Higgs pseudo-observables in $e^+e^-\to h\gamma$

Abstract

We analyse the process $e^+e^-\to h\gamma$ in the Standard Model (SM) and in generic extensions with heavy new physics, using the framework of Higgs pseudo-observables. We decompose the one-loop amplitude into separately gauge-invariant photon-pole, $Z$-pole, and non-pole contributions. For heavy new physics, we show that the leading deviations from the SM are necessarily encoded in the $h\gamma\gamma$ and $hZ\gamma$ on-shell effective couplings. Genuine $e^+e^-h\gamma$ contact interactions arise only at higher orders in a derivative expansion, both in SMEFT and HEFT. Although Higgs decays constrain the magnitudes of these pseudo-observables, they leave their relative sign undetermined. Measurements of $e^+e^-\to h\gamma$ at different FCC-ee energies can resolve this ambiguity. Deviations incompatible with this description would instead point towards relatively light new degrees of freedom, as illustrated by a simplified $Z'$ model.