Measurement of the $\mathrm{{t\bar{t}}}$ production cross section in pp collisions at $\sqrt{s}=7$ TeV using the kinematic properties of events with leptons and jets

Abstract

A measurement of the tt production cross section in proton-proton collisions at a centre-of-mass energy of 7 TeV has been performed at the LHC with the CMS detector. The analysis uses a data sample corresponding to an integrated luminosity of 36 pb −1 and is based on the reconstruction of the final state with one isolated, high transversemomentum electron or muon and three or more hadronic jets. The kinematic properties of the events are used to separate the tt signal from W+jets and QCD multijet background events. The measured cross section is 173 +39 −32 (stat.+ syst.) pb, consistent with standard model expectations.

Measurement of the mass difference between top and antitop quarks

“…We quote 30% of the total shift observed in ∆m t when we remove W+jets (−0.26 ± 0.20 GeV), Z+jets (0.05 ± 0.04 GeV) and single top-quark production (0.05 ± 0.02 GeV), and 100% for the background from multijet events (−2 ± 5 MeV). The sizes of these variations represent approximately the uncertainties on the rates of the corresponding background processes observed in previous analyses [22,23] with very similar event selection, taking into account the fact that in the current analysis the background rates are fixed to the predictions from simulation. For each contribution, we take the larger of the observed shift in ∆m t or its statistical uncertainty and add the four sources in quadrature.…”

“…Uncertainties are purely statistical and do not include contributions from production cross sections, integrated luminosity, detector acceptance, or selection efficiencies, as discussed in refs. [22,23].…”

“…An incorrect fraction of tt signal events in the simulation would, in principle, bias the calibration procedure. Previous analyses based on a very similar selection [22,23] constrain the signal fraction with an uncertainty around 20%. We vary the relative signal fraction by ±20% keeping the background composition fixed; this yields an effect of ∓ 0.02 ± 0.01 GeV on ∆m t .…”

Weak-triplet, color-octet scalars, and the CDF dijet excess

“…6). Nevertheless, there are some measurements without neural networks or b-tags of the t " t cross section in the lepton-plus-jets channel at the LHC [38], so these constraints must be checked more carefully. Simulating octotriplet production with MADGRAPH, PYTHIA, and PGS and imposing cuts from both the ATLAS and CMS collaborations [38], we find that, for B 3 ¼ 40% and M Â ¼ 155 GeV, the octo-triplet contribution to events with three or more jets is within the statistical error bars at 95% confidence.…”

“…Nevertheless, there are some measurements without neural networks or b-tags of the t " t cross section in the lepton-plus-jets channel at the LHC [38], so these constraints must be checked more carefully. Simulating octotriplet production with MADGRAPH, PYTHIA, and PGS and imposing cuts from both the ATLAS and CMS collaborations [38], we find that, for B 3 ¼ 40% and M Â ¼ 155 GeV, the octo-triplet contribution to events with three or more jets is within the statistical error bars at 95% confidence. This approach is conservative and takes the ATLAS and CMS background simulations and extracted t " t signal at face value; since these measurements are not dedicated new-physics searches, potential octo-triplet decays would be bundled with the t " t signal, which would make the statistical bound unrealistically constraining.…”