CMS-TOP-16-001

CMS-TOP-16-001 ; CERN-EP-2016-266
Search for CP violation in $\mathrm{ t \bar{t} }$ production and decay in proton-proton collisions at $\sqrt{s} =$ 8 TeV
CMS Collaboration
28 November 2016
JHEP 03 (2017) 101
Abstract: The results of a first search for CP violation in the production and decay of top quark-antiquark ( $\mathrm{ t \bar{t} }$ ) pairs are presented. The search is based on asymmetries in T-odd, triple-product correlation observables, where T is the time-reversal operator. The analysis uses a sample of proton-proton collisions at $\sqrt{s} =$ 8 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 19.7 fb $^{-1}$ . Events are selected having one electron or muon and at least four jets. The T-odd observables are measured using four-momentum vectors associated with $\mathrm{ t \bar{t} }$ production and decay. The measured asymmetries exhibit no evidence for CP-violating effects, consistent with the expectation from the standard model.
Links: e-print arXiv:1611.08931 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The measured invariant mass distributions from data (points) of (upper) hadronically and (lower) semileptonically decaying top quark candidates in the (left) electron and (right) muon channels, compared to the predictions for the signal and various backgrounds from simulation (filled histograms). The QCD background is found to be negligible. The overflow events are collected in the last bins. The vertical bars on the data points and the hatched bands indicate the statistical uncertainties in the data and simulation, respectively. The lower panels show the relative fractional difference between the data and simulation, with its statistical uncertainty.
png pdf	Figure 1-a: The measured invariant mass distribution from data (points) of hadronically decaying top quark candidates in the electron channel, compared to the predictions for the signal and various backgrounds from simulation (filled histograms). The QCD background is found to be negligible. The overflow events are collected in the last bin. The vertical bars on the data points and the hatched bands indicate the statistical uncertainties in the data and simulation, respectively. The lower panel shows the relative fractional difference between the data and simulation, with its statistical uncertainty.
png pdf	Figure 1-b: The measured invariant mass distribution from data (points) of hadronically decaying top quark candidates in the muon channel, compared to the predictions for the signal and various backgrounds from simulation (filled histograms). The QCD background is found to be negligible. The overflow events are collected in the last bin. The vertical bars on the data points and the hatched bands indicate the statistical uncertainties in the data and simulation, respectively. The lower panel shows the relative fractional difference between the data and simulation, with its statistical uncertainty.
png pdf	Figure 1-c: The measured invariant mass distribution from data (points) of semileptonically decaying top quark candidates in the electron channel, compared to the predictions for the signal and various backgrounds from simulation (filled histograms). The QCD background is found to be negligible. The overflow events are collected in the last bin. The vertical bars on the data points and the hatched bands indicate the statistical uncertainties in the data and simulation, respectively. The lower panel shows the relative fractional difference between the data and simulation, with its statistical uncertainty.
png pdf	Figure 1-d: The measured invariant mass distribution from data (points) of semileptonically decaying top quark candidates in the muon channel, compared to the predictions for the signal and various backgrounds from simulation (filled histograms). The QCD background is found to be negligible. The overflow events are collected in the last bin. The vertical bars on the data points and the hatched bands indicate the statistical uncertainties in the data and simulation, respectively. The lower panel shows the relative fractional difference between the data and simulation, with its statistical uncertainty.
png pdf	Figure 2: Distribution of the invariant mass $M_{\ell \mathrm{ b } }$ of the semileptonically decaying top quark candidates for the (left) electron and (right) muon channels, in comparison to the results of the fit described in the text. Overflow events are collected in the last bins. The vertical bars on the data points indicate the statistical uncertainties. The hatched bands shows the combined statistical and systematic uncertainties in the fit results added in quadrature. The lower panels show the relative fractional difference between the data and simulation, with vertical bars that indicate the total uncertainties from the fit.
png pdf	Figure 2-a: Distribution of the invariant mass $M_{\ell \mathrm{ b } }$ of the semileptonically decaying top quark candidates for the electron channel, in comparison to the results of the fit described in the text. Overflow events are collected in the last bin. The vertical bars on the data points indicate the statistical uncertainties. The hatched bands shows the combined statistical and systematic uncertainties in the fit results added in quadrature. The lower panel shows the relative fractional difference between the data and simulation, with vertical bars that indicate the total uncertainties from the fit.
png pdf	Figure 2-b: Distribution of the invariant mass $M_{\ell \mathrm{ b } }$ of the semileptonically decaying top quark candidates for the muon channel, in comparison to the results of the fit described in the text. Overflow events are collected in the last bin. The vertical bars on the data points indicate the statistical uncertainties. The hatched bands shows the combined statistical and systematic uncertainties in the fit results added in quadrature. The lower panel shows the relative fractional difference between the data and simulation, with vertical bars that indicate the total uncertainties from the fit.
png pdf	Figure 3: The results from simulation of the asymmetry correction procedure using a dilution factor for the four different CPV observables. The circular markers show the output ${ A_\mathrm {CP}^\prime }$ measurements for each generated ${A_\mathrm {CP}}$ value. The dashed lines are the result of linear fits to the ${ A_\mathrm {CP}^\prime }$ points. The triangular markers give the corrected ${A_\mathrm {CP}}$ values, obtained after applying the dilution factor. The solid lines are the result of linear fits to the corrected ${A_\mathrm {CP}}$ points. The statistical uncertainties in both sets of asymmetries are smaller than the markers.
