We propose a new measurement of the ratio of positron-proton to electron-proton, elastic scattering at DESY to determine the contributions beyond single-photon exchange, which are essential to the QED description of the most fundamental process in hadronic physics. A 20 cm long liquid hydrogen target together with the extracted beam from the DESY synchrotron would yield an average luminosity of 2.12 × 10 35 cm −2 •s −1 •sr −1 (∼ 200 times the luminosity achieved by OLYM-PUS). A commissioning run at 2 GeV followed by measurements at 3 GeV would provide new data up to Q 2 = 4.6 (GeV/c) 2 (twice the range of current measurements). Lead tungstate calorimeters would be used to detect the scattered leptons at polar angles of 30°, 50°, 70°, 90°, and 110°. The measurements could be scheduled to not interfere with the operation of PETRA. We present rate estimates and simulations for the planned measurements including background considerations. Initial measurements at the DESY test beam facility using prototype lead tungstate calorimeters in 2019, 2021, and 2022 were made to check the Monte Carlo simulations and the performance of the calorimeters. These tests also investigated different readout schemes (triggered and streaming). Various upgrades are possible to shorten the running time and to make higher beam energies and thus greater Q 2 ranges accessible.

Tests of a prototype lead tungstate calorimeter were made over two weeks at the end of September, 2019, at the DESY II Test Beam Facility in Hamburg, Germany. The purpose of these tests was to gain experience with the construction, operation, and performance of a simple lead tungstate calorimeter, and also to compare a traditional triggered readout scheme with a streaming readout approach. These tests are important for the proposed Two-Photon Exchange experiment at the DESY test beam facility and for work towards a future electromagnetic calorimeter that could be used in an Electron-Ion Collider detector. Details on all aspects of the test, the subsequent analysis, and the results are presented.