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| CMS-TOP-20-005 ; CERN-EP-2022-051 | ||
| Search for CP violation using $\mathrm{t\bar{t}}$ events in the lepton+jets channel in pp collisions at $\sqrt{s}=$ 13 TeV | ||
| CMS Collaboration | ||
| 4 May 2022 | ||
| JHEP 06 (2023) 081 | ||
| Abstract: Results are presented on a search for CP violation in the production and decay of top quark-antiquark pairs in the lepton+jets channel. The search is based on data from proton-proton collisions at $\sqrt{s}=$ 13 TeV, collected with the CMS detector, corresponding to an integrated luminosity of 138 fb$^{-1}$. Possible CP violation effects are evaluated by measuring uncorrected asymmetries in observables constructed from linearly independent four-momentum vectors of the final-state particles. The dimensionless chromoelectric dipole moment of the top quark obtained from the observed asymmetries is measured to be 0.04 $\pm$ 0.10 (stat) $\pm$ 0.07 (syst), and the asymmetries exhibit no evidence for CP-violating effects, consistent with expectations from the standard model. | ||
| Links: e-print arXiv:2205.02314 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Distributions of the CP observables ${O_3}$ (upper left), ${O_6}$ (upper right), ${O_{12}}$ (lower left), and ${O_{14}}$ (lower right), normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for electron events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 1-a:
Distribution of the CP observable ${O_3}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for electron events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 1-b:
Distribution of the CP observable ${O_6}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for electron events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 1-c:
Distribution of the CP observable ${O_{12}}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for electron events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 1-d:
Distribution of the CP observable ${O_{14}}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for electron events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 2:
Distributions of the CP observables ${O_3}$ (upper left), ${O_6}$ (upper right), ${O_{12}}$ (lower left), and ${O_{14}}$ (lower right), normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for muon events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 2-a:
Distribution of the CP observable ${O_3}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for muon events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 2-b:
Distribution of the CP observable ${O_6}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for muon events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
png pdf |
Figure 2-c:
Distribution of the CP observable ${O_{12}}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for muon events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 2-d:
Distribution of the CP observable ${O_{14}}$, normalized with respect to ${m_{\mathrm{t}}^3}$, from data (points) and from the various sources in simulation (colored histograms) for muon events in the signal region. The solid-blue line shows the CEDM simulated signal normalized to the data with the CP-odd parameter $ {d_{\mathrm{t} \mathrm {G}}} = +$3. The vertical bars on the data points indicate the statistical uncertainties in the data, and the hatched bands show the quadrature sum of the statistical and systematic uncertainties in the simulation. The lower two panels display the ratio of the data to the sum of the MC predictions and the ratio of the CEDM to the SM predictions for $ {d_{\mathrm{t} \mathrm {G}}} = +$3 (red lines) and $-$3 (dark-blue lines). |
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Figure 3:
The normalized ${m_{\ell \mathrm{b}}}$ distributions for the electron (left) and muon (right) channels. The upper two plots compare the background-enriched distributions from data (solid line) to the MC predictions (dotted-red line). The lower two plots give the background-enriched distributions from data (solid line) and the MC predictions for the distributions from the background in the signal events. |
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Figure 3-a:
The normalized ${m_{\ell \mathrm{b}}}$ distribution for the electron channel. The plot compares the background-enriched distributions from data (solid line) to the MC predictions (dotted-red line). |
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Figure 3-b:
The normalized ${m_{\ell \mathrm{b}}}$ distribution for the muon channel. The plot compares the background-enriched distributions from data (solid line) to the MC predictions (dotted-red line). |
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Figure 3-c:
The normalized ${m_{\ell \mathrm{b}}}$ distribution for the electron channel. The plot gives the background-enriched distributions from data (solid line) and the MC predictions for the distributions from the background in the signal events. |
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Figure 3-d:
The normalized ${m_{\ell \mathrm{b}}}$ distribution for the muon channel. The plot gives the background-enriched distributions from data (solid line) and the MC predictions for the distributions from the background in the signal events. |
