CMS-PAS-TOP-20-005

CMS-PAS-TOP-20-005
Search for CP violation in top quark pair events in the lepton+jets channel at $\sqrt{s}=$ 13 TeV
CMS Collaboration
July 2021

Abstract: The results of a search for CP violation in the production and decay of top quark-antiquark pairs using the semileptonic decay channel are presented. The search uses data from proton-proton collisions at $\sqrt{s}=$ 13 TeV, collected with the CMS detector, corresponding to an integrated luminosity of 137 fb $^{-1}$ . Asymmetries that probe CP violating interactions are measured in observables constructed from linearly independent four-momentum vectors associated with the final-state particles. The CP violating effect is evaluated using the measured asymmetries in each observable. The measured effective asymmetries achieved a precision of 10 $^{-3}$ and exhibit no evidence for CP violating effects, consistent with the expectation from the standard model.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Comparison of data and simulation in the signal region for the four observables in the electron channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 1-a: Comparison of data and simulation in the signal region for the four observables in the electron channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 1-b: Comparison of data and simulation in the signal region for the four observables in the electron channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 1-c: Comparison of data and simulation in the signal region for the four observables in the electron channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 1-d: Comparison of data and simulation in the signal region for the four observables in the electron channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 2: Comparison of data and simulation in the signal region for the four observables in the muon channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 2-a: Comparison of data and simulation in the signal region for the four observables in the muon channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 2-b: Comparison of data and simulation in the signal region for the four observables in the muon channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 2-c: Comparison of data and simulation in the signal region for the four observables in the muon channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 2-d: Comparison of data and simulation in the signal region for the four observables in the muon channel. The dashed line shows the CEDM simulated signal with CP-odd CEDM parameter set to 3 and normalized to data. The vertical bars on the data points indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the overall uncertainty in the expected yield, including all systematic uncertainties described in Section 5, except that due to changing background template. The orange line shows the ratio $(d_{tG} = -3) / (d_{tG} = 0)$ of CEDM samples at the generator level. The green line shows the ratio $(d_{tG} = 3) / (d_{tG} = 0)$ of CEDM samples at the generator level.
png pdf	Figure 3: Distribution of the invariant mass $M_{lb}$ in the electron (left) and muon (right) channel. CR in the label means W+jets control region and SR in the label means the signal region. PDF in the y-axis title means probability density function. The upper two figures show that the W+jets enriched control region is background-like. The lower two figures show little difference between the simulated background in the signal region and the W+jets-enriched control region, which are considered as one of our systematic uncertainties. For signal-region simulated background samples, we consider the Drell-Yan, single top, diboson, and W+jets processes.
png pdf	Figure 3-a: Distribution of the invariant mass $M_{lb}$ in the electron (left) and muon (right) channel. CR in the label means W+jets control region and SR in the label means the signal region. PDF in the y-axis title means probability density function. The upper two figures show that the W+jets enriched control region is background-like. The lower two figures show little difference between the simulated background in the signal region and the W+jets-enriched control region, which are considered as one of our systematic uncertainties. For signal-region simulated background samples, we consider the Drell-Yan, single top, diboson, and W+jets processes.
png pdf	Figure 3-b: Distribution of the invariant mass $M_{lb}$ in the electron (left) and muon (right) channel. CR in the label means W+jets control region and SR in the label means the signal region. PDF in the y-axis title means probability density function. The upper two figures show that the W+jets enriched control region is background-like. The lower two figures show little difference between the simulated background in the signal region and the W+jets-enriched control region, which are considered as one of our systematic uncertainties. For signal-region simulated background samples, we consider the Drell-Yan, single top, diboson, and W+jets processes.
png pdf	Figure 3-c: Distribution of the invariant mass $M_{lb}$ in the electron (left) and muon (right) channel. CR in the label means W+jets control region and SR in the label means the signal region. PDF in the y-axis title means probability density function. The upper two figures show that the W+jets enriched control region is background-like. The lower two figures show little difference between the simulated background in the signal region and the W+jets-enriched control region, which are considered as one of our systematic uncertainties. For signal-region simulated background samples, we consider the Drell-Yan, single top, diboson, and W+jets processes.
