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.
The CMS Physics Analysis Summary is available on CDS.
Figure 1a: Comparison of Δ 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} (red) summed over all E_{parton} and η_{parton} for b jets. PDF  PNG 

Figure 1b: Comparison of Δ 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} (red) summed over all E_{parton} and η_{parton} for light jets. PDF  PNG 

Figure 2: The χ^{2} 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. PDF  PNG 

Figure 3a: 2lnλ_{event} 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. PDF  PNG 

Figure 3b: 2lnλ_{event} 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. PDF  PNG 

Figure 4: The 2lnλ_{sample} 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 2lnλ_{sample} observed in data. PDF  PNG 

Figure 5: Result of the template fit to the 2lnλ_{event} 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 dashdot curve is the background contribution. PDF  PNG 

Figure 6: The 2lnλ_{sample} 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 2lnλ_{sample} 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. PDF  PNG 
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. PDF  PNG 

Table 2: Parameters of the twodimensional calibration function f_{calibrated} = p_{0} + p_{1} f'_{obs} + p_{2} f'_{obs} β'_{obs} with f'_{obs} = f_{obs}  0.502 and β'_{obs} = β_{obs}  0.150. The correlation between the parameters is below 10 %. PDF  PNG 

Table 3: Systematic uncertainty contributions to the fraction f of events with SM spin correlation. PDF  PNG 
lnP_{H=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. PDF  PNG 
AndreasMeyer  20150709