TOP-14-007

Search for anomalous Wtb couplings and top FCNC in t-channel single-top-quark events

Abstract

Single-top-quark events in the t-channel are used to probe $Wtb$ anomalous couplings and to search for top quark Flavor Changing Neutral Current (FCNC) interactions in proton-proton collisions at $\sqrt{s}=7$ TeV. The analyzed data correspond to an integrated luminosity of 5 fb-1. Events with the top quark decaying into a muon, neutrino and b-quark are selected. A Bayesian neural network is used to discriminate between signal and backgrounds. The observed event yields are consistent with SM prediction, and exclusion limits at 95% C.L. are determined. The exclusion limits on anomalous right vector and left tensor couplings of the $Wtb$ vertex are found to be $|f_{V}^{R}|< 0.34$ and $|f_{T}^{L}|<0.09$. In the scenarios with FCNC $tcg$ and $tug$ couplings, limits on the coupling strengths are found to be $\kappa_{tug}/\Lambda < 1.8 \cdot 10^{-2}~ \mathrm{TeV^{-1}},  \kappa_{tcg}/\Lambda < 5.6 \cdot 10^{-2} ~ \mathrm{TeV^{-1}}$ which corresponds to limits on the branching ratios $Br(t~\rightarrow~u+g)~<~3.55\times10^{-4},Br(t~\rightarrow~c+g)~<~3.44\times10^{-3}$.

Public Document

The document CMS-PAS-TOP-14-007 can be found on the CERN Document Server, here.

Control plots and BNN distributions

Figure/Table Caption
bnn_QCD.png The discriminant of the QCD BNN used for the QCD-background rejection for the simulation and the data, with statistical error bars only. The cut value is shown as a vertical line.

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MtW.png The transverse W boson mass distribution, with statistical error bars only.

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bnn_QCD_shapes.png Separation power of the QCD BNN discriminator for the training (”train”) and testing (”exam”) events.

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rocs_all.png Comparison of efficiencies of cuts for QCD background suppression. The blue curve shows efficiencies of cuts on the transverse W boson mass, the red one represents QCD BNN discriminator cuts. The green curve shows the signal efficiency w.r.t. to different QCD BNN cuts. A selection on the BNN rather than a selection on the transverse W boson mass allows to significantly increase the signal efficiency by 10%, while keeping a similar background rejection. With the cut on the QCD BNN about 90% of the QCD background is suppressed, while loosing only about 20% of signal events.

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bnn_SM_ttbar.png Data-to-simulation agreement of the SM BNN discriminant in the ttbar (4 jets with 1 tag) control region. The simulation normalized to theoretical cross sections. The data points are shown with statistical error bars only. The comparison shows a good agreement in control regions.

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bnn_SM_zerotag.png Data-to-simulation agreement of the SM BNN discriminant in the W + jets (no b-tagged jets) control region. The simulation normalized to theoretical cross sections. The data points are shown with statistical error bars only. The comparison shows a good agreement in control regions.

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bnn_SM_2j1t.png Data-to-simulation agreement of the SM BNN discriminant in the part of the signal region: two jets and one of them is b-tagged (2j1t). The simulation normalized to theoretical cross sections. The data points are shown with statistical error bars only. The comparison shows a good agreement in signal region.

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bnn_SM_3j1t.png Data-to-simulation agreement of the SM BNN discriminant in the part of the signal region: three jets and one of them is b-tagged (3j1t). The simulation normalized to theoretical cross sections. The data points are shown with statistical error bars only. The comparison shows a good agreement in signal region.

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bnn_SM_3j2t.png Data-to-simulation agreement of the SM BNN discriminant in the part of the signal region: three jets and two of them are b-tagged (3j2t). The simulation normalized to theoretical cross sections. The data points are shown with statistical error bars only. The comparison shows a good agreement in signal region.

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bnn_SM_theta.png The SM BNN discriminant after the statistical analysis and evaluation of all the uncertainties. The hashed band corresponds to the systematic uncertainty. The points in the ratio plot are shown with statistical errors only.

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bnn_LVRV.png Data and model comparison of BNN anomalous $Wtb$ discriminator for the scenario with nonzero left and right vector couplings. The BNN aWtb was trained to separate events with right-handed vector operator in the $Wtb$ interaction and SM single top quark. The hashed band corresponds to systematic uncertainty.

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bnn_LVLT.png Data and model comparison of BNN aWtb discriminant for the scenario with nonzero left vector and tensor couplings. The BNN aWtb was trained to separate possible events with left tensor coupling in the $Wtb$ interaction and SM events. The hashed band corresponds to the systematic uncertainty.

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fcnc_feynman.gif Representative Feynman diagrams for FCNC $tcg$ processes; the diagrams for FCNC $tug$ processes are similar.

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bnn_fcnc_tug.png Data and model agreement for the $tug$ FCNC BNN discriminant. The $tug$ FCNC BNN is trained to separate $tug$ events from all SM processes. The hashed band corresponds to systematic uncertainty.

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bnn_fcnc_tcg.png Data and model agreement for the $tcg$ FCNC BNN discriminant. The $tcg$ FCNC BNN is trained to separate $tcg$ events from all SM processes. The hashed band corresponds to systematic uncertainty.

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Exclusion plots

lvrv_fit.png Exclusion limits in two-dimensions on left and right vector couplings at 68% and 95% C.L. for the observed and expected limits.

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lvlt_fit.png Exclusion limits in two-dimensions on left vector and tensor couplings at 68% and 95% C.L. for the observed and expected limits.

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kukc_fit.png Exclusion upper limits in two-dimensions on FCNC $tug$ and $tcg$ couplings at 68% and 95% C.L.

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Tables

table1.png The event yields for an integrated luminosity of 5 inv. fb before and after the QCD BNN cut. Cross sections used for the MC normalization and theoretical uncertainties are also shown.

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table2.png Uncertainties on the background normalization introduced to the statistical model; these variations of parameters are the same as for the previous CMS SM measurement of the single top quark production cross section

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table3.png Input variables of BNNs used in the analysis. Sign V marks the variables used for the particular BNN

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-- AndreasMeyer - 21 May 2014

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