Neutron spin rotation measurements

Abstract

The neutron spin rotation (NSR) collaboration used parity-violating spin rotation of transversely polarized neutrons transmitted through a 0.5 m liquid helium target to constrain weak coupling constants between nucleons. While consistent with theoretical expectation, the upper limit set by this measurement on the rotation angle is limited by statistical uncertainties. The NSR collaboration is preparing a new measurement to improve this statistically-limited result by about an order of magnitude. In addition to using the new high-flux NG-C beam at the NIST Center for Neutron Research, the apparatus was upgraded to take advantage of the larger-area and more divergent NG-C beam. Significant improvements are also being made to the cryogenic design. Details of these improvements and readiness of the upgraded apparatus are presented. We also comment on how recent theoretical work combining effective field theory techniques with the 1/N_c expansion of QCD along with previous NN weak measurements can be used to make a prediction for dϕ/dz in ⁴He. An experiment using the same apparatus with a room-temperature target was carried out at LANSCE to place limits on parity-conserving rotations from possible fifth-force interactions to complement previous studies. We sought this interaction using a slow neutron polarimeter that passed transversely polarized slow neutrons by unpolarized slabs of material arranged so that this interaction would tilt the plane of polarization and develop a component along the neutron momentum. The results of this measurement and its impact on the neutron-matter coupling g_A² from such an interaction are presented. The NSR collaboration is also preparing a new measurement that uses an upgraded version of the room-temperature target to be run on the NG-C beamline; and it is expected to constrain g_A² by at least two additional orders of magnitude for λ_c between 1 cm and 1 μm.