We study the most general scalar potential of the Georgi-Machacek model, which adds isospintriplet 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-

We update the indirect constraints on the Georgi-Machacek model from B-physics and electroweak precision observables, including new constraints from b → sγ and B 0 s → µ + µ − . 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 non-standard Higgs decays are present.

Models in which the Higgs sector is extended by a single electroweak scalar multiplet X can possess an accidental global U(1) symmetry at the renormalizable level if X has isospin T greater or equal to 2. We show that all such U(1)-symmetric models are excluded by the interplay of the cosmological relic density of the lightest (neutral) component of X and its direct detection cross section via Z exchange. The sole exception is the T=2 multiplet, whose lightest member decays on a few-day to few-year timescale via a Planck-suppressed dimension-5 operator.Comment: 20 pages, 5 figures. Additional constraints from Higgs decay, typos corrected in Eqs. 26 and A3, refs added, version accepted by PR

The Georgi-Machacek model adds scalar triplets to the Standard Model Higgs sector in such a way as to preserve custodial SU(2) symmetry in the scalar potential. This allows the triplets to have a nonnegligible vacuum expectation value while satisfying constraints from the ρ parameter. Depending on the parameters, the 125 GeV neutral Higgs particle can have couplings to W W and ZZ larger than in the Standard Model due to mixing with the triplets. The model also contains singly-and doubly-charged Higgs particles that couple to vector boson pairs at tree level (W Z and like-sign W W , respectively).GMCALC is a self-contained FORTRAN program that, given a set of input parameters, calculates the particle spectrum and tree-level couplings in the Georgi-Machacek model, checks theoretical and indirect constraints, and computes the branching ratios and total widths of the scalars. It also generates a param card.dat file for MadGraph5 to be used with the corresponding FeynRules model implementation.

We study the phenomenology of the Georgi-Machacek model at next-to-leading order (NLO) in QCD matched to parton shower, using a fully-automated tool chain based on MadGraph5 aMC@NLO and FeynRules. We focus on the production of the fermiophobic custodial fiveplet scalars H through vector boson fusion (VBF), associated production with a vector boson (V H5), and scalar pair production (H5H5). For these production mechanisms we compute NLO corrections to production rates as well as to differential distributions. Our results demonstrate that the Standard Model (SM) overall K-factors for such processes cannot in general be directly applied to beyond-the-SM distributions, due both to differences in the scalar electroweak charges and to variation of the K-factors over the differential distributions.