TOP-14-023 Measurements of ttbar Spin Correlations and Top-Quark Polarization Using Dilepton Final States in pp Collisions at √s = 8 TeV

Abstract

Spin correlations and polarization are measured in top quark-antiquark pairs produced in pp collisions at the LHC at √s = 8 TeV. The data correspond to an integrated luminosity of 19.5 fb-1 collected with the CMS detector. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a b quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, unfolded to the parton level, both inclusively and differentially with respect to the invariant mass, rapidity, and transverse momentum of the top-antitop system. All measurements are found to be in agreement with predictions of the standard model of particle physics. A search for new physics in the form of a top quark chromo-magnetic anomalous coupling is performed, and the measured value of the real part of the chomo-magnetic coupling Re(μt) is found to be -0.013 0.032 with a corresponding 95% confidence interval of -0.050 < Re(μt ) < 0.076.

Results

The measured values of the asymmetries are summarized in Table 5. Correlation coefficient values obtained from the measured asymmetries are Chel = 0.278 0.084 and D = 0.205 0.035. The top quark polarization is found to be P = −0.022 0.059. All measurements are in agreement with the standard model expectations, and help constrain theories beyond the standard model.

In addition to the results summarized in Table 5, we extract fSM (fraction of events with SM-like spin correlations) from the two-dimensional differential cross section in Δφ and Mtt, because binning the acceptance correction in Mtt results in a significant reduction in the uncertainty from top quark pT modelling. The result is fSM = 1.16 0.15.

Public Document

The CMS Physics Analysis Summary is available on CDS.

Figures

Figure 1a: The reconstructed Mtt distributions from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The first and last bins include underflow and overflow events, respectively. The error bars on the data points represent the statistical uncertainties.

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Figure 1b: The reconstructed pTtt distributions from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The first and last bins include underflow and overflow events, respectively. The error bars on the data points represent the statistical uncertainties.

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Figure 1c: The reconstructed ytt, distributions from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The first and last bins include underflow and overflow events, respectively. The error bars on the data points represent the statistical uncertainties.

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Figure 2a: The reconstructed distribution of ∆φl+l- from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data.The error bars on the data points represent the statistical uncertainties.

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Figure 2b: The reconstructed distribution of cosθ*l+ from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data.The error bars on the data points represent the statistical uncertainties.

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Figure 2c: The reconstructed distribution of cosθ*l- from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data.The error bars on the data points represent the statistical uncertainties.

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Figure 2d: The reconstructed distribution of cosθ*l+*cosθ*l- from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data.The error bars on the data points represent the statistical uncertainties.

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Figure 2e: The reconstructed distribution of cos φ from data (points) and simulation (histogram). All the dilepton flavour combinations are included. The simulated signal yield is normalized to that of the background-subtracted data.The error bars on the data points represent the statistical uncertainties.

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Figure 3a: Background-subtracted and unfolded differential measurement of ∆φl+l- normalized to unit area (points), parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The ratio of the measured bins with the MC@NLO prediction is shown in the bottom panel, and the statistical and systematic uncertainties are represented by the error bars and the hatched band, respectively.

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Figure 3b: Background-subtracted and unfolded differential measurement of cosθ*l normalized to unit area (points), parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histogram). The ratio of the measured bins with the MC@NLO prediction is shown in the bottom panel, and the statistical and systematic uncertainties are represented by the error bars and the hatched band, respectively.

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Figure 3c: Background-subtracted and unfolded differential measurement of cosθ*l+*cosθ*l- normalized to unit area (points), parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The ratio of the measured bins with the MC@NLO prediction is shown in the bottom panel, and the statistical and systematic uncertainties are represented by the error bars and the hatched band, respectively.

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Figure 3d: Background-subtracted and unfolded differential measurement of cos φ normalized to unit area (points), parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The ratio of the measured bins with the MC@NLO prediction is shown in the bottom panel, and the statistical and systematic uncertainties are represented by the error bars and the hatched band, respectively.

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Figure 4a: Dependence of the asymmetry variable AΔφ obtained from the unfolded distributions (points) on Mtt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4b: Dependence of the asymmetry variable AΔφ obtained from the unfolded distributions (points) on ytt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4c: Dependence of the asymmetry variable AΔφ obtained from the unfolded distributions (points) on pTtt and parton-level predictions from MC@NLO (red histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The last bin of each plot includes overflow events.

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Figure 4d: Dependence of the asymmetry variable AP obtained from the unfolded distributions (points) on Mtt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histogram). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4e: Dependence of the asymmetry variable AP obtained from the unfolded distributions (points) on ytt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histogram). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4f: Dependence of the asymmetry variable AP obtained from the unfolded distributions (points) on pTtt and parton-level predictions from MC@NLO (red histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The last bin of each plot includes overflow events.

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Figure 4g: Dependence of the asymmetry variable Ac1c2 obtained from the unfolded distributions (points) on Mtt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4h: Dependence of the asymmetry variable Ac1c2 obtained from the unfolded distributions (points) on ytt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4i: Dependence of the asymmetry variable Ac1c2 obtained from the unfolded distributions (points) on pTtt and parton-level predictions from MC@NLO (red histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The last bin of each plot includes overflow events.

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Figure 4j: Dependence of the asymmetry variable Acos φ obtained from the unfolded distributions (points) on Mtt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4k: Dependence of the asymmetry variable Acos φ obtained from the unfolded distributions (points) on ytt, parton-level predictions from MC@NLO (red histograms), and theoretical predictions at NLO+EW (blue histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The theoretical factorization and renormalization scale uncertainties are represented by the pale blue hatched band. The last bin of each plot includes overflow events.

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Figure 4l: Dependence of the asymmetry variable Acos φ obtained from the unfolded distributions (points) on pTtt and parton-level predictions from MC@NLO (red histograms). The statistical and systematic uncertainties of the measured bins are represented by the error bars and the grey hatched band, respectively. The last bin of each plot includes overflow events.

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Figure 5: Theoretically calculated contribution from new physics with a non-zero CMDM to (1/σ)(dσ/dΔφl+l-) at the LHC (8 TeV), for Re(μt ) = 1. A negative cross section arises from a negative interference term with SM tt production.

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Figure 5: (1/σ)(dσ/d∆φl+l-) differential cross section. The red line corresponds to the result of the fit, and the blue lines show the SM NLO+EW predictions for renormalization and factorization scales of mtop, 2mtop and mtop/2.

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Tables

Table 1: Summary of the scale factors derived in control regions and applied to the background samples.

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Table 2: The predicted background and observed event yields after applying the event selection criteria and normalization described in the text. Uncertainties are statistical only.

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Table 3: Reconstructed uncorrected inclusive asymmetry variables. AP+ (AP−) corresponds to the asymmetry of the distribution of positively (negatively) charged leptons. The errors represent the statistical uncertainties.

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Table 4: Systematic uncertainties on the inclusive asymmetry variables.

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Table 5: The inclusive asymmetry measurements obtained from the angular distributions unfolded to the parton level and parton-level predictions from the MC@NLO simulation and from NLO+EW calculations. For the data, the first uncertainty is statistical and the second is systematic. For the simulated results, the uncertainties are statistical. The uncertainties in the NLO+EW calculations come from varying the factorization and renormalization scales up and down by a factor of two, which for Ac1c2 and Acos φ are summed in quadrature with the difference between the expanded and unexpanded predictions.

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