This preview shows page 1. Sign up to view the full content.
Unformatted text preview: tributions each of
which is generated with a different pair of f0 and fþ values.
Several ambiguities arise in reconstructing cosÃ . These include the assignment of the reconstructed jets to the quarks
from W decay (for the hadronic side of ‘ þ jets events) and
the impact from the unmeasured value for the neutrino Hobbs, Neubauer, and Willenbrock: Tests of the standard electroweak model at . . . 1509 "
TABLE XXIV. W boson helicity fractions in tt decay determined
from ﬁts to data. The ﬁrst two lines are the results from simultaneous ﬁts for f0 and fþ . The third line is the result from the ﬁt for
f0 with fþ ¼ 0, and the fourth is from the ﬁt for fþ with f0 set to
the SM value. In all cases, the statistical uncertainty is shown ﬁrst
and the second uncertainty is the systematic uncertainty.
CDF
f0
fþ
f 0 , fþ ¼ 0
fþ , f0 ¼ 0:70 D0 0:66 Æ 0:16 Æ 0:05
À0:03 Æ 0:06 Æ 0:03
0:62 Æ 0:10 Æ 0:05
À0:03 Æ 0:04 Æ 0:03 0:425 Æ 0:166 Æ 0:102
0:119 Æ 0:090 Æ 0:053
0:619 Æ 0:090 Æ 0:052
À0:002 Æ 0:047 Æ 0:047 arising in leptonic W decay. These issues are handled differently in the three (two CDF, one D0) analyses. The CDF
analyses use only the leptonic W decay. The D0 analysis also
uses the jets from the hadronic W , but only j cosÃ j is
reconstructed for these, not cosÃ . Although this loses the
ability to distinguish fþ from fÀ for the hadronic decay, f0 is
better constrained.
The results are extracted using likelihood ﬁts to test cosÃ
distributions. For one of the CDF methods and the D0
analysis, predicted reconstructed cosÃ distributions including signal and background contributions are generated using
simulated events. The distributions are generated for a range
of f0 and fþ values, and best values are extracted using
likelihood comparisons to the data. For the second CDF
method, the result is compared to a distribution generated
by convoluting the true cosÃ distribution divided into six
bins with a migration function which gives the probability
that an event generated in a given true bin ends up as a given
reconstructed bin. This function includes the effects of resolution and acceptance. Effects of helicity on the acceptance
are considered in all cases.
The experimental results are summarized in Table XXIV.
The results are consistent with each other and with the SM
prediction. They are generally limited by the statistical precision of the data set and are expected to improve by a factor
between Â2 and Â10 by the end of Run II, depending on the
analysis and machine performance achieved. Even if there is
no improvement in the systematic uncertainty, the ﬁnal
results will still be statistics limited, but the statistical and
systematic uncertainties will be similar. The dominant sys"
tematic uncertainty in the D0 result comes from the tt production and decay model which was tested by comparing
results from events generated using the PYTHIA generator
(Sjostrand et al., 2001) and with the default...
View
Full
Document
This document was uploaded on 09/28/2013.
 Fall '13
 Energy

Click to edit the document details