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Measurements of the t-tbar production cross section in lepton+jets final states in pp collisions at 8 TeV and ratio of 8 to 7 TeV cross sections

CMS Collaboration

TL;DR

This CMS study measures the tt̄ production cross section in pp collisions at √s = 8 TeV using 19.6 fb^{-1} in the lepton+jets channel, with concurrent analysis of 7 TeV data to form a precise 8/7 cross-section ratio. A binned likelihood fit to discriminants M_{l b} and M3, aided by data-driven calibrations of b-tagging and jet energy scale, yields σ_{t\bar{t}}^{vis} = 3.80 ± 0.06 (stat) ± 0.18 (syst) ± 0.10 (lumi) pb and σ_{t\bar{t}} = 228.5 ± 3.8 (stat) ± 13.7 (syst) ± 6.0 (lumi) pb, in agreement with NNLO predictions. The 8/7 ratio is measured as R^{8/7} = 1.43 ± 0.04 (stat) ± 0.07 (syst) ± 0.05 (lumi), reflecting cancellation of many systematics. An alternative M3-based analysis corroborates the result, and MT-dependence studies enable cross-compatibility across assumed mt values. Overall, the results provide a high-precision test of perturbative QCD and contribute to constraining SM parameters and PDFs.

Abstract

A measurement of the top quark pair production (t-tbar) cross section in proton-proton collisions at the centre-of-mass energy of 8 TeV is presented using data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.6 inverse femtobarns. This analysis is performed in the t-tbar decay channels with one isolated, high transverse momentum electron or muon and at least four jets, at least one of which is required to be identified as originating from hadronization of a b quark. The calibration of the jet energy scale and the efficiency of b jet identification are determined from data. The measured t-tbar cross section is 228.5 +/- 3.8 (stat) +/- 13.7 (syst) +/- 6.0 (lumi) pb. This measurement is compared with an analysis of 7 TeV data, corresponding to an integrated luminosity of 5.0 inverse femtobarns, to determine the ratio of 8 TeV to 7 TeV cross sections, which is found to be 1.43 +/- 0.04 (stat) +/- 0.07 (syst) +/- 0.05 (lumi). The measurements are in agreement with QCD predictions up to next-to-next-to-leading order.

Measurements of the t-tbar production cross section in lepton+jets final states in pp collisions at 8 TeV and ratio of 8 to 7 TeV cross sections

TL;DR

This CMS study measures the tt̄ production cross section in pp collisions at √s = 8 TeV using 19.6 fb^{-1} in the lepton+jets channel, with concurrent analysis of 7 TeV data to form a precise 8/7 cross-section ratio. A binned likelihood fit to discriminants M_{l b} and M3, aided by data-driven calibrations of b-tagging and jet energy scale, yields σ_{t\bar{t}}^{vis} = 3.80 ± 0.06 (stat) ± 0.18 (syst) ± 0.10 (lumi) pb and σ_{t\bar{t}} = 228.5 ± 3.8 (stat) ± 13.7 (syst) ± 6.0 (lumi) pb, in agreement with NNLO predictions. The 8/7 ratio is measured as R^{8/7} = 1.43 ± 0.04 (stat) ± 0.07 (syst) ± 0.05 (lumi), reflecting cancellation of many systematics. An alternative M3-based analysis corroborates the result, and MT-dependence studies enable cross-compatibility across assumed mt values. Overall, the results provide a high-precision test of perturbative QCD and contribute to constraining SM parameters and PDFs.

Abstract

A measurement of the top quark pair production (t-tbar) cross section in proton-proton collisions at the centre-of-mass energy of 8 TeV is presented using data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.6 inverse femtobarns. This analysis is performed in the t-tbar decay channels with one isolated, high transverse momentum electron or muon and at least four jets, at least one of which is required to be identified as originating from hadronization of a b quark. The calibration of the jet energy scale and the efficiency of b jet identification are determined from data. The measured t-tbar cross section is 228.5 +/- 3.8 (stat) +/- 13.7 (syst) +/- 6.0 (lumi) pb. This measurement is compared with an analysis of 7 TeV data, corresponding to an integrated luminosity of 5.0 inverse femtobarns, to determine the ratio of 8 TeV to 7 TeV cross sections, which is found to be 1.43 +/- 0.04 (stat) +/- 0.07 (syst) +/- 0.05 (lumi). The measurements are in agreement with QCD predictions up to next-to-next-to-leading order.

Paper Structure

This paper contains 15 sections, 9 equations, 3 figures, 8 tables.

Table of Contents

  1. Introduction
  2. The CMS detector
  3. Data and simulation
  4. Reconstruction and event selection
  5. Visible and total cross section measurements
  6. Acceptance
  7. Selection efficiency
  8. Systematic uncertainties
  9. Results and combination
  10. Results at sqrt(s) = 8 TeV
  11. Dependence on the top quark mass at sqrt(s) = 8 TeV
  12. Results at sqrt(s) = 7 TeV and cross section ratio
  13. Alternative approach at sqrt(s) = 8 TeV using M3
  14. Summary
  15. The CMS Collaboration

Figures (3)

  • Figure 1: Transverse momentum distributions of the first- and second-leading jet (top), the muon and $E_{\mathrm{T}}^{\text{miss}}$ distribution (bottom) for all relevant processes in the muon+jets channel with the requirement of at least one ${b}$-tagged jet. The simulation is normalized to the standard model cross section values and $p_{\mathrm{T}}$-reweighting is applied to the ${t}\overline{{t}}$ contribution. The multijet background is negligible and not shown. The distributions are already corrected for the ${b}$ tagging efficiency scale factor. The hashed area shows the uncertainty in the luminosity measurement and the ${b}$ tagging systematic uncertainty. The last bin includes the overflow. The ratio between data and simulation is shown in the lower panels for bins with non-zero entries.
  • Figure 2: Distributions of the lepton-jet mass in the muon+jets (left) and electron+jets (right) channels, rescaled to the fit results.
  • Figure 3: Distributions of the $M_3$ mass in the 8$\,\text{Te\spaceV}$ data, for the muon+jets (left) and electron+jets (right) channels, rescaled to the template likelihood fit results. The last filled bin includes the overflow.