Measurement of $\mathcal{B}r(H \to Z\gamma)$ at the $250$ GeV ILC

The $e^+e^- \to HZ$ process with the subsequent decay of the Higgs boson $H \to Z\gamma$ is studied, where both $Z$-bosons are reconstructed in the final states with two jets. The analysis is performed using Monte Carlo data samples obtained with detailed ILD detector simulation assuming an integrated luminosity of $2ab^{-1}$, beam polarizations of ${\mathcal{P}}_{e^-e^+} = (-0.8, +0.3)$, and center-of-mass energy of $\sqrt{s} = 250$ GeV for the electron–positron International Linear Collider being currently designed. The analysis is repeated for the case of two $0.9 ab^{-1}$ data samples with polarizations ${\mathcal{P}}_{e^-e^+} = (\mp0.8, \pm0.3)$. Contributions of the potential background processes are studied using all available ILD MC event samples. The largest background comes from the $e^+e^- \to W^+W^-$ process supplemented by an energetic photon produced by initial state radiation. To suppress this background, we require that at least one of the $Z$ bosons decays to $b$-jets. To reduce the jet reconstruction uncertainties the $M_{\Delta} = M(jj\gamma) - M(jj) + M(Z_{\rm nom})$ variable is used, where $M(Z_{\rm nom}) = 91.2$ GeV. The $M_{\Delta}$ distributions are obtained for the studied signal and backgrounds to estimate the expected accuracy of the ${\mathcal B}r(H \to Z\gamma)$ measurement. The accuracy is $22\%$ for the option of the single polarization sample described above and deteriorate to $24\%$ in case of the sample with two polarizations. The proposed method can be applied at any future $e^+e^-$ collider.