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Figure 4:
The ${m_{\ell \mathrm{b}}}$ invariant mass distributions in the electron (left) and muon (right) channels from data (points). The results of the fit to the $ \mathrm{t\bar{t}} $ and background templates are shown by the red and green histograms, respectively. The vertical bars on the data points in the upper panels indicate the statistical uncertainties in the data and the hatched bands show the combined statistical and systematic uncertainties in the simulation. The lower panels give the ratio of the data to the sum of the fitted MC predictions. The blue bands represent the systematic uncertainties in the expected yield in the simulation for all sources of systematic uncertainty (Section 6). |
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Figure 4-a:
The ${m_{\ell \mathrm{b}}}$ invariant mass distributions in the electron channel from data (points). The results of the fit to the $ \mathrm{t\bar{t}} $ and background templates are shown by the red and green histograms, respectively. The vertical bars on the data points in the upper panel indicate the statistical uncertainties in the data and the hatched bands show the combined statistical and systematic uncertainties in the simulation. The lower panel gives the ratio of the data to the sum of the fitted MC predictions. The blue band represents the systematic uncertainties in the expected yield in the simulation for all sources of systematic uncertainty (Section 6). |
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Figure 4-b:
The ${m_{\ell \mathrm{b}}}$ invariant mass distributions in the muon channel from data (points). The results of the fit to the $ \mathrm{t\bar{t}} $ and background templates are shown by the red and green histograms, respectively. The vertical bars on the data points in the upper panel indicate the statistical uncertainties in the data and the hatched bands show the combined statistical and systematic uncertainties in the simulation. The lower panel gives the ratio of the data to the sum of the fitted MC predictions. The blue band represents the systematic uncertainties in the expected yield in the simulation for all sources of systematic uncertainty (Section 6). |
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Figure 5:
The measured CP-violating asymmetries in simulation as a function of the generator-level pseudo-asymmetry for the CP observables ${O_3}$ (upper left), ${O_6}$ (upper right), ${O_{12}}$ (lower left), and ${O_{14}}$ (lower right). The red circles and blue diamonds give the ${A'_\text {CP}}$ and ${A_\text {CP}}$ values, respectively, with the red-dotted and blue-solid lines showing the results of linear fits to those corresponding values. The ${A_\text {CP}}$ value is obtained by dividing the ${A'_\text {CP}}$ value by the dilution factor determined from simulation for that CP observable. The statistical uncertainties in the ${A'_\text {CP}}$ values are smaller than the markers. |
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Figure 5-a:
The measured CP-violating asymmetry in simulation as a function of the generator-level pseudo-asymmetry for the CP observable ${O_3}$. The red circles and blue diamonds give the ${A'_\text {CP}}$ and ${A_\text {CP}}$ values, respectively, with the red-dotted and blue-solid lines showing the results of linear fits to those corresponding values. The ${A_\text {CP}}$ value is obtained by dividing the ${A'_\text {CP}}$ value by the dilution factor determined from simulation for that CP observable. The statistical uncertainties in the ${A'_\text {CP}}$ values are smaller than the markers. |
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Figure 5-b:
The measured CP-violating asymmetry in simulation as a function of the generator-level pseudo-asymmetry for the CP observable ${O_6}$. The red circles and blue diamonds give the ${A'_\text {CP}}$ and ${A_\text {CP}}$ values, respectively, with the red-dotted and blue-solid lines showing the results of linear fits to those corresponding values. The ${A_\text {CP}}$ value is obtained by dividing the ${A'_\text {CP}}$ value by the dilution factor determined from simulation for that CP observable. The statistical uncertainties in the ${A'_\text {CP}}$ values are smaller than the markers. |
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Figure 5-c:
The measured CP-violating asymmetry in simulation as a function of the generator-level pseudo-asymmetry for the CP observable ${O_{12}}$. The red circles and blue diamonds give the ${A'_\text {CP}}$ and ${A_\text {CP}}$ values, respectively, with the red-dotted and blue-solid lines showing the results of linear fits to those corresponding values. The ${A_\text {CP}}$ value is obtained by dividing the ${A'_\text {CP}}$ value by the dilution factor determined from simulation for that CP observable. The statistical uncertainties in the ${A'_\text {CP}}$ values are smaller than the markers. |
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Figure 5-d:
