CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

the Tevatron). Figure 2.4 shows the Feynman diagram for these processes.
(a) (b) (c) (d)
Figure 2.4: Feynman diagrams for top production processes at lowest level: (a), (b), and (c) gluon-gluon
scattering diagrams, (d) quark-antiquark diagram.
The t¯t production cross section depends on the exact value of the top quark mass, mt, and the collision
energy. Figure 2.5 summarizes the results of the t¯t cross-section measurement by both ATLAS and CMS
experiments, in different decay channels. These were done with 2011 collision data, at √ s = 7 TeV. The
combinations of the different channels yields σt¯t = 177 ± 3(stat.) +8
−7 (syst.) ± 7(lumi.) pb for ATLAS [21] and
σ t¯t = 166 ± 2(stat.) ± 11(syst.) ± 8(lumi.) pb for CMS [22]. Measurements for the 2012 collision energy,
√ s = 8 TeV, are on-going. Additionally, top quarks are produced via single-top quark production mech-
anisms. These, however have smaller production rates compared to the top pairs. Three subprocesses
contribute to single-top quark pair production: the exchange of a virtual W-boson in the t-channel, or in
the s-channel, and the associated production of a top quark and an on-shell W-boson. The t-channel mode
is the process with highest cross section at the Tevatron and at the LHC.
The top quarks decay almost exclusively to t → Wb since the CKM matrix element Vtb is close to unity.
The Feynman diagram for this decay is depicted in Figure 2.6(a). This means that the final state topo-
logy of processes involving top quarks is determined by the decay mode of the W-boson. As mentioned in
Section 2.2.3 the hadronic final states have larger branching fractions, but in an environment with copious
multijet production, such as the LHC, the leptonic modes provide cleaner signatures.
In addition to the SM favored decay, there are other decay channels, predicted to be smaller by several
orders of magnitude in the SM. The second most likely decays are the CKM non-diagonal decays t → Ws
and t → Wd. The branching ratios (BR) are in the order of BR(t → Ws) ∼ 1.6 × 10 −3 and BR(t → Wd) ∼
15