Measurement of the Singlet Anomalous Magnetic Moment of the Proton using Parity Violating Elastic Electron Scattering

The SAMPLE experiment was designed to measure the contribution of strange quarks to the magnetic moment of the proton.  The experiment was proposed in 1989, and ran in the Bates North Experimental Hall from 1998 to 2001.  The collaboration included members from Caltech, the University of Illinois, Virginia Tech, MIT, William and Mary, Kentucky, Argonne and Louisiana Tech.  SAMPLE measured the asymmetry in the scattering of longitudinally polarized electrons from hydrogen; that is, the difference in scattering rate depended on whether the incident electrons were left- or right-handed.  The expected asymmetry was less than 10 parts per million, requiring an intense beam of polarized electrons and a dense liquid hydrogen target.  SAMPLE also took data on a liquid deuterium target to separate out the axial vector electron-nucleon form factor from the strange magnetic form factor.

SAMPLE used an electron beam energy of 200 MeV and a beam current of about 40 µA on a 40 cm long liquid hydrogen or deuterium target.  The target was cooled by a 715 W helium gas refrigerator.  Data were also taken at a beam energy of 125 MeV on deuterium.  The beam polarization was measured in a Møller polarimeter.  Due to the very small asymmetry, careful control and measurement of helicity-correlated beam parameters (energy, current, position, etc.) were required.  The scattered electrons were detected in a large solid angle air Čerenkov detector set at backwards angles from the target.  The Čerenkov light from the electrons was focused with large ellipsoidal mirrors onto 8-inch photomultiplier tubes shielded by cylinders containing 10 cm of lead.

The experimental results showed the contribution of strange quarks to the proton’s magnetic moment are (0.1+/-5)%.  The SAMPLE collaboration published several papers on the results, including an article in Science magazine.  Three PhD theses were written based on SAMPLE results.  A number of subsequent parity-violation experiments at Jefferson Laboratory have built on techniques developed for SAMPLE.