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_γ with respect to the neutron spin direction of γ rays emitted in the reaction n+p→d+γ 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_γ in n-+p→d+γ is dominated by a ΔI=1, 3S₁-3P₁ 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 _γ=[-1.2±2.1(stat.)±0.2(sys.)]×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 _γ,LR=[-1.8±1.9(stat.)±0.2(sys.)]×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 5×10^-8 and the aim of the NPDGamma collaboration is to measure it to 1×10^-8. The second phase of the measurement will be performed at the Spallation Neutron Source at Oak Ridge National Laboratory.