GEn-Recoil Polarimetry Experiment Using the SBS Experimental Setup at Jefferson Lab

A series of experiments to measure nucleon electromagnetic form factors (EMFFs) to high four-momentum transfer $Q^2$ values is underway 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 experiment in this program is GEn-RP: a measurement of the neutron EMFF ratio $G_E^n/G_M^n$ using recoil polarimetry at $Q^2 = 4.5$\,(GeV/c)$^2$. In GEn-RP, electrons elastically scatter off neutrons in a liquid deuteron target. The electrons are detected by the BigBite (BB) spectrometer while the neutron and its polarization are determined in the SBS. Two polarimetry techniques are employed: charge-exchange $np$ → $pn$ and conventional $np$ → $np$ scattering. This is the first proof-of-principle validation of the charge-exchange method against the conventional approach through direct comparison. The primary trackers in both BB and SBS are gaseous electron multipliers (GEMs). A combination of ''X--Y" and new, rotated-basis ''U--V" GEMs 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.