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 3He ( n , p )3H, using the capture of polarized cold neutrons in an unpolarized active 3He 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 APV = [ 1.55 ± 0.97 ( stat ) ± 0.24 ( sys ) ] × 10−8, which has the smallest uncertainty of any hadronic parity-violating asymmetry measurement so far.