CMS-TOP-12-006

CMS-TOP-12-006 ; CERN-EP-2016-016
Measurements of the $\mathrm{ t \bar{t} }$ 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
29 February 2016
EPJC 77 (2017) 15
Abstract: A measurement of the top quark pair production ( $\mathrm{ t \bar{t} }$ ) 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 fb $^{-1}$ . This analysis is performed in the $\mathrm{ t \bar{t} }$ 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 $\mathrm{ t \bar{t} }$ cross section is 228.5 $\pm$ 3.8 (stat) $\pm$ 13.7 (syst) $\pm$ 6.0 (lumi) pb. This measurement is compared with an analysis of 7 TeV data, corresponding to an integrated luminosity of 5.0 fb $^{-1}$ , to determine the ratio of 8 TeV to 7 TeV cross sections, which is found to be 1.43 $\pm$ 0.04 (stat) $\pm$ 0.07 (syst) $\pm$ 0.05 (lumi). The measurements are in agreement with QCD predictions up to next-to-next-to-leading order.
Links: e-print arXiv:1602.09024 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Transverse momentum distributions of the first- and second-leading jet (a,b), the muon and ${E_{\mathrm {T}}^{\text {miss}}}$ distribution (c,d) 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 $\mathrm{ t \bar{t} }$ 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 ratio between data and simulation is shown in the lower panels for bins with non-zero entries.
png pdf	Figure 1-a: Transverse momentum distributions of the first- and second-leading jet (a,b), the muon and ${E_{\mathrm {T}}^{\text {miss}}}$ distribution (c,d) 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 $\mathrm{ t \bar{t} }$ 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 ratio between data and simulation is shown in the lower panels for bins with non-zero entries.
png pdf	Figure 1-b: Transverse momentum distributions of the first- and second-leading jet (a,b), the muon and ${E_{\mathrm {T}}^{\text {miss}}}$ distribution (c,d) 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 $\mathrm{ t \bar{t} }$ 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 ratio between data and simulation is shown in the lower panels for bins with non-zero entries.
png pdf	Figure 1-c: Transverse momentum distributions of the first- and second-leading jet (a,b), the muon and ${E_{\mathrm {T}}^{\text {miss}}}$ distribution (c,d) 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 $\mathrm{ t \bar{t} }$ 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 ratio between data and simulation is shown in the lower panels for bins with non-zero entries.
png pdf	Figure 1-d: Transverse momentum distributions of the first- and second-leading jet (a,b), the muon and ${E_{\mathrm {T}}^{\text {miss}}}$ distribution (c,d) 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 $\mathrm{ t \bar{t} }$ 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 ratio between data and simulation is shown in the lower panels for bins with non-zero entries.
png pdf	Figure 2: Distributions of the lepton-jet mass in the muon+jets (a) and electron+jets (b) channels, rescaled to the fit results.
png pdf	Figure 2-a: Distributions of the lepton-jet mass in the muon+jets (a) and electron+jets (b) channels, rescaled to the fit results.
png pdf	Figure 2-b: Distributions of the lepton-jet mass in the muon+jets (a) and electron+jets (b) channels, rescaled to the fit results.
png pdf	Figure 3: Distributions of the $M_3$ mass in the 8 TeV data, for the muon+jets (a) and electron+jets (b) channels, rescaled to the template likelihood fit results.
png pdf	Figure 3-a: Distributions of the $M_3$ mass in the 8 TeV data, for the muon+jets (a) and electron+jets (b) channels, rescaled to the template likelihood fit results.
png pdf	Figure 3-b: Distributions of the $M_3$ mass in the 8 TeV data, for the muon+jets (a) and electron+jets (b) channels, rescaled to the template likelihood fit results.

