Parity Violetion Experiments - JLab12

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Parity Violetion Experiments

Physics program > Standard Model tests

Measurements of parity violation in electron scattering (PVES) have provided a wealth of information on the nucleon and the electroweak interaction. In this kind of experiments the parity violating asymmetry Apv, that is the fractional difference in cross sections for positive and negative helicity electrons is measured. Measuring Apv one can probe the weak force and the structure of the nucleon. A very exciting PVES program was performed at JLab taking advantage of the exceptional features of the CEBAF electron beam. The experiments HAPPEX-I, HAPPEX-II and HAPPEX-III in Hall A and the experiment G0 in Hall C used PVES to explore the effects of intrinsic strangeness in the proton while the experiments PVDIS in Hall A and QWeak in Hall C measured parity violation in electron-parity scattering as a probe of electroweak couplings. In the PVES experiments performed in Hall A the scattered electrons were focused onto detectors in twin High-Resolution Spectrometers (HRS). The electron helicity was held constant for periods referred to as ‘‘windows’’. The right-left helicity asymmetry in the integrated detector response, normalized to the beam intensity, was computed for sets of complementary helicity windows to form the raw asymmetry. Correcting the raw asymmetry for the noise generated from random fluctuations in beam position and energy and correcting further for background sources like beam charge normalization, detector nonlinearity,  etc.

Apv was obtained. HAPPEX series experiments obtained very important results: they showed that the strange contributions to the nucleon form factors are consistent with zero and not more than a few percent of the proton form factors. For the upgrade of the CEBAF beam energy to a maximum of 12 GeV, the new spectrometer SOLID and the experiment MOLLER have been proposed. SOLID will allow measurements in deep inelastic scattering (DIS) simultaneously covering a large kinematic range in both xBj and Q2 and providing consequently new information on a variety of topics including charge symmetry violation, higher twist contribution to proton structure and electroweak coupling constants. The MOLLER experiment will measure the weak charge of the electron with a precision of 2.3% by measuring the parity violating asymmetry in electron-electron (Møller) scattering. This measurement will provide a very precise measurement of the weak mixing angle sin2θW  with a fractional accuracy in the determination of sin2θW at the level of 0.1% probing physics beyond the Standard Model with sensitivities to mass scales of new physics up to 7.5 GeV.

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