The continuing story of two-photon exchange: results from the OLYMPUS experiment

Abstract

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.