I will begin with an overview of the neutron physics program at NIST and will then focus on a representative experiment to test time-reversal symmetry. The existence of related charge-parity (CP) symmetry violation is necessary to explain the dominance of matter over antimatter in the early universe. Thus far, the observed CP violation can be entirely accounted for by a phase in the Cabbibo-Kobayashi-Maskawa matrix, however this phase is insufficient to account for the known baryon asymmetry in the context of Big Bang cosmology and there is good reason to search for CP and time-reversal violation in other systems. The emiT experiment tests time reversal symmetry in the β-decay of polarized free neutrons by searching for the T-odd, P-even triple correlation Dσn⋅pexpν, where σ and p are the neutron spin and decay product momenta, respectively. The detection of this correlation above the small calculable effect from final state interactions would be a direct indication of time reversal symmetry violation. The D coefficient is the most sensitive probe of the phase, φAV, between the axial-vector (A) and vector (V) currents and is sensitive to scalar and tensor interactions that could arise due to beyond-Standard-Model physics. A blind analysis and extensive study of systematic effects has been completed with the result D = (-0.96±1.89(stat)±1.01(sys))x10-4, representing the most sensitive test of time- reversal invariance in beta decay. Within the Standard Model, the result can be interpreted as a measure of the phase φAV = (180.013±0.028)°.
Argonne Physics Division Colloquium Schedule