Abstract
Neutron spin rotation is expected from quark-quark weak interactions in the standard model, which induce weak interactions among nucleons that violate parity. We present the results from an experiment searching for the effect of parity violation via the spin rotation of polarized neutrons in a liquid 4He medium. The value for the neutron spin rotation angle per unit length in 4He, rad/m, is consistent with zero. The result agrees with the best current theoretical estimates of the size of nucleon-nucleon weak amplitudes from other experiments and with the expectations from recent theoretical approaches to weak nucleon-nucleon interactions. In this paper we review the theoretical status of parity violation in the 4He system and discuss details of the data analysis leading to the quoted result. Analysis tools are presented that quantify systematic uncertainties in this measurement and that are expected to be essential for future measurements.
Highlights
Neutron spin rotation is expected from quark-quark weak interactions in the standard model, which induce weak interactions among nucleons that violate parity
The hadronic weak interaction (HWI) in general and the NN weak interaction amplitudes in particular are scientifically interesting for several reasons [2,3,4,5,6]
Both nonleptonic weak kaon decays and nonleptonic weak decays of hyperons exhibit deviations from the relative sizes of weak amplitudes expected by chiral symmetry whose dynamical source is still not fully understood [7]
Summary
Neutron spin rotation is expected from quark-quark weak interactions in the standard model, which induce weak interactions among nucleons that violate parity. Recent work relating this amplitude via a chiral theorem to a fourquark operator contribution to the neutron-proton mass difference [53,54] may make it possible to perform a reliable lattice calculation despite the presence of the disconnected quark diagrams in this case If both of these lattice efforts succeed, two of the five low-energy pionless EFT parameters will be determined from the standard model. The parity odd amplitudes involved in n + 4He spin rotation can be treated to an excellent approximation as perturbations in this and all low energy NN weak interaction processes They can be estimated in meson-exchange (i.e., DDH) or other QCD models, parametrized using EFT, or calculated from the standard model using lattice gauge theory. Two new measurements of the low energy s-wave scattering length for neutrons on 4He, which is important input for any calculation of neutron spin rotation as a check on the strong interaction component of the calculation, have been conducted and are being analyzed [83,84]
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