CMS-TOP-22-008

CMS-TOP-22-008 ; CERN-EP-2023-146
Evidence for tWZ production in proton-proton collisions at $ \sqrt{s} = $ 13 TeV in multilepton final states
CMS Collaboration
19 December 2023
Submitted to Phys. Lett. B
Abstract: The first evidence for the standard model production of a top quark in association with a W boson and a Z boson is reported. The measurement is performed in multilepton final states, where the Z boson is reconstructed via its decays to electron or muon pairs and the W boson decays either to leptons or hadrons. The analysed data were recorded by the CMS experiment at the CERN LHC in 2016-2018 in proton-proton collisions at $ \sqrt{s}= $ 13 TeV, and correspond to an integrated luminosity of 138 fb$ ^{-1} $. The measured cross section is 354 $ \pm $ 54 (stat) $ \pm $ 95 (syst) fb, and corresponds to a statistical significance of 3.4 standard deviations.
Links: e-print arXiv:2312.11668 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagrams for the tWZ production at NLO accuracy in QCD (left), and for the $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ production arising when simulating the tWZ process at NLO accuracy in QCD (right). The two diagrams share the same final state, and are different in the number of resonant top quarks.
png pdf	Figure 1-a: Feynman diagram for the tWZ production at NLO accuracy in QCD.
png pdf	Figure 1-b: Feynman diagram for the $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ production arising when simulating the tWZ process at NLO accuracy in QCD.
png pdf	Figure 2: Pre-fit examples of the input features for the two DNN trainings: the $ {p_{\mathrm{T}}}_{\ell{\mathrm{j}} }^{\text{max}} $ (upper left) and $ m_{\text{sys}} $ (upper right) observables in $ \text{SR}_{3\ell,3{\mathrm{j}} } $, as well as the $ p_{\mathrm{T}} $ of the leading lepton (lower left) and leading jet (lower right) in $ \text{SR}_{3\ell,2{\mathrm{j}} } $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) before the fit.
png pdf	Figure 2-a: Pre-fit distribution for $ {p_{\mathrm{T}}}_{\ell{\mathrm{j}} }^{\text{max}} $ in $ \text{SR}_{3\ell,3{\mathrm{j}} } $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) before the fit.
png pdf	Figure 2-b: Pre-fit distribution for $ m_{\text{sys}} $ in $ \text{SR}_{3\ell,3{\mathrm{j}} } $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) before the fit.
png pdf	Figure 2-c: Pre-fit distribution for the $ p_{\mathrm{T}} $ of the leading lepton in $ \text{SR}_{3\ell,2{\mathrm{j}} } $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) before the fit.
png pdf	Figure 2-d: Pre-fit distribution for the $ p_{\mathrm{T}} $ of the leading jet in $ \text{SR}_{3\ell,2{\mathrm{j}} } $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) before the fit.
png pdf	Figure 3: Score of the tWZ output node from the multiclass classifier in $ \text{SR}_{3\ell,3{\mathrm{j}} } $ for events with exactly one b jet (upper left), and of the $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ output node in $ \text{SR}_{3\ell,3{\mathrm{j}} } $ for events with more than one b jet (upper right); score of the tWZ output node of the binary classifier in $ \text{SR}_{3\ell,2{\mathrm{j}} } $ (lower left), and the b jet multiplicity in $ \text{SR}_{4\ell} $ (lower right). The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) after the fit.
png pdf	Figure 3-a: Score of the tWZ output node from the multiclass classifier in $ \text{SR}_{3\ell,3{\mathrm{j}} } $ for events with exactly one b jet. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) after the fit.
png pdf	Figure 3-b: Score of the $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ output node in $ \text{SR}_{3\ell,3{\mathrm{j}} } $ for events with more than one b jet. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) after the fit.
png pdf	Figure 3-c: Score of the tWZ output node of the binary classifier in $ \text{SR}_{3\ell,2{\mathrm{j}} } $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) after the fit.
png pdf	Figure 3-d: b jet multiplicity in $ \text{SR}_{4\ell} $. The VV(V) group in the legend denotes the VVV, WW, and W in association with jets backgrounds. The dashed band shows the total uncertainty (statistical and systematic) after the fit.
png pdf	Figure 4: Two-dimensional likelihood scan of the signal strenghts of tWZ ($ \mu_{\mathrm{t}\mathrm{W}\mathrm{Z}} $) vs. $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ ($ \mu_{{\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z}} $). The blue lines show the 68%, 95%, and 99% confidence level contours. The black cross represents the best-fit value, while the red diamond the SM expectation.

Tables
png pdf	Table 1: Expected yields for signal and background processes and observed number of events in the signal regions.
png pdf	Table 2: Expected yields for signal and background processes and observed number of events in the control regions.

Summary

A measurement of the tWZ process is performed in final states with three or four leptons, using the data collected by the CMS experiment at the LHC in 2016-2018, at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. To assess backgrounds and establish the signal, the measurement heavily relies on multivariate techniques and the data are exploited in multiple regions and categories. The challenge of the tWZ signal overlapping with the $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ background is overcome using the latest advancements in the tWZ modeling. We find the signal to have an observed statistical significance of 3.4 standard deviations, corresponding to a measured cross section of 354 $ \pm $ 54 (stat) $ \pm $ 95 (syst) fb that is two standard deviations above the standard model prediction. This is the first evidence of the tWZ process.

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