Tables
png pdf	Table 1: Average acceptance values for the muon and electron channels obtained with MADGRAPH and POWHEG at $\sqrt s =$ 7 and 8 TeV, without and with top quark ${p_{\mathrm {T}}}$ -reweighting applied. The statistical uncertainty is 0.0004, i.e. below 3%. The theoretical uncertainties are at the level of 2%, as discussed in the text.
png pdf	Table 2: Signal selection efficiencies, at $\sqrt s =$ 8 TeV, determined from simulation using MADGRAPH. The non-reweighted acceptance from Table 1 is used. The relative statistical uncertainty on these numbers is below 3%.
png pdf	Table 3: Components (in %) of the JES uncertainty at 8 TeV in the muon and electron channels. The correlation coefficients used in their combination are also shown.
png pdf	Table 4: Overview of the systematic uncertainties in the measurement of the $\mathrm{ t \bar{t} }$ cross sections at 8 TeV, both for the total and the visible cross sections. For the ``signal modeling'' uncertainty the larger between the matrix element (ME) and parton shower (PS) uncertainties is taken, as explained in Section 6. The correlations assumed for the combination of the muon and electron channels are also given.
png pdf	Table 5: Overview of the systematic uncertainties in the measurement of the $\mathrm{ t \bar{t} }$ cross sections at 7 TeV, both for the total and the visible cross sections. For the ``signal modeling'' uncertainty the larger between the matrix element (ME) and parton shower (PS) uncertainties is taken, as explained in Section 6. The correlations assumed for the combination of the muon and electron channels are also shown.
png pdf	Table 6: Visible cross section measurements at $\sqrt s =$ 7 and 8 TeV with the reference analysis $M_{\ell \mathrm{b} }$ and the alternative analysis $M_3$ (described in Section 6). Results obtained for $m_{\mathrm{t} }=$ 172.5 GeV with MADGRAPH and with POWHEG are shown. The uncertainties are in the order: statistical, systematic, and due to the luminosity determination.
png pdf	Table 7: Total cross section measurements at $\sqrt s =$ 7 and 8 TeV with the reference analysis $M_{\ell \mathrm{b} }$ and the alternative analysis $M_3$ (described in Section 6). Results obtained for $m_{\mathrm{t} }=$ 172.5 GeV with MADGRAPH and with POWHEG are shown. The uncertainties are in the order: statistical, systematic, and due to the luminosity determination.
png pdf	Table 8: Slope values for the muon and electron channels obtained with linear fits to the cross section values at $\sqrt s =$ 8 TeV as a function of the top quark mass. The MADGRAPH generator is used. The change in sign is due to the acceptance $A$ .

Summary

A measurement of the

$\mathrm{ t \bar{t} }$ production cross section at

$\sqrt{s} =$ 8 TeV is presented, using the data collected with the CMS detector and corresponding to an integrated luminosity of 19.6 fb

$^{-1}$ . The analysis is performed in the

$\mathrm{ t \bar{t} }$ semileptonic decay channel with one muon or electron and at least four jets in the final state with at least one b-tagged jet. The

$\mathrm{ t \bar{t} }$ cross section is extracted using a binned maximum-likelihood fit of templates from simulated events to the data sample. The results from the two semileptonic channels are combined using the BLUE method. Techniques based on control samples in data are used to determine the b tagging efficiency and to calibrate the jet energy scale. These techniques allow for a better determination of the corresponding systematic uncertainties, particularly for the JES, which is a dominant source of experimental uncertainty.

In the kinematic range defined in the simulation with exactly one muon or electron with

$p_{\mathrm{T}} >$ 32 GeV and

$| \eta | <$ 2.1, one neutrino with

$p_{\mathrm{T}} >$ 40 GeV, and at least four jets with

$p_{\mathrm{T}} >$ 40 GeV and

$| \eta | <$ 2.5, the measured visible

$\mathrm{ t \bar{t} }$ cross section at

$\sqrt{s} =$ 8 TeV is 3.80

$\pm$ 0.06 (stat)

$\pm$ 0.18 (syst)

$\pm$ 0.10 (lumi) pb.

Using the MADGRAPH generator for the extrapolation to the full phase space, the total

$\mathrm{ t \bar{t} }$ cross section at 8 TeV is 228.5

$\pm$ 3.8 (stat)

$\pm$ 13.7 (syst)

$\pm$ 6.0 (lumi) pb. The result of an alternative analysis, which makes use of the observable M3, is in good agreement with this value. Furthermore, the analysis performed using data at

$\sqrt{s} =$ 7 TeV, yields a total cross section measurement of 161.7

$\pm$ 6.0 (stat)

$\pm$ 12.0 (syst)

$\pm$ 3.6 (lumi) pb. The measured cross section ratio, where a number of experimental uncertainties cancel out, is 1.43

$\pm$ 0.04 (stat)

$\pm$ 0.07 (syst)

$\pm$ 0.05 (lumi).

All measurements are in agreement with the NNLO theoretical predictions.

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