Measurement of Spin Correlations in $t\overline{t}$ Systems in the muon+jets Channel using a Matrix Element Method with the CMS Detector at the LHC.

In this work, the consistency of the spin correlation strength in top quark pair events with the Standard Model (SM) prediction is tested in the muon+jets final state. The events are obtained from $\mathrm{pp}$ collisions at a centre-of-mass energy of $\sqrt{s}\,=$ 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. Using a Matrix Element Method, event likelihoods are calculated under two different top quark pair hypotheses: the SM hypothesis and a hypothesis which predicts zero spin correlation. Using the event likelihoods, a variable sensitive to the spin correlation strength is constructed. In a hypothesis testing procedure, the data are found to favour the SM hypothesis. Using a template fit method, the fraction of events that show SM spin correlations is measured to be $f\,=\,0.72\,\pm\,0.08\,\mathrm{(stat)}{}^{+0.15}_{-0.13}\,\mathrm{(syst)}$, representing the most precise measurement of this quantity in the lepton+jets channel to date.