The decoupling limit in the Georgi-Machacek model

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 O(v(2) / M-new(2)), where v is the SM Higgs vacuum expectation value and M-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 O(v(4) / M-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-new as large as 800 GeV.