Optimizing the Design of a Cherenkov Detector for a New Two-Photon Exchange Experiment

The discrepancy between polarized and unpolarized measurements of the proton’s form factor ratio may be caused by two-photon exchange (TPE), though the evidence has so far been inconclusive. Recent experiments, including OLYMPUS, extracted the TPE contribution to elastic ep scattering by measuring the e⁺p to e^-p cross section ratio, but generally lacked the energy and precision to probe kinematics where the discrepancy is large. The TPEX Experiment, proposed to be run at DESY,  aims to measure the same cross section ratio to sub-percent precision at momentum transfers exceeding 4 GeV²/c², sufficient for a decisive test. Monitoring the relative e⁺ and e^- luminosities will be crucial, and TPEX will employ a pair of quartz Cherenkov counters to monitor the Møller/Bhabha rates at forward angles. I will present the results of Geant4 simulation studies performed to optimize the design and placement of the counters in order to minimize the eventual systematic uncertainty. These studies show that the proposed monitor design is robust to offsets in beam and collimator alignment, the two primary systematics for the OLYMPUS experiment luminosity monitor.