Limit on parity-violating neutron spin rotation in 4He
A. Opper 1*, For the Neutron Spin Rotation Collaboration
1 The George Washington University
The nucleon-nucleon (NN) weak interaction is one of the more poorly understood areas of the Standard Model. Present theoretical models are essentially phenomenological, and experiments in few-nucleon systems do not determine all of the weak NN amplitudes. We have performed the first phase of a high precision measurement of parity-violating (PV) neutron spin rotation at the US National Institute of Standards and Technology (NIST Gaithersburg) Center for Neutron Research. The rotation angle of transversely polarized cold neutrons passing through a liquid 4He target was measured to be d(phi)/dz = (+1.7 +/- 9.1 (stat) +/- 1.4(sys)) X 10-7 rad/m, which is the most precise measurement of PV spin rotation to date. The uncertainty is smaller than current estimates of the range of possible values of d(phi)/dz in the n-4He system. The result will be presented along with a discussion of the theoretical motivation, experimental design, analysis for data and systematic effects, and plans for the next phase of the experiment.
The isoscaler and isovector weak N-N coupling constants of the DDH framework with the DDH "reasonable range" indicated by the square box and the limits on those coupling constants imposed by this measurement shown by the diagonal red lines. Possible limits of a future measurement at NIST are shown by the black diagonal lines.