Analyzing the Energy Resolution of a Lead-Tungstate Calorimeter for a Proposed 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 so far has been inconclusive. Recent attempts, including by the OLYMPUS Experiment, extracted the TPE contribution to elastic ep scattering by measuring the e⁺p to e^-p cross section ratio, but have generally lacked both the energy and luminosity to probe kinematics where the discrepancy is large. The TPEX Experiment, proposed to run at DESY, aims to measure the same cross section ratio at momentum transfers exceeding 4.6 GeV²/c² with a 3 GeV incident beam energy where the discrepancy is clear. TPEX will measure scattered leptons with a set of ten lead glass calorimeter arrays. These calorimeters will need sufficient energy resolution to distinguish elastic scattering events from pion electro-production, the principal background. I will present the results from the TPEX Spring 2022 Test Beam to extract the energy resolution of a prototype lead-tungstate calorimeter array as a function of temperature. I will incorporate these results into a GEANT4 simulation to demonstrate the background rejection performance of the TPEX calorimeters. These studies will indicate the feasibility of TPEX utilizing lead-tungstate crystals to reach the desired systematic goals, whether or not future test-beam experiments are required, and convey the status of the TPEX experiment.