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emiT: A Search for Time Reversal Invariance Violation in Polarized Neutron Decay

Collaborators

T.E. Chupp and K.P. Coulter - University of Michigan
M.S. Dewey, A.K. Thompson, and J.S. Nico - NIST
S.J. Freedman and B.K. Fujikawa -University of California at Berkeley
A. Garcia, H.P. Mumm, R.G.H. Robertson, and J.F. Wilkerson - CENPA/University of Washington
G.L. Jones - Hamilton College
A.K. Komives - DePauw University
F.E. Wietfeldt - Tulane University

Motivation

For more than 35 years, neutral kaons were the only system in which charge-parity (CP) violation was observed. Although it can be explained in the Standard Model using a Kobayashi-Maskawa phase, this phase is small and its effects are highly suppressed in processes involving only the lighter quarks. Recently, additional experiments have reported measuring CP violation in the decay of neutral B mesons. The Standard Model also accommodates the mixing in the B system quite well. This makes it amenable to measurement, but it also has the effect of making any departures from the Standard Model more difficult to observe.

The observed violation of CP symmetry coupled with the CPT theorem requires the existence of time (T) violation. Hence, experiments searching for time-reversal invariance violation provide information that complements direct CP tests. These T-violation searches span a wide number of systems, including atoms, nuclei, and elementary particles, each having quite different sensitivities to time reversal effects predicted in the various theoretical models. The information from different T-violation experiments coupled with direct CP violation tests offers the best hope to untangle the CP-violation puzzle.

With its small Standard-Model values of time-reversal violating observables, neutron beta decay provides a excellent laboratory in which to search for interactions beyond the Standard Model. Leptoquark, left-right symmetric models, and "exotic" fermion models can all lead to violations of time reversal symmetry at potentially measurable levels. The experiment of the emiT collaboration searches for evidence of T violation in the beta decay of polarized, free neutrons.

The technique employed by the emiT involves the measurement of the triple-correlation of the neutron spin, electron momentum, and neutrino momentum

Eq. 1 $\sigma_n \cdot p_e \times p_\nu$

observed in the beta decay of polarized neutrons

Eq. 2 $n \rightarrow p+e + \nu_e$

This triple correlation term, appearing with strength D, is odd under time reversal, while even under the parity transformation. This is an important distinction between electric dipole moment (EDM) experiments whose interaction is both time-reversal and parity violating. The physics behind these two experiments is manifestly different and allows neutron decay experiments to place limits on new physics in cases where the EDM measurements are not sensitive. It has been argued that T-violating, P-conserving constraints can be derived from EDM measurements in spite of the fact that EDMs are P-violating. EDM and neutron decay correlation searches may provide complimentary information.

1997 Data Run

emiT experimental setup
Figure 1

A beam of cold neutrons is provided by the the NIST Center for Neutron Research. The emiT detector and associated beamline and spin transport, as shown in Figure 1, were assembled at the NG-6 fundamental physics experimental station. The detector consists of four 50 cm long electron detectors and four 30 cm long proton detectors arranged octagonally around the neutron beam, as illustrated in Figure 2. The octagonal geometry, maximizes the experiment's sensitivity to D by balancing the sine dependence of the cross product with the large angles between the proton and electron momenta that are favored by kinematics. The decay protons drift in a field free region 20 cm across before being focussed by a -35 kV potential into semiconductor detectors. With a maximum recoil energy of 750 eV, most of the protons arrive between 0.5 µs to 2 µs after the betas, as shown in Figure 3. Approximately 1.4 × 106 events were acquired during several months of running in 1997.

emiT detector
Figure 2

Analysis of the data yielded a reduced uncertainty on the value of D of [-0.6 ± 1.2 (stat) ± 0.5 (sys)] × 10-3 (see L.J. Lising et al., Phys. Rev. C 62, (2000) 055501). This value can be combined with earlier measurements to produce a new world average for the neutron D coefficient of (-0.6 ± 1.0) × 10-3 and constrains the phase of  gA/gV  to phi = 180.08° ± 0.13°. This represents a 33% improvement over the current world average and further constrains Standard Model extensions incorporating leptoquarks.

coincidence data from the experiment
Figure 3

Current Status

Although this result sets an improved upper bound for the neutron D coefficient, the uncertainty is dominated by the statistical uncertainty, but it is not limited by the NG-6 beam fluence rate. This is a consequence of the poor performance of the ion-planted detectors and high voltage-related breakdowns during the run. Several modifications and upgrades to the apparatus to rectify these problems are nearing completion. These upgrades should result in a reduction by a factor of five in the uncertainty of D. In addition, measurements show an increase of approximately 1.5 in the cold neutron fluence rate resulting from a cold source upgrade for the NIST reactor. emiT is now running and will take data until the latter part of 2003. The improvements outlined here are expected to produce a factor of ten higher data rate with significantly reduced systematic effects, and it is feasible to reduce the statistical uncertainty on D to 2 × 10-4.

Other Links

More information about the emiT experiment is available at http://ewiserver.npl.washington.edu/emit/


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Online: October 2003