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using ALPGEN (Mangano et al., 2003). The CDF result does
not have a single dominant source of systematic uncertainty.
Although the contributions vary somewhat, the uncertainties
arising from jet calibration, the background prediction, and
modeling of ﬁnal-state radiation are roughly equivalent to
each and provide most of the systematic uncertainty.
C. Single-top production and Vtb The top quark was ﬁrst observed in reactions mediated by
the strong interaction process pp ! tt (Abachi et al. (1995)
and Abe et al. (1995), and nearly all measurements of the
top-quark properties have been made using such events.
Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 FIG. 45. The s-channel (left) and t-channel (right) diagrams for
single-top production. However, top quarks can also be produced singly via the
s-channel (Cortese and Petronzio, 1991; Stelzer and
Willenbrock, 1995; Heinson et al., 1997) and t-channel
(Willenbrock and Dicus, 1986; Yuan, 1990; Ellis and Parke,
1992; Heinson et al., 1997) electroweak diagrams shown in
Fig. 45. These production modes involve the Wtb coupling
and therefore cross-section measurements provide previously
unmeasured information on the electroweak sector, particularly the CKM matrix element Vtb . In addition to providing
information about Vtb , detecting these events demonstrates
progress toward Higgs sensitivity for the WH associated
production mode with the subsequent decay H ! bb.
The NLO cross sections for these processes are s ¼
1:12 Æ 0:05 pb and t ¼ 2:34 Æ 0:13 pb (Smith and
Willenbrock, 1996; Stelzer, Sullivan, and Willenbrock,
et al., 2002; Campbell, Ellis, and
Tramontano, 2004; Sullivan, 2004; Cao et al., 2005; Cao,
Schwienhorst, and Yuan, 2005; Frixione et al., 2006;
Kidonakis, 2006; Campbell et al., 2009) for the s-channel
and t-channel reactions, respectively, assuming jVtb j ¼ 1 and
a top-quark mass of Mt ¼ 175 GeV=c2 . While the total
single-top cross section is slightly more than 40% of the tt
cross section, the reduced parton multiplicity in the single-top
events implies a signal-to-noise ratio which is roughly
25 times lower than in tt making these events much more
difﬁcult to identify. In the SM with three generations, existing
measurements of jVub j and jVcb j tightly constrain jVtb j to
almost exactly unity (Amsler et al., 2008). If there are more
than three generations, however, jVtb j is almost entirely
Both D0 and CDF have long histories of searching for
single-top production. The most recent results from both D0
(Abazov et al., 2007d, 2008h) and CDF (Aaltonen et al.,
2008d) give clear evidence for single-top production and
include results for the cross section and the CKM element
jVtb j. The D0 analysis uses a data sample corresponding
Ldt ¼ 2:3 fbÀ1 . The CDF analysis uses up to
Ldt ¼ 3:2 fbÀ1 .
Candidate single top events are selected in the lepton þ
jets channel in which the W decays via...
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