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| CMS-TOP-14-023 ; CERN-PH-EP-2015-333 | ||
| Measurements of $\mathrm{ t \bar{t} }$ spin correlations and top quark polarization using dilepton final states in pp collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 6 January 2016 | ||
| Phys. Rev. D 93 (2016) 052007 | ||
| Abstract: Measurements of the top quark-antiquark ($\mathrm{ t \bar{t} }$) spin correlations and the top quark polarization are presented for $\mathrm{ t \bar{t} }$ pairs produced in pp 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 $\mathrm{ t \bar{t} }$ 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. | ||
| Links: e-print arXiv:1601.01107 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1-a:
Reconstructed $M_{ {\mathrm{ t \bar{t} } } }$, $y_{ {\mathrm{ t \bar{t} } } }$, and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) 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 $M_{ {\mathrm{ t \bar{t} } } }$ and $ {{p_{\mathrm {T}}}^{ {\mathrm{ 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 1-b:
Reconstructed $M_{ {\mathrm{ t \bar{t} } } }$, $y_{ {\mathrm{ t \bar{t} } } }$, and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) 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 $M_{ {\mathrm{ t \bar{t} } } }$ and $ {{p_{\mathrm {T}}}^{ {\mathrm{ 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 1-c:
Reconstructed $M_{ {\mathrm{ t \bar{t} } } }$, $y_{ {\mathrm{ t \bar{t} } } }$, and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) 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 $M_{ {\mathrm{ t \bar{t} } } }$ and $ {{p_{\mathrm {T}}}^{ {\mathrm{ 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-a:
Reconstructed angular distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) and background distributions shown separately. All three dilepton flavor combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The vertical bars on the data points represent the statistical uncertainties. The lower panels show the ratio of the numbers of events from data and simulation. |
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Figure 2-b:
Reconstructed angular distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) and background distributions shown separately. All three dilepton flavor combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The vertical bars on the data points represent the statistical uncertainties. The lower panels show the ratio of the numbers of events from data and simulation. |
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Figure 2-c:
Reconstructed angular distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) and background distributions shown separately. All three dilepton flavor combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The vertical bars on the data points represent the statistical uncertainties. The lower panels show the ratio of the numbers of events from data and simulation. |
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Figure 2-d:
Reconstructed angular distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) and background distributions shown separately. All three dilepton flavor combinations are included. The simulated signal yield is normalized to that of the background-subtracted data. The vertical bars on the data points represent the statistical uncertainties. The lower panels show the ratio of the numbers of events from data and simulation. |
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Figure 2-e:
Reconstructed angular distributions from data (points) and simulation (histogram), with the expected signal ($ { {\mathrm{ t \bar{t} } } \to \ell ^{+}\ell ^{-}} $) 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-a:
Normalized differential cross section as a function of $ {| \Delta \phi _{\ell ^+\ell ^-} | }$, $\cos\varphi $, $\cos\theta ^{\star }_{\ell ^+} \cos\theta ^{\star }_{\ell ^-}$, and $\cos\theta ^{\star }_\ell $ from data (points); parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] with (SM) and without (no spin corr.) spin correlations (solid and dotted histograms, respectively). For the $\cos\theta ^{\star }_\ell $ distribution, CP conservation is assumed in the combination of the $\cos\theta ^{\star }_{\ell ^\pm }$ 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. |
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Figure 3-b:
Normalized differential cross section as a function of $ {| \Delta \phi _{\ell ^+\ell ^-} | }$, $\cos\varphi $, $\cos\theta ^{\star }_{\ell ^+} \cos\theta ^{\star }_{\ell ^-}$, and $\cos\theta ^{\star }_\ell $ from data (points); parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] with (SM) and without (no spin corr.) spin correlations (solid and dotted histograms, respectively). For the $\cos\theta ^{\star }_\ell $ distribution, CP conservation is assumed in the combination of the $\cos\theta ^{\star }_{\ell ^\pm }$ 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. |
png pdf |
Figure 3-c:
Normalized differential cross section as a function of $ {| \Delta \phi _{\ell ^+\ell ^-} | }$, $\cos\varphi $, $\cos\theta ^{\star }_{\ell ^+} \cos\theta ^{\star }_{\ell ^-}$, and $\cos\theta ^{\star }_\ell $ from data (points); parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] with (SM) and without (no spin corr.) spin correlations (solid and dotted histograms, respectively). For the $\cos\theta ^{\star }_\ell $ distribution, CP conservation is assumed in the combination of the $\cos\theta ^{\star }_{\ell ^\pm }$ 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. |
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Figure 3-d:
Normalized differential cross section as a function of $ {| \Delta \phi _{\ell ^+\ell ^-} | }$, $\cos\varphi $, $\cos\theta ^{\star }_{\ell ^+} \cos\theta ^{\star }_{\ell ^-}$, and $\cos\theta ^{\star }_\ell $ from data (points); parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] with (SM) and without (no spin corr.) spin correlations (solid and dotted histograms, respectively). For the $\cos\theta ^{\star }_\ell $ distribution, CP conservation is assumed in the combination of the $\cos\theta ^{\star }_{\ell ^\pm }$ 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. |
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Figure 4-a:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
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Figure 4-b:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
