Measuring high-precision luminosity at the CEPC

Abstract

Purpose: Luminosity measurement at the Circular Electron-Positron Collider (CEPC) is required to achieve 10^{-4} precision when operating at the center-of-mass energy of the Z-pole. Approximately 10^{12} 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. Results: The luminosity measurement by counting Bhabha events depends on the accuracy of the lower acceptance angle (θ_{acc}) at the detector's fiducial edge. The beam-pipe design incorporates lowmass windows of 1 mm thick beryllium (Be) layers to reduce multiple scattering effects. The LumiCal has pixelated silicon detectors with better than 5 um resolution and LYSO crystals segmented into 3x3 mm2, which enhances the capability for detecting radiative Bhabha events. To achieve a precision level of 10^{-4}, it is crucial to monitor the interaction point (IP) of colliding beams and the positions of detectors with the error on mean of better than 1 urad. Conclusion: The LumiCal measures Bhabha scattering events using Si-detectors and finely segmented LYSO arrays. Its design is optimized for detecting radiative photons that are separated from electrons by a sufficiently large opening angle. This measurement aims to detect higher order corrections to the Bhabha interaction. Emphasis is placed on steering the beams for IP distribution and survey monitoring of detector positions to achieve high-precision luminosity measurements.lation with GEANT predicts measurements of scattered electrons, positrons, and radiation photons.