TOP-13-015 Measurement of spin correlations of top-quark pairs using the matrix element method in events with one muon and jets in the final state at √s = 8 TeV

Abstract

The consistency of the top quark pair events produced with standard model (SM) correlated spins is tested using a Matrix Element Method in the muon+jets final state. The events are obtained from pp collisions at a center of mass energy of √s = 8 TeV, corresponding to an integrated luminosity of 19.7 fb-1, collected with the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC). The data agree with the SM hypothesis within 2.2 σ (standard deviation) and agree with the uncorrelated hypothesis within 2.9 σ based on a hypothesis testing procedure. 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 ± 0.09 (stat) +0.15-0.13 (syst), representing the most precise measurement of this quantity in the lepton+jets channel at present.

Public Document

The CMS Physics Analysis Summary is available on CDS.

Figures

Figure 1a: Comparison of Δ E distributions (Eparton - Ejet) based on the values obtained from simulation (black) and based on the determined transfer function distribution W(Eparton,Ejet) at Eparton (red) summed over all Eparton and ηparton for b jets.

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Figure 1b: Comparison of Δ E distributions (Eparton - Ejet) based on the values obtained from simulation (black) and based on the determined transfer function distribution W(Eparton,Ejet) at Eparton (red) summed over all Eparton and ηparton for light jets.

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ChiProbability.png Figure 2: The χ2 probability distribution of the selected solutions of the kinematic fit in the μ+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. The uncertainty bars only consider statistical uncertainties. For the analysis, we will only consider events with a probability larger than 0.08.

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Figure 3a: -2lnλevent distribution in the muon channel. The SM correlated t‾t simulation was used as signal. Both data and simulation are normalised to unity. The bottom plot shows the ratio of data over simulation. The uncertainty bars only consider statistical uncertainties, while the hatched uncertainty band in the top plot and the green band in the ratio plot includes both statistical and systematic uncertainties.

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Figure 3b: -2lnλevent distribution in the muon channel. The uncorrelated t‾t simulation was used as signal. Both data and simulation are normalised to unity. The bottom plot shows the ratio of data over simulation. The uncertainty bars only consider statistical uncertainties, while the hatched uncertainty band in the top plot and the green band in the ratio plot includes both statistical and systematic uncertainties.

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Figure 4: The -2lnλsample distribution is shown in simulation, 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‾t simulation and the red dashed Gaussian using uncorrelated t‾t simulation. The green arrow indicates the -2lnλsample observed in data.

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Figure 5: Result of the template fit to the -2lnλevent distribution to data. The black dots are the data, the blue solid curve is the overall 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 -2lnλsample distribution is shown in simulation, 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‾t simulation and the red dashed Gaussian using uncorrelated t‾t simulation. The green arrow indicates the -2lnλsample observed in data. The magenta dotted curve shows a mixture of 72 % SM correlated t‾t events and 28 % uncorrelated t‾t events, as observed in the template fit.

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Tables

Table 1: Event yield after event selection. The contribution from various physics processes is given with a comparison between the data and the total simulation at the bottom.

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Table 2: Parameters of the two-dimensional calibration function fcalibrated = p0 + p1 f'obs + p2 f'obs β'obs with f'obs = fobs - 0.502 and β'obs = βobs - 0.150. The correlation between the parameters is below 10 %.

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Table 3: Systematic uncertainty contributions to the fraction f of events with SM spin correlation.

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Extra figures

AbsLikelihood.png -lnPH=C distribution of the event likelihoods calculated under the SM hypothesis in the muon channel. The t‾t simulation is the MC@NLO correlated sample. Both data and simulation are normalised to unity. The bottom plot shows the ratio of data over simulation. The uncertainty bars only consider statistical uncertainties, while the hatched uncertainty band in the top plot and the green band in the ratio plot includes both statistical and systematic uncertainties.

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AndreasMeyer - 2015-07-09

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