NPDGamma: a measurement of the parity-violating gamma asymmetry Aγ in n-p capture

The NPDGamma Experiment measures the parity-violating correlation A(gamma) between neutron spin and photon momentum in the reaction (n) over right arrow + p -> d + gamma. Knowledge of A(gamma) and other parity-violating observables in few-body nuclear systems will provide constraints for a parameterized description of Delta S = 0 parity-violating phenomena free from complications of nuclear structure. The NPDGamma experiment uses a polarized cold pulsed neutron beam, a liquid parahydrogen target, and a cylindrical array of 48 CsI(T1) scintillation detectors operated in current mode to search for the asymmetry. NPDGamma recently completed the first phase of the program to measure A(gamma) at the Los Alamos Neutron Science Center with the preliminary result A(gamma) = (-1.2 +/- 2.1(stat.) +/- 0.1(sys.)) x 10(-7), reproducing the previous upper limit from a measurement at a reactor facility. We discuss the theoretical background and experimental method and report on preliminary analysis of the LANSCE data. The second phase of the program to measure A(gamma) is in progress at the Spoliation Neutron Source at Oak Ridge National Laboratory.