Measurement of the Charge-Averaged Elastic Lepton-Proton Scattering Cross Section by the OLYMPUS Experiment

TL;DR

This paper presents the first measurement of the charge-averaged elastic lepton-proton scattering cross section, reducing uncertainties related to two-photon exchange effects, and offers new insights into proton electromagnetic form factors.

Contribution

It introduces a novel measurement of the charge-averaged cross section using the OLYMPUS experiment, providing data less affected by two-photon exchange and aiding future form factor analyses.

Findings

First measurement of charge-averaged elastic lepton-proton cross section

Data constrains proton electromagnetic form factor models

Results distinguish between existing form factor fits

Abstract

We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of M{\o}ller, Bhabha and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.