Measurement of the Top Quark Polarization in the Top Quark Pair Production using the Dilepton Final State Events at the Tevatron

Abstract

In this thesis, we present a measurement of the top quark polarization in the $t\bar{t}$ pair production in $p\bar{p}$ collisions at the Fermilab Tevatron collider at $\sqrt{s}$=1.96 TeV. We use the full Run II data sample corresponding to 9.1 fb$^{-1}$ of integrated luminosity collected with the CDF detector. We select the _x000C_final state events containing two high transverse momentum leptons (electron or muon), two jets and large missing E$_{t}$.<br/> We measure the top quark polarization through the two dimensional distribution of lepton angles along the spin quantization axis. In order to reconstruct the angular distribution of top quark decay products, we perform full kinematic reconstruction using predicted distributions of P$_{z}^{t\bar{t}}$, P$_{T}^{t\bar{t}}$ and M$_{t\bar{t}}$. We make signal templates using NLO powheg Monte Carlo simulation and background templates from admixture of Monte Carlo simulations and data-based background modellings. Then, we perform binned likelihood _x000C_t of two dimensional angular distribution of data to signal and background templates and obtain a measurement of the top quark polarization.<br/> We consider two di_x000B_erent axes for the top quark polarization measurement: the helicity axis, and the transverse axis. Two measurements of _x000B_$\alpha$P in each basis, the product of the leptonic spin analyzing power and the top quark polarization, are performed assuming that the polarization is introduced by either a CP conserving (CPC) or a CP violating (CPV) production process. We measure the top quark polarizations which are consistent with standard model predictions of negligible polarization. of<br/> <br/> $\alpha P_{hel}^{CPC}$ $= -0.130 ~_{-0.109}^{+0.114}(stat.) ~\pm 0.111(syst.)$ <br/> $\alpha P_{hel}^{CPV}$ $= -0.046 ~\pm 0.123(stat.) ~\pm 0.040(syst.)$ <br/> $\alpha P_{trans}^{CPC}$ $= -0.077 ~\pm 0.177(stat.) ~\pm 0.098(syst.)$ <br/> $\alpha P_{trans}^{CPV}$ $= -0.111 ~_{-0.145}^{+0.146}(stat.) ~_{-0.056}^{+0.055}(syst.)$<br/> <br/> which are consistent with standard model predictions of negligible polarization.