Limit on parity-violating neutron spin rotation in a liquid 4He target

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. The rotation angle of transversely polarized cold neutrons passing through a liquid $^4$He target was measured to be $d\phi/dz = (+1.7 \pm 9.1 (stat) \pm 1.4(sys)) \times10^{-7}$ 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-$^4$He 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.