C. Trull

New limit on time-reversal violation in beta decay.

We report the results of an improved determination of the triple correlation DP·(p(e)×p(v)) that can be used to limit possible time-reversal invariance in the beta decay of polarized neutrons and constrain extensions to the standard model. Our result is D=[-0.96±1.89(stat)±1.01(sys)]×10(-4). The corresponding phase between gA and gV is ϕAV=180.013°±0.028° (68% confidence level). This result represents the most sensitive measurement of D in nuclear β decay.

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

emiT: An apparatus to test time reversal invariance in polarized neutron decay

We describe an apparatus used to measure the triple-correlation term (Dσn⋅pe×pν) in the beta decay of polarized neutrons. The D coefficient is sensitive to possible violations of time reversal invariance. The detector has an octagonal symmetry that optimizes electron–proton coincidence rates and reduces systematic effects. A beam of longitudinally polarized cold neutrons passes through the detector chamber, where a small fraction undergo beta decay. The final-state protons are accelerated and focused onto arrays of cooled semiconductor diodes, while the coincident electrons are detected using panels of plastic scintillator. Details regarding the design and performance of the proton detectors, beta detectors, and the electronics used in the data collection system are presented. The neutron beam characteristics, the spin-transport magnetic fields, and polarization measurements are also described.

A backscatter-suppressed beta spectrometer for neutron decay studies

We describe a beta electron spectrometer for use in an upcoming experiment that will measure the beta-antineutrino correlation coefficient (a coefficient) in neutron beta decay. Electron energy is measured by a thick plastic scintillator detector. A conical array of plastic scintillator veto detectors is used to suppress events where the electron is backscattered. A Monte Carlo simulation of this device in the configuration of the a coefficient experiment is presented. The design, construction, and testing of a full-scale prototype device is described. We discuss the performance of this spectrometer with respect to its suitability for the experiment.

aCORN: An experiment to measure the electron-antineutrino correlation coefficient in free neutron decay

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

The aCORN Backscatter-Suppressed Beta Spectrometer

Backscatter of electrons from a beta spectrometer, with incomplete energy deposition, can lead to undesirable effects in many types of experiments. We present and discuss the design and operation of a backscatter-suppressed beta spectrometer that was developed as part of a program to measure the electronantineutrino correlation coefficient in neutron beta decay (aCORN). An array of backscatter veto detectors surrounds a plastic scintillator beta energy detector. The spectrometer contains an axial magnetic field gradient, so electrons are efficiently admitted but have a low probability for escaping back through the entrance after backscattering. The design, construction, calibration, and performance of the spectrometer are discussed.

A Backscatter Suppressed Electron Detector for the Measurement of "a".

A new method of measuring the electron-antineutrino angular correlation coefficient, little “a”, from neutron decay—to be performed at the National Institute of Standards and Technology—will require an electron spectrometer that strongly suppresses backscattered electrons. A prototype consisting of six trapezoidal veto detectors arranged around a plastic scintillator has been tested with an electron beam produced by a Van de Graaff accelerator. The results of this test and its implications for the little “a” measurement are discussed.

A new limit on time-reversal violation in beta decay: Results of the emiTII experiment

We have measured the triple correlation D〈Jn〉 · (pe × pv) with a polarized cold-neutron beam at the NIST Center for Neutron Research by observing proton-electron 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 10−5 and can be calculated with precision of 1% or better. The D coefficient is sensitive to the phase, of λ the ratio of axial-vector and vector amplitudes as well as to scalar and tensor interactions that could arise due to beyond-Standard-Model physics such as leptoquarks. Over 300 million proton-electron coincidence events were used in a blind analysis with the result D = [−0.96±1.89(stat)±1.01(sys)]×10−4.

Proposed Measurement of the Beta-Neutrino Correlation in Neutron Decay

Currently, the beta-neutrino asymmetry has the largest uncertainty (4 %) of the neutron decay angular correlations. Without requiring polarimetry this decay parameter can be used to measure λ (ga/gv), test Cabibbo-Kobayashi-Maskawa (CKM) unitarity limit scalar and tensor currents, and search for Charged Vector Current (CVC) violation. We propose to measure the beta-neutrino asymmetry coeffcient, a, using time-of-flight for the recoil protons. We hope to achieve a systematic uncertainty of σa / a ≈ 1.0 %. After tests at Indiana University’s Low Energy Neutron Source (LENS), the apparatus will be moved to the National Institute of Standards and Technology (NIST) where the measurement can achieve a statistical uncertainty of 1 % to 2 % in about 200 beam days.