The measured CP-violating asymmetry in simulation as a function of the generator-level pseudo-asymmetry for the CP observable ${O_{14}}$. The red circles and blue diamonds give the ${A'_\text {CP}}$ and ${A_\text {CP}}$ values, respectively, with the red-dotted and blue-solid lines showing the results of linear fits to those corresponding values. The ${A_\text {CP}}$ value is obtained by dividing the ${A'_\text {CP}}$ value by the dilution factor determined from simulation for that CP observable. The statistical uncertainties in the ${A'_\text {CP}}$ values are smaller than the markers. |
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Figure 6:
The measured effective asymmetries ${A'_\text {CP}}$ for each CP observable from the electron, muon, and combined lepton+jets channels. The results from the previous CMS measurement at $\sqrt {s}=$ 8 TeV [12] are shown by the green circles. The inner vertical bars on the symbols represent the statistical uncertainty, and the outer bars the statistical and systematic uncertainties added in quadrature. |
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Figure 7:
The measured dimensionless CEDM ${d_{\mathrm{t} \mathrm {G}}}$ for each CP observable (blue squares) in the lepton+jets channel and the combined result (red point). The inner horizontal bars on the points represent the statistical uncertainty and the outer bar the combined statistical and systematic uncertainties added in quadrature. |
| Tables | |
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Table 1:
The predicted $ \mathrm{t\bar{t}} $ signal and background contributions to the signal events from simulation for the electron and muon channels. |
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Table 2:
The fitted number of $ \mathrm{t\bar{t}} $ signal and $ \mathrm{t\bar{t}} $ background events (fitted $ \mathrm{t\bar{t}} $) and other background events (fitted background) in the electron and muon channels, along with the $ \mathrm{t\bar{t}} $ purities. Although the fit is performed for $ {m_{\ell \mathrm{b}}} < $ 500 GeV, the event yields are given for $ {m_{\ell \mathrm{b}}} < $ 150 GeV. The uncertainties shown are statistical only. |
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Table 3:
The dilution factor {D} determined from simulation and its systematic uncertainty for each CP observable. The statistical uncertainty is negligible compared to the systematic uncertainty. |
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Table 4:
The ${A'_\text {CP}}$ values and their statistical uncertainties in percent for each CP observable from the electron and muon event-mixing samples and their combination, used to search for detector or reconstruction bias. |
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Table 5:
The sources and values of the systematic uncertainties in ${A'_\text {CP}}$ for each of the CP observables in percent, averaged over the two lepton-flavor channels. The experimental sources are listed first and then the theoretical ones. |
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Table 6:
The measured effective asymmetries ${A'_\text {CP}}$ in percent for each of the CP observables for the electron, muon, and combined data samples. The first uncertainty is statistical and the second is systematic. The statistical uncertainties are the same for each lepton type because the numbers of signal events are the same. |
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Table 7:
The measured ${A_\text {CP}}$ and corresponding ${d_{\mathrm{t} \mathrm {G}}}$ values for each of the CP observables using the SM simulation predictions for the dilution factor {D} in the combined lepton+jets channel. The first uncertainty is statistical and the second is systematic. |
| Summary |
| The results of a search have been presented for the combined charge conjugate and parity (CP) violation effects in top quark-antiquark events performed in the electron+jets and muon+jets final states. The top quark and antiquark are each assumed to decay into a bottom quark and a W boson, with one W boson decaying hadronically and the other leptonically into an electron or muon and accompanying neutrino. This study uses data collected by the CMS experiment at the LHC from proton-proton collisions at $\sqrt{s}=$ 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The CP-violating asymmetries are obtained with four different triple-product T-odd observables, where T is the time-reversal operator, constructed using linearly independent four-momentum vectors associated with the final-state particles. The uncorrected asymmetries are computed using the fitted signal. There are no statistically significant indications of CP violation, with all the CP asymmetries being consistent with the standard model expectations. The resulting combined measurement of the dimensionless chromoelectric dipole moment gives $d_{\mathrm{tG}} =$ 0.04 $\pm$ 0.10 (stat) $\pm$ 0.07 (syst) and exhibits no evidence for CP-violating effects, consistent with expectations from the standard model. These results are compatible with a previous study performed by CMS in data from pp collisions at $\sqrt{s}=$ 8 TeV [12] with uncertainties improved by roughly a factor of 3. |
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