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Unformatted text preview: t al. (2008). Both of these include a subset of the
measurements listed.
Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 B. W helicity from topquark decays In the SM, top quarks decay with a V À A interaction
through the process t ! W þ b with Bðt ! W þ bÞ % 1. The
SM predicts a fraction f0 ¼ 0:697 Æ 0:012 of topquark
decays gives a longitudinally polarized W and a fraction
fþ ¼ 3:6 Â 10À4 gives a righthanded W . This leaves fÀ ¼
1 À f0 À fþ giving lefthanded W s. Compared to the decay
of lower mass quarks, the fraction of decays with longitudinally polarized W is signiﬁcantly increased because of the
large Yukawa coupling between the top quark and Higgs
boson. Changes to the Lorentz (V À A) structure of the decay
will result in changed values for the polarization fractions.
Indirect results from b ! s measurements (Cho and Misiak,
1994; Fujikawa and Yamada, 1994) constrain fþ to less than
a few percent.
The angular distribution of the electrontype (I3 ¼ À1=2)
W decay product in t ! Wb is given by
!ðcÞ ¼ 3½2ð1 À c2 Þf0 þ ð1 À cÞ2 fÀ þ ð1 þ cÞ2 fþ
8 all jets, CDF [k] 160 For this combination, which includes preliminary results, the
systematic uncertainty is signiﬁcantly larger than the statistical uncertainty. The dominant components of the systematic
uncertainty arise from uncertainty on the in situ jet energy
"
calibration, uncertainty related to the tt production and decay
model, and uncertainty arising for color recombination.
These contribute 0.48, 0.49, and 0:41 GeV=c2 to the uncertainty, respectively.
An alternative method of measuring the topquark mass
makes use of the dependence of the production cross section
on the mass. The topquark MS mass may be extracted
directly from such a method (Langenfeld, Moch, and Uwer,
2009).
By the end of the Tevatron running, the experiments expect
to increase the data set by at least a factor of 4 compared to
the published results and at least a factor of 2 for the data sets
used in the combination. Thus, the statistical precision can be
improved, but the systematic uncertainties will also need to
be improved if the full statistical power of the ﬁnal data set is
to be realized. in which c cosÃ is the cosine of the decay angle in the W
rest frame measured with respect to the topquark direction.
Both CDF (Aaltonen et al., 2009i) and D0 (Abazov et al.,
2008g) measured the helicity fractions in topquark decay.
R
The CDF analysis uses Ldt ¼ 1:9 fbÀ1 , and the D0 result
R
is based on Ldt ¼ 1:0 fbÀ1 .
Both experiments select events in the ‘ þ jets topology,
and D0 also uses the ‘‘ þ X topology. The initial selections
are similar to those described earlier in Sec. IV.A. CDF uses a
second selection with somewhat different requirements to
perform an independent analysis. The CDF results are from
the combination of the two methods.
All results rely on reconstructing the cosÃ distribution and
comparing that to a set of predicted dis...
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