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

TL;DR

This study estimates the precision of measuring the Higgs decay to Z gamma at the 250 GeV ILC using detailed simulations, background suppression, and a novel mass variable, achieving about 22-24% accuracy.

Contribution

It introduces a detailed simulation-based method for measuring Br(H→Zγ) at the ILC, including background suppression techniques and a new mass variable to improve accuracy.

Findings

Expected measurement accuracy of 22-24% for Br(H→Zγ)

Effective background suppression using Z→b-jet decays

A novel mass variable M_Δ improves measurement precision

Abstract

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 (The International Large Detector) detector simulation assuming an integrated luminosity of 2 ab$^{-1}$, beam polarizations of ${\cal{P}}_{e^-e^+} = (-0.8, +0.3)$, and center-of-mass energy of $\sqrt{s}$ = 250 GeV. The analysis is repeated for the case of two 0.9 ab$^{-1}$ data samples with polarizations ${\cal{P}}_{e^-e^+} = (\mp0.8, \pm0.3)$. 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 ${\cal B}r(H \to Z\gamma)$ 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.