CMS-TOP-13-013 ; CERN-PH-EP-2015-189 | ||
Measurement of the charge asymmetry in top quark pair production in pp collisions at $\sqrt{s} =$ 8 TeV using a template method | ||
CMS Collaboration | ||
16 August 2015 | ||
Phys. Rev. D93 (2016) 034014 | ||
Abstract: The charge asymmetry in the production of top quark and antiquark pairs is measured in proton-proton collisions at a center-of-mass energy of 8 TeV. The data, corresponding to an integrated luminosity of 19.6 fb$^{-1}$, were collected by the CMS experiment at the LHC. Events with a single isolated electron or muon, and four or more jets, at least one of which is likely to have originated from hadronization of a bottom quark, are selected. A template technique is used to measure the asymmetry in the distribution of differences in the top quark and antiquark absolute rapidities. The measured asymmetry is $A_c^y = $ [ 0.33 $\pm$ 0.26 (stat) $\pm$ 0.33 (syst) ]%, which is the most precise result to date. The results are compared to calculations based on the standard model and on several beyond-the-standard-model scenarios. | ||
Links: e-print arXiv:1508.03862 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
The (left) symmetric $\vec{x}^+$ and (right) antisymmetric $\vec{x}^-$ components of the binned probability distributions in the observable $ {\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} $, constructed using POWHEG [20] with CT10 PDFs [21], for $ {\mathrm {t}\overline {\mathrm {t}}} $ production from $ {{{\mathrm {g}} {\mathrm {g}}}} $, $ {{\mathrm {q}} {\overline {\mathrm {q}}}} $, $ {{{\mathrm {q}} {\mathrm {g}}}} $, and $ {{{\overline {\mathrm {q}}} {\mathrm {g}}}} $ initial states. |
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Figure 1-a:
The symmetric $\vec{x}^+$ components of the binned probability distributions in the observable $ {\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} $, constructed using POWHEG [20] with CT10 PDFs [21], for $ {\mathrm {t}\overline {\mathrm {t}}} $ production from $ {{{\mathrm {g}} {\mathrm {g}}}} $, $ {{\mathrm {q}} {\overline {\mathrm {q}}}} $, $ {{{\mathrm {q}} {\mathrm {g}}}} $, and $ {{{\overline {\mathrm {q}}} {\mathrm {g}}}} $ initial states. |
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Figure 1-b:
The antisymmetric $\vec{x}^-$ components of the binned probability distributions in the observable $ {\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} $, constructed using POWHEG [20] with CT10 PDFs [21], for $ {\mathrm {t}\overline {\mathrm {t}}} $ production from $ {{{\mathrm {g}} {\mathrm {g}}}} $, $ {{\mathrm {q}} {\overline {\mathrm {q}}}} $, $ {{{\mathrm {q}} {\mathrm {g}}}} $, and $ {{{\overline {\mathrm {q}}} {\mathrm {g}}}} $ initial states. |
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Figure 2:
The median value of the logarithm of the ratio of parton energy to measured energy, as a function of measured $ {p_{\mathrm {T}}} $ in three bins of $ | \eta | $, for (left) b jets from top quark decay, (center) jets from W boson decay, and (right) other jets. |
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Figure 2-a:
The median value of the logarithm of the ratio of parton energy to measured energy, as a function of measured $ {p_{\mathrm {T}}} $ in three bins of $ | \eta | $, for b jets from top quark decay. |
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Figure 2-b:
The median value of the logarithm of the ratio of parton energy to measured energy, as a function of measured $ {p_{\mathrm {T}}} $ in three bins of $ | \eta | $, for jets from W boson decay. |
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Figure 2-c:
The median value of the logarithm of the ratio of parton energy to measured energy, as a function of measured $ {p_{\mathrm {T}}} $ in three bins of $ | \eta | $, for other jets. |
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Figure 3:
The conditional probability densities of the CSV b tagging discriminant from simulation for jets from b quarks, jets from W boson decay, and other jets. |
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Figure 4:
The two-dimensional probability density from simulation of jet invariant masses from W boson ($m_{\hat{c}\hat{d}}$) and top quark ($m_{\hat{b}\hat{c}\hat{d}}$) decay is shown (top), along with contours in standard deviations (MSD) of the corresponding Gaussian approximation. Probability densities for correct and incorrect jet assignments (middle) are shown (left) for MSD and (right) for $\sqrt {\chi _a^2}$ of the leptonically decaying top quark reconstruction. The corresponding likelihood ratios are shown below. |
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Figure 4-a:
