Abstract
Two-photon exchange is believed to be responsible for the discrepancies in the proton electric to magnetic form factor ratio found with the Rosenbluth and polarization transfer methods. If this explanation is correct, one expects significant di erences in the lepton-proton cross sections between positrons and electrons. The OLYMPUS experi- ment at DESY in Hamburg, Germany was designed to measure the ratio of unpolarized positron-proton and electron-proton elastic scattering cross sections over a wide kine- matic range with high precision, in order to quantify the e ect of two-photon exchange. The experiment used intense beams of electrons and positrons stored in the DORIS ring at 2.0 GeV interacting with an internal windowless hydrogen gas target. The current status of OLYMPUS will be discussed.
Highlights
Measurements of the proton electric to magnetic elastic form factor ratio with the polarization transfer method at Jefferson Lab [1, 2] have revealed an unexpected and large discrepancy with the elastic form factor ratio obtained using the Rosenbluth separation technique in unpolarized cross section measurements [3]
The discrepancy has been explained as the effect of hard two-photon exchange beyond the usual one-photon exchange approximation in the calculation of the elastic electron-proton scattering cross section [4,5,6,7,8,9,10]
The results from both methods are mostly based on the single photon exchange assumption including standard radiative corrections [11], which account for two-photon exchange only to the extent that one of the photons is soft
Summary
Measurements of the proton electric to magnetic elastic form factor ratio with the polarization transfer method at Jefferson Lab [1, 2] have revealed an unexpected and large discrepancy with the elastic form factor ratio obtained using the Rosenbluth separation technique in unpolarized cross section measurements [3]. The discrepancy has been explained as the effect of hard two-photon exchange beyond the usual one-photon exchange approximation in the calculation of the elastic electron-proton scattering cross section [4,5,6,7,8,9,10].
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