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CMS-TOP-24-009 ; CERN-EP-2025-205
Observation of tWZ production at the CMS experiment
Submitted to Phys. Rev. Lett.
Abstract: The first observation of single top quark production in association with a W and a Z boson in proton-proton collisions is reported. The analysis uses data at center-of-mass energies of 13 and 13.6 TeV recorded with the CMS detector at the CERN LHC, corresponding to a total integrated luminosity of 200 fb$ ^{-1} $. Events with three or four charged leptons, which can be electrons or muons, are selected. Advanced machine-learning algorithms and improved reconstruction methods, compared to an earlier analysis, result in an unprecedented sensitivity to tWZ production. The measured cross sections for tWZ production are 248 $ \pm $ 52 fb and 244 $ \pm $ 74 fb for $ \sqrt{s}= $ 13 and 13.6 TeV, respectively. The signal is established with a statistical significance of 5.8 standard deviations, with 3.5 expected, compared to the background-only hypothesis.
Figures Summary References CMS Publications
Figures

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Figure 1:
Example Feynman diagrams for tWZ production in the SM (left) and for anomalous tWZ production through an effective interaction (right). Charge conjugate versions of these diagrams are implied.

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Figure 1-a:
Example Feynman diagrams for tWZ production in the SM (left) and for anomalous tWZ production through an effective interaction (right). Charge conjugate versions of these diagrams are implied.

png pdf
Figure 1-b:
Example Feynman diagrams for tWZ production in the SM (left) and for anomalous tWZ production through an effective interaction (right). Charge conjugate versions of these diagrams are implied.

png pdf
Figure 2:
Distributions in the classifier output nodes for the tWZ (upper left) and $ \mathrm{t} \overline{\mathrm{t}} \mathrm{Z} $ (upper right) nodes in $ \text{SR}_{3\ell} $, and in the classifier output node in $ \text{SR}_{4\ell} $ (lower) after a fit to the data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulation is shown below the distributions.

png pdf
Figure 2-a:
Distributions in the classifier output nodes for the tWZ (upper left) and $ \mathrm{t} \overline{\mathrm{t}} \mathrm{Z} $ (upper right) nodes in $ \text{SR}_{3\ell} $, and in the classifier output node in $ \text{SR}_{4\ell} $ (lower) after a fit to the data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulation is shown below the distributions.

png pdf
Figure 2-b:
Distributions in the classifier output nodes for the tWZ (upper left) and $ \mathrm{t} \overline{\mathrm{t}} \mathrm{Z} $ (upper right) nodes in $ \text{SR}_{3\ell} $, and in the classifier output node in $ \text{SR}_{4\ell} $ (lower) after a fit to the data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulation is shown below the distributions.

png pdf
Figure 2-c:
Distributions in the classifier output nodes for the tWZ (upper left) and $ \mathrm{t} \overline{\mathrm{t}} \mathrm{Z} $ (upper right) nodes in $ \text{SR}_{3\ell} $, and in the classifier output node in $ \text{SR}_{4\ell} $ (lower) after a fit to the data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulation is shown below the distributions.

png pdf
Figure 3:
Likelihood scan of the signal strengths for tWZ and $ \mathrm{t} \overline{\mathrm{t}} \mathrm{Z} $ production. The black cross shows the best fit value, while the black diamond indicates the SM expected value. The contours in different shades of blue correspond to the 68, 95, and 99% confidence level.

png pdf
Figure A1:
Distribution in the $ p_{\mathrm{T}} $ of the $ 3^{rd} $ leading lepton for the $ \text{CR}_{\mathrm{DY}} $ (left) and $ \text{CR}_{{\mathrm{t}\overline{\mathrm{t}}} } $ (right) after a fit to data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulations is shown below the distributions.

png pdf
Figure A1-a:
Distribution in the $ p_{\mathrm{T}} $ of the $ 3^{rd} $ leading lepton for the $ \text{CR}_{\mathrm{DY}} $ (left) and $ \text{CR}_{{\mathrm{t}\overline{\mathrm{t}}} } $ (right) after a fit to data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulations is shown below the distributions.

