Top Quark Spin Correlations with the CMS Detector

This dissertation presents precision measurements of top quark polarizations and top quark pair spin correlations, which probe the independent coefficients of the top-spin components of the top quark pair production density matrix, targeting all channels (ee, eµ,  µµ) of the top quark pair dileptonic decay mode with final states containing two oppositely charged leptons, and using 137.7 fb⁻¹ of data recorded by the CMS experiment at the LHC with 13 TeV center-of-mass energy, during 2016, 2017, and 2018.

All measured observables are corrected for detector efficiencies, acceptances, and migrations, unfolded to parton-level, and extrapolated to the full phase space using a regularized unfolding procedure.

Spin-density coefficients are extracted from the unfolded distributions and compared to theoretical predictions and predictions from Monte Carlo simulations with next-to-leading-order matrix element accuracy interfaced with parton-shower algorithms.

The measurements are performed both in the full phase-space and differentially as a function of top quark pair invariant mass.

The measured coefficients showed decent agreement with the MC predictions, and better agreement when compared to QCD perturbative calculations for top quark pair production at NLO with electroweak corrections, and the measurement precision for one-dimensional normalized unfolded cross-sections and extracted coefficients were improved by as much as a factor of two compared to previous measurements.