Measurements of the Two-Photon Exchange Contributions to the $e-n$ Elastic Scattering Cross-section on the SBS at JLab and Related GEM Crosstalk Analysis*

The nTPE experiment ran as part of the first segment of JLab's SBS program. nTPE is a pioneering measurement of the two-photon exchange contribution to the electron-neutron elastic scattering cross section. The experiment had a four-momentum transfer of $Q^2 = 4.5$\,(GeV/c)$^2$. Comparisons of proton form factor measurements acquired through polarization transfer and the Rosenbluth technique show discrepancies, especially as the four-momentum transfer, $Q^{2}$, increases above 2\,(GeV/c)$^2$. 

Two photon exchange is strongly favored as the reason for this discrepancy and the experiment assesses the contribution of two-photon exchange in $e$-$N$ elastic scattering processes by elastically scattering electrons off neutrons in a deuterium target. The scattered electrons were detected in the BigBite spectrometer --- which is equipped with a Gas Electron Multiplier (GEM) detector-based tracker --- while the scattered nucleons were detected in a hadron calorimeter located behind the SBS magnet. Measurement of the ratio D(e,e'n) over D(e,e'p) production rates allows us to reach required low systematics.

High background rates in BigBite's tracker (100 kHz/cm$^{2}$) were present during the experiment and require rejection and clean-track reconstruction techniques for proper extraction of physics results. This talk will discuss the nTPE experiment, GEM detector background rejection techniques, and some preliminary analysis results.