Search for single top-quark production via flavour changing neutral currents at 8 TeV with the ATLAS detector
- Aad, Georges et al
- arXiv:1509.00294CERN-PH-EP-2015-172
| Leading-order Feynman diagram for FCNC top-quark production in the $qg \rightarrow t$ mode followed by the decay of the top quark into a $b$-quark and a $W$ boson, where the $W$ boson decays into a lepton and a neutrino. |
| The transverse momentum of the lepton versus the azimuthal angle between the lepton and the jet. The colours indicate the number of events in data after the simulated backgrounds except the multi-jet contribution have been subtracted and before the cut given by \Eqn{\protect\ref{eq:antiqcd}} is applied. The solid black line shows the cut. |
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| Fitted distributions of the missing transverse momentum $\MET$ for \subref{subfig:missetfit_CR_ele}~central electrons and \subref{subfig:missetfit_CR_muo}~muons in the control region and for \subref{subfig:missetfit_SR_ele}~central electrons and \subref{subfig:missetfit_SR_muo}~muons in the signal region. The last histogram bin includes overflow events and the hatched error bands contain the MC statistical uncertainty combined with the normalisation uncertainty on the multi-jet background.Caption not extracted |
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| Kinematic distributions in the control region for the combined electron and muon channels. All processes are normalised to the result of the binned maximum-likelihood fit used to determine the fraction of multi-jet events. Shown are: \subref{subfig:CR_leppt} the transverse momentum and \subref{subfig:CR_lepeta} pseudorapidity of the lepton, \subref{subfig:CR_jetpt} the transverse momentum and \subref{subfig:CR_jeteta} pseudorapidity of the jet, \subref{subfig:CR_met} the missing transverse momentum and \subref{subfig:CR_mtw} $W$-boson transverse mass. The last histogram bin includes overflow events and the hatched error bands contain the statistical and systematic uncertainties.Caption not extracted |
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| Probability densities of the three most important discriminating variables: \subref{subfig:Shape_mttop} the transverse mass of the reconstructed top quark; \subref{subfig:Shape_ptlep} the transverse momentum of the charged lepton; and \subref{subfig:Shape_drleptop} the distance in the $\eta$--$\phi$ plane between the charged lepton and the reconstructed top quark. The last histogram bin includes overflows. |
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| Distributions of three important discriminating variables (except for the transverse momentum of the lepton): \subref{subfig:CR_mttop} and \subref{subfig:SR_mttop} the top-quark transverse mass in the control and signal regions; \subref{subfig:CR_drleptop} and \subref{subfig:SR_drleptop} the $\Delta R$ between the lepton and the reconstructed top quark in the control and signal regions; \subref{subfig:CR_dphijettop} and \subref{subfig:SR_dphijettop} the $\Delta \phi$ between the jet and the reconstructed top quark. All processes are normalised using the scale factors obtained in the binned maximum-likelihood fit to the \MET~distribution. The FCNC signal cross-section is scaled to \SI{50}{\pb} and overlayed on the distributions in the signal region. The last histogram bin includes overflow events and the hatched error bands denote the combined statistical and systematic uncertainties.Caption not extracted |
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| Probability density of the neural-network output distribution for the signal and the most important background processes. |
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| Neural-network output distribution \subref{subfig:CR_nn} in the control region and \subref{subfig:SR_nn} in the signal region. The shape of the signal scaled to \SI{50}{\pb} is shown in \subref{subfig:SR_nn}. All background processes are shown normalised to the result of the binned maximum-likelihood fit used to determine the fraction of multi-jet events. The hatched band indicates the combined statistical and systematic uncertainties.Caption not extracted |
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| \subref{subfig:NNlimitcontrol} Neural-network output distribution in the signal region and \subref{subfig:NNlimitsignal} in the signal region with neural network output above 0.1. In both figures the signal contribution scaled to the observed upper limit is shown. The hatched band indicates the total posterior uncertainty as obtained from the limit calculation.Caption not extracted |
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| \subref{subfig:coupling} Upper limit on the coupling constants $\kappa_{ugt}$ and $\kappa_{cgt}$ and \subref{subfig:branching} on the branching fractions $\BR(t \rightarrow ug)$ and $\BR(t \rightarrow cg)$. The shaded band shows the one standard deviation variation of the expected limit.Caption not extracted |