Time reversal in neutron beta decay - the emiT experiment

The theory of neutron beta decay provides for the possibility of a violation of time-reversal (T) invariance due to the presence of a manifestly T-odd term. This term is a triple correlation involving the neutron spin and the momenta of the electron and neutrino decay products. The degree of correlation (usually denoted by the coefficient D) may be measured using a beam of cold, polarized neutrons and detecting beta-decay electrons in coincidence with recoil protons. The emiT experiment will improve on previous neutron-based measurements of D by employing PIN diode proton detectors in an octagonal detector array at the Cold Neutron Research Facility, NIST. The expected sensitivity to D is 3 x 10-4, which may permit restrictions to be placed on several extensions to the Standard Model that allow values of D near 10-3. The detector system is currently nearing completion and preliminary runs will be made this summer at NIST