Upper bound on parity-violating neutron spin rotation in 4He

We report an upper bound on parity-violating neutron spin rotation in He-4. This experiment is the most sensitive search for neutron-weak optical activity yet performed and represents a significant advance in precision in comparison to past measurements in heavy nuclei. The experiment was performed at the NG-6 slow-neutron beamline at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Our result for the neutron spin rotation angle per unit length in He-4 is d phi/dz = [+ 1.7 +/- 9.1(stat.) +/- 1.4(sys.)] x 10(-7) rad/m. The statistical uncertainty is smaller than current estimates of the range of possible values of d phi/dz in n+He-4.