Measurements of Two-Photon Exchange Contributions to the Electron-Neutron Elastic Scattering Cross-Section Using the SBS Experimental Setup at Jefferson Lab*

The first two experiments to measure nucleon electromagnetic form factors to high four-momentum transfer $Q^2$ have recently concluded (Feb. 2022) at Jefferson Lab (JLab) using the Super BigBite Spectrometer (SBS). SBS is a large-acceptance spectrometer designed to handle high fluxes and high-momentum particles produced by JLab's 11\,GeV/c electron beam. One of them is nTPE: a measurement of the two-photon exchange contribution to the electron-neutron elastic scattering cross section at a four-momentum transfer of $Q^2 = 4.5$\,(GeV/c)$^2$. This experiment will assess the two photon contribution by Rosenbluth separation -- with precision at least 20-50 times greater than previously achieved -- and with accurate values of \nicefrac{$G^{n}_{E}$}{$G^{n}_{M}$} for the neutron found in companion SBS experiments. The experimental goal is achieved by measuring the ratio of rates \nicefrac{$e$-$n$}{$e$-$p$} for determination of the ratio of cross sections, and the precisely known Rosenbluth ratio for the proton. The trackers in BigBite and SBS are gaseous electron multipliers (GEMs). GEMs of various types and configurations are used to break analytical ambiguity created by high rates and improve tracking efficiencies. These GEMs are critical tracking components for all present and future high-impact experiments of the SBS program.