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Figure 4-c:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-d:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-e:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-f:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
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Figure 4-g:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-h:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-i:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-j:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-k:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
png pdf |
Figure 4-l:
Dependence of the four asymmetry variables from data (points) on $M_{ {\mathrm{ t \bar{t} } } }$ (a), $ {| y_{ {\mathrm{ t \bar{t} } } } | }$ (middle), and $ {{p_{\mathrm {T}}}^{ {\mathrm{ t \bar{t} } } }} $ (b), obtained from the unfolded double-differential distributions; parton-level predictions from mc@nlo (dashed histograms); and theoretical predictions at NLO [63,4] 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ simultaneously up and down by a factor of 2. The last bin of each plot includes overflow events. |
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Figure 5-a:
a : 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 $(1/\sigma )( {\mathrm {d}}\sigma / {\mathrm {d}} {| \Delta \phi _{\ell ^+\ell ^-} | })$, for $\mathrm{Re} (\hat{\mu }_{\mathrm{ t } }) \ll 1$. b : 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ equal to $m_{\mathrm{ t } }$, $2m_{\mathrm{ t } }$, and $m_{\mathrm{ t } }/2$. The vertical bars on the data points represent the total uncertainties. |
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Figure 5-b:
a : 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 $(1/\sigma )( {\mathrm {d}}\sigma / {\mathrm {d}} {| \Delta \phi _{\ell ^+\ell ^-} | })$, for $\mathrm{Re} (\hat{\mu }_{\mathrm{ t } }) \ll 1$. b : 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 $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ equal to $m_{\mathrm{ t } }$, $2m_{\mathrm{ t } }$, and $m_{\mathrm{ t } }/2$. The vertical bars on the data points represent the total uncertainties. |
| Tables | |
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Table 1:
Descriptions of the various control regions, their intended background process, and the scale factors derived from them, including either the statistical and systematic uncertainties or the total uncertainty. The last row gives the scale factor used for all the remaining backgrounds, whose contributions are estimated from simulation alone. |
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Table 2:
Predicted background and observed event yields, with their statistical uncertainties, after applying the event selection criteria and normalization described in the text. |
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Table 3:
Values of the uncorrected inclusive asymmetry variables from simulation and data, prior to background subtraction. The uncertainties shown are statistical. |
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Table 4:
Sources and values of the systematic uncertainties in the inclusive asymmetry variables. |
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Table 5:
Inclusive asymmetry measurements obtained from the angular distributions unfolded to the parton level, and the parton-level predictions from the mc@nlo simulation and from NLO calculations with (SM) and without (no spin corr.) spin correlations [63,4]. For the data, the first uncertainty is statistical and the second is systematic. For the mc@nlo results and NLO calculations, the uncertainties are statistical and theoretical, respectively. |
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Table 6:
Values of $f_{\mathrm {SM}}$, the strength of the measured spin correlations relative to the SM prediction, derived from the numbers in Table {tab:ResultsUnfolded}. The last row shows an additional measurement of $f_{\mathrm {SM}}$ made from the projection in $ {| \Delta \phi _{\ell ^+\ell ^-} | }$ of the normalized double-differential cross section as a function of $ {| \Delta \phi _{\ell ^+\ell ^-} | }$ and $M_{ {\mathrm{ t \bar{t} } } }$. The uncertainties shown are statistical, systematic, and theoretical, respectively. The total uncertainty in each result, found by adding the individual uncertainties in quadrature, is shown in the last column. |
| Summary |
|
Measurements of the $\mathrm{ t \bar{t} }$ spin correlations and the top quark polarization have been presented in the $\mathrm{ t \bar{t} }$ dilepton final states (${\mathrm{ e }^-\mathrm{ e }^+} $, ${\mathrm{ e }^\pm \mu^\mp} $, and ${\mu^+ \mu^-} $), using angular distributions unfolded to the parton level and as a function of the $\mathrm{ t \bar{t} } $-system variables $M_{\mathrm{ t \bar{t} }}$, $\abs{y_{\mathrm{ t \bar{t} }}}$, and ${{p_{\mathrm{T}}}^{\mathrm{ t \bar{t} }}} $. The data sample corresponds to an integrated luminosity of 19.5 fb$^{-1}$ from pp collisions at $\sqrt{s}= $ 8 TeV, collected by the CMS experiment at the LHC. For the spin correlation coefficients, we measure $C_{\mathrm{hel}} =$ 0.278 $\pm$ 0.084 and $D =$ 0.205 $\pm$ 0.031. The measurements sensitive to spin correlations are translated into determinations of $f_{\mathrm{SM}}$, the strength of the spin correlations relative to the SM prediction. The most precise result comes from the measurement of $A_{\Delta\phi} =$ 0.095 $\pm$ 0.006 (stat) $\pm$ 0.007 (syst), yielding $f_{\mathrm{SM}} =$ 1.12$^{+\:0.12}_{-\:0.15}$. The SM (CP-conserving) top quark polarization is measured to be $P = -0.022 \pm 0.058 $, while the CP-violating component is found to be $P^{\mathrm{CPV}} = 0.000 \pm 0.016 $. All measurements are in agreement with the SM expectations, and help constrain theories of physics beyond the SM. The measured top quark spin observables are compared to theoretical predictions in order to search for hypothetical top quark anomalous couplings. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment $\mathrm{Re} ({\mu}_{\mathrm{t}})$ and the imaginary part of the chromo-electric dipole moment $\mathrm{Im} ({d}_{\mathrm{t}})$ are evaluated. Values outside the intervals $-0.053<\mathrm{Re} ({\mu}_{\mathrm{t}})< $ 0.026 and $-0.068< \mathrm{Im} ({d}_{\mathrm{t}}) < $ 0.067 are excluded at the 95% confidence level, the first such measurements to date. |
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