Undergraduate Research at Jefferson Lab

Two-Photon Exchange in Electron-Nucleon Scattering

Student: Jesse Ashworth

School: University of Washington

Mentored By: Wally Melnitchouk

The electric and magnetic form factors of the nucleon describe its fundamental charge and current distributions. These form factors have traditionally been determined by computing elastic electron-nucleon scattering cross sections, to first order expansion in the fine structure constant, α - encompassing a process called one-photon exchange. Experimental discrepancies in the proton's electric-to-magnetic form factor ratio have prompted a need to compute cross sections to second order expansion in α, involving two-photon exchange (TPE) interactions. Two methods for calculating TPE cross section contributions exist: one based on hadronic degrees of freedom (suitable at low Q²) and the other on partonic degrees of freedom (applicable at high Q²). Both methods have been claimed to principally account for the form factor discrepancy. However, ambiguities exist in the separation of the soft and hard parts of the partonic cross sections. This work aims to resolve such ambiguities and ultimately pave the way toward a unified description of TPE effects at all Q² values.