Measurement of the top quark-antiquark pair production in association with a W or Z boson in pp collisions at $\sqrt{s}$ = 8 TeV

Abstract

A measurement of the cross section for the production of top quark-antiquark pairs ($\textrm{t}\bar{\textrm{t}}$) in association with a vector boson (W or Z) in proton-proton collisions at $\sqrt{s}$ = 8TeV is presented. The results are based on a dataset corresponding to an integrated luminosity of $19.5\textrm{fb}^{-1}$ recorded with the CMS detector at the LHC. The measurement is performed in three leptonic (e and $\mu$) channels: a same-sign dilepton analysis targeting $\textrm{t}\bar{\textrm{t}}\textrm{W}$ events, and trilepton and four-lepton analyses designed for $\textrm{t}\bar{\textrm{t}}\textrm{Z}$ events. In the same-sign dilepton channel, the $\textrm{t}\bar{\textrm{t}}$W cross section is measured as $\sigma_{\textrm{t}\bar{\textrm{t}}\textrm{W}} = 170 ^{+90}_{-80} \mathrm{(stat.)} ^{+70}_{-70} \mathrm{(syst.)}~\mathrm{fb}$ with a significance of 1.6 standard deviations. Combining the trilepton and four-lepton channels, a direct measurement of the $\textrm{t}\bar{\textrm{t}}$Z cross section, $\sigma_{\textrm{t}\bar{\textrm{t}}\textrm{Z}} = 200  ^{+80}_{-70} \mathrm{(stat.)} ^{+40}_{-30} \mathrm{(syst.)}~\mathrm{fb}$, is obtained with a significance of 3.2 standard deviations. The measured cross sections are compatible with standard model predictions within their experimental uncertainties. The inclusive $\textrm{t}\bar{\textrm{t}}\textrm{V}$ process, where V stands for W or Z, is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels.

Introduction

Figure (left) Figure (right) Caption
The dominant leading-order Feynman diagrams for ttW and ttZ production in pp collisions. The charge conjugate of the diagrams shown is implied.

Same-sign dilepton analysis

Figure (left) Figure (right) Caption
Event yields in data after final dilepton selection requirements, compared to the background estimates and signal expectations. Contributions separated by final states (left) and trijets mass distribution for the hadronic top-quark candidate (right).The combination of statistical and systematic uncertainties is denoted by the shaded area.
Event yields in data after final dilepton selection requirements, compared to the background estimates and signal expectations. HT distribution (left) and the leading-lepton $p_T$ distribution (right). The combination of statistical and systematic uncertainties is denoted by the shaded area.

Table Caption
Estimated backgrounds, expected signal, and observed number of events for the $\mu^{\pm}\mu^{\pm}$, $e^{\pm}\mu^{\pm}$, and $e^{\pm}e^{\pm}$ channels. Uncertainties include both the statistical and the systematic components.The systematic uncertainty in the signal contribution does not include the theoretical uncertainty in the signal production cross section.

Trilepton analysis

Figure (left) Figure (right) Caption
Event yields in data after final trilepton selection requirements, compared to the background estimates and signal expectations. Contributions separated by final states where the two leptons consistent with the Z boson are inside parenthesis on the bin labels (left) and trijets mass distribution for the hadronic top-quark candidate (right). The combination of statistical and systematic uncertainties is denoted by the shaded area.
Event yields in data after final trilepton selection requirements, compared to the background estimates and signal expectations. Z-boson candidate dilepton mass distribution (left) and the distribution of the number of b-tagged jets passing medium operating point of the b-tagger (right). The combination of statistical and systematic uncertainties is denoted by the shaded area.

Table Caption
Estimated backgrounds, expected signal, and observed number of events for the trilepton channel. Uncertainties include both the statistical and the systematic components.The systematic uncertainty in the signal contribution does not include the theoretical uncertainty in the signal production cross section.

Four-lepton analysis

Figure (left) Figure (right) Caption
Event yields in data after final four-lepton selection requirements, compared to the background estimates and signal expectations. Contributions in the 1~b-tagged jet and 2~b-tagged jets signal regions (left) and inclusive MET distribution (right). The ZZ component of the background is shown separately from the rest of the irreducible processes. The combination of statistical and systematic uncertainties is denoted by the shaded area.

Table Caption
Estimated backgrounds, expected signal, and observed number of events for the four-lepton channel. Uncertainties include both the statistical and the systematic components. The systematic uncertainty in the signal contribution does not include the theoretical uncertainty in the signal production cross section. The ZZ component of the background is shown separately from the rest of the irreducible processes.

Systematic uncertainties in signal selection efficiency

Table Caption
Systematic uncertainties in the signal selection efficiency for the three considered channels: ttW in dilepton (2$\ell$) final state; ttZ in trilepton (3$\ell$) and four-lepton (4$\ell$) final states.

Results

Table/Figure Caption
Results of the extraction of cross sections, from single and combined channels.
Results for the two dimensional fit of the ttW and ttZ cross sections.
The result of the two-dimensional best fit for ttW and ttZ cross sections (cross symbol) is shown along with its 68 and 95% confidence level contours. The result of this fit is superimposed with the separate ttW and ttZ cross-section measurements, and the corresponding $1\sigma$ bands, obtained from the dilepton, and the trilepton/four-lepton channels, respectively. The figure also shows the predictions from theory and the corresponding uncertainties.

Topic attachments
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PDFpdf ObsPred_FinalSel_ALL_HT_preliminary.pdf r1 manage 16.0 K 2014-05-30 - 23:16 BorisMangano  
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PDFpdf ObsPred_pp_mm_TotalBin_preliminary.pdf r1 manage 32.5 K 2014-05-30 - 23:17 BorisMangano  
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PDFpdf comb1.pdf r1 manage 106.0 K 2014-05-30 - 23:48 BorisMangano  
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PDFpdf fit_2d.pdf r2 r1 manage 55.8 K 2014-05-31 - 00:02 BorisMangano  
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Topic revision: r5 - 2014-06-26 - AndreasMeyer
 
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