Measuring high-precision luminosity at the CEPC

Purpose Luminosity measurement at the Circular Electron-Positron Collider (CEPC) is required to achieve 10⁻⁴ 10 - 4 precision when operating at the center-of-mass energy of the Z-pole. Approximately 10 ¹2 1 2 Z-bosons will be collected to refine measurements of Standard Model processes. The design of the luminosity calorimeter (LumiCal) takes into account the geometry of the Machine-Detector-Interface (MDI) for detection of Bhabha events. The detector simulation with GEANT predicts measurements of scattered electrons, positrons, and radiation photons. Methods Bhabha events are generated using the BHLUMI program. Electrons arriving within the LumiCal fiducial coverage are counted. The scattering angles of electrons are measured, taking into account deviations caused by multiple scattering and detector resolution. The error of the mean for events passing the lower fiducial edge in polar angle, θ acc θ acc , shall be better than 1 μ μ rad, which corresponds to 10⁻⁴ 10 - 4 on the Bhabha cross section. In each beam direction, the LumiCal design features two silicon position detectors to measure electron impact positions, and 2\, X₀ 2 X 0 LYSO bars behind for e^± /γ e ± / γ identification. The 13\, X₀ 13 X 0 LYSO calorimeter, mounted further downstream in front of the quadrupole magnet, is used to measure energy of electrons. Results The luminosity measurement by counting Bhabha events depends on the accuracy of the lower acceptance angle ( θ acc θ acc ) at the detector’s fiducial edge. The beam-pipe design incorporates low-mass windows of 1 mm thick beryllium (Be) layers to reduce multiple scattering effects. The LumiCal has pixelated silicon detectors with better than 5 μ μ m resolution and LYSO crystals segmented into 3× 3 3 × 3 mm ²