Top results from CMS

The top quark has a special place in the Standard Model (SM). It is the heaviest elementary particle known to date, its mass [1] being very close to the scale of electroweak symmetry breaking. As such it plays a special role in many models of new physics beyond the SM. In the SM, the top quark decays almost exclusively to a W b pair. At the LHC [2], the top quark is predominantly produced in pairs, via the strong interaction, with a total cross-section of 158 −24 [3] at the NLO . From an experimental point of view, tt pairs can be classified in three channels according to the decay of the two W bosons originated by the top and anti-top decays: dileptonic channel, when both W decay leptonically; all-hadronic channel, when both W decay to quarks; semi-leptonic channel, when one W decays to leptons and the other to quarks. The three channels have a branching fraction of about 10, 48 and 42 %, respectively. An additional production mechanism is the single-top production, via the electro-weak interaction. The theory of electroweak interactions predicts three different production mechanisms for single top quarks in hadron-hadron collisions: t channel, s channel, and tW (or W-associated). In 7 TeV proton-proton collisions the t-channel is by far the most abundant of the three mechanisms with a NLO cross section of 59.1 −4.0pb in the 4-flavour scheme and 62.3 +2.3 −2.4pb in the 5-flavour [4], for a top mass of 172.5 GeV/c. The top quark has a paramount importance at the LHC. Final states originated from top decays typically include jets, leptons and missing energy, hence typically involve almost all subdetectors and allow thorough tests of the performances of the detector. In addition, studies of top-quark production and properties represent an important tool to verify SM predictions and QCD calculations in the LHC environment. Several extensions of the SM foresee a preferential coupling to the third generation and in particular to the top-quark sector. For example, new resonances may exist decaying to top-antitop pairs.