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Measurement of the Parity Non-Conserving Neutron Spin Rotation (PNC) in Liquid Helium


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 neutronstraversing a 42-cm liquid helium target from the period of January through June of 2008. The apparatus included a neutron spin flipper, 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. 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 three reactor cycles (about 18 weeks) of data and recently 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 party-nonconserving influences) as a function of the run. The result from the final analysis for the rotation angle is (+1.7 +/-9.2) 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 considered for the new high flux NG-C beamline.

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. At  low energies these processes are best described by an effective meson theory. Experiments measuring PNC spin rotations in low A nuclei are one of the few ways to access and test these fundamental theories.


Photograph/Graft Captions:

Figure 1: Photograph of the apparatus on the NG-6 beam line. The neutron beam exits after traversing the LHe targets and the polarization is analyzed in with the super-mirror and 3He ion chamber.

Figure 2: Results of the pi-coil on runs for the neutron spin rotation experiment. The rotation angle from each run is plotted as function of the time of the run throughout the entire experiment. The results of this experiment produced the best limit on the party-nonconserving neutron spin rotation in liquid helium.

Measurement of the Parity Non_Conserving (PNC) Neutron Spin Rotation in Liquid Helium.
Photograph by: Neutron Physics Group

Lead Organizational Unit:



Neutron Physics Group

Jeffrey S. Nico

Hans (Pieter) Mumm

NIST Associates:

William M. Snow
Indiana University

Erik Swanson
University of Washington