The two-dimensional probability density from simulation of jet invariant masses from W boson ($m_{\hat{c}\hat{d}}$) and top quark ($m_{\hat{b}\hat{c}\hat{d}}$) decay is shown, along with contours in standard deviations (MSD) of the corresponding Gaussian approximation. |
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Figure 4-b:
Probability densities for correct and incorrect jet assignments are shown for MSD of the leptonically decaying top quark reconstruction. |
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Figure 4-c:
Probability densities for correct and incorrect jet assignments are shown for $\sqrt {\chi _a^2}$ of the leptonically decaying top quark reconstruction. |
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Figure 4-d:
Likelihood ratios for correct and incorrect jet assignments are shown for MSD of the leptonically decaying top quark reconstruction. |
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Figure 4-e:
Likelihood ratios for correct and incorrect jet assignments are shown for $\sqrt {\chi _a^2}$ of the leptonically decaying top quark reconstruction. |
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Figure 5:
The angle $\pi {\Delta} $ of the resultant sum of three vectors spaced at equal angles, in which the magnitude of each is the likelihood of the respective population. The dashed arrows are translations of the $ {\mathrm {e}}^{2i\pi /3}$ and $ {\mathrm {e}}^{-2i\pi /3}$ vectors which illustrate the construction of the sum. The circle is shown to indicate the relative scale. |
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Figure 6:
The probability distribution of the discriminant $ {\Delta} $ for (top left) selected e+jets events and (top right) selected $\mu$+jets events, for the simulated Wj and $ {\mathrm {t}\overline {\mathrm {t}}} $ populations, and for the Mj population, which is modeled from the sideband data with simulated contributions subtracted. The probability distributions in each observable used to construct the discriminant are shown for (middle) e+jets and (bottom) $\mu$+jets channels. The overflow is included in the rightmost bin of the $ {M_\mathrm {T}} $ distributions. |
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Figure 6-a:
The probability distribution of the discriminant $ {\Delta} $ for selected e+jets events, for the simulated Wj and $ {\mathrm {t}\overline {\mathrm {t}}} $ populations, and for the Mj population, which is modeled from the sideband data with simulated contributions subtracted. |
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Figure 6-b:
The probability distribution of the discriminant $ {\Delta} $ for selected $\mu$+jets events, for the simulated Wj and $ {\mathrm {t}\overline {\mathrm {t}}} $ populations, and for the Mj population, which is modeled from the sideband data with simulated contributions subtracted. |
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Figure 6-c:
The probability distribution in an observable used to construct the discriminant are shown for e+jets channel. The overflow is included in the rightmost bin. |
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Figure 6-d:
The probability distribution in an observable used to construct the discriminant are shown for e+jets channel. |
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Figure 6-e:
The probability distribution in an observable used to construct the discriminant are shown for e+jets channel. |
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Figure 6-f:
The probability distribution in an observable used to construct the discriminant are shown for $\mu$+jets channel. The overflow is included in the rightmost bin. |
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Figure 6-g:
The probability distribution in an observable used to construct the discriminant are shown for $\mu$+jets channel. |
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Figure 6-h:
The probability distribution in an observable used to construct the discriminant are shown for $\mu$+jets channel. |
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Figure 7:
The (left) symmetric and (right) antisymmetric components of the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ probability distribution for selected $ {\mathrm {t}\overline {\mathrm {t}}} $ simulation events in the e+jets and $\mu$+jets channels. The vertical bars show the statistical uncertainties, while the horizontal bars display the bin widths. |
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Figure 7-a:
The symmetric component of the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ probability distribution for selected $ {\mathrm {t}\overline {\mathrm {t}}} $ simulation events in the e+jets and $\mu$+jets channels. The vertical bars show the statistical uncertainties, while the horizontal bars display the bin widths. |
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Figure 7-b:
