Measurement of parity-violating γ-ray asymmetry in the capture of polarized cold neutrons on protons

The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry A(gamma) with respect to the neutron spin direction of gamma rays emitted in the reaction (n) over right arrow + p -> d + gamma using the capture of polarized cold neutrons on the protons in a liquid parahydrogen target. One expects parity-odd effects in the hadronic weak interaction between nucleons to be induced by the weak interaction between quarks. A(gamma) in (n) over right arrow + p -> d + gamma is dominated by a Delta I = 1, S-3(1)-P-3(1) parity-odd transition amplitude in the n-p system. The first phase of the measurement was completed at the Los Alamos Neutron Science Center spallation source (LANSCE), with the result A(gamma) = [-1.2 +/- 2.1 (stat.) +/- 0.2 (sys.)] x 10(-7). We also report the first measurement of an upper limit for the parity-allowed left-right asymmetry in this reaction, with the result A(gamma,LR) = [-1.8 +/- 1.9 (stat.) +/- 0.2 (sys.)] x 10(-7). In this paper we give a detailed report on the theoretical background, experimental setup, measurements, extraction of parity-odd and parity-allowed asymmetries, analysis of potential systematic effects, and LANSCE results. The asymmetry has an estimated size of 5 x 10(-8) and the aim of the NPDGamma collaboration is to measure it to 1 x 10(-8). The second phase of the measurement will be performed at the Spallation Neutron Source at Oak Ridge National Laboratory.