CMS-TOP-13-015

CMS-TOP-13-015 ; CERN-PH-EP-2015-289
Measurement of spin correlations in $\mathrm{ t \bar{t} }$ production using the matrix element method in the muon+jets final state in pp collisions at $ \sqrt{s} = $ 8 TeV
CMS Collaboration
19 November 2015
Phys. Lett. B 758 (2016) 321
Abstract: The consistency of the spin correlation strength in top quark pair production with the standard model (SM) prediction is tested in the muon+jets final state. The events are selected from pp collisions, collected by the CMS detector, at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The data are compared with the expectation for the spin correlation predicted by the SM and with the expectation of no correlation. Using a template fit method, the fraction of events that show SM spin correlations is measured to be 0.72 $\pm$ 0.08 (stat) $^{+0.15}_{-0.13}$ (syst), representing the most precise measurement of this quantity in the muon+jets final state to date.
Links: e-print arXiv:1511.06170 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $\chi ^{2}$ probability distribution of the selected solutions of the kinematic fit in the $\mu $+jets channel, showing a shape comparison between data and simulation including the statistical uncertainties. The relative contributions in simulation are calculated using the theoretical cross sections with the total yield normalised to data. For the analysis, we only consider events with a probability larger than 0.08, as indicated by the arrow.
png pdf	Figure 2-a: $\Delta E$ distributions based on the values obtained from simulation (circles) compared to the $\Delta E$ distribution obtained by folding the $E_\text {parton}$ spectrum of matched partons with the transfer function (squares) summed over all values of $E_\text {parton}$ and $ {\| \eta _\text {parton} \| }$. The mean and RMS shown on the plots are obtained from simulation. The figure is shown for b quark jets (a) and for light quark jets (b).
png pdf	Figure 2-b: $\Delta E$ distributions based on the values obtained from simulation (circles) compared to the $\Delta E$ distribution obtained by folding the $E_\text {parton}$ spectrum of matched partons with the transfer function (squares) summed over all values of $E_\text {parton}$ and $ {\| \eta _\text {parton} \| }$. The mean and RMS shown on the plots are obtained from simulation. The figure is shown for b quark jets (a) and for light quark jets (b).
png pdf	Figure 3-a: Distribution of $-2\ln\lambda _\text {event}$. The SM $ {\mathrm{ t \bar{t} } } $ simulation is used in the (a) plot and the uncorrelated $ {\mathrm{ t \bar{t} } } $ simulation in the (b) plot. Both data and simulation are normalised to unity. The hatched uncertainty band includes statistical and systematic uncertainties. The error bars in the ratio plot at the bottom only consider statistical uncertainties (of both data and simulation), while the uncertainty band covers both statistical and systematic uncertainties. Systematic uncertainties are described in Section 8. The overlap of the green uncertainty band, which is constructed around the marker position, with the ratio value of 1 indicates agreement between the data and the simulation within the total uncertainty.
png pdf	Figure 3-b: Distribution of $-2\ln\lambda _\text {event}$. The SM $ {\mathrm{ t \bar{t} } } $ simulation is used in the (a) plot and the uncorrelated $ {\mathrm{ t \bar{t} } } $ simulation in the (b) plot. Both data and simulation are normalised to unity. The hatched uncertainty band includes statistical and systematic uncertainties. The error bars in the ratio plot at the bottom only consider statistical uncertainties (of both data and simulation), while the uncertainty band covers both statistical and systematic uncertainties. Systematic uncertainties are described in Section 8. The overlap of the green uncertainty band, which is constructed around the marker position, with the ratio value of 1 indicates agreement between the data and the simulation within the total uncertainty.
png pdf	Figure 4: The expected $-2\ln\lambda _\mathrm {sample}$ distribution estimated using simulation, evaluated at the data sample size. The samples in simulation contain signal and background mixed according to the theoretical cross sections, with the solid Gaussian function using SM $ {\mathrm{ t \bar{t} } } $ simulation and the dashed Gaussian function using uncorrelated $ {\mathrm{ t \bar{t} } } $ simulation. From this figure, the separation power can be assessed in the case when systematic effects are not considered.
png pdf	Figure 5: Result of the template fit to data. The squares represent the data with the statistical uncertainty smaller than the marker size, the dotted curve is the overall result of the fit, the solid curve is the contribution of the SM signal template to the fit, the dashed curve is the contribution of the uncorrelated signal, and the dash-dot curve is the background contribution.
png pdf	Figure 6: The $-2\ln\lambda _\mathrm {sample}$ distribution in simulation, evaluated for the data set size. The samples in simulation contain signal and background mixed according to the theoretical cross sections, with the solid distribution obtained using SM $ {\mathrm{ t \bar{t} } } $ simulation and the dashed distribution obtained using uncorrelated $ {\mathrm{ t \bar{t} } } $ simulation, including systematic uncertainties. The arrow indicates the $-2\ln\lambda _\mathrm {sample}$ observed in data. The dotted curve shows a mixture of 72% SM $ {\mathrm{ t \bar{t} } } $ events and 28% uncorrelated $ {\mathrm{ t \bar{t} } } $ events.

Tables
png pdf	Table 1: Event yield after event selection, with the statistical uncertainties. The contributions from various physics processes are given, with a comparison between the data and the total simulation at the bottom.
png pdf	Table 2: Fit parameters of the 2D calibration function. The residual correlation between the fit parameters is below 10% and is ignored.
png pdf	Table 3: Sources of systematic uncertainty in the fraction $f$ of events with the SM spin correlation. There is no downward variation for the $ {p_{\mathrm {T}}} ^\mathrm {t}$ modeling.

Summary

The hypothesis that $\mathrm{ t \bar{t} }$ events are produced with correlated spins as predicted by the SM is tested using a matrix element method in the $\mu$+jets final state at $ \sqrt{s} = $ 8 TeV, using pp collisions corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The data agree with the uncorrelated hypothesis within 2.9 standard deviations, whereas agreement with the SM is within 2.2 standard deviations. Our hypotheses are only considered up to NLO effects in the simulation, with LO matrix elements in the likelihood calculations.

Using a template fit method, the fraction of events which show SM spin correlations has been extracted. This fraction is measured to be $f =$ 0.72 $\pm$ 0.08 (stat) $^{+0.15}_{-0.13}$ (syst), leading to a spin correlation strength of $A^\text{measured}_\text{hel} =$ 0.23 $\pm$ 0.03 (stat) $^{+0.05}_{-0.04}$ (syst) using the value obtained in simulation which is compatible with the theoretical prediction for $A^\mathrm{SM}_\text{hel}$ from [51,52]. The result is the most precise determination of this quantity in the muon+jets final state to date and is competitive with the most accurate result in the dilepton final state [9].

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