New Scalar Contributions to $h\to Z\gamma$

We calculate the Higgs decay rate of $h\to Z\gamma$ 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 $h\to Z\gamma$ matches with that for $h\to \gamma\gamma$ in the limit of $m_Z=0$, but it is different from those in the literature. To illustrate our result, by taking the current $2\sigma$ excess of the $h\to \gamma\gamma$ rate measured by the LHC, we examine the corresponding shift for the $Z\gamma$ decay channel due to the new scalar. We show that the enhancement or reduction of the $h\to Z\gamma$ rate only depends on the relative size of $T$ and the absolute value of $Y$. Explicitly, we predict $0.76<R_{Z\gamma}\equiv \Gamma(h\to Z\gamma)/\Gamma_{SM}(h\to Z\gamma)<2.05$ by imposing the observed range of $1.5<R_{\gamma\gamma}\equiv \Gamma(h\to Z\gamma\gamma)/\Gamma_{SM}(h\to\gamma\gamma)<2$, 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.