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 ${}^3\mathrm{He}(n,p{)}^3\mathrm{H}$, using the capture of polarized cold neutrons in an unpolarized active ${}^3\mathrm{He}$ 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 $A_{\mathrm{PV}}=[1.55\pm 0.97(\mathrm{stat})\pm 0.24(\mathrm{sys})]\times {10}^{-8}$, which has the smallest uncertainty of any hadronic parity-violating asymmetry measurement so far.

• Received 24 April 2020
• Accepted 4 August 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.131803