We calculate the Higgs decay rate of h→Zγ by including the contributions from new scalars with arbitrary quantum numbers of the weak isospin (T) and hypercharge (Y) in the standard model. We find that our general formula for the decay rate of h→Zγ matches with that for h→γγ in the limit of mZ=0, but it is different from those in the literature. To illustrate our result, by taking the current 2σ excess of the h→γγ rate measured by the LHC, we examine the corresponding shift for the Zγ decay channel due to the new scalar. We show that the enhancement or reduction of the h→Zγ rate only depends on the relative size of T and the absolute value of Y. Explicitly, we predict 0.76<RZγ≡Γ(h→Zγ)/ΓSM(h→Zγ)<2.05 by imposing the observed range of 1.5<Rγγ≡Γ(h→Zγγ)/ΓSM(h→γγ)<2, which is independent of the number of multiplets and the couplings to the Higgs particle as long as the scalars are heavier than 200 GeV. This result provides a clear signature for the future LHC measurements to test physics beyond the standard model.
