Search for a T-odd, P-even Triple Correlation in Neutron Decay

Search for a T-odd, P-even Triple Correlation in Neutron Decay T.E. Chupp, 1 R.L. Cooper, 1 K.P. Coulter, 1 S.J. Freedman, 2 B.K. Fujikawa, 2 A. Garc´ia, 3, 4 G.L. Jones, 5 H.P. Mumm, 6 J.S. Nico, 6 A.K. Thompson, 6 C.A. Trull, 7 F.E. Wietfeldt, 7 and J.F. Wilkerson 3, 8, 9 University of Michigan, Ann Arbor, Michigan 48104, USA Physics Department, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA CENPA and Physics Department, University of Washington, Seattle, WA 98195 USA Department of Physics, University of Notre Dame, Notre Dame, IN 46556 USA Physics Department, Hamilton College, Clinton, NY 13323, USA National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Physics Department, Tulane University, New Orleans, LA 70118, USA Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599, USA Oak Ridge National Lab, Oak Ridge, TN, 37831 USA Background: Time-reversal-invariance violation, or equivalently CP violation, may explain the observed cosmological baryon asymmetry as well as signal physics beyond the Standard Model. In the decay of polarized neutrons, the triple correlation D J n ·(p e ×p ν ) is a parity-even, time-reversal- odd observable that is uniquely sensitive to the relative phase of the axial-vector amplitude with respect to the vector amplitude. The triple correlation is also sensitive to possible contributions from scalar and tensor amplitudes. Final-state effects also contribute to D at the level of 10 −5 and can be calculated with a precision of 1% or better. Purpose: We have improved the sensitivity to T-odd, P-even interactions in nuclear beta decay. Methods: We measured proton-electron coincidences from decays of longitudinally polarized neutrons with a highly symmetric detector array designed to cancel the time-reversal-even, parity-odd Standard-Model contributions to polarized neutron decay. Over 300 million proton-electron coincidence events were used to extract D and study systematic effects in a blind analysis. Results: We find D = [−0.94 ± 1.89(stat) ± 0.97(sys)] × 10 −4 . Conclusions: This is the most sensitive measurement of D in nuclear beta decay. Our result can be interpreted as a measurement of the phase of the ratio of the axial-vector and vector coupling constants (C A /C V = |λ|e iφ AV ) with φ AV = 180.012 ◦ ±0.028 ◦ (68% confidence level) or to constrain time-reversal violating scalar and tensor interactions that arise in certain extensions to the Standard Model such as leptoquarks. This paper presents details of the experiment, analysis, and systematic- error corrections. PACS numbers: 24.80.+y, 11.30.Er, 12.15.Ji, 13.30.Ce DISCLAIMER: This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of Cal- ifornia, nor any of their employees, makes any warranty, express or implied, or assumes any le- gal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, prod- uct, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommen- dation, or favoring by the United States Govern- ment or any agency thereof, or the Regents of the University of California. The views and opin- ions of authors expressed herein do not necessar- ily state or reflect those of the United States Gov- ernment or any agency thereof or the Regents of the University of California. I. INTRODUCTION The symmetries of physical processes under the trans- formations of charge conjugation (C), parity (P), and time reversal (T) have played a central role in the de- velopment of the Standard Model of elementary-particle interactions [1]. Time-reversal-symmetry violation (or T violation), which is equivalent to CP violation assum- ing CPT symmetry, has been of particular interest be- cause it is sensitive to many kinds of new physics. The CP-violating parameters of the Standard Model are the Cabibbo-Kobayashi-Maskawa (CKM) phase, which en- ters in the mixing of three generations of quarks, and the parameter θ QCD . The effect of the CKM phase is strongly suppressed in the permanent electric dipole mo- ments (EDMs) of the neutron [2] and heavy atoms [3, 4], and recent EDM results combine to set upper limits on θ QCD . All laboratory measurements to date are consis- tent with a single source of CP violation, i.e. the phase in the CKM matrix. An exception may be the 3.2 sigma deviation observed recently as an asymmetry in the pro- duction of pairs of like-sign muons reported by the D0