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 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.

Figures (12)

• Figure 1: The cut-view on one side of the IP is shown for the MDI region containing the LumiCal modules mounted before the flange of the race-track beam-pipe and before the quadrupole magnet. The LumiCa modules located before the flange consist of two Si-wafers and $2\; X_0$ LYSO bars arranged above and below the race-track beam pipe. The green-colored Be layers serve as low-mass windows in conjunction with dual-layer Al pipes for water cooling.
• Figure 2: The LumiCal design in the GEANT simulation is depicted in two views: a) the $x$-$y$ perspective of the LYSO bars in front of the flange and behind it, with dimensions of $3\times 3$ mm$^2$ and $10\times 10$ mm$^2$, respectively, positioned above and below the race-track pipe; b) the $x$-$z$ view of a 50 GeV electron shower in the Si-wafers and LYSO bars, along with the flange and bellow.
• Figure 3: Distributions in theta of scattered electrons of BHLUMI are plotted, which are generated (in 10 to 120 mRad) in the center-of-mass frame and are boosted for the beam crossing of 33 mRad. The data points in red are electrons selected outside the beam pipe with $\theta >25$ mRad to the beam pipe centers (and $|y|>25$ mm in between, projected at $|z|=1$ m). Bhabha events with both electrons and positrons detected in detector fiducial of $|y|>25$ mm (projected at $|z|=1$ m) are plotted in blue.
• Figure 4: The green points are electron hits generated using BHLUMI (in $\theta$ range of $10-120$ mRad) which are boosted and projected at $|z| \!=\! 1$ m. The blue boxes are hits with $e^+$ and $e^-$ selected in the LumiCal fiducial region (in red dashed lines), for the threshold of $25 <\theta_{lab}<80$ mRad to the beam pipe centers and the joint rectangles in between, excluding the areas below the beam pipe of $|y|>25$ mm.
• Figure 5: The distributions of scattered $e^+, e^-$ opening angle, generated with the BHLUMI, are plotted in the center-of-mass (CM) frame and the boosted laboratory frame. Events with radiative photons cause a deviation from the back-to-back angle of $e^+$ and $e^-$.