Precision Electron-Beam Polarimetry at 1 GeV Using Diamond Microstrip Detectors

Abstract

We report on the highest precision yet achieved in the measurement of the polarization of a lowenergy, O(1 GeV), 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 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 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 Q weak 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.

Mapping electromagnetic fields structure in plasma using a spin polarized electron beam

“…One can also use an optical polarimetry scheme where polarized electrons excite atomic target to high order states and then radiate circularly polarized fluorescence 43 . By using these methods, the variation of the beam spin polarization [44][45][46][47] in the level of 10 −2 can be measured. However in our scheme the spin variation is in the 10 −4 level, a higher resolution spin detector should be used.…”

A test of local Lorentz invariance with Compton scattering asymmetry

“…Details on the Compton polarimeter can be found in Ref. [48,50,51]. The results discussed here are from the electron detector only and were collected during the period between November, 2011 and May, 2012.…”

“…The measured asymmetry was built from these yields as described in Ref. [48,51]. A statistical precision of < 1% per hour was routinely achieved.…”

“…This work represents the first use of Compton scattering asymmetry to test Lorentz invariance. Polarimeters based on Compton scattering are routinely used to continuously monitor the electron beam polarization at electron accelerators [35][36][37][38][39][40][41][42][43][44][45][46][47][48]. These polarimeters involve measuring a known QED double-spin asymmetry in electron scattering from a photon beam of known polarization.…”

“…The Comptonscattered electrons and photons can be independently measured and analyzed to determine the polarization of the electron beam. Recently, the Compton polarimeter at Hall C in Jefferson Lab (JLab) demonstrated sub-1% precision [48]. This high degree of precision of Compton polarimeters allows precise measurement of the vacuum refractive index of the Compton-scattered photon.…”