• A novel comparison of Moller and Compton electron-beam polarimeters

      Magee, JA; Narayan, A; Jones, D; Beminiwattha, R; Cornejo, JC; Dalton, MM; Deconinck, W; Dutta, D; Gaskell, D; Martin, JW; Paschke, KD; Tvaskis, V; Asaturyan, A; Benesch, J; Cates, G; Cavness, BS; Dillon-Townes, LA; Hays, G; Hoskins, J; Ihloff, E; Jones, R; King, PM; Kowalski, S; Kurchaninov, L; Lee, L; McCreary, A; McDonald, M; Micherdzinska, A; Mkrtchyan, A; Mkrtchyan, H; Nelyubin, V; Page, S; Ramsay, WD; Solvignon, P; Storey, D; Tobias, WA; Urban, E; Vidal, C; Waidyawansa, B; Wang, P; Zhamkotchyan, S (2017-03-10)
      We have performed a novel comparison between electron-beam polarimeters based on M{\o}ller and Compton scattering. A sequence of electron-beam polarization measurements were performed at low beam currents ($<$ 5 $\mu$A) during the $Q_{\rm weak}$ experiment in Hall C at Jefferson Lab. These low current measurements were bracketed by the regular high current (180 $\mu$A) operation of the Compton polarimeter. All measurements were found to be consistent within experimental uncertainties of 1% or less, demonstrating that electron polarization does not depend significantly on the beam current. This result lends confidence to the common practice of applying M{\o}ller measurements made at low beam currents to physics experiments performed at higher beam currents. The agreement between two polarimetry techniques based on independent physical processes sets an important benchmark for future precision asymmetry measurements that require sub-1% precision in polarimetry.
    • Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

      Narayan, A; Jones, D; Cornejo, JC; Dalton, MM; Deconinck, W; Dutta, D; Gaskell, D; Martin, JW; Paschke, KD; Tvaskis, V; Asaturyan, A; Benesch, J; Cates, G; Cavness, BS; Dillon-Townes, LA; Hays, G; Ihloff, E; Jones, R; King, PM; Kowalski, S; Kurchaninov, L; Lee, L; McCreary, A; McDonald, M; Micherdzinska, A; Mkrtchyan, A; Mkrtchyan, H; Nelyubin, V; Page, S; Ramsay, WD; Solvignon, P; Storey, D; Tobias, A; Urban, E; Vidal, C; Waidyawansa, B; Wang, P; Zhamkotchyan, S (2016-02-16)
      We report on the highest precision yet achieved in the measurement of the polarization of a low energy, $\mathcal{O}$(1 GeV), electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall~C at Jefferson Lab. A number of technical innovations were necessary, including a novel method for precise control of the laser polarization in a cavity and a novel diamond micro-strip detector which was able to capture most of the spectrum of scattered electrons. The data analysis technique exploited track finding, the high granularity of the detector and its large acceptance. The polarization of the $180~\mu$A, $1.16$~GeV electron beam was measured with a statistical precision of $<$~1\% per hour and a systematic uncertainty of 0.59\%. This exceeds the level of precision required by the \qweak experiment, a measurement of the vector weak charge of the proton. Proposed future low-energy experiments require polarization uncertainty $<$~0.4\%, and this result represents an important demonstration of that possibility. This measurement is also the first use of diamond detectors for particle tracking in an experiment.