The Liquid Hydrogen Target for TPEX

The notable discrepancies between the Rosenbluth separation method and the polarization transfer technique in determining the proton form factor ratio is still not resolved. The two-photon exchange (TPE) contribution is the current best explanation in resolving this discrepancy. The three recent experiments (VEPP-3, CLAS and OLYMPUS), which have reported on direct measurements of TPE, show little evidence for large contributions of hard two-photon exchange up to Q² = 2.5 (GeV/c)². The Two-Photon Exchange experiment (TPEX) at DESY proposes to determine the hard two-photon contributions by measuring the ratio of positron-proton to electron-proton elastic scattering at beam energies of 2 and 3 GeV. This will extend the momentum transfer range up to Q² = 4.7 (GeV/c)², where the TPE effect is expected to be more significant. For this new experiment, we propose to build a liquid hydrogen target that will yield a luminosity about a factor of 200 times higher than that of the OLYMPUS experiment. This higher luminosity will greatly shorten the run time needed at 2 GeV and help to make up for the lower cross section at 3 GeV beam energies. In this presentation, the latest updates on the requirements, technical design and implementation of the liquid hydrogen target for TPEX will be presented and discussed.