First Precision Measurement of the Parity Violating Asymmetry in Cold Neutron Capture on 3He

We report the first precision measurement of the parity-violating asymmetry in the direction of proton momentum with respect to the neutron spin, in the reaction He-3(n, p)H-3, using the capture of polarized cold neutrons in an unpolarized active He-3 target. The asymmetry is a result of the weak interaction between nucleons, which remains one of the least well-understood aspects of electroweak theory. The measurement provides an important benchmark for modern effective field theory and potential model calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is A(PV) = [1.55 +/- 0.97(stat) +/- 0.24(sys)] x 10(-8), which has the smallest uncertainty of any hadronic parity-violating asymmetry measurement so far.