Gas Ring Imaging Cherenkov (GRINCH) Detector for the SuperBigBite Experiments

The SuperBigbite (SBS) program at Jefferson Lab will use a large acceptance 2-arm spectrometer to determine the neutron electromagnetic form factor ratio $G_E^n/G_M^n$ for $Q^2$ up to $13.5$ $(GeV/c)^2$, extracted using several polarimetry techniques across 3 approved experiments ($G_M^n$, $G_E^n$, and $G_E^n$-RP). This ratio is largely unexplored for the neutron at $Q^2 > 3.5$ $(GeV/c)^2$. Probing this region would shed light on the surprising result of the corresponding proton form factor ratio measured using recoil polarization, which appears to drop at higher $Q^2$ (diverging with the Rosenbluth result), as well as isolate the contributions from up/down valence quarks, which would have a profound impact on our understanding of nucleon structure.

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To make these small cross section measurements, luminosities up to $10^{37}/cm^{2}s$ are needed. A sophisticated detector package has therefore been constructed, which can be configured for a variety of kinematic settings to detect quasi-elastically scattered electrons from a $4.4-11GeV$ electron beam hitting a liquid deuterium target, while rejecting the high-rate background. To aid in this background rejection, a gas Cherenkov detector (GRINCH) with a highly-segmented PMT array was developed to identify electrons, with an expected pion rejection rate of $95\%$.