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.