure in the right-handed Majorana masses is responsible for the large mixing angles, whereas the small mixing angle $θ_{13}$ originates from the Dirac Yukawa couplings in analogy to the quark sector. To obtain $θ_{13} \approx 0.15$ we study a possible $SU(5)$ grand unified theory realization with a $U(1) \times \mathbb{Z}'_2 \times \mathbb{Z}''_2 \times \mathbb{Z}'''_2$ flavor symmetry and include a complex perturbation parameter in the Dirac mass matrix. The consequences for $CP$ violating phases and effects on leptogenesis are investigated.