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Register a new userStudy of the $hgamma Z$ coupling at the ILC
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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 $hgamma Z$ 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 $P(e^-,e^+)=(-80%,+30%)$.
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We studied the $e^+e^- to h gamma $ process at the International Linear Collider (ILC) at $sqrt{s}=250$ GeV, based on the full detector simulation of the International Large Detector (ILD). This process is loop-induced in the Standard Model (SM) and is sensitive to new physics which alters $h gamma gamma$ or $h gamma Z$ coupling. We performed the analysis by employing the leading signal channels with $h to b bar{b}$ and $h to WW^*$ and including full SM background processes. The results are obtained for two scenarios of beam polarisations each with an integrated luminosity of 900 fb$^{-1}$. We found the expected significance of the SM signal is 0.40$sigma$ for $P(e^-,e^+)=(-0.8,+0.3)$ (the left-handed polarisation), and 0.06$sigma$ for $P(e^-,e^+)=(+0.8,-0.3)$ (the right-handed polarisation). The bounds on new physics effects are reported as the 95% C.L. upper limit for the cross-section of $e^+e^- to h gamma$: $sigma_{hgamma}^L <$ 1.8 fb and $sigma_{hgamma}^R <$ 0.5 fb respectively for left- and right-handed polarisations. The constraints on effective $hgamma Z$ couplings are to be further studied.
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