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 $\mathrm{n⃗}+p\to d+\gamma$ 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 $\mathrm{n⃗}+p\to d+\gamma$ is dominated by a $\Delta I=1$, ${}^3{S}_1$-${}^3{P}_1$ 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 $A_{\gamma }=[-1.2\pm 2.1\hspace{0.5em}(\mathrm{stat}.)\pm 0.2\hspace{0.5em}(\mathrm{sys}.)]\times {10}^{-7}$. We also report the first measurement of an upper limit for the parity-allowed left-right asymmetry in this reaction, with the result $A_{\gamma ,\mathrm{LR}}=[-1.8\pm 1.9\hspace{0.5em}(\mathrm{stat}.)\pm 0.2\hspace{0.5em}(\mathrm{sys}.)]\times {10}^{-7}$. 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 $5\times {10}^{-8}$ and the aim of the NPDGamma collaboration is to measure it to $1\times {10}^{-8}$. The second phase of the measurement will be performed at the Spallation Neutron Source at Oak Ridge National Laboratory.

• Received 1 March 2010

DOI:https://doi.org/10.1103/PhysRevC.83.015505