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Fundamental Physics: Experiments with a Precision Neutron Polarimeter


The Neutron Spin Rotation (NSR) collaboration has developed a neutron spin polarimeter that has allowed the precise measurement of spin rotations at a level of less than 10-6 rad/m. This sensitive polarimeter was used with a liquid helium target as a test of parity non-conservation (PNC) in the nucleon-nucleon weak interaction. 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 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. In addition, the NSR polarimeter was more recently used to place a stringent limit on possible long-range, spin-dependent interactions through the exchange of an exotic vector boson. Upgrades to the apparatus should enable significantly improved limits on both quantities.


We 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 apparatus is shown in Fig. 1 assembled on the NG-6 beamline at the NIST Center for Neutron Research. 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 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.

NG-6 beam line
Figure 1. Photograph of the apparatus on the NG-6 beam line.
Credit: NIST

Upon completion of the running, both the statistical and systematic analysis of the data were performed. 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). The result from the final analysis for the rotation angle is (+2.1 ± 8.8) × 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 upgraded 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 to improve the limit on PNC rotation to 2 × 10−7 rad/m, which will be sufficient to test recent theoretical predictions.

Distribution of measured parity non-conserving rotation angles per meter
Figure 2: Distribution of measured parity non-conserving rotation angles per meter. The error bars represent one-sigma statistical uncertainty. The solid lines are fits to a Gaussian function, and the residuals are shown above the data (from Swanson et al., 2019).
Credit: NIST

Since the last operation of the polarimeter, many of the components were upgraded, which enabled a novel method for searching for short-range forces. Specifically, one could search for the possible existence of new interactions in nature with ranges of mesoscopic scale (millimeters to microns), corresponding to exchange boson masses in the 1 meV to 1 eV range and with very weak couplings to matter. 

The improved apparatus was outfitted with a room-temperature target using multiple flat plates containing a large mass density gradient across the gaps traversed by the polarized neutrons. The test masses were arranged in four quadrant regions each containing eight open channels for the neutrons separated by two plate thicknesses (see Fig. 3). The precision polarimetry was performed in a manner analogous to that of the liquid helium target: the target was designed such that in any of the four possible target states two diagonally opposed quadrants would not be sensitive to any short-range force while the remaining two would produce rotations of opposite signs from opposite mass-gradients. Rotating the target into different states reverses the mass gradient, and thus the signs of the rotations in each quadrant, without affecting the signs of rotations from magnetic fields. This allows one to isolate rotations from systematic effects versus those potentially arising from short-range forces.

spin rotation apparatus
Figure 3: Target assembly used in the spin rotation apparatus used to search for exotic neutron spin-dependent interactions at Los Alamos Neutron Science Center.

The experiment was carried out on the FP12 neutron beamline at the Los Alamos Neutron Scattering Center (LANSCE) at Los Alamos National Laboratory. The result improved limits on possible long-range, spin-dependent interactions through the exchange of an exotic vector boson, as shown in Fig. 4. Prospects for future improvement in the sensitivity are very good. The intensity of the cold neutron beam at the NG-C beam at NIST is higher by about two orders of magnitude. Combined with a longer running time and the use of a denser target material such as tungsten in place of the copper used in this work, one can envision a further improvement in the statistical sensitivity in future measurements of more than two orders of magnitude assuming that there are no limitations from unanticipated systematic uncertainties. 


measured neutron spin rotation angle
Figure 4: (left) The measured neutron spin rotation angle as a function of 30-minute production runs at LANSCE, each run containing data from all target orientations. (right) An upper limit on the value of the exotic coupling parameter is drawn at 95% confidence in blue, including both statistical and systematic uncertainties. For details on the measured parameters shown in the plots and the experiments using different methodologies which produced exclusion lines labeled (a) and (b), see the reference of Haddock et al. in Physics Letters B 2018.
Credit: NIST
Created February 3, 2011, Updated March 5, 2021