2,383 research outputs found
Atomic Hydrogen Cleaning of Polarized GaAs Photocathodes
Atomic hydrogen cleaning followed by heat cleaning at 450C was used
to prepare negative-electron-affinity GaAs photocathodes. When hydrogen ions
were eliminated, quantum efficiencies of 15% were obtained for bulk GaAs
cathodes, higher than the results obtained using conventional 600C heat
cleaning. The low-temperature cleaning technique was successfully applied to
thin, strained GaAs cathodes used for producing highly polarized electrons. No
depolarization was observed even when the optimum cleaning time of about 30
seconds was extended by a factor of 100
Observation of Target Electron Momentum Effects in Single-Arm M\o ller Polarimetry
In 1992, L.G. Levchuk noted that the asymmetries measured in M\o ller
scattering polarimeters could be significantly affected by the intrinsic
momenta of the target electrons. This effect is largest in devices with very
small acceptance or very high resolution in laboratory scattering angle. We use
a high resolution polarimeter in the linac of the polarized SLAC Linear
Collider to study this effect. We observe that the inclusion of the effect
alters the measured beam polarization by -14% of itself and produces a result
that is consistent with measurements from a Compton polarimeter. Additionally,
the inclusion of the effect is necessary to correctly simulate the observed
shape of the two-body elastic scattering peak.Comment: 29 pages, uuencoded gzip-compressed postscript (351 kb). Uncompressed
postscript file (898 kb) available to DECNET users as
SLC::USER_DISK_SLC1:[MORRIS]levpre.p
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A polarization study of strained GaAs photocathode structures
The polarized electron source at SLAC has performed extremely well during recent years supplying electrons having a spin polarization of 78% (85%) for high (low) current operation with beam current limited primarily by experimental requirements. However, there is room for improvement in the electron polarization. The less-than-ideal polarizations are a result of both imperfections and depolarizing mechanisms within the photocathode. The structure of the photocathode used at SLAC in the polarized electron source is a single-strained emitting layer structure grown atop a GaAs substrate. Here, the properties of several types of strained GaAs and GaAsP photocathodes have been studied using x-ray diffraction and photoemission
Recent Progress at SLAC Extracting High Charge from Highly-Polarized Photocathodes for Future-Collider Applications
Future colliders such as NLC and JLC will require a highly-polarized
macropulse with charge that is more than an order of magnitude beyond that
which could be produced for the SLC. The maximum charge from the SLC
uniformly-doped GaAs photocathode was limited by the surface charge limit
(SCL). The SCL effect can be overcome by using an extremely high (>1019 cm-3)
surface dopant concentration. When combined with a medium dopant concentration
in the majority of the active layer (to avoid depolarization), the surface
concentration has been found to degrade during normal heat cleaning (1 hour at
600 C). The Be dopant as typically used in an MBE-grown superlattice cathode is
especially susceptible to this effect compared to Zn or C dopant. Some relief
can be found by lowering the cleaning temperature, but the long-term general
solution appears to be atomic hydrogen cleaning.Comment: 11 pages, 3 figures, 1 table, contributed to 10th Workshop on
Polarized Sources and Targets, Novosibirsk, Sept. 22-26, 2003, to be
submitted to Nucl. Instrum. and Meth.
Precision Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetries A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 0.7
< Q^2 < 20 GeV^2 by scattering 29.1 and 32.3 GeV longitudinally polarized
electrons from transversely polarized NH3 and 6LiD targets. Our measured g2
approximately follows the twist-2 Wandzura-Wilczek calculation. The twist-3
reduced matrix elements d2p and d2n are less than two standard deviations from
zero. The data are inconsistent with the Burkhardt-Cottingham sum rule if there
is no pathological behavior as x->0. The Efremov-Leader-Teryaev integral is
consistent with zero within our measured kinematic range. The absolute value of
A2 is significantly smaller than the sqrt[R(1+A1)/2] limit.Comment: 12 pages, 4 figures, 2 table
Measurement of the Proton and Deuteron Spin Structure Functions g2 and Asymmetry A2
We have measured the spin structure functions g2p and g2d and the virtual
photon asymmetries A2p and A2d over the kinematic range 0.02 < x < 0.8 and 1.0
< Q^2 < 30(GeV/c)^2 by scattering 38.8 GeV longitudinally polarized electrons
from transversely polarized NH3 and 6LiD targets.The absolute value of A2 is
significantly smaller than the sqrt{R} positivity limit over the measured
range, while g2 is consistent with the twist-2 Wandzura-Wilczek calculation. We
obtain results for the twist-3 reduced matrix elements d2p, d2d and d2n. The
Burkhardt-Cottingham sum rule integral - int(g2(x)dx) is reported for the range
0.02 < x < 0.8.Comment: 12 pages, 4 figures, 1 tabl
Precision Determination of the Neutron Spin Structure Function g1n
We report on a precision measurement of the neutron spin structure function
using deep inelastic scattering of polarized electrons by polarized
^3He. For the kinematic range 0.014<x<0.7 and 1 (GeV/c)^2< Q^2< 17 (GeV/c)^2,
we obtain at an average . We find relatively large negative
values for at low . The results call into question the usual Regge
theory method for extrapolating to x=0 to find the full neutron integral
, needed for testing quark-parton model and QCD sum rules.Comment: 5 pages, 3 figures To be published in Phys. Rev. Let
Evidence for an excess of B -> D(*) Tau Nu decays
Based on the full BaBar data sample, we report improved measurements of the
ratios R(D(*)) = B(B -> D(*) Tau Nu)/B(B -> D(*) l Nu), where l is either e or
mu. These ratios are sensitive to new physics contributions in the form of a
charged Higgs boson. We measure R(D) = 0.440 +- 0.058 +- 0.042 and R(D*) =
0.332 +- 0.024 +- 0.018, which exceed the Standard Model expectations by 2.0
sigma and 2.7 sigma, respectively. Taken together, our results disagree with
these expectations at the 3.4 sigma level. This excess cannot be explained by a
charged Higgs boson in the type II two-Higgs-doublet model. We also report the
observation of the decay B -> D Tau Nu, with a significance of 6.8 sigma.Comment: Expanded section on systematics, text corrections, improved the
format of Figure 2 and included the effect of the change of the Tau
polarization due to the charged Higg
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