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The Beta-Asymmetry in Neutron Decay Jeffery W.
Martin W. K. Kellogg Radiation Laboratory,
California Institute of Technology, Pasadena, CA
91125 for the UCNA Collaboration T. J. Bowles1
(co-PI), R. Carr2, B. W. Filippone2, A. Garcia3,
P. Geltenbort4, R. E. Hill1, S. A. Hoedl3, G. E.
Hogan1, T. M. Ito2, S. K. Lamoreaux1, C.-Y. Liu1,
M. Makela5, R. Mammei5, J. W. Martin2, R. D.
McKeown2, F. Merrill1, C. L. Morris1, M. Pitt5,
B. Plaster2, K. Sabourov6, A. Sallaska3, A.
Saunders1, A. Serebrov7, S. Sjue3, E. Tatar8, R.
B. Vogelaar5, Y.-P. Xu6, A. R. Young6 (co-PI),
and J. Yuan2 1Los Alamos National Laboratory, Los
Alamos, NM 87545 2W. K. Kellogg Radiation
Laboratory, California Institute of Technology,
Pasadena, CA 91125 3Center for Experimental
Nuclear Physics and Astrophysics, University of
Washington, Seattle, WA 98195 4Institut
Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9,
France. 5Virginia Polytechnic Institute and
State University, Blacksburg, VA 24061 6North
Carolina State University, Raleigh, NC 27695.
7St.-Petersburg Nuclear Physics Institute,
Russian Academy of Sciences, 188350 Gatchina,
Leningrad District, Russia 8Idaho State
University, Pocatello, ID 83209.
Neutron Beta-Decay
A New Way to Make Ultracold Neutrons
The UCNA Experiment

• Decay Rate
• The neutron decays via the weak interaction into
  an electron, a proton, and an electron
  anti-neutrino.
• The decay rate depends on the polarization of the
  neutron, and the momenta of the outgoing electron
  and anti-neutrino.
• Measuring the total rate (the lifetime) and any
  of the above correlation parameters determines
  the vector (GV) and axial-vector (GA) couplings.
• The beta-asymmetry parameter A is the most
  sensitive of the correlation parameters to GA.
  This parameter represents correlation between the
  electron momentum and the neutron polarization.

Figure Experimental Schematic for UCNA.
Polarized UCN exit the polarizer magnet and enter
the beta-spectrometer. While inside the
spectrometer, a fraction of the UCN decay. The
emitted electrons spiral around the magnetic
field lines and are sensed in MWPC/scintillator
detectors. The asymmetry in rates between the
detectors at each end gives the decay parameter
A.
previous record ILL
Improvements for a determination of A All
previous experiments on A used a reactor source
of cold neutrons, and used supermirror polarizers
to polarize the neutrons. Using UCN , we can
achieve reduced backgrounds We use a
spallation source to produce neutrons, which can
be pulsed to reduce prompt backgrounds, and can
be switched off to determine remaining
backgrounds. higher neutron polarization We
expect neutron polarizations at the level 99.9
and hence smaller systematic corrections.
The Standard Model

• In the standard model
• By the conserved vector current theorem (CVC),
  the neutron coupling constant GV is related to
  the fundamental quark coupling. GV is therefore
  related to the element Vud of the
  Cabibbo-Kobayashi-Maskawa (CKM) matrix.
• The matrix must be unitary, so, theoretically
• With precise measurements of Vud and Vus, we can
  test this!
• Status (2002)

Figure The new UCN source for UCNA as
construction finishes at Los Alamos April 2004.
The UCN source also underwent successful
commissioning in April 2004.
Future programme Upgrades for UCNA are planned.
One upgrade involves the use of silicon detectors
in place of the existing scintillators. The
silicon detectors would provide significantly
improved energy resolution, allowing for more
accurate beta-spectroscopy. Silicon detectors
make it possible to extract energy-dependent
recoil-order terms in the asymmetry, sensitive to
weak magnetism and to extract the
energy-dependence of the unpolarized decay
spectrum, sensitive to Fierz interference.
Detectors for Accurate Beta-Spectroscopy

The UCNA experiment uses a combination of a
multi-wire proportional counter (MWPC) with a
scintillation counter to detect the emitted
betas. The scintillation counter provides a
measurement of the full energy deposition and
fast timing. The MWPC provides position
sensitivity, background rejection, and a low
detector threshold.
Figure prototype silicon detector for UCNA.
Another planned upgrade involves the use of
proton detectors. Protons would be accelerated
into a thin foil producing secondary electrons.
The secondaries would be sensed in the silicon
detectors. Proton detection allows the
possibility of extracting the remaining
correlation parameters B and a.
With the level of accuracy to be attained,
systematic uncertainties relating to the accurate
detection of the decay electrons become
important. We used accelerator and radioactive
sources to precisely calibrate the beta
detectors. (see Refs. 5 and 6 and Fig.)
MWPC Anode
Scintillator sum
counts
References
channels
channels
Figure UCNA beta-detector ready for insertion
into spectrometer.
Figure Digitized detector response for 120 keV
incident electron beam from electron gun.

• The Caltech UCNA website, http//www.krl.caltech.e
  du/ucn. My email address jmartin_at_krl.caltech.edu
  .
• T. Bowles and A. R. Young (co-principal
  investigators), A proposal for an accurate
  measurement of the neutron spin, electron angular
  correlation in polarized neutron beta-decay with
  ultracold neutrons (2000).
• C. L. Morris et al, Measurements of
  Ultracold-Neutron Lifetimes in Solid Deuterium,
  Phys. Rev. Lett. 89, 272501 (2002).
• A. Saunders et al, Demonstration of a
  Solid-Deuterium Source of Ultracold Neutrons,
  nucl-ex/0312021, submitted to Phys. Lett.
• J. W. Martin et al, Measurement of Electron
  Backscattering in the Energy Range of Neutron
  Beta-Decay, Phys. Rev. C 68, 055503 (2003).
• J. Yuan et al, A Double-Focusing Helmholtz-Coil
  Spectrometer, Nucl. Instrum. Methods Phys. Res.
  A 465, 404 (2001).
• This work is supported by the National Science
  Foundation and the Department of Energy.

Systematic effects due to backscattering of
electrons from the MWPC windows and the
scintillator face will necessitate a correction
on the determination of A at the level 0.1. To
gain confidence in this correction, we have
carried out detailed measurements of
backscattering in the low-energy regime (see Ref.
5).