Measurement of the transverse polarization of electrons emitted in neutron decay: A search for weak exotic couplings

Abstract

First results are presented from an experiment that aims at the simultaneous determination of both transversal polarization components of electrons emitted in the decay of free neutrons—two observables that have never before been addressed experimentally. A non-zero value of the R correlation coefficient, due to the e - polarization component perpendicular to the plane spanned by the spin of the decaying neutron and the electron momentum, would signal a violation of time reversal invariance. The value of the N correlation coefficient, given by the transverse e - polarization component within that plane, is expected to be finite. The measurement of N, both, probes the Standard Model and serves as an important systematic check of the experimental apparatus with respect to the R correlation measurement. Data taking using the polarized cold neutron beam FUNSPIN from the SINQ spallation source at the Paul Scherrer Institute, Villigen, Switzerland, has been completed. Preliminary results of the data analysis are presented. Two options for a possible future experiment leading to the accuracy of about 3 × 10 - 4 are discussed.