The Q_weak Experimental Apparatus
Abstract
The Jefferson Lab Q_weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 microA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using dedicated low-current (~100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
Cite
@article{arxiv.1409.7100,
title = {The Q_weak Experimental Apparatus},
author = {Qweak Collaboration and T. Allison and M. Anderson and D. Androic and D. S. Armstrong and A. Asaturyan and T. D. Averett and R. Averill and J. Balewski and J. Beaufait and R. S. Beminiwattha and J. Benesch and F. Benmokhtar and J. Bessuille and J. Birchall and E. Bonnell and J. Bowman and P. Brindza and D. B. Brown and R. D. Carlini and G. D. Cates and B. Cavness and G. Clark and J. C. Cornejo and S. Covrig Dusa and M. M. Dalton and C. A. Davis and D. C. Dean and W. Deconinck and J. Diefenbach and K. Dow and J. F. Dowd and J. A. Dunne and D. Dutta and W. S. Duvall and J. R. Echols and M. Elaasar and W. R. Falk and K. D. Finelli and J. M. Finn and D. Gaskell and M. T. W. Gericke and J. Grames and V. M. Gray and K. Grimm and F. Guo and J. Hansknecht and D. J. Harrison and E. Henderson and J. R. Hoskins and E. Ihloff and K. Johnston and D. Jones and M. Jones and R. Jones and M. Kargiantoulakis and J. Kelsey and N. Khan and P. M. King and E. Korkmaz and S. Kowalski and A. Kubera and J. Leacock and J. P. Leckey and A. R. Lee and J. H. Lee and L. Lee and Y. Liang and S. MacEwan and D. Mack and J. A. Magee and R. Mahurin and J. Mammei and J. W. Martin and A. McCreary and M. H. McDonald and M. J. McHugh and P. Medeiros and D. Meekins and J. Mei and R. Michaels and A. Micherdzinska and A. Mkrtchyan and H. Mkrtchyan and N. Morgan and J. Musson and K. E. Mesick and A. Narayan and L. Z. Ndukum and V. Nelyubin and Nuruzzaman and W. T. H. van Oers and A. K. Opper and S. A. Page and J. Pan and K. D. Paschke and S. K. Phillips and M. L. Pitt and M. Poelker and J. F. Rajotte and W. D. Ramsay and W. R. Roberts and J. Roche and P. W. Rose and B. Sawatzky and T. Seva and M. H. Shabestari and R. Silwal and N. Simicevic and G. R. Smith and S. Sobczynski and P. Solvignon and D. T. Spayde and B. Stokes and D. W. Storey and A. Subedi and R. Subedi and R. Suleiman and V. Tadevosyan and W. A. Tobias and V. Tvaskis and E. Urban and B. Waidyawansa and P. Wang and S. P. Wells and S. A. Wood and S. Yang and S. Zhamkochyan and R. B. Zielinski},
journal= {arXiv preprint arXiv:1409.7100},
year = {2015}
}
Comments
48 pages, 36 figures. Accepted by Nuclear Instruments and Methods A