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| CMS-TOP-24-012 ; CERN-EP-2025-113 | ||
| Search for CP violation in events with top quarks and Z bosons at $ \sqrt{s}= $ 13 and 13.6 TeV | ||
| CMS Collaboration | ||
| 27 May 2025 | ||
| Submitted to Phys. Lett. B | ||
| Abstract: A search for the violation of the charge-parity (CP) symmetry in the production of top quarks in association with Z bosons is presented, using events with at least three charged leptons and additional jets. The search is performed in a sample of proton-proton collision data collected by the CMS experiment at the CERN LHC in 2016-2018 at a center-of-mass energy of 13 TeV and in 2022 at 13.6 TeV, corresponding to a total integrated luminosity of 173 fb$ ^{-1} $. For the first time in this final state, observables that are odd under the CP transformation are employed. Also for the first time, physics-informed machine-learning techniques are used to construct these observables. While for standard model (SM) processes the distributions of these observables are predicted to be symmetric around zero, CP-violating modifications of the SM would introduce asymmetries. Two CP-odd operators $ \mathcal{O}_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ \mathcal{O}_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ in the SM effective field theory are considered that may modify the interactions between top quarks and electroweak bosons. The obtained results are consistent with the SM prediction within two standard deviations, and exclusion limits on the associated Wilson coefficients of $ -$2.7 $ < c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} < $ 2.5 and $ -$0.2 $ < c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} < $ 2.0 are set at 95% confidence level. The largest discrepancy is observed in $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ where data is consistent with positive values, with an observed local significance with respect to the SM hypothesis of 2.5 standard deviations, when only linear terms are considered. | ||
| Links: e-print arXiv:2505.21206 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
png pdf |
Figure 1:
Example Feynman diagrams for $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ (left ) and $ \mathrm{t}\mathrm{Z}\mathrm{q} $ (right ) production with vertices that can be modified by $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $) highlighted in red (blue) circle. |
png pdf |
Figure 1-a:
Example Feynman diagrams for $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ (left ) and $ \mathrm{t}\mathrm{Z}\mathrm{q} $ (right ) production with vertices that can be modified by $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $) highlighted in red (blue) circle. |
png pdf |
Figure 1-b:
Example Feynman diagrams for $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ (left ) and $ \mathrm{t}\mathrm{Z}\mathrm{q} $ (right ) production with vertices that can be modified by $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $) highlighted in red (blue) circle. |
png pdf |
Figure 2:
left (right ): Distribution of the discretized $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $) score for events in the $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $-like ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like) category in $ \mathrm{t}\mathrm{Z}\mathrm{q} $ ($ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $) events. The contributions from the SM, linear, and quadratic terms when each Wilson coefficient is set to unity are plotted separately. For better visibility the interference contribution in the $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like category has been scaled by a factor of 4. |
png pdf |
Figure 2-a:
left (right ): Distribution of the discretized $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $) score for events in the $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $-like ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like) category in $ \mathrm{t}\mathrm{Z}\mathrm{q} $ ($ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $) events. The contributions from the SM, linear, and quadratic terms when each Wilson coefficient is set to unity are plotted separately. For better visibility the interference contribution in the $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like category has been scaled by a factor of 4. |
png pdf |
Figure 2-b:
left (right ): Distribution of the discretized $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $) score for events in the $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $-like ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like) category in $ \mathrm{t}\mathrm{Z}\mathrm{q} $ ($ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $) events. The contributions from the SM, linear, and quadratic terms when each Wilson coefficient is set to unity are plotted separately. For better visibility the interference contribution in the $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like category has been scaled by a factor of 4. |
png pdf |
Figure 3:
left (right ): Distribution of the discretized $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $) score for events in the $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $-like ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like) category, compared with the predictions obtained when all fit parameters are set to their maximum likelihood value in the linear fit, where $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ are determined simultaneously. The lower panels show the ratio between data (black dots), the prediction of the linear (blue line), and quadratic (green line) fits, over the prefit value. The red, blue, and green bands show the prefit uncertainty and the postfit uncertainties of the linear and quadratic fits, respectively. |
png pdf |
Figure 3-a:
