$h\to \mathrm{{\rm Y}}\gamma$ Decay: Smoking Gun Signature of Wrong-Sign $hb\bar{b}$ Coupling

We perform a model-independent study of new physics effects in the Higgs decay $h\to \mathrm{{\rm Y}}\gamma$, focusing on scenarios that spoil the accidental cancellation between the direct and indirect amplitudes. After imposing all existing constraints from Higgs production and decay measurements, we find that a wrong-sign $hb\bar{b}$ coupling is the only viable scenario capable of enhancing the $h\to \mathrm{{\rm Y}}\gamma$ decay width by nearly two orders of magnitude. Therefore, an observation of a significantly enhanced $h\to \mathrm{{\rm Y}}\gamma$ rate at the LHC or future colliders would provide unambiguous evidence for a wrong-sign $hb\bar{b}$ coupling, directly pointing to the presence of an extended Higgs sector.