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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 \cdot(p_{e}\timesp_{\nu}) 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 1e-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\pm1.89(stat)\pm0.97(sys)]e-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 (CA/CV= |{\lambda}|exp(i{\phi}_AV)) with {\phi}_AV = 180.012{\deg} \pm0.028{\deg} (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.

A New Limit on Time-Reversal Violation in Beta Decay H.P. Mumm, 1, 2 T.E. Chupp, 3 R.L. Cooper, 3 K.P. Coulter, 3 S.J. Freedman, 4 B.K. Fujikawa, 4 A. Garc a, 2, 5 G.L. Jones, 6 J.S. Nico, 1 A.K. Thompson, 1 C.A. Trull, 7 J.F. Wilkerson, 8, 2 and F.E. Wietfeldt 7 National Institute of Standards and Technology, Gaithersburg, MD 20899 CENPA and Physics Department, University of Washington, Seattle, WA 98195 Physics Department, University of Michigan, Ann Arbor, MI, 48104 Physics Department, University of California at Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Department of Physics, University of Notre Dame, Notre Dame, IN 46556 Physics Department, Hamilton College, Clinton, NY 13323 Physics Department, Tulane University, New Orleans, LA 70118 Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599 @htedX wy TD PHIIA e report the results of n improved determintion of the triple orreltion D P ¡ @ p e ¢ p v A tht n e used to limit possile timeEreversl invrine in the et dey of polrized neutrons nd onstrin extensions to the tndrd wodelF yur result is D a @ H : WT ¦ I : VW@ stat A ¦ I : HI@ sys AA ¢ IH 4 F he orresponding phse etween g A nd g V is AV a IVH : HIQ ¦ H : HPV @TV7 ondene levelAF his result represents the most sensitive mesurement of D in nuler et deyF PACS numbers: 24.80.1y, 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 specic 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 reect those of the United States Gov- ernment or any agency thereof or the Regents of the University of California. The existence of charge-parity (CP) symmetry viola- tion in nature is particularly important in that it is nec- essary to explain the preponderance of matter over an- timatter in the universe [1]. Thus far, CP violation has been observed only in the K and B meson systems [2{ 4] and can be entirely accounted for by a phase in the Cabbibo-Kobayashi-Maskawa matrix in the electroweak Lagrangian. This phase is insucient to account for the known baryon asymmetry in the context of Big Bang cosmology [5], so there is good reason to search for CP violation in other systems. As CP and time-reversal (T) violation can be related to each other through the CPT theorem, experimental limits on electric dipole moments and T-odd observables in nuclear beta decay place strict constraints on some, but not all, possible sources of new CP violation. The decay probability distribution for neutron beta de- cay, dW , can be written in terms of the beam polariza- tion P and the momenta (energies) of the electron p e ( E e ) and antineutrino p ( E ) as [6] dW p G 1 + a p e ¡ E + b m e + E E p P ¡ A e e E e e + B p E + D p e ¢ p E e E A contribution of the parity-even triple correlation D P ¡ ( p e ¢ p ) above the level of calculable nal-state inter- actions (FSI) implies T-violation. The PDG average of recent measurements is D = ( 4 ¦ 6) ¢ 10 4 [7{9], while the FSI for the neutron are $ 10 5 [10, 11]. Comple- mentary limits can be set on other T-violating correla- tions, and recently a limit on R has been published [12]. Various theoretical models that extend the SM, such as left-right symmetric theories, leptoquarks, and certain exotic fermions could give rise to observable eects that are as large as the present experimental limits [13]. Cal- culations performed within the Minimal Supersymmetric Model, however, predict D . 10 7 [14]. In the neutron rest frame, the triple correlation can be expressed as D P ¡ ( p p ¢ p e ), where p p is the proton mo- mentum. Thus one can extract D from the spin depen- dence of proton-electron coincidences in the decay of cold polarized neutrons. Our measurement was carried out at the National Institute of Standards and Technology Center for Neutron Research [15]. The detector, shown schematically in Fig. 1, consisted of an octagonal array of four electron-detection planes and four proton-detection