The antisymmetric component of the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ probability distribution for selected $ {\mathrm {t}\overline {\mathrm {t}}} $ simulation events in the e+jets and $\mu$+jets channels. The vertical bars show the statistical uncertainties, while the horizontal bars display the bin widths. |
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Figure 8:
The distribution of the statistical uncertainty in $A_c^y$ from measurements using pseudo-experiments, with an expected value of 0.258%. The statistical uncertainty extracted from the data is marked by the arrow. |
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Figure 9:
The bias in the measured charge asymmetry for SM simulations and alternative $ {\mathrm {t}\overline {\mathrm {t}}} $ models, based on extended POWHEG SM templates, versus the charge asymmetry in each sample. The beyond-SM samples are MadGraph simulations of Z' bosons and axigluons with masses of 200 GeV and 2 TeV. Uncertainty in the bias of the extended POWHEG model is dominated by the number of pseudo-experiments used, while the uncertainty in the bias of each alternative model is dominated by the statistical uncertainty in the sample. The hatched area shows the systematic uncertainty in the measurement of $A_c^y$ from sources related to the modeling, including simulation statistical uncertainty, renormalization and factorization scales, choice of $ {\mathrm {t}\overline {\mathrm {t}}} $ generator, top quark mass, and PDFs. |
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Figure 10:
Sample composition is measured using the discriminant $ {\Delta} $ distribution (top), in a model with contributions from $ {\mathrm {t}\overline {\mathrm {t}}} $, Wj, Mj, and St+DY. With the sample composition subsequently fixed, the amplitude of the antisymmetric $ {\mathrm {t}\overline {\mathrm {t}}} $ contribution is measured in the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distribution, shown decomposed into symmetric (middle) and antisymmetric (bottom) components. The thick line shows the antisymmetric component of the fit model. The measurements are performed independently on the (left) e+jets and (right) $\mu$+jets samples. |
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Figure 10-a:
Sample composition is measured using the discriminant $ {\Delta} $ distribution, in a model with contributions from $ {\mathrm {t}\overline {\mathrm {t}}} $, Wj, Mj, and St+DY. The measurements are performed on the e+jets sample. |
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Figure 10-b:
Sample composition is measured using the discriminant $ {\Delta} $ distribution, in a model with contributions from $ {\mathrm {t}\overline {\mathrm {t}}} $, Wj, Mj, and St+DY. The measurements are performed on the $\mu$+jets sample. |
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Figure 10-c:
With the sample composition subsequently fixed, the amplitude of the antisymmetric $ {\mathrm {t}\overline {\mathrm {t}}} $ contribution is measured in the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distribution, symmetric component. The measurements are performed on the e+jets sample. |
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Figure 10-d:
With the sample composition subsequently fixed, the amplitude of the antisymmetric $ {\mathrm {t}\overline {\mathrm {t}}} $ contribution is measured in the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distribution, symmetric component. The measurements are performed on the $\mu$+jets sample. |
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Figure 10-e:
With the sample composition subsequently fixed, the amplitude of the antisymmetric $ {\mathrm {t}\overline {\mathrm {t}}} $ contribution is measured in the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distribution, antisymmetric component. The thick line shows the antisymmetric component of the fit model. The measurements are performed on the e+jets sample. |
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Figure 10-f:
With the sample composition subsequently fixed, the amplitude of the antisymmetric $ {\mathrm {t}\overline {\mathrm {t}}} $ contribution is measured in the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distribution, antisymmetric component. The thick line shows the antisymmetric component of the fit model. The measurements are performed on the $\mu$+jets sample. |
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Figure 11:
