Abstract

We report on the highest precision yet achieved in the measurement of the polarization of a low-energy, O(1GeV) , continuous-wave (CW) 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 microstrip detector that 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−μA , 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 weak vector 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 the first use of diamond detectors for particle tracking in an experiment. It demonstrates the stable operation of a diamond-based tracking detector in a high radiation environment, for two years.

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Plain Language Summary

Electron beams can be produced with the electron spins aligned along a fixed direction, and experiments that use these kinds of beams rely on the precise knowledge of the fraction of electrons that are aligned (i.e., the “polarization” of the beam). One way of measuring beam polarization is Compton scattering, the collision of the beam electrons with photons from a laser, where the scattered electrons and/or the backscattered light are detected. The difference in the Compton scattering rates for electrons with spins oriented the same way or opposite to the spins of the laser photons is very well known. For a known laser polarization, the electron-beam polarization can be deduced by comparing the expected with the measured rate difference. Here, we share results of a new Compton polarimeter at our laboratory that has achieved the highest precision ever for low-energy (about 1 GeV) electron beams.

Our polarimeter incorporates key innovations such as ensuring that the laser beam is 100% polarized and a novel diamond-based electron detector that images the paths of the scattered electrons and can withstand very high radiation doses given the experimental duration of 200 days. Furthermore, this polarimeter can be operated without disrupting the electron beam. We have used this new polarimeter for the Qweak experiment, a high-precision test of the standard model of particle physics that requires an electron-beam polarization precision better than 1%. Our results significantly surpass this requirement.

We anticipate that our results will pave the way for upcoming standard-model tests with even more stringent (<0.4% ) requirements on the precision of the beam polarization.

Details

Title
Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors
Author
Narayan, A
Publication year
2016
Publication date
Jan-Mar 2016
Publisher
American Physical Society
e-ISSN
21603308
Source type
Scholarly Journal
Language of publication
English
ProQuest document ID
2550309348
Copyright
© 2016. This work is licensed under http://creativecommons.org/licenses/by/3.0/ (the “License”). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.