First Precision Measurement of the Parity Violating Asymmetry in Cold Neutron Capture on ${}^3$He

Abstract

We report the first precision measurement of the parity-violating asymmetry in the direction of proton emission with respect to the neutron spin, in the reaction ${}^3\mathrm{He}(\mathrm{n},\mathrm{p}{)}^3\mathrm{H}$, 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 most poorly understood aspects of electro-weak theory. The measurement provides an important benchmark for modern effective field theory (EFT) calculations. Measurements like this are necessary to determine the spin-isospin structure of the hadronic weak interaction. Our asymmetry result is $A_{PV}=\left(1.58\pm 0.97(\mathrm{stat})\pm 0.24(\mathrm{sys})\right)\times 10^{-8}$, which has the smallest uncertainty of any parity-violating asymmetry measurement so far.

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• Signature of Einstein-Cartan theory
• The HIBEAM Instrument at the European Spallation Source
• Towards a unified treatment of $\Delta S=0$ parity violation in low-energy nuclear processes
• QCD Analysis of $\Delta S=0$ Hadronic Parity Violation
• $\Delta S=0$ Hadronic Parity Violation in Next-to-Leading Order QCD: Anomalous Dimension Matrices and Their Implications