The Qweak tracking system
The Q weak P experiment will measure the parity-violating elastic e-p scattering asymmetry to extract the weak charge of the proton. The experiment employs a toroidal magnet to focus electrons scattered at 8° ± 2°, corresponding to Q2 ∼ 0.03 (GeV/c)2, on eight Čerenkov detectors located in the focal plane of the spectrometer. Since the asymmetry is proportional to Q2, it is crucial to obtain an accurate measure of the acceptance-averaged value of Q2. A tracking system will be used in a low-rate counting mode, allowing individual events to be observed. This will enable a determination of the average Q2 by measuring the scattering angle and interaction vertex, for mapping the response across the surface of the Čerenkov detectors, and for the dilution of the Čerenkov detector signal by background.