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The Standard Model has been remarkably successful in describing weak interactions between leptons, leptons and hadrons, and in flavor-changing decays of hadrons. However, it has proven difficult both experimentally and theoretically to test the Standard Model with the nucleon-nucleon weak interaction. Strong and electromagnetic processes dominate at low energy so investigations are limited to parity non-conserving (PNC) observables, where weak currents must play a role. Experiments measuring PNC spin-rotations in low-A nuclei are one of the few ways to access and test these fundamental theories.
We recently completed a successful run of an experiment to study the strong interaction using weak interaction properties of the neutron. The neutron spin-rotation experiment is based on the principle that a transversely polarized neutron beam will experience a parity-violating rotation of its polarization vector about its momentum axis in the target due to the weak interaction component of the forward scattering amplitude. To measure the small rotation angle, a neutron polarimeter was used in which the horizontal-component of the neutron beam polarization was measured for a neutron beam initially polarized along the vertical axis. The challenge was to distinguish small parity-violating rotations from rotations that arise from residual magnetic fields.
The collaboration acquired data on the rotation angle of neutrons traversing a 42-cm liquid helium target from the period of January through June of 2008. The apparatus included neutron polarizers, input and output guides made from float glass, magnetic shielding, cryogenic targets, a data acquisition system, and a segmented 3He ion chamber. It is shown in Figure 1 assembled on the NG-6 beamline at the NIST Center for Neutron Research. The target was divided into four quadrants, front and back and side-to-side. This allowed one to remove the beam fluctuations by operating two simultaneous experiments side-by-side and also to minimize the effect of magnetic field drifts by inserting between the upstream and downstream targets a magnetic pi-coil that rotates the spins by 180 degrees. The position of the targets was changed by pumping the liquid helium using a non-magnetic centrifugal pump. Data were acquired in three pi-coil states: off, +180 degree rotation, and -180 degree rotation.
We acquired about 18 weeks of data and completed both the statistical and systematic analysis of the data. We calculated rotation angles for each of the pi-coil and target states. Figure 2 shows the PNC angle with the pi-coil on (i.e., when it is sensitive to PNC influences) as a function of the run. The result from the final analysis for the rotation angle is (+1.7 ± 9.2) × 10−7 rad/m, which is the best limit on spin rotation in liquid helium. The uncertainty is dominated by counting statistics, and hence, an improved experiment is being developed for the new high-flux NG-C beamline. The experiment will take advantage of the large area of the beam by redesigning the LHe target, using supermirror guides, and purchasing improved polarizers. The goal of the new experiment is improve the limit on PNC rotation by a factor of five to 2 × 10−7 rad/m.
Figure 1. Photograph of the apparatus on the NG-6 beam line.
Figure 2. Distribution of measured parity non-conserving rotation angles per meter. The solid line is a fit to a Gaussian distribution with a constant background.