We propose a minimal extension of the standard model in which neutrinos are Dirac particles and their tiny masses are explained without requiring tiny Yukawa couplings. A second Higgs doublet with a tiny vacuum expectation value provides neutrino masses while simultaneously improving the naturalness of the model by allowing a heavier standard-model-like Higgs boson consistent with electroweak precision data. The model predicts a $\mu \to e\gamma$ rate potentially detectable in the current round of experiments, as well as distinctive signatures in the production and decay of the charged Higgs $H^{+}$ of the second doublet which can be tested at future colliders. Neutrinoless double beta decay is absent.

We study the most general scalar potential of the Georgi-Machacek model, which adds isospin-triplet scalars to the Standard Model (SM) in a way that preserves custodial SU(2) symmetry. We show that this model possesses a decoupling limit, in which the predominantly-triplet states become heavy and degenerate while the couplings of the remaining light neutral scalar approach those of the SM Higgs boson. We find that the SM-like Higgs boson couplings to fermion pairs and gauge boson pairs can deviate from their SM values by corrections as large as $\mathcal{O}({v}^{2}/{M}_{\text{new}}^{2})$, where $v$ is the SM Higgs vacuum expectation value and ${M}_{\text{new}}$ is the mass scale of the predominantly-triplet states. In particular, the SM-like Higgs boson couplings to $W$ and $Z$ boson pairs can decouple much more slowly than in two Higgs doublet models, in which they deviate from their SM values like $\mathcal{O}({v}^{4}/{M}_{\text{new}}^{4})$. Furthermore, near the decoupling limit the SM-like Higgs boson couplings to $W$ and $Z$ pairs are always larger than their SM values, which cannot occur in two Higgs doublet models. As such, a precision measurement of Higgs couplings to $W$ and $Z$ pairs may provide an effective method of distinguishing the Georgi-Machacek model from two Higgs doublet models. Using numerical scans, we show that the coupling deviations can reach 10% for ${M}_{\text{new}}$ as large as 800 GeV.

We update the indirect constraints on the Georgi-Machacek model from $B$-physics and electroweak precision observables, including new constraints from $b\to s\gamma$ and $B_s^0\to {\mu }^{+}{\mu }^{-}$. We illustrate the effect of these constraints on the couplings of the Standard-Model-like Higgs boson by performing scans using the most general scalar potential, subject to vacuum stability and perturbativity constraints. We find that simultaneous enhancements of all the Higgs production cross sections by up to 39% are still allowed after imposing these constraints. LHC rate measurements on the Higgs pole could be blind to these enhancements if unobserved nonstandard Higgs decays are present.

We determine the constraints on the isospin and hypercharge of a scalar electroweak multiplet from partial wave unitarity of tree-level scattering diagrams. The constraint from SU(2)L interactions yields T$\le${}7/2 (i.e., n$\le${}8) for a complex scalar multiplet and T$\le${}4 (i.e., n$\le${}9) for a real scalar multiplet, where n=2T+1 is the number of isospin states in the multiplet.

The Georgi-Machacek model adds two $\mathrm{SU}(2{)}_L$-triplet scalars to the Standard Model in such a way as to preserve custodial SU(2) symmetry. We study the generalizations of the Georgi-Machacek model to $\mathrm{SU}(2{)}_L$ representations larger than triplets. Perturbative unitarity considerations limit the possibilities to models containing only $\mathrm{SU}(2{)}_L$ quartets, quintets, or sextets. These models are phenomenologically interesting because they allow the couplings of the 125 GeV Higgs boson to $WW$ and $ZZ$ to be larger than their values in the Standard Model. We write down the most general custodial SU(2)-preserving scalar potentials for these models and outline their phenomenology. We find that experimental and theoretical constraints on the fermiophobic custodial-fiveplet states present in each of the models lead to absolute upper bounds on the 125 GeV Higgs boson coupling strength to $WW$ and $ZZ$.