Speaker
Description
Summary
Over the past two decades, a discrepancy has emerged between two
different techniques for measuring the proton's electromagnetic
form factors. Unpolarized electron-proton cross section measurements
paint a picture of the proton's internal structure that is incompatible
with measurements from polarization transfer experiments. The leading
hypothesis is that the discrepancy is caused by a typically neglected
radiative correction, hard two-photon exchange (TPE), which would affect
the two measurement techniques in different ways. There is no model independent
way to calculate hard TPE, but it can be measured experimentally by
looking for an asymmetry between the positron-proton and electron-proton
elastic cross sections. Three recent experiments have attempted to quantify
this asymmetry, and, just last month, the third of these, called OLYMPUS,
released its results (arXiv:1611.04685). The OLYMPUS experiment
collected data in 2012 at DESY, alternating between 2 GeV electron and
positron beams, directed through a hydrogen gas target. The scattered
lepton and recoiling proton were detected in coincidence with a
large acceptance toroidal spectrometer. The relative luminosity between
the two beam species was monitored with three independent systems, and
the results comprise 3 inverse fb of integrated luminosity, exceeding by
a factor of three the other two TPE experiments combined. In this talk,
the case for the TPE hypothesis will be presented, the OLYMPUS experiment
will be described, and the results of all three experiments will be compared.
