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Spin-polarized UCN transported into 3-m long guide situated within
a 1-Tesla superconducting solenoidal spectrometer 
Fraction of the UCNs undergo β-decay
Emitted decay electrons spiral around the field lines and are
detected in (identical) MWPC/scintillator detector packages 
located at both ends of the spectrometer
Measured asymmetry in the rates in the two detector packages yields
a value for A
The differential probability that a polarized neutron will decay
into an electron, a proton, and an electron anti-neutrino with
momenta in specified directions relative to the neutron spin is
given by 
The neutron-spin, electron-momentum angular correlation parameter,
A, (the “beta asymmetry”) can be expressed in terms of the ratio of
the axial-vector and vector coupling constants, λ = gA / gV , as
Measurements of A provide the definitive value for gA, the weak
axial-vector coupling constant of the nucleon. Further,
measurements of the neutron lifetime, τn , and A, determine the CKM
matrix element Vud as 
solid deuterium (< 6 K)
What are ultra-cold neutrons (UCN) ?
Neutrons with speeds < 8 m/s (< 350 nano-eV) that can undergo
total external reflection at all angles from various material
surfaces and can be reflected (μ·B interaction) from ~few Tesla
How do we produce UCN ?
Short (few hundred μs long) pulse of 800 MeV protons incident on a
tungsten spallation target produces a flux of MeV neutrons that are
moderated to < 100 K
Downscattered into UCN regime (< 4 mK) via phonon interactions
in a ~2-liter solid deuterium source maintained at < 6 K
How do we transport and polarize UCN ?
Transported along cylindrical guides coated with 58Ni or a
diamond-like film evaporated onto the surface of the guides
Spin-polarized via transport through a 7-Tesla magnetic field
UCN production in solid deuterium was pioneered at LANL in a
prototype solid deuterium source .
Plastic scintillator for measurement of energy and timing
MWPC for position information and background (gamma) rejection
(coincidence between scintillator and MWPC)
Detector cart for the MWPC/scintillator on the floor prior to
insertion into the spectrometer.
1-Tesla solenoidal spectrometer
UCN from source
 J.D. Jackson, S.B. Treiman, and H.W. Wyld, Jr., Phys. Rev. 106,
 S. Gardner and C. Zhang, Phys. Rev. Lett. 86, 5666
 A. Czarnecki, W.J. Marciano, and A. Sirlin, Phys. Rev. D 70,
 R.W. Pattie et al., Phys. Rev. Lett. 102, 012301 (2009).
 J. Liu et al., Phys. Rev. Lett. 105, 181803 (2010).
 H. Abele et al., Phys. Rev. Lett. 88, 211801 (2002).
 A. Serebrov et al., Phys. Lett. B 605, 72 (2005).
 I.S. Towner and J.C. Hardy, Rep. Prog. Phys. 73, 046301
 C.L. Morris et al., Phys. Rev. Lett. 89, 272501 2002); A.
Saunders et al., Phys. Lett. B 593, 55 (2004).
 B. Plaster et al., Nucl. Instrum. Methods Phys. Res. A 595,
 T.M. Ito et al., Nucl. Instrum. Methods Phys. Res. A 571, 676
PDG 2010 Values
walls of source coated with 58Ni
Past measurements of A with beams of polarized cold neutrons
suffered from significant discrepancies.
The UCNA experiment has provided the first-ever measurement of A
with stored ultracold neutrons (UCN). The use of UCN has
significant advantages in terms of the neutron polarization and
neutron-generated backgrounds. The first UCNA results [4,5] are in
good agreement with the most recent, and most precise, cold
neutron-based experiment, PERKEO II .
Current (2010) status of A, lifetime, and neutron-based Vud results
: UCNA Proof-of-Principle
: PERKEO II
S/B ~ 40:1 from 275–625 keV
Binned values for A after corrections for backscattering and β cosθ
Size of systematic corrections for backscattering and β cosθ
acceptance for different experimental configurations
Precision measurement of the neutron β-asymmetry A with
spin-polarized ultracold neutrons
B.W. Filippone, K.P. Hickerson, T.M. Ito, J. Liu, J.W. Martin, M.
Mendenhall, A. Pérez Galván, B. Plaster, R. Schmid, B. Tipton, and
W.K. Kellogg Radiation Laboratory, California Institute of
Technology, Pasadena, CA 91125 USA
and the UCNA Collaboration Institutions
Duke University, Idaho State University, Indiana University,
Institut Laue-Langevin, Los Alamos National Laboratory, North
Carolina State University, Shanghai Jiao Tong University, Texas
A&M University, Triangle Universities Nuclear Laboratory,
University of Kentucky, University of Washington, University of
Winnipeg, Virginia Tech
Spin-polarized neutron β-decay
UCNA experimental setup