png pdf	Figure 3-d: Distribution of the invariant mass $M_{lb}$ in the electron (left) and muon (right) channel. CR in the label means W+jets control region and SR in the label means the signal region. PDF in the y-axis title means probability density function. The upper two figures show that the W+jets enriched control region is background-like. The lower two figures show little difference between the simulated background in the signal region and the W+jets-enriched control region, which are considered as one of our systematic uncertainties. For signal-region simulated background samples, we consider the Drell-Yan, single top, diboson, and W+jets processes.
png pdf	Figure 4: The distribution of the invariant mass $M_{lb}$ of the leptonically decaying top quark candidates in the electron (left) and muon (right) channels. The vertical bars on the data points in the lower panels indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the systematic uncertainties in the expected yield in the simulation, including all sources of systematic uncertainty (Section 5), except for the uncertainty due to changing the background template.
png pdf	Figure 4-a: The distribution of the invariant mass $M_{lb}$ of the leptonically decaying top quark candidates in the electron (left) and muon (right) channels. The vertical bars on the data points in the lower panels indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the systematic uncertainties in the expected yield in the simulation, including all sources of systematic uncertainty (Section 5), except for the uncertainty due to changing the background template.
png pdf	Figure 4-b: The distribution of the invariant mass $M_{lb}$ of the leptonically decaying top quark candidates in the electron (left) and muon (right) channels. The vertical bars on the data points in the lower panels indicate the statistical uncertainties in the data and the hatched bands indicate the statistical uncertainties combined with the systematic uncertainties in the simulation. The blue bands represent the systematic uncertainties in the expected yield in the simulation, including all sources of systematic uncertainty (Section 5), except for the uncertainty due to changing the background template.
png pdf	Figure 5: Model dependency for the dilution factor $D_{12}$ . The points show the ratio between the value obtained by re-weigthing events according to different CEDM models with the baseline results obtained in the SM simulation. It should be noted that, although all the points are correlated, the deviations from unity are much smaller than the statistical uncertainty represented by the error bars.
png pdf	Figure 6: Asymmetries as a function of a varying input true asymmetry for each observable. The circular markers representing the $A'_{CP}$ are fitted to the dashed red lines, and the diamond markers representing the $A_{CP}$ are fitted to the solid blue lines. The value of each $A_{CP}$ value is obtained after applying the dilution factor to the corresponding $A'_{CP}$ value. The slopes of the solid blue lines are larger than the dashed red lines, demonstrating the magnitude of the dilution effect. The statistical uncertainties in the values of the asymmetries are smaller than the markers.
png pdf	Figure 6-a: Asymmetries as a function of a varying input true asymmetry for each observable. The circular markers representing the $A'_{CP}$ are fitted to the dashed red lines, and the diamond markers representing the $A_{CP}$ are fitted to the solid blue lines. The value of each $A_{CP}$ value is obtained after applying the dilution factor to the corresponding $A'_{CP}$ value. The slopes of the solid blue lines are larger than the dashed red lines, demonstrating the magnitude of the dilution effect. The statistical uncertainties in the values of the asymmetries are smaller than the markers.
png pdf	Figure 6-b: Asymmetries as a function of a varying input true asymmetry for each observable. The circular markers representing the $A'_{CP}$ are fitted to the dashed red lines, and the diamond markers representing the $A_{CP}$ are fitted to the solid blue lines. The value of each $A_{CP}$ value is obtained after applying the dilution factor to the corresponding $A'_{CP}$ value. The slopes of the solid blue lines are larger than the dashed red lines, demonstrating the magnitude of the dilution effect. The statistical uncertainties in the values of the asymmetries are smaller than the markers.
png pdf	Figure 6-c: Asymmetries as a function of a varying input true asymmetry for each observable. The circular markers representing the $A'_{CP}$ are fitted to the dashed red lines, and the diamond markers representing the $A_{CP}$ are fitted to the solid blue lines. The value of each $A_{CP}$ value is obtained after applying the dilution factor to the corresponding $A'_{CP}$ value. The slopes of the solid blue lines are larger than the dashed red lines, demonstrating the magnitude of the dilution effect. The statistical uncertainties in the values of the asymmetries are smaller than the markers.
png pdf	Figure 6-d: Asymmetries as a function of a varying input true asymmetry for each observable. The circular markers representing the $A'_{CP}$ are fitted to the dashed red lines, and the diamond markers representing the $A_{CP}$ are fitted to the solid blue lines. The value of each $A_{CP}$ value is obtained after applying the dilution factor to the corresponding $A'_{CP}$ value. The slopes of the solid blue lines are larger than the dashed red lines, demonstrating the magnitude of the dilution effect. The statistical uncertainties in the values of the asymmetries are smaller than the markers.
png pdf	Figure 7: The results of the effective asymmetries $A'_{CP}$ for each observable are shown for the separate electron and muon channels, as well as for the combined lepton+jets channel. The inner bars represent the statistical uncertainties, and the outer bars represent the combined statistical and systematic uncertainties added in quadrature. The 13 TeV results reduce the uncertainties by a factor of 3 compared with the 8 TeV results.