png pdf	Figure 3-a: The results from simulation of the asymmetry correction procedure using a dilution factor for the " $O_2$ in lepton+jets" observable. The circular markers show the output ${ A_\mathrm {CP}^\prime }$ measurements for each generated ${A_\mathrm {CP}}$ value. The dashed lines is the result of a linear fit to the ${ A_\mathrm {CP}^\prime }$ points. The triangular markers give the corrected ${A_\mathrm {CP}}$ values, obtained after applying the dilution factor. The solid line is the result of a linear fit to the corrected ${A_\mathrm {CP}}$ points. The statistical uncertainties in both sets of asymmetries are smaller than the markers.
png pdf	Figure 3-b: The results from simulation of the asymmetry correction procedure using a dilution factor for the " $O_3$ in lepton+jets" observable. The circular markers show the output ${ A_\mathrm {CP}^\prime }$ measurements for each generated ${A_\mathrm {CP}}$ value. The dashed lines is the result of a linear fit to the ${ A_\mathrm {CP}^\prime }$ points. The triangular markers give the corrected ${A_\mathrm {CP}}$ values, obtained after applying the dilution factor. The solid line is the result of a linear fit to the corrected ${A_\mathrm {CP}}$ points. The statistical uncertainties in both sets of asymmetries are smaller than the markers.
png pdf	Figure 3-c: The results from simulation of the asymmetry correction procedure using a dilution factor for the " $O_4$ in lepton+jets" observable. The circular markers show the output ${ A_\mathrm {CP}^\prime }$ measurements for each generated ${A_\mathrm {CP}}$ value. The dashed lines is the result of a linear fit to the ${ A_\mathrm {CP}^\prime }$ points. The triangular markers give the corrected ${A_\mathrm {CP}}$ values, obtained after applying the dilution factor. The solid line is the result of a linear fit to the corrected ${A_\mathrm {CP}}$ points. The statistical uncertainties in both sets of asymmetries are smaller than the markers.
png pdf	Figure 3-d: The results from simulation of the asymmetry correction procedure using a dilution factor for the " $O_7$ in lepton+jets" observable. The circular markers show the output ${ A_\mathrm {CP}^\prime }$ measurements for each generated ${A_\mathrm {CP}}$ value. The dashed lines is the result of a linear fit to the ${ A_\mathrm {CP}^\prime }$ points. The triangular markers give the corrected ${A_\mathrm {CP}}$ values, obtained after applying the dilution factor. The solid line is the result of a linear fit to the corrected ${A_\mathrm {CP}}$ points. The statistical uncertainties in both sets of asymmetries are smaller than the markers.
png pdf	Figure 4: Distributions of the four CPV observables given in Eq.(2), determined from the combined electron and muon channels from data (points) and simulated signal and background (filled histograms). The simulated $\mathrm{ t \bar{t} }$ and background samples are normalized to the fitted yields. The overflow events are collected in the first and last bins. Each observable is given in units of $m_{\mathrm{ t } }^3$ , where $m_{\mathrm{ t } }=$ 172.5 GeV. The vertical bars represent the statistical uncertainties in the data. The hatched bands give the combined statistical and systematic uncertainties added in quadrature.
png pdf	Figure 4-a: Distribution of the "O_2" CPV observable given in Eq.(2), determined from the combined electron and muon channels from data (points) and simulated signal and background (filled histograms). The simulated $\mathrm{ t \bar{t} }$ and background samples are normalized to the fitted yields. The overflow events are collected in the first and last bins. Each observable is given in units of $m_{\mathrm{ t } }^3$ , where $m_{\mathrm{ t } }=$ 172.5 GeV. The vertical bars represent the statistical uncertainties in the data. The hatched bands give the combined statistical and systematic uncertainties added in quadrature.
png pdf	Figure 4-b: Distribution of the "O_3" CPV observable given in Eq.(2), determined from the combined electron and muon channels from data (points) and simulated signal and background (filled histograms). The simulated $\mathrm{ t \bar{t} }$ and background samples are normalized to the fitted yields. The overflow events are collected in the first and last bins. Each observable is given in units of $m_{\mathrm{ t } }^3$ , where $m_{\mathrm{ t } }=$ 172.5 GeV. The vertical bars represent the statistical uncertainties in the data. The hatched bands give the combined statistical and systematic uncertainties added in quadrature.
png pdf	Figure 4-c: Distribution of the "O_4" CPV observable given in Eq.(2), determined from the combined electron and muon channels from data (points) and simulated signal and background (filled histograms). The simulated $\mathrm{ t \bar{t} }$ and background samples are normalized to the fitted yields. The overflow events are collected in the first and last bins. Each observable is given in units of $m_{\mathrm{ t } }^3$ , where $m_{\mathrm{ t } }=$ 172.5 GeV. The vertical bars represent the statistical uncertainties in the data. The hatched bands give the combined statistical and systematic uncertainties added in quadrature.
png pdf	Figure 4-d: Distribution of the "O_7" CPV observable given in Eq.(2), determined from the combined electron and muon channels from data (points) and simulated signal and background (filled histograms). The simulated $\mathrm{ t \bar{t} }$ and background samples are normalized to the fitted yields. The overflow events are collected in the first and last bins. Each observable is given in units of $m_{\mathrm{ t } }^3$ , where $m_{\mathrm{ t } }=$ 172.5 GeV. The vertical bars represent the statistical uncertainties in the data. The hatched bands give the combined statistical and systematic uncertainties added in quadrature.
png pdf	Figure 5: Summary of the uncorrected (corrected) CP asymmetries ${ A_\mathrm {CP}^\prime }$ ( ${A_\mathrm {CP}}$ ) for the observables defined in Eq.(2). The results for ${ A_\mathrm {CP}^\prime }$ are shown for the electron and muon channels separately and for their combination. The results for ${A_\mathrm {CP}}$ are shown for the combined electron and muon channels, using the dilution factors from SM simulation of $\mathrm{ t \bar{t} }$ production. The inner bars represent the statistical uncertainties, and the outer bars the combined statistical and systematic uncertainties added in quadrature.

