Measurement of electroweak asymmetries in vector electron positron scattering at 0.1 less than Q(2) less than 0.4 (GeV/c)(2).

摘要

This work is based on the first engineering run of the G 0 experiment from October 2002 though January 2003 in Hall C at Jefferson Lab. The G0 experiment will be the first to measure the weak neutral form factors: GEZ( Q2), GMZ( Q2) and GAe( Q2) and to extract the proton's strange form factors: GEs(Q2) and GMs(Q 2) via a Rosenbluth separation over a range of Q 2 (0.1--1.0 (GeV/c)2). This will require four sets of measurements: forward angle measurements with a proton target, and three sets of backward angle measurements with a hydrogen and deuterium target. The measurements are made of the parity-violating asymmetries in elastic electron scattering.;The G0 experiment is a major installation at Jefferson Lab's Hall C with a new dedicated spectrometer. The superconducting magnet is made up of 8 coils with a maximum field of 1.6 T·m. The scintillator detector array (detector solid angle between 0.4--0.9 sr) detects recoiled protons in the forward angle measurement (where thetap = 70° +/- 10° corresponding to scattered electron angles of a few degrees) and to detect scattered electrons in the backward angle measurements. This detector array is made up of a set of 16 pairs of scintillators arranged in 8 sectors around the beam line. Custom electronics handle the high data rate (approximately 1 MHz per detector). The target is a 20 cm long liquid hydrogen cryotarget. Besides the check-out of the new hardware, G 0 has stringent requirements on the performance of the polarized electron beam in order to minimize false asymmetries. Further complicating this fact, in the forward mode, was the requirement that the time structure of the JLab beam had to be changed from 499 MHz to 31 MHz in order to count the recoiled protons in the spectrometer. Data was collected over a 12 day period at the end of the engineering run. These data were analyzed for a Q2 range of 0.1--0.4 (GeV/c) 2 corresponding to measured electroweak asymmetries that ranged from (-4.4 +/- 1.6 +/- 1.6 ppm) to (-8.5 +/- 2.8 +/- 2.5 ppm).