left (right ): Distribution of the discretized $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $) score for events in the $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $-like ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like) category, compared with the predictions obtained when all fit parameters are set to their maximum likelihood value in the linear fit, where $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ are determined simultaneously. The lower panels show the ratio between data (black dots), the prediction of the linear (blue line), and quadratic (green line) fits, over the prefit value. The red, blue, and green bands show the prefit uncertainty and the postfit uncertainties of the linear and quadratic fits, respectively. |
png pdf |
Figure 3-b:
left (right ): Distribution of the discretized $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $) score for events in the $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $-like ($ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $-like) category, compared with the predictions obtained when all fit parameters are set to their maximum likelihood value in the linear fit, where $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ are determined simultaneously. The lower panels show the ratio between data (black dots), the prediction of the linear (blue line), and quadratic (green line) fits, over the prefit value. The red, blue, and green bands show the prefit uncertainty and the postfit uncertainties of the linear and quadratic fits, respectively. |
png pdf |
Figure 4:
Likelihood scans as a function of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ (upper row) and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ (lower row), separately for cases in which the other coefficient is set to zero (dark black solid line) or profiled (red dashed line). Plots in the left (right) column represent the linear (quadratic) fit. Light gray lines represent the quantiles of the test statistics. |
png pdf |
Figure 4-a:
Likelihood scans as a function of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ (upper row) and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ (lower row), separately for cases in which the other coefficient is set to zero (dark black solid line) or profiled (red dashed line). Plots in the left (right) column represent the linear (quadratic) fit. Light gray lines represent the quantiles of the test statistics. |
png pdf |
Figure 4-b:
Likelihood scans as a function of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ (upper row) and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ (lower row), separately for cases in which the other coefficient is set to zero (dark black solid line) or profiled (red dashed line). Plots in the left (right) column represent the linear (quadratic) fit. Light gray lines represent the quantiles of the test statistics. |
png pdf |
Figure 4-c:
Likelihood scans as a function of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ (upper row) and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ (lower row), separately for cases in which the other coefficient is set to zero (dark black solid line) or profiled (red dashed line). Plots in the left (right) column represent the linear (quadratic) fit. Light gray lines represent the quantiles of the test statistics. |
png pdf |
Figure 4-d:
Likelihood scans as a function of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ (upper row) and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ (lower row), separately for cases in which the other coefficient is set to zero (dark black solid line) or profiled (red dashed line). Plots in the left (right) column represent the linear (quadratic) fit. Light gray lines represent the quantiles of the test statistics. |
png pdf |
Figure 5:
Likelihood scans as functions of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $, including linear contributions only (left ) and both linear and quadratic contributions (right ). |
png pdf |
Figure 5-a:
Likelihood scans as functions of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $, including linear contributions only (left ) and both linear and quadratic contributions (right ). |
png pdf |
Figure 5-b:
Likelihood scans as functions of $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $, including linear contributions only (left ) and both linear and quadratic contributions (right ). |
| Summary |
| We have presented a search for additional sources of the charge-parity (CP) symmetry violation in the associated production of top quarks and a Z boson, in particular, for $ {\mathrm{t}\overline{\mathrm{t}}} \mathrm{Z} $ and $ \mathrm{t}\mathrm{Z}\mathrm{q} $ production in final states with at least three leptons. The search is performed in a sample of proton-proton collision data at center-of-mass energies of 13 and 13.6 TeV corresponding to a total integrated luminosity of 173 fb$ ^{-1} $. The measurement uses, for the first time in this topology, CP-odd observables which are constructed using physics-informed machine-learning techniques. These observables are predicted by the standard model (SM) to be symmetrically distributed whereas asymmetries could arise from CP-violating effects. The results are generally consistent with the SM prediction. We set limits on the Wilson coefficients $ c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} $ and $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $ of $ -2.7 < c_{\mathrm{t}\mathrm{W}}^{\mathrm{I}} < $ 2.5 and $ -0.2 < c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} < $ 2.0, respectively, at 95% confidence level, considering the effect of both linear and quadratic terms, and profiling the other coefficient. The largest discrepancy is observed in $ c_{\mathrm{t}\mathrm{Z}}^{\mathrm{I}} $, for which data is consistent with positive values. When considering only linear terms, the observed local significance with respect to the SM hypothesis corresponds to 2.5 standard deviations. |
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