This paper presents distributions of topological observables in inclusive three- and four-jet events produced in pp collisions at a centre-of-mass energy of 7 TeV with a data sample collected by the CMS experiment corresponding to a luminosity of 5.1 inverse femtobarns. The distributions are corrected for detector effects, and compared with several event generators based on two- and multi-parton matrix elements at leading order. Among the considered calculations, MADGRAPH interfaced with PYTHIA6 displays the best overall agreement with data.

The top-quark pair production cross section in 7 TeV center-of-mass energy proton–proton collisions is measured using data collected by the CMS detector at the LHC. The measurement uses events with one jet identified as a hadronically decaying τ lepton and at least four additional energetic jets, at least one of which is identified as coming from a b quark. The analyzed data sample corresponds to an integrated luminosity of 3.9 fb(−1) recorded by a dedicated multijet plus hadronically decaying τ trigger. A neural network has been developed to separate the top-quark pairs from the W+jets and multijet backgrounds. The measured value of is consistent with the standard model predictions.

The W-boson helicity fractions in top-quark decays are measured with ttbar events in the lepton+jets final state, using proton-proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 5.0 inverse femtobarns. The measured fractions of longitudinal, left-, and right-handed helicity are F0 = 0.682 +/- 0.030 (stat.) +/- 0.033 (syst.), FL = 0.310 +/- 0.022 (stat.) +/- 0.022 (syst.), and FR = 0.008 +/- 0.012 (stat.) +/- 0.014 (syst.), consistent with the standard model predictions. The measured fractions are used to probe the existence of anomalous Wtb couplings. Exclusion limits on the real components of the anomalous couplings gL, gR are also derived.

Measurements are reported of the normalized differential cross sections for top quark pair production with respect to four kinematic event variables: the missing transverse energy; the scalar sum of the jet transverse momentum (pT); the scalar sum of the pT of all objects in the event; and the pT of leptonically decaying W bosons from top quark decays. The data sample, collected using the CMS detector at the LHC, consists of 5.0 inverse femtobarns of proton-proton collisions at sqrt(s) = 7 TeV and 19.7 inverse femtobarns at sqrt(s) = 8 TeV. Top quark pair events containing one electron or muon are selected. The results are presented after correcting for detector effects to allow direct comparison with theoretical predictions. No significant deviations from the predictions of several standard model event simulation generators are observed.

Single top quark events produced in the t channel are used to set limits on anomalous Wtb couplings and to search for top quark flavour-changing neutral current (FCNC) interactions. The data taken with the CMS detector at the LHC in proton-proton collisions at sqrt(s) = 7 and 8 TeV correspond to integrated luminosities of 5.0 and 19.7 inverse femtobarns, respectively. The analysis is performed using events with one muon and two or three jets. A Bayesian neural network technique is used to discriminate between the signal and backgrounds, which are observed to be consistent with the standard model prediction. The 95% confidence level (CL) exclusion limits on anomalous right-handed vector, and left- and right-handed tensor Wtb couplings are measured to be |f[V]^R| < 0.16, |f[T]^L| < 0.057, and -0.049 < f[T]^R < 0.048, respectively. For the FCNC couplings kappa[tug] and kappa[tcg], the 95% CL upper limits on coupling strengths are |kappa[tug]|/Lambda < 4.1E-3 TeV-1 and |kappa[tcg]|/Lambda < 1.8E-2 TeV-1, where Lambda is the scale for new physics, and correspond to upper limits on the branching fractions of 2.0E-5 and 4.1E-4 for the decays t to ug and t to cg, respectively.

Measurements are presented of differential cross-sections for top quark pair production in pp collisions at sqrt(s) = 7 TeV relative to the total inclusive top quark pair production cross-section. A data sample of 2.05/fb recorded by the ATLAS detector at the Large Hadron Collider is used. Relative differential cross-sections are derived as a function of the invariant mass, the transverse momentum and the rapidity of the top quark pair system. Events are selected in the lepton (electron or muon) + jets channel. The background-subtracted differential distributions are corrected for detector effects, normalized to the total inclusive top quark pair production cross-section and compared to theoretical predictions. The measurement uncertainties range typically between 10% and 20% and are generally dominated by systematic effects. No significant deviations from the Standard Model expectations are observed.

This paper presents a search for a new heavy particle produced in association with a top or antitop quark. Two models in which the new heavy particle is a color singlet or a color triplet are considered, decaying respectively to tbarq or tq, leading to a resonance within the ttbar + jets signature. The full 2011 ATLAS pp collision dataset from the LHC (4.7 fb-1) is used to search for ttbar events produced in association with jets, in which one of the W bosons from the top quarks decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation, and a new particle with mass below 430 GeV for both W boson and color triplet models is excluded at 95% confidence level, assuming unit right-handed coupling.

A search is presented for the pair production of light scalar top quarks in sqrt(s) = 7 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. This analysis uses the full data sample collected during 2011 that corresponds to a total integrated luminosity of 4.7 fb-1. Light scalar top quarks are searched for in events with two opposite-sign leptons (e, mu), large missing transverse momentum and at least one jet in the final state. No excess over Standard Model expectations is found, and the results are interpreted under the assumption that the light scalar top decays to a b-quark in addition to an on-shell chargino whose decay occurs through a virtual W boson. If the chargino mass is 106 GeV, light scalar top quark masses up to 130 GeV are excluded for neutralino masses below 70 GeV.

The results of a search for pair production of light top squarks are presented, using 4.7 fb^-1 of sqrt(s) = 7 TeV proton-proton collisions collected with the ATLAS detector at the Large Hadron Collider. This search targets top squarks with masses similar to, or lighter than, the top quark mass. Final states containing exclusively one or two leptons (e, mu), large missing transverse momentum, light-jets and b-jets are used to reconstruct the top squark pair system. Global mass scale variables are used to separate the signal from a large ttbar background. No excess over the Standard Model expectations is found. The results are interpreted in the framework of the Minimal Supersymmetric Standard Model, assuming the top squark decays exclusively to a chargino and a b-quark. Light top squarks with masses between 123-167 GeV are excluded for neutralino masses around 55 GeV.

A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at sqrt(s) = 7 TeV. The measurement is based on 4.7 fb-1 of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with Standard Model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.