At top, the negative logarithm of the likelihood is shown as a function of $\alpha $ (upper axis) and $A_c^y$ (lower axis), for e+jets (closed circles) and $\mu$+jets (open circles) measurements. The statistical uncertainty in each is given by the intersections of the parabolas with $-\log{L}=$ 0.5, which are marked by arrows. The 68% confidence interval of the combined $A_c^y$ measurement is compared with those of the SM predictions by POWHEG, Kuhn and Rodrigo [8], and Bernreuther and Si [9]. At bottom, the antisymmetric component of the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distributions in data and the model are shown for (left) e+jets and (right) $\mu$+jets, for the central value (solid), and for the upper (dashed) and lower (dotted) limits of the 68% statistical confidence intervals. |
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Figure 11-a:
At top, the negative logarithm of the likelihood is shown as a function of $\alpha $ (upper axis) and $A_c^y$ (lower axis), for e+jets (closed circles) and $\mu$+jets (open circles) measurements. The statistical uncertainty in each is given by the intersections of the parabolas with $-\log{L}=$ 0.5, which are marked by arrows. The 68% confidence interval of the combined $A_c^y$ measurement is compared with those of the SM predictions by POWHEG, Kuhn and Rodrigo [8], and Bernreuther and Si [9]. |
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Figure 11-b:
The antisymmetric component of the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distributions in data and the model are shown for e+jets, for the central value (solid), and for the upper (dashed) and lower (dotted) limits of the 68% statistical confidence intervals. |
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Figure 11-c:
The antisymmetric component of the $ {{\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} ^{\text {rec}}} $ distributions in data and the model are shown for $\mu$+jets, for the central value (solid), and for the upper (dashed) and lower (dotted) limits of the 68% statistical confidence intervals. |
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Figure 12:
Comparison of results from this analysis (template) with those of the CMS 8 TeV unfolding analysis [19], and SM predictions from theoretical calculations of K{u}hn and Rodrigo [8], Bernreuther and Si [9], POWHEG, and mc@nlo. The shaded bands correspond to 68% and 95% confidence intervals of the current measurement. The inner bars on the CMS measurements indicate the statistical uncertainty, the outer bars the statistical and systematic uncertainty added in quadrature. |
Tables | |
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Table 1:
The $ {\mathrm {t}\overline {\mathrm {t}}} $ initial-state fractions and charge asymmetries in the observable $ {\varUpsilon _{{\mathrm {t}\overline {\mathrm {t}}}}} $, calculated with POWHEG using the CT10 PDFs. The statistical uncertainty in the last digits is indicated in parentheses. |
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Table 2:
Uncertainty in the combined measurement of $A_c^y$ from systematic sources, ordered by decreasing magnitude. |
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Table 3:
Results from the fit of the sample composition, in thousands of events, for the e+jets and $\mu$+jets channels. The statistical uncertainty in the last digits is indicated in parentheses. The results of the simultaneous fit in both channels are included only for comparison and are not used in the measurement of $A_c^{y}$. |
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Table 4:
Comparison of charge asymmetry measurements and predictions. |
Summary |
The forward-central $\mathrm{t\bar{t}} $ charge asymmetry in proton-proton collisions at 8 TeV center-of-mass energy has been measured using lepton+jets events from data corresponding to an integrated luminosity of 19.6 fb$^{-1}$ . Novel techniques in top quark reconstruction and background discrimination have been employed, which are likely to be of interest in future analyses. The measurement utilizes a template technique based on a parametrization of the SM. The result, $A_c^y = $ [ 0.33 $\pm$ 0.26 (stat) $\pm$ 0.33 (syst) ]%, is the most precise to date. It is consistent with SM predictions, but does not rule out the alternative models considered. The forward-central $\mathrm{ t \bar{t} }$ charge asymmetry in proton-proton collisions at 8 TeV center-of-mass energy has been measured using lepton+jets events from data corresponding to an integrated luminosity of 19.6 fb$^{-1}$. Novel techniques in top quark reconstruction and background discrimination have been employed, which are likely to be of interest in future analyses. The measurement utilizes a template technique based on a parametrization of the SM. The result, $A^y_c =$ 0.33 $\pm$ 0.26 (stat) $\pm$ 0.33 (syst) %, is the most precise to date. It is consistent with SM predictions, but does not rule out the alternative models considered. |
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Compact Muon Solenoid LHC, CERN |