Combined SLD Measurement of Ab at the Z0 Resonance using Various Techniques Page: 3 of 9
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reconstructed polar angle. The statistical correlations between analyses for this dataset was
then determined from the overlapping event fractions, the fractions of events where different
tags assigned the same (opposite) quark directions, and the individual event weights. This
statistical correlation was then diluted to account for the fact that not all analyses use the
The statistical correlations extracted range from - 10 - 30% depending on the pair of
analyses considered. The largest correlation (28%) was observed between the jet-charge and
vertex-charge analyses, as expected; due to its statistical power the jet-charge analysis has
significant overlap with all three other analyses. The smallest correlation (8%) was between
the lepton tag and vertex charge analyses.
Correlations between analyses due to common systematic error sources have been treated
in the standardized fashion developed by the LEP Electroweak Working Group . Since
three of the four analyses (all but the lepton tag) use self-calibration techniques based on
the data, most of the quoted systematic errors are in fact dominated by data statistics and
thus (mostly) uncorrelated. For the purposes of this combination, we assume A, is fixed at
its Standard Model value.
The analyses are then combined in a weighted average using the individual analysis er-
rors and the statistical correlation matrix. Each analysis receives a weight in the overall
combination based on its statistical and uncorrelated systematic error. Statistical and un-
correlated systematic errors are combined in quadrature and correlated systematic errors
are combined linearly. The final analysis weights are 38% (jet-charge), 30% (leptons), 22%
(vertex-charge), and 10% (kaons). The combined SLD preliminary result obtained with this
Ab = 0.905 0.017(stat) 0.020(syst) (combined). (5)
This result differs slightly from the LEP Electroweak Working Group fit [9, 10] of the same
data due to correlations between the Ab and A, results which enter here primarily through
the lepton-tag analysis. We explicitly ignore such correlations in our average whereas the
LEP global fits include them.
Our average result for Ab agrees well with the Standard Model expectation of 0.935, and
also with that derived from the current combination of LEP results (0.892 0.024) used in
the global electroweak fit. The combined LEP and SLD results, however, imply that Ab
deviates from the Standard Model at the - 2.5- level; this intriguing situation has persisted
since 1996 despite significant improvements in statistical and systematic errors. One recent
analysis  of the world's Ab data shows no evidence of systematic bias or underestimated
errors. Thus the experimental question of possible anomalies in the Zbb coupling remains
 D. Jackson, Nucl. Instrum. Methods A388, 247 (1997).
 K. Abe et al. (SLD Collaboration), Phys. Rev. Lett. 81, 942 (1998);
 N. deGroot, to appear in Proceedings of the XXXIV Rencontres de Moriond, Les Arcs,
France, Mar 13-20 1999.
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Crawford, Glen D. Combined SLD Measurement of Ab at the Z0 Resonance using Various Techniques, report, July 27, 1999; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc620132/m1/3/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.