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| CMS-PAS-TOP-13-015 | ||
| Measurement of spin correlations in top pair events in the lepton+jets channel with the matrix element method at 8 TeV | ||
| CMS Collaboration | ||
| June 2015 | ||
| Abstract: The consistency of the top quark pair events produced with standard model (SM) correlated spins is tested in the muon+jets final state. The events are obtained from pp collisions at a center of mass energy of $\sqrt{s} =$ 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$, collected with the CMS detector at the CERN LHC. The data agree with the SM hypothesis within 2.2$\sigma$ (standard deviation) and agree with the uncorrelated hypothesis within 2.9$\sigma$ based on a hypothesis testing procedure. In addition, using a template fit method, the fraction of events which show SM spin correlations $f$, is extracted. This fraction is measured to be $f =$ 0.72 $\pm$ 0.09 (stat) $^{+0.15}_{-0.13}$ (syst), representing the most precise measurement of this quantity in the lepton+jets channel at present. | ||
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Links:
CDS record (PDF) ;
Public twiki page ;
CADI line (restricted) ; Figures are also available from the CDS record. These preliminary results are superseded in this paper, PLB 758 (2016) 321. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1-a:
Comparison of $\Delta E$ distributions ($E_{parton}-E_{jet}$) based on the values obtained from simulation (black) and based on the determined transfer function distribution W($E_{parton}$,$E_{jet}$) at $E_{parton}$ and unconstrained $E_{jet}$ (red) summed over all $E_{parton}$ and $|\eta _{parton}|$. |
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Figure 1-b:
Comparison of $\Delta E$ distributions ($E_{parton}-E_{jet}$) based on the values obtained from simulation (black) and based on the determined transfer function distribution W($E_{parton}$,$E_{jet}$) at $E_{parton}$ and unconstrained $E_{jet}$ (red) summed over all $E_{parton}$ and $|\eta _{parton}|$. |
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Figure 2:
The $\chi ^{2}$ probability distribution of the selected solutions of the kinematic fit in the $\mu $+jets channel, showing a comparison between data and simulation. The relative contributions in simulation are calculated using the theoretical cross sections with the simulation normalised to data. For the analysis, we only consider events with a probability larger than 0.08. |
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Figure 3-a:
$-2 \ln\lambda _{event}$ distribution in the muon channel. In (a) the SM correlated $t\overline {t}$ simulation was used and in (b) the uncorrelated $t\overline {t}$ simulation. Both data and simulation are normalised to unity. The hatched uncertainty band includes statistical and systematic uncertainties. The uncertainty bars in the ratio plot at the bottom only consider statistical uncertainties, while the uncertainty band covers both statistical and systematic uncertainties. |
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Figure 3-b:
$-2 \ln\lambda _{event}$ distribution in the muon channel. In (a) the SM correlated $t\overline {t}$ simulation was used and in (b) the uncorrelated $t\overline {t}$ simulation. Both data and simulation are normalised to unity. The hatched uncertainty band includes statistical and systematic uncertainties. The uncertainty bars in the ratio plot at the bottom only consider statistical uncertainties, while the uncertainty band covers both statistical and systematic uncertainties. |
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Figure 4:
The $-2\ln\lambda _{sample}$ distributions obtained using simulated events are shown, evaluated at the data sample size in the muon channel. The samples in simulation contain signal and background mixed according to the theoretical cross sections, with the blue solid Gaussian using SM correlated $t\overline {t}$ simulation and the red dashed Gaussian using uncorrelated $t\overline {t}$ simulation. The green arrow indicates the $-2\ln\lambda _{sample}$ observed in data. |
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Figure 5:
Result of the template fit to data. The black dots are the data, the blue solid curve is the overal result of the fit, the red dashed curve is the contribution of the SM signal template to the fit, the green dotted curve is the contribution of the uncorrelated signal, while the orange dash-dot curve is the background contribution. |
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Figure 6:
The $-2\ln\lambda _{sample}$ distributions using simulated events, evaluated at the data sample size in the muon channel. The samples in simulation contain signal and background mixed according to the theoretical cross sections, with the blue solid distribution obtained using SM correlated $t\overline {t}$ simulation and the red dashed distribution obtained using uncorrelated $t\overline {t}$ simulation including systematic uncertainties. The green arrow indicates the $-2\ln\lambda _{sample}$ observed in data. The magenta dotted curve shows a mixture of 72 % SM correlated $t\overline {t}$ events and 28 % uncorrelated $t\overline {t}$ events. |
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Compact Muon Solenoid LHC, CERN |
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