Tables
png pdf	Table 1: The expected composition of signal (semileptonic and dileptonic ${\mathrm{t} {}\mathrm{\bar{t}}}$ ) and different background processes in the signal region, estimated using simulation.
png pdf	Table 2: Fitted number of events in the electron and muon channels, along with the ${\mathrm{t} {}\mathrm{\bar{t}}}$ (semileptonic+dileptonic) fraction in percent. Although the fit is implemented over the full mass range, the fitted results are shown for $M_{lb} <$ 150 GeV.
png pdf	Table 3: The mean value of asymmetries of mixed data samples and the standard deviation on the mean value. The shift from the mean value in the bias measurement or the standard deviation on the shift is taken as the systematic uncertainty from possible detector bias. The results show no significant intrinsic detector bias within about one standard deviation of the statistical uncertainty in both the electron and muon channels and their combined value.
png pdf	Table 4: Systematic uncertainties in the $A'_{CP}$ measurement in both channels. The uncertainties due to experimental sources are in the upper part of the list, and theoretical sources are in the lower part. The $A'_{CP}$ values are obtained after a template fit with full signal region selection.
png pdf	Table 5: The measured dilution factor $D$ with their uncertainties for each observable. The first uncertainties are statistical and the second are systematic. The systematic uncertainties listed in Table are taken into consideration, except for the uncertainties from changing the background template.
png pdf	Table 6: The results of the effective asymmetries $A'_{CP}$ for each observable are presented in both electron and muon channels, as well as for the combined lepton+jets channel. The first uncertainties are statistical, which comes from data and is the same for all observables. The second uncertainties are systematic uncertainties, which are dominated by W+HF modeling and the ME-PS matching. The systematic uncertainties combine detector bias and all the sources are listed in Table 4.

Summary

A search for CP violating effects in top quark-antiquark events has been presented. The search has been performed in the electron + jets and muon + jets final states, with one top quark assumed to decay hadronically and the other leptonically. This study uses the data from

$\sqrt{s}=$ 13 TeV pp collisions collected with the CMS detector during Run 2. The CP violating asymmetries are measured with the triple-product T-odd observables, constructed using linearly independent four-momentum vectors associated with the final state particles, where T is the time-reversal operator. A data control sample is used to model the shape of the background in the asymmetry observables. The background contribution in the signal region is estimated from a fit to the mass distribution

$M_{lb}$ associated with the leptonically decaying top quarks. The effective asymmetries are computed using the fitted signal events. The measured effective asymmetries exhibit no evidence for CPV effects, consistent with the expectation from the standard model.

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