Diphoton rate in the inert doublet model with a 125 GeV Higgs boson

An improved analysis of the diphoton decay rate of the Higgs boson in the inert doublet model is presented together with a critical discussion of the results existing in the literature. For a Higgs boson mass Mₕ of 125 GeV and taking into account various constraints---vacuum stability, existence of the inert vacuum, perturbative unitarity, electroweak precision tests, and the LEP bounds---we find regions in the parameter space where the diphoton rate is enhanced. The resulting regions are confronted with the allowed values of the dark matter mass. We find that a significant enhancement in the two-photon decay of the Higgs boson is only possible for constrained values of the scalar couplings λ₃~hH⁺H^-, λ₃₄₅~hHH and the masses of the charged scalar and the dark matter particle. The enhancement above 1.3 demands that the masses of H^± and H be less than 135 GeV (and above 62.5 GeV) and -1.46<λ₃, λ₃₄₅<-0.24. In addition, we analyze the correlation of the diphoton and Zγ rates.