Top quark physics: from Tevatron to LHC Page: 2 of 9
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The most precise measurements of the tt cross section have been achieved so far in the f +jets
channel which has a good rate and manageable background dominated by the production of W
bosons in association with heavy and light flavor jets (W+jets). To discriminate signal from
background two approaches are used. The first approach makes use of the distinct kinematic
features of a tt event arising from its large mass. The second approach requires that at least one
of the jets per event is identified as a b-jet. DO performed t4 cross section measurement with 4.3
fb-' dataset using both methods 2. The measured cross section using b-jet identification yields
at = 7.93' .4(stat + syst + lumi) pb. The measurement that combines kinematic event infor-
mation into a discriminant using Boosted Decision Trees and performs a fit to data in the same
dataset gives a more precise result: a5 = 7.700 (stat + syst + lumi) pb. For both methods
the measurements are dominated by the systematic uncertainties with the largest contribution
coming from the determination of integrated luminosity (6.1%).
The CDF collaboration significantly reduces the dependence on the luminosity measure-
ment and its associated large systematic uncertainty by exploiting the correlation between
the luminosity measurements for Z boson and t4 production 3. In the CDF analysis, the
ratio of the t4 to Z boson cross section, measured using the same triggers and dataset, is
computed and multiplied by the precisely known theoretical Z cross section. Thus, the lu-
minosity uncertainty is replaced with the smaller theoretical and experimental uncertainties
on Z cross section. Using this approach in the dataset of 4.6 fb-1 the CDF collaboration
measures the cross section of a5 = 7.82 0.38(stat) 0.37(syst) 0.15(theory) pb combin-
ing kinematic event information into a neural network and performing a fit to the data and
at5 = 7.32 0.36(stat) 0.59(syst) 0.14(theory) pb using b-jet identification. Combination of
these two measurements yields the most precise top quark cross section measurement to date of
at = 7.70 0.52 pb. All cross sections quoted above assume top quark mass mt = 172.5 GeV.
For the measurements using b-jet identification important uncertainty comes from the flavor
composition of W+jets background which is not well modeled by existing event generators. CDF
collaboration performed a simultaneous measurement of the tt cross section and the normaliza-
tion of different W+jets background components 4. The latter are consistent with the existing
measurements using other procedures. This approach has an additional advantage of measuring
many systematic uncertainties in-situ which inversely scale with integrated luminosity similarly
to statistical uncertainties.
Up to now tt production cross sections were measured at Tevatron in all channels except for
the one with two hadronic taus in the final state. All measurements are in agreement with the
2.2 Single top quark
Electroweak production of the single top quarks was predicted 5 ten years before the discovery
of the top quark, but it was observed only recently by the CDF and DO collaborations 6 due
to its small production rate and high backgrounds. Single top production allows to probe Wtb
interaction since its rate is proportional to the Vtb mixing matrix element. At the Tevatron single
top quarks are produced by either a t-channel exchange of a virtual W boson which produces a
top quark via interaction with a b-quark, or by an s-channel exchange of an off-shell W boson
which decays into a top and a b quark. At the LHC single top production via a b-quark or a
t-quark exchange, so called Wt channel, also becomes important.
Following the observation of the combined s- and t-channel production, the DO collabo-
ration published 4.8a evidence for the t-channel production 7 with a cross section of Utch=
3.140%-8 pb. Figure 1 (left) summarizes combined s+t channel single top cross section measure-
ments 6, and Fig. 1 (right) shows the result of a simultaneaous s- and t-channel cross section
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Shabalina, E.K. & /Gottingen U., II. Phys. Inst. Top quark physics: from Tevatron to LHC, article, October 1, 2010; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc1012260/m1/2/: accessed April 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.