Probing the Effective HZ$γ$ Coupling via H$γ$ Production at FCC-ee

Abstract

This work presents the first dedicated study of the $e^+e^-\rightarrow Hγ$ production mode at the FCC-ee. The analysis exploits all foreseen FCC-ee running scenarios at and above the WW threshold. This study demonstrates that the $e^+e^-\rightarrow Hγ$ process provides a novel and direct handle on the effective $Hγγ$ and $HZγ$ couplings through production, complementary to measurements based on Higgs decays. By combining measurements of multiple Higgs decay channels and center-of-mass energies, we show that an overall precision of 15% is achievable on the effective $HZγ$ coupling. In addition, unlike decay observables, this process is also sensitive to the relative sign of the coupling and allows a full resolution of the corresponding sign ambiguity. We compare the expected sensitivity to the projected bounds from direct measurements of the $γγ$ and $Zγ$ branching ratios at the HL-LHC and FCC-ee, as well as projections from global fits in the SMEFT framework.