Triple-GEM based Polarimeter Tracker for the High-Q² Proton Form Factor Measurement (GEp-V) at Jefferson Lab

The study of the internal structure of the nucleon demands precise experimental measurement of elastic electromagnetic form factors. At the forefront of the Super Bigbite Spectrometer Program at the Jefferson Lab, the GEp-V Experiment uses the recoil polarization method for an accurate measurement of the proton electromagnetic form factor ratio G_Ep/ G_Mpup to a Q² of 11 GeV². The nature of the experiment demands a large-acceptance spectrometer tracker that could handle unprecedented background rates. The Gas Electron Multiplier Detector-based tracker developed for GEp-V consists of the Front Tracker and the Focal Plane Polarimeter (FPP) tracker, each with 8 individual GEM layers and a CH2 analyzer between them. The scattering of a longitudinally polarized electron beam from the liquid hydrogen target results in a polarization transfer to the recoil proton. In the one-photon approximation, the ratio between perpendicular and parallel components of the transferred polarization is directly proportional to the form factor ratio. The recoil proton undergoes spin-dependent secondary scattering in the CH2 analyzer, resulting in azimuthal asymmetry on FPP. The operation of GEM detectors at such high rates required optimization. An overview of the development, GEM detector optimization, and operation of the polarimeter tracker, as well as the process of performing polarimetry, will be presented.