TOP-13-010 - Measurement of the cross section ratio σ(t t-bar b b-bar) / σ(t t-bar jj) in pp collisions at √s = 8 TeV

arXiv:1411.5621 - CERN-PH-EP-2014-249 - submitted to JHEP
Inspire - HepData - RIVET

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Abstract

The first measurement of the cross section ratio $\sigma_{t\bar{t} b\bar{b}}$/$\sigma_{t\bar{t}jj}$ is presented using a data sample corresponding to an integrated luminosity of 19.6 inverse femtobarns collected in pp collisions at sqrt(s) = 8 TeV with the CMS detector at the LHC. Events with two leptons (e or mu) and four reconstructed jets, including two identified as b quark jets, in the final state are selected. The ratio is determined for a minimum jet transverse momentum pt of both 20 and 40 GeV. The measured ratio is 0.022 +/- 0.003 (stat) +/- 0.005 (syst) for pt > 20 GeV. The absolute cross sections $\sigma_{t\bar{t} b\bar{b}}$ and $\sigma_{t\bar{t}jj}$ are also measured. The measured ratio for pt > 40 GeV is compatible with a theoretical quantum chromodynamics calculation at next-to-leading order.

Results

A measurement of the cross section ratio $\sigma_{t\bar{t} b\bar{b}}$ / $\sigma_{t\bar{t}jj}$ has been presented by the CMS experiment, using a data sample of pp collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.6 ${\rm fb}^{-1}$. The individual cross sections $\sigma_{t\bar{t} b\bar{b}}$ and $\sigma_{t\bar{t}jj}$ have also been determined. The cross section ratio was measured in a visible phase space region using the dilepton decay mode of $t\bar{t}$ events and corrected to the particle level, corresponding to the detector acceptance. The measured cross section ratio in the visible phase space is $\sigma_{t\bar{t} b\bar{b}}$/$\sigma_{t\bar{t}jj}$= 0.022 $\pm$ 0.003 (stat.) $\pm$ 0.005 (syst.) with a minimum pt for the particle-level jets of 20 GeV. The cross section ratio has also been measured in the full phase space with minimum parton-jet pt thresholds of pt > 20 and > 40 GeV in order to compare with a NLO QCD calculation of the cross section ratio. The measurement is compatible within 1.6 standard deviations with the theoretical prediction. These are the first measurements of the cross sections $\sigma_{t\bar{t} b\bar{b}}$ and $\sigma_{t\bar{t}jj}$, and their ratio. The result will provide important information about the main background in the search for $t\bar{t}H$ and as a figure of merit for testing the validity of NLO QCD calculations.

Figures

Figure 1a: Normalized distribution of the b jet discriminator for the third jet in an event, sorted in decreasing order of b-tagging discriminator value, after the full event selection. The histograms are obtained from MC simulation and are separated according to jet flavour.

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Figure 1b: Normalized distribution of the b jet discriminator for the fourth jet in an event, sorted in decreasing order of b-tagging discriminator value, after the full event selection. The histograms are obtained from MC simulation and are separated according to jet flavour.

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Figure 2a: Distribution of b jet discriminator for the third jet in events in decreasing order of b-tagging discriminator value, after the full event selection. Points are from data and stacked histograms from MC simulation using results from the fit to data. The ratio of the number of data events to the total number of MC events after the fit is shown in the lower panels.

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Figure 2b: Distribution of b jet discriminator for the fourth jet in events in decreasing order of b-tagging discriminator value, after the full event selection. Points are from data and stacked histograms from MC simulation using results from the fit to data. The ratio of the number of data events to the total number of MC events after the fit is shown in the lower panels.

PDF - PNG - EPS
Figure 3: Distribution of b jet multiplicity after the four-jet requirement but without the b-tagging requirement. Points are from data and stacked histograms from MC simulation using results from the fit to data. The ratio of the number of data events to the total number of MC events after the fit is shown in the lower panel.

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Tables

Table Caption
Table 1: The number of events for each physics process and for each dilepton category after fitting to the data, their total, and the observed total number of events. The results are after the final event selection. The $Z/\gamma^* \to \ell\ell$ uncertainty is from data, while all other uncertainties include only the statistical uncertainties in the MC samples.

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Table 2: Summary of the systematic uncertainties from various sources contributing to $\sigma_{t\bar{t} b\bar{b}}$, $\sigma_{t\bar{t}jj}$, and the ratio $\sigma_{t\bar{t} b\bar{b}}$/$\sigma_{t\bar{t}jj}$ for a jet pt threshold of pt > 20 GeV in the visible phase space.

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Table 3: The measured cross sections $\sigma_{t\bar{t} b\bar{b}}$ and $\sigma_{t\bar{t}jj}$and their ratio are given for the visible phase space (PS) defined as two leptons with pt > 20 GeV and abs(eta) < 2.4 plus four jets, including two b jets with pt > 20 GeV and abs(eta) < 2.5, and the full phase space, corrected for acceptance and branching fractions. The full phase-space results are given for jet thresholds of pt > 20 and 40 GeV. The uncertainties shown are statistical and systematic, respectively. The predictions of a NLO theoretical calculation for the full phase space and pt > 40 GeV are also given.

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AndreasMeyer - 2015-04-12

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