png pdf
Figure A1-b:
Distribution in the $ p_{\mathrm{T}} $ of the $ 3^{rd} $ leading lepton for the $ \text{CR}_{\mathrm{DY}} $ (left) and $ \text{CR}_{{\mathrm{t}\overline{\mathrm{t}}} } $ (right) after a fit to data. The dashed band shows the postfit uncertainty. The results from fits to 13 and 13.6 TeV data are combined for illustration purposes. The ratio of data to the predictions from MC simulations is shown below the distributions.

png pdf
Figure A2:
Values of the signal strengths for tWZ production obtained from the different data-taking periods (left), and from events with different lepton flavors (right): three electrons ($ \mathrm{e}\mathrm{e}\mathrm{e} $), two electrons and one muon ($ \mathrm{e}\mathrm{e}\mu $), one electron and two muons ($ \mathrm{e}\mu\mu $), and three muons ($ \mu\mu\mu $). If there are more than three leptons in an event, the three $ p_{\mathrm{T}} $-leading leptons are used to determine the flavor. The combined results from 13 TeV data (Run 2) and from 13.6 TeV data (Run 3) are also shown. The result from the combined fit to 13 and 13.6 TeV data is shown as a red line with the corresponding uncertainty shown as a red band. The SM prediction is shown as a gray band.

png pdf
Figure A2-a:
Values of the signal strengths for tWZ production obtained from the different data-taking periods (left), and from events with different lepton flavors (right): three electrons ($ \mathrm{e}\mathrm{e}\mathrm{e} $), two electrons and one muon ($ \mathrm{e}\mathrm{e}\mu $), one electron and two muons ($ \mathrm{e}\mu\mu $), and three muons ($ \mu\mu\mu $). If there are more than three leptons in an event, the three $ p_{\mathrm{T}} $-leading leptons are used to determine the flavor. The combined results from 13 TeV data (Run 2) and from 13.6 TeV data (Run 3) are also shown. The result from the combined fit to 13 and 13.6 TeV data is shown as a red line with the corresponding uncertainty shown as a red band. The SM prediction is shown as a gray band.

png pdf
Figure A2-b:
Values of the signal strengths for tWZ production obtained from the different data-taking periods (left), and from events with different lepton flavors (right): three electrons ($ \mathrm{e}\mathrm{e}\mathrm{e} $), two electrons and one muon ($ \mathrm{e}\mathrm{e}\mu $), one electron and two muons ($ \mathrm{e}\mu\mu $), and three muons ($ \mu\mu\mu $). If there are more than three leptons in an event, the three $ p_{\mathrm{T}} $-leading leptons are used to determine the flavor. The combined results from 13 TeV data (Run 2) and from 13.6 TeV data (Run 3) are also shown. The result from the combined fit to 13 and 13.6 TeV data is shown as a red line with the corresponding uncertainty shown as a red band. The SM prediction is shown as a gray band.
Summary
In summary, a measurement of tWZ production in proton-proton (pp) collisions has been presented in events with three and four charged leptons. The analysis is performed using data recorded with the CMS detector at center-of-mass energies of 13 and 13.6 TeV, corresponding to a total integrated luminosity of 200 fb$ ^{-1} $. Advanced machine-learning algorithms are employed to significantly improve the sensitivity of the analysis compared to an earlier analysis. Analyzing the same data, an expected significance of 3.0 standard deviations (s.d.) is achieved, compared to 1.4 s.d. obtained previously. Combining the 13 and 13.6 TeV data, a significance of 5.8 s.d. is observed. The measured signal strength $ \mu_{\mathrm{t}\mathrm{W}\mathrm{Z}}= $ 1.77 $ \pm $ 0.32 (tot) agrees with the standard model prediction within 2.3 s.d. The measured cross sections are 248 $ \pm $ 52 fb and 242 $ \pm $ 77 fb for $ \sqrt{s}= $ 13 and 13.6 TeV, respectively. This is the first observation of tWZ production in pp collisions. This measurement represents a significant step forward in our understanding of rare processes involving the top quark and electroweak bosons. It opens up the possibility for future studies of the top quark electroweak interactions in the context of the standard model and its extensions.
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Compact Muon Solenoid
LHC, CERN