Full parameter scan of the Zee model: exploring Higgs lepton flavor violation

Abstract

We study the general Zee model, which includes an extra Higgs scalar doublet and a new singly-charged scalar singlet. Neutrino masses are generated at one-loop level, and in order to describe leptonic mixing, both the Standard Model and the extra Higgs scalar doublets need to couple to leptons (in a type-III two-Higgs doublet model), which necessarily generates large lepton flavor violating signals, also in Higgs decays. Imposing all relevant phenomenological constraints and performing a full numerical scan of the parameter space, we find that both normal and inverted neutrino mass orderings can be fitted, although the latter is disfavored with respect to the former. In fact, inverted ordering can only be accommodated if $θ_{23}$ turns out to be in the first octant. A branching ratio for $h \to τμ$ of up to $10^{-2}$ is allowed, but it could be as low as $10^{-6}$. In addition, if future expected sensitivities of $τ\to μγ$ are achieved, normal ordering can be almost completely tested. Also, $μe$ conversion is expected to probe large parts of the parameter space, excluding completely inverted ordering if no signal is observed. Furthermore, non-standard neutrino interactions are found to be smaller than $10^{-6}$, which is well below future experimental sensitivity. Finally, the results of our scan indicate that the masses of the additional scalars have to be below $2.5$ TeV, and typically they are lower than that and therefore within the reach of the LHC and future colliders.