Measurements of $t\overline{t}$ spin correlations and top quark polarization using dilepton final states in $pp$ collisions at $\sqrt{s}=8\text{ }\text{ }\mathrm{TeV}$

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Measurements of $t \bar{t}$ spin correlations and top quark polarization using dilepton final states in $p p$ collisions at $\sqrt{s} = 8 TeV$

V. Khachatryan et al. (CMS Collaboration)

Phys. Rev. D 93, 052007 – Published 9 March 2016

Abstract

Measurements of the top quark-antiquark ( $t \bar{t}$ ) spin correlations and the top quark polarization are presented for $t \bar{t}$ pairs produced in $p p$ collisions at $\sqrt{s} = 8 TeV$ . The data correspond to an integrated luminosity of $19.5 {fb}^{- 1}$ collected with the CMS detector at the LHC. 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 bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the $t \bar{t}$ system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated.

Received 6 January 2016

DOI:https://doi.org/10.1103/PhysRevD.93.052007

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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Particle interactions

Leptons Top quark

Particles & Fields

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Vol. 93, Iss. 5 — 1 March 2016

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Figure 1
Reconstructed $M_{t \bar{t}}$ , $y_{t \bar{t}}$ , and ${p_{T}}^{t \bar{t}}$ distributions from data (points) and simulation (histogram), with the expected signal ( $t \bar{t} \to ℓ^{+} ℓ^{-}$ ) and background distributions shown separately. All three dilepton flavor combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The last bins of the $M_{t \bar{t}}$ and ${p_{T}}^{t \bar{t}}$ distributions include overflow events. The vertical bars on the data points represent the statistical uncertainties. The lower panels show the ratio of the numbers of events from data and simulation.
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Figure 2
Reconstructed angular distributions from data (points) and simulation (histogram), with the expected signal ( $t \bar{t} \to ℓ^{+} ℓ^{-}$ ) and background distributions shown separately. All three dilepton flavor combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The vertical bars on the data points represent the statistical uncertainties. The lower panels show the ratio of the numbers of events from data and simulation.
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Figure 3
Normalized differential cross section as a function of $| Δ ϕ_{ℓ^{+} ℓ^{-}} |$ , $\cos φ$ , $\cos θ_{ℓ^{+}}^{⋆} \cos θ_{ℓ^{-}}^{⋆}$ , and $\cos θ_{ℓ}^{⋆}$ from data (points); parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at $NLO$ [4, 63] with (SM) and without (no spin corr.) spin correlations (solid and dotted histograms, respectively). For the $\cos θ_{ℓ}^{⋆}$ distribution, $C P$ conservation is assumed in the combination of the $\cos θ_{ℓ^{\pm}}^{⋆}$ measurements from positively and negatively charged leptons. The ratio of the data to the mc@nlo prediction is shown in the lower panels. The inner and outer vertical bars on the data points represent the statistical and total uncertainties, respectively. The hatched bands represent variations of $μ_{R}$ and $μ_{F}$ simultaneously up and down by a factor of 2.
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Figure 4
Dependence of the four asymmetry variables from data (points) on $M_{t \bar{t}}$ (left), $| y_{t \bar{t}} |$ (middle), and ${p_{T}}^{t \bar{t}}$ (right), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at $NLO$ [4, 63] with (SM) and without (no spin corr.) spin correlations (solid and dotted histograms, respectively). The inner and outer vertical bars on the data points represent the statistical and total uncertainties, respectively. The hatched bands represent variations of $μ_{R}$ and $μ_{F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events.
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Figure 5
Top : theoretical prediction from Ref. [4] (points) and polynomial parametrization (line) for the contribution from new physics with a nonzero CMDM to the normalized differential cross section $(1 / σ) (d σ / d | Δ ϕ_{ℓ^{+} ℓ^{-}} |)$ , for $Re ({\hat{μ}}_{t}) ≪ 1$ . Bottom : normalized differential cross section from data (points). The solid line corresponds to the result of the fit to the form given in Eq. (3), and the dashed lines show the parametrized SM $NLO$ predictions for $μ_{R}$ and $μ_{F}$ equal to $m_{t}$ , $2 m_{t}$ , and $m_{t} / 2$ . The vertical bars on the data points represent the total uncertainties.
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Physical Review D

covering particles, fields, gravitation, and cosmology

Measurements of $t \bar{t}$ spin correlations and top quark polarization using dilepton final states in $p p$ collisions at $\sqrt{s} = 8 TeV$

V. Khachatryan et al. (CMS Collaboration)

Phys. Rev. D 93, 052007 – Published 9 March 2016

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Physical Review D

covering particles, fields, gravitation, and cosmology

Measurements of tt¯ spin correlations and top quark polarization using dilepton final states in pp collisions at s=8 TeV

V. Khachatryan et al. (CMS Collaboration)

Phys. Rev. D 93, 052007 – Published 9 March 2016

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Measurements of $t \bar{t}$ spin correlations and top quark polarization using dilepton final states in $p p$ collisions at $\sqrt{s} = 8 TeV$