NIST logo

Neutron Spin Rotation in Liquid Helium:  A neutron spin-rotation experiment is based on the principle that a transversely polarized neutron beam will experience a party-nonconserving rotation of its polarization vector about its momentum axis in a target due to the weak interaction component of the forward scattering amplitude. To measure the small PNC rotation, a neutron polarimeter is used, and a component of the transverse polarization is measured for a neutron beam initially polarized along the vertical axis.  Measurement of the spin rotation of polarized neutrons traversing a liquid helium target provides information on parity-violating nucleon-nucleon forces.

Data on this rotation angle were acquired in a recent run on the NG-6 beamline, and the analysis of that data is in progress.  The analysis will be completed soon and produce a final value for the rotation angle, which is expected to be at the level of approximately 10-7 rad/m. In addition, the systematic effects that can alter the measured rotation should be completely quantified shortly. Magnet fields are expected to be the dominant systematic effects, and focus will be on quantifying their contributions.  We anticipate that this measurement will produce the most precisely measured party-violating rotation angle. The sensitivity produced by this measurement should tightly constrain weak nucleon-nucleon couplings.

Thomas Gentile