Study of the $hγZ$ coupling at the ILC

Abstract: We study the $h \gamma Z$ coupling, which is a loop induced coupling in the Standard Model (SM), to probe new physics. In a global fit based on the SM Effective Field Theory, measurement of the SM $h \gamma Z$ coupling can provide a very useful constraint, in particular for the precise determination of $hZZ$ and $hWW$ couplings. At the International Linear Collider (ILC), there are two direct ways to study the $h \gamma Z$ coupling: one is to measure the branching ratio of the $h \to \gamma Z$ decay and the other to measure the cross section for the $e^+e^- \to h \gamma$ process. We have performed a full simulation study of the $e^+e^- \to h \gamma$ process at the 250 GeV ILC, assuming 2 ab$^{-1}$ data collected by the International Large Detector (ILD). The expected 1$\sigma$ bound on the effective $h\gamma Z$ coupling ($\zeta_{AZ}$) combining measurements of the cross section for $e^+e^- \to h \gamma$ followed by $h \to b \bar{b}$ and the $h \to \gamma Z$ branching ratio is $-0.0015<\zeta_{AZ}<0.0015$. The expected significance for the signal cross section in the fully hadronic $h \to WW^*$ channel is 0.09 $\sigma$ for beam polarizations of $P(e^-,e^+)=(-80\%,+30\%)$.