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Measurement of the Spin-Proton Asymmetry from Polarized Neutron DecayCollaborators
M.S. Dewey, T.R. Gentile, J.S. Nico, and A.K. Thompson - NISTDetermination of the ratio of the axial vector to vector weak coupling coefficients,  , provides a key test
of the Standard Model, in particular the unitarity of the CKM matrix. Although
the spin-electron asymmetry coefficient (A) is
the most sensitive to , the lack of
consistency in the determinations of
from measurements of A is problematic. We
propose a complementary measurement for determining
using the spin-proton asymmetry
observable, C, rather than the spin-electron
asymmetry. C is proportional to
A+B, where B
is the spin-neutrino asymmetry, and is related to , via C = k / (1+3 2). It has the advantage that it allows for a
determination of via an independent
approach as compared to previous determinations. The only attempt to measure
C was unsuccessful due to a large gamma ray
background. In recent developmental work, we have already demonstrated that we
can achieve adequate levels of signal-to-background. We plan to perform this
experiment with minor modifications to the NIST beam lifetime apparatus and the
implementation of spin filter techniques for accurate neutron polarimetry.
The relative uncertainty in
assuming
The proposed experiment involves using a 5 T solenoid, an electrostatic mirror to reflect decaying protons, and a proton detector to the side of the neutron beam, shown schematically in Figure 1. Longitudinally polarized neutrons enter the solenoid and the number of decay protons emitted parallel versus antiparallel to the neutron polarization yields the proton asymmetry. This apparatus exists at NIST and has previously been used in the Penning-trap neutron lifetime measurement. The Penning trap itself will be replaced with an electrostatic mirror to allow detection of protons emitted in the opposite direction from the proton detector held at a negative high voltage. Neutrons will be polarized parallel to the beam axis; the polarization will be periodically flipped to measure the asymmetry. At the end of the beamline will be a thick 3He analyzer to measure the neutron polarization. Although supermirrors can provide a highly polarized beam with high transmission, their use for analyzing polarization has been a major source of uncertainty in past measurements of A and B. Recent studies at the Institut Laue-Langevin have demonstrated that 3He-based neutron spin filters can be used to measure neutron polarization with an uncertainty better than 0.2 %.
We anticipate that the neutron polarimetry will be the most difficult aspect of the experiment, requiring a significant fraction of the effort. We plan to use a nearly opaque spin filter, which is possible because of the high beam intensity and allows for an analyzing power close to unity. Using the capability of reversing the direction of the 3He polarization in this analyzer, transmission measurements will be used to determine both the beam polarization and the spin flipper efficiency. Because of the variation of the transmission of the 3He with wavelength, a chopper will be employed for wavelength selection, and cells with different 3He pressures will be required. The neutron polarization will be measured before and after the experiment and monitored continuously throughout the run.
Recently, tests were performed to determine the gamma ray background
in the silicon detector and the expected proton rates.
Figure 2 shows the proton spectrum with the
detector biased to 32.5 kV resulting in a signal-to-noise ratio of 5.
The proton rate was approximately 5 s-1. This rate will be
increased by simply opening up the highly collimated beam that was in
place for the Penning-trap lifetime apparatus. Using this
demonstrated capability in proton counting and assuming reasonable
estimates for the neutron polarization of 0.960 ± 0.002, a spin
flip efficiency of 0.990 ± 0.001, and a proton asymmetry of -0.24,
a 0.5 % measurement of
 Inquiries or
comments: david.gilliam@nist.govOnline: October 2003 |
