Paper detail

Measuring high-precision luminosity at the CEPC

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 the detection of Bhabha events. The detector simulation with GEANT predicts measurements of scattered electrons, positrons, and radiation photons. Results: The luminosity measurement derived from Bhabha event counting relies on the low-θ fiducial edge with a mean of better than 1 μRad. Both the beam monitoring on the interaction point (IP) and the LumiCal Si-wafer positions shall be monitored to a mean of better than 1 μm. The beam-pipe design is optimized with a low-mass window of less than 2 mm thick Be window for calibration of multiple scattering. With Si-layers capable of 5 μm resolution, the error on the mean of fiducial edges is measured to 1 μm. The detector displacement requires survey monitoring to sub-micron precision. Conclusion: The scattered electrons at IP are measured with the LumiCal Si-wafers and high granularity of LYSO bars. The accompanying photon with larger opening angles can be identified and studied for radiative Bhabha events. The NLO calculations for the Bhabha interaction are achieving 10^-4. With the LumiCal design of silicon detectors and LYSO calorimeters, the precision is pursued for IP and detector positions being monitored, to achieve the goal of 10^-4 precision on luminosity measurement.