The expected measurement precision of the branching ratio of the Higgs decaying to the di-photon at the CEPC

Abstract: This paper presents the prospects of measuring $\sigma(e^{+}e^{-}\to ZH)\times Br(H \to \gamma\gamma)$ in 3 $Z$ decay channels $Z \to q\bar{q} / \mu^{+} \mu^{-} / \nu\bar{\nu}$ using the baseline detector with $\sqrt{s} = 240 GeV$ at the Circular Electron Positron Collider (CEPC) . The simulated Monte Carlo events are generated and scaled to an integrated luminosity of 5.6 $ab^{-1}$ to mimic the data. Extrapolated results to 20 $ab^{-1}$ are also shown. The expected statistical precision of this measurement after combining 3 channels of $Z$ boson decay is 7.7\%. With some preliminary estimation on the systematical uncertainties, the total precision is 7.9\%. The performance of CEPC electro-magnetic calorimeter (ECAL) is studied by smearing the photon energy resolution in simulated events in $e^{+}e^{-} \to ZH \to q\bar{q}\gamma\gamma$ channel. In present ECAL design, the stochastic term in resolution plays the dominant role in the precision of Higgs measurements in $H \to \gamma\gamma$ channel. The impact of the resolution on the measured precision of $\sigma(ZH)\times Br(ZH \to q\bar{q}\gamma\gamma)$ as well as the optimization of ECAL constant term and stochastic term are studied for the further detector design.