Dr Hans Pieter Mumm (NIST)
We have measured the D-coefficient in the triple correlation of the neutron spin with proton and electron momenta by observing coincidences in the decay of polarized neutrons. A non-zero value of D can arise due to parity-even-time-reversal-odd interactions that imply CP violation due to the CPT theorem. (Final-state effects also contribute to D at the level of 1e-5 and can be calculated with precision of 1% or better . The D coefficient is uniquely sensitive to the phase, φAV, of the ratio of axial-vector (A) and vector (V) amplitudes: λ=gA/gV as well as to scalar and tensor interactions that could arise due to beyond-Standard-Model physics such as leptoquarks . The experiment was performed with the NG-6 cold-neutron beam at the NIST Center for Neutron Research in Gaithersburg, Maryland. The neutron beam is polarized, passes through a spin flipper and is collimated into a spectrometer, which measures proton-electron coincidences in an octagonal detector array concentric with the neutron beam. The recoil protons were accelerated to ~28 keV and detected by surface barrier detectors. The electrons were detected in plastic scintillators. The detector is highly segmented, allowing the triple correlation to be isolated and separated from a variety of systematic effects due to the parity-odd-time-reversal even correlations . A 14-month run in 2002-2003 produced a sample of over 300 million proton- electron coincidence events. A blind analysis and extensive study of all significant systematic effects has recently been completed with the result D = (-0.96±1.89 (stat)±1.01(sys))e-4. The corresponding upper limit on D is a factor of three improvement over the previous upper limit for neutron decay [4,5] and over the upper limit measured in 19Ne decay , and thus our result represents the most sensitive test of time- reversal invariance in beta decay. Assuming only vector and axial vector interactions in beta decay, the result can be interpreted as a measure of the phase φAV = (180.013±0.028)°. This result also improves constrains on certain non-VA interactions. 1. S. Ando et al. Phys. Lett. B 667, 109 (2009). 2. P. Herczeg, in Proc. of the 6th Int. PASCOS-98, (1998). 3. H.P. Mumm et al., Rev. Sci. Inst. 75, 5343 (2004). 4. L.J. Lising et al. Phys. Rev. C 62, 055501 (2000). 5. T. Soldner et al. Physics Letters B 581 49 (2004). 6. A.L. Hallin et al., Phys. Rev. Lett. 52, 1054 (1984).
Dr Alan Thompson (NIST) Prof. Alejandro Garcia (University of Washington) Dr Brian Fujikawa (Lawrence Berkeley Lab) Dr C.A. Trull (Tulane University) Prof. Fred Wietfeldt (Tulane University) Prof. Gordon Jones (Hamilton College) Dr Jeffrey Nico (NIST) Prof. John Wilkerson (University of North Carolina) Dr Kevin Coulter (University of Michigan) Dr Robert Cooper (University of Michigan) Prof. Stuart Freedman (University of California)