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| CMS-TOP-14-008 ; CERN-EP-2017-112 | ||
| Measurement of the semileptonic $\mathrm{ t \bar{t} }$+$\gamma$ production cross section in pp collisions at $\sqrt{s}=$ 8 TeV | ||
| CMS Collaboration | ||
| 25 June 2017 | ||
| JHEP 10 (2017) 006 | ||
| Abstract: A measurement of the cross section for top quark-antiquark ($\mathrm{ t \bar{t} }$) pairs produced in association with a photon in proton-proton collisions at $\sqrt{s}=$ 8 TeV is presented. The analysis uses data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The signal is defined as the production of a $\mathrm{ t \bar{t} }$ pair in association with a photon having a transverse energy larger than 25 GeV and an absolute pseudorapidity smaller than 1.44. The measurement is performed in the fiducial phase space corresponding to the semileptonic decay chain of the $\mathrm{ t \bar{t} }$ pair, and the cross section is measured relative to the inclusive $\mathrm{ t \bar{t} }$ pair production cross section. The fiducial cross section for associated $\mathrm{ t \bar{t} }$ pair and photon production is found to be 127 $\pm$ 27 (stat+syst) fb per semileptonic final state. The measured value is in agreement with the theoretical prediction. | ||
| Links: e-print arXiv:1706.08128 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Examples of Feynman diagrams for the $\mathrm{ t } \mathrm{ \bar{t} } $+$ \gamma $ signal process in the semileptonic final state where the $ \mathrm{ t } {}\mathrm{ \bar{t} } $ pair is produced through gluon-gluon fusion with a photon emitted from one of the top quarks (left), and through quark-antiquark annihilation with a photon emitted from one of the initial partons (right). |
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Figure 1-a:
Example of Feynman diagram for the $\mathrm{ t } \mathrm{ \bar{t} } $+$ \gamma $ signal process in the semileptonic final state where the $ \mathrm{ t } {}\mathrm{ \bar{t} } $ pair is produced through gluon-gluon fusion with a photon emitted from one of the top quarks. |
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Figure 1-b:
Example of Feynman diagram for the $\mathrm{ t } \mathrm{ \bar{t} } $+$ \gamma $ signal process in the semileptonic final state where the $ \mathrm{ t } {}\mathrm{ \bar{t} } $ pair is produced through quark-antiquark annihilation with a photon emitted from one of the initial partons. |
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Figure 2:
Normalized distributions of the $ {M_3} $ variable for $\mathrm{ t } \mathrm{ \bar{t} } $+$ \gamma $ , $ \mathrm{ t } {}\mathrm{ \bar{t} } $ , W+$\gamma$, and other background processes in a combination of the e+jets and $\mu$+jets final state after the photon selection. |
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Figure 3:
Distribution of the $ {M_3} $ variable in data and simulation, scaled to the result of the fit in a combination of the e+jets and $\mu$+jets channels, for events passing the photon selection. The lower panel shows the ratio of the data to the prediction from simulation. The uncertainty band is a combination of statistical and systematic uncertainties in the simulation. |
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Figure 4:
Shapes of isolated (left) and nonprompt (right) photon templates of the photon charged-hadron isolation, comparing templates derived from data to the distributions found from simulation in a combination of the e+jets and $\mu$+jets final states. The lower panel shows the ratio of the distributions derived from data to those found from simulation. |
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Figure 4-a:
Shape of isolated photon template of the photon charged-hadron isolation, comparing template derived from data to the distribution found from simulation in a combination of the e+jets and $\mu$+jets final states. The lower panel shows the ratio of the distributions derived from data to those found from simulation. |
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Figure 4-b:
Shape of nonprompt photon template of the photon charged-hadron isolation, comparing template derived from data to the distribution found from simulation in a combination of the e+jets and $\mu$+jets final states. The lower panel shows the ratio of the distributions derived from data to those found from simulation. |
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Figure 5:
Result of the fit to the photon charged-hadron isolation in a combination of the e+jets and $\mu$+jets final states. The uncertainty band shows the statistical uncertainties in the templates derived from data. The lower panel shows the ratio of the distribution observed in data to the sum of the templates scaled to the fit result. |
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Figure 6:
Result of the fit to the invariant mass of the electron and photon for events passing the modified event selection with the b tagging requirement relaxed. Distributions are shown scaled to the results of the fit for $\mathrm{ Z } \to \mathrm{ e } \mathrm{ e } (\mathrm{ e } \to \gamma )$ and all other simulated samples (dashed lines), as well as the sum of the two samples (solid line). The lower panel shows the ratio of the data to the simulation scaled to the fit results. |
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Figure 7:
Distribution of the transverse momentum of the photon in data and simulation, scaled to the result of the likelihood fit in a combination of the e+jets and $\mu$+jets channels for events passing the photon selection. The lower panel shows the ratio of the data to the prediction from simulation. The uncertainty band is a combination of statistical and systematic uncertainties in the simulation. |
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Figure 8:
Distribution of the absolute value of the pseudorapidity of the photon in data and simulation, scaled to the result of the likelihood fit in a combination of the e+jets and $\mu$+jets channels for events passing the photon selection. The lower panel shows the ratio of the data to the prediction from simulation. The uncertainty band is a combination of statistical and systematic uncertainties in the simulation. |
| Tables | |
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Table 1:
Simulated samples categorized by reconstructed photon origin, after photon selection in the e+jets channel. The data-based multijet sample is not expected to have signal photons or electrons. All uncertainties combine statistical and systematic contributions. |
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Table 2:
Simulated samples categorized by reconstructed photon origin, after photon selection in the $\mu$+jets channel. The data-based multijet sample is not expected to have signal photons or electrons. All uncertainties combine statistical and systematic contributions. |
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Table 3:
Kinematic acceptance and efficiency of the $\mathrm{ t } \mathrm{ \bar{t} } $+$ \gamma $ selection in the e+jets and $\mu$+jets final states. |
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Table 4:
Uncertainties in the cross section ratio $R$ for the combination of the e+jets and $\mu$+jets final states. |
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Table 5:
Cross section ratios, as well as fiducial and total cross sections per semileptonic final state. |
| Summary |
|
The results of a measurement of the production of a top quark-antiquark ($\mathrm{ t \bar{t} }$) pair produced in association with a photon have been presented. The measurement is performed using 19.7 fb$^{-1}$ of data collected by the CMS detector at a center-of-mass energy of 8 TeV. The analysis has been performed in the semileptonic e+jets and $\mu$+jets decay channels. The ratio of the ${\mathrm{ t \bar{t} } }$+$ \gamma$ to $\mathrm{ t \bar{t} } $ production cross sections has been measured to be $R= \sigma_{ {\mathrm{ t \bar{t} } } {+} \gamma} /\sigma_{\mathrm{ t \bar{t} }}={( 5.2 \pm 1.1 ) \times 10^{-4}}$. By multiplying the measured ratio by the previously measured value of the $\mathrm{ t \bar{t} }$ cross section, the fiducial cross section for $ \mathrm{ t \bar{t} } $+$ \gamma$ production of 127 $\pm$ 27 fb has been found for events in the e+jets and $\mu$+jets final states. The measured values are in agreement with the theoretical predictions. |
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