Eur. Phys. J. A (2021) 57: 144
https://doi.org/10.1140/epja/s10050-021-00462-3

Regular Article - Experimental Physics

Determination of two-photon exchange via $e^{+}p/e^{-}p$ scattering with CLAS12

¹ Center for Frontiers in Nuclear Science, Stony Brook University, Stony Book, NY, USA
² Riken BNL Research Center, Brookhaven National Laboratory, Upton, NY, USA
³ Thomas Jefferson National Accelerator Facility, Newport News, VA, USA
⁴ The George Washington University, Washington, DC, USA

^a jan.bernauer@stonybrook.edu

Received: 5 March 2021
Accepted: 6 April 2021
Published online: 23 April 2021

Abstract

The proton elastic form factor ratio shows a discrepancy between measurements using the Rosenbluth technique in unpolarized beam and target experiments and measurements using polarization degrees of freedom. The proposed explanation of this discrepancy is uncorrected hard two-photon exchange (TPE), a type of radiative correction that is conventionally neglected. The effect size and agreement with theoretical predictions has been tested recently by three experiments. While the results support the existence of a small two-photon exchange effect, they cannot establish that theoretical treatments are valid. At larger momentum transfers, theory remains untested. This proposal aims to measure two-photon exchange over an extended and so far largely untested $Q^2$ and $\epsilon$ range with high precision using the CLAS12 experiment. Such data are crucial to clearly confirm or rule out TPE as the driver for the discrepancy as well as test several theoretical approaches, believed valid in different parts of the tested $Q^2$ range.