emiT
Electroweak Interaction Research at the University of Washington
Time Reversal in Neutron Beta Decay---The First Run of emiT
The emiT experiment is a search for a violation
of time-reversal (T) invariance in the beta decay of free neutrons. The
experiment utilizes a beam of cold (2.7 meV) polarized neutrons from
the Cold Neutron Research Facility at the
National Institute of Standards and Technology (NIST)
in Gaithersburg, MD.
A sizable team of scientists has been assembled to perform this experiment
from Los Alamos National Laboratory,
NIST, the University
of California at Berkeley/Lawrence Berkeley National Laboratory,
the University of Michigan, the University
of Notre Dame, and the University of Washington's Nuclear
Physics Laboratory (NPL).
emiT probes the T-odd triple correlation (between the neutron spin and
the momenta of the neutrino and electron decay products) in the neutron
beta-decay distribution. The coefficient of this correlation,
D, is measured by detecting decay electrons in coincidence with recoil
protons while controlling the neutron polarization. Technological advances
in neutron polarization and an improved detector geometry should allow emiT to
eventually attain a sensitivity to D of 3x10-4). This level of
sensitivity represents a factor of five improvement over previous neutron T
tests, and may permit restrictions to be placed on several extensions to the
Standard Model that allow values of D near 10-3.
emiT is the first neutron T test to make use of a "supermirror" neutron
polarizer. Thus, emiT achieves a polarization of ~95 +/- 2%, as opposed to
the 65--85% polarizations typical in previous experiments.
The emiT detector consists of four plastic scintillator paddles for electron
detection and four arrays of large-area PIN diodes to detect the protons.
The eight detector segments are arranged in an alternating octagonal array
about the neutron beam so that each segment of one type lies at an angle of
135 degrees relative to two segments of the other type. This geometry
takes advantage of the fact that the electron--proton angular distribution
is strongly peaked due to the disparate masses of the decay products. When
compared to the 90-degree geometry used in previous experiments,
this octagonal geometry results in an increase in signal rate which is the
equivalent of roughly a factor of three increase in neutron beam flux.
The emiT experiment was installed on the NG-6 beamline at NIST from November
of 1996 until September of 1997. Roughly two months were spent carefully
characterizing the neutron beam and performing the initial shakedown of the
detector. Data were then collected during five six-week reactor cycles
starting in January and continuing into September of 1997. In general the
emiT detector performed well; a total of roughly 15 million
coincidence events were recorded and the maximum sustained
coincidence rate observed was 7 Hz. Analysis of these data is now complete.
(see L.J. Lising et al., Phys. Rev. C, 62, 055501 (2000).
The first run resulted in the best-ever measurement of D, and a slight improvement on the
world-average value. However, the data were dominated by systematic uncertainties. The emiT detector
has been shipped to NPL and solutions to problems that occured during the
first run are aggressively being sought. After hardware upgrades it is
expected that a second run of emiT will occur during 2001.
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Please send comments to: kheeger@u.washington.edu
Last update: October 8, 2000