Tables
png pdf	Table 1: The uncorrected CP asymmetry ${A_\mathrm {CP}^\prime }$ , measured in percent, obtained from the control sample as described in the text for each of the four observables. Results are given for the electron and muon channels separately and for their combination. For the separate electron and muon channels, the uncertainties are statistical. For the combined results, the first uncertainty is statistical and the second systematic.
png pdf	Table 2: The observed and fitted number of events in the electron and muon channels as well as the fitted $\mathrm{ t \bar{t} }$ fraction (purity) in percent. While the fit is performed over the full mass range, the fitted and observed results are for $M_{\ell \mathrm{ b } } <$ 200 GeV. The first uncertainty is statistical and the second systematic.
png pdf	Table 3: For each of the four CPV observables, the fraction $k$ of wrong-sign events and the associated dilution factor $\mathcal {D}$ computed from $k$ , determined from simulated $\mathrm{ t \bar{t} }$ events. The first uncertainty is statistical and the second is systematic.
png pdf	Table 4: The uncorrected (corrected) CP asymmetry ${ A_\mathrm {CP}^\prime }$ ( ${A_\mathrm {CP}}$ ), measured in percent, for each of the four CPV observables. Results for ${ A_\mathrm {CP}^\prime }$ are given for the electron and muon channels separately and for their combination. For the ${ A_\mathrm {CP}^\prime }$ results, the first uncertainty is statistical and the second systematic. The ${A_\mathrm {CP}}$ values assume the dilution factors found from the SM simulation. The uncertainties in the ${A_\mathrm {CP}}$ results are the combined statistical and systematic terms added in quadrature.

Summary

The first search for CP-violating effects in top quark-antiquark events has been presented. The search is performed in the electron + jets and muon + jets final states, with one top quark assumed to decay hadronically and the other semileptonically. The search is based on a sample of proton-proton collision data collected at

$\sqrt{s} =$ 8 TeV with the CMS detector in 2012, corresponding to an integrated luminosity of 19.7 fb

$^{-1}$ . The CP-violating asymmetries are measured using four T-odd, triple-product observables, where T is the time-reversal operator. A data control sample is used to verify that no significant spurious CP asymmetry is introduced by background processes, and to model the shape of the background in the asymmetry observables. The normalization of the background contribution in the signal region is determined from a fit to the mass distribution

$M_{\ell\mathrm{ b }}$ associated with the semileptonically decaying top quarks. The background-subtracted distributions of the observables are used to compute the uncorrected asymmetries. The corrected asymmetries are obtained by using a multiplicative dilution factor derived from simulation. Both the uncorrected and corrected asymmetries are consistent with zero, in agreement with the expectation from the standard model.

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