Measurement of Spin Correlations in Top/Anti-Top Events from pp Collisions at sqrt(s) = 7 TeV in the Lepton+Jets Final State with the ATLAS Detector

Abstract

The top quark decays before it hadronises. Before its spin state can be changed in a process of strong interaction, it is directly transferred to the top quark decay products. The top quark spin can be deduced by studying angular distributions of the decay products. The Standard Model predicts the top/anti-top quark (tt) pairs to have correlated spins. The degree is sensitive to the spin and the production mechanisms of the top quark. Measuring the spin correlation allows to test the predictions. New physics e ects can be reflected in deviations from the prediction. In this thesis the spin correlation of tt pairs, produced at a centre-of-mass energy of sqrt(s) = 7 TeV and reconstructed with the ATLAS detector, is measured. The dataset corresponds to an integrated luminosity of 4.6 fb-1. tt pairs are reconstructed in the lepton+jets channel using a kinematic likelihood fit offering the identi cation of light up- and down-type quarks from the t->bQ->bqq decay. The spin correlation is measured via the distribution of the azimuthal angle between two top quark spin analyzers in the laboratory frame. It is expressed as the degree of tt spin correlation predicted by the Standard Model, fSM. The results of fSM(DeltaPhi(charged lepton, down-type quark)) = 1.53 ± 0.14 (stat.) ± 0:32 (syst.); fSM(DeltaPhi(charged lepton, b-quark)) = 0.53 ± 0.18 (stat.) ± 0.49 (syst.); fSM(DeltaPhi(combined)) = 1.12 ± 0.11 (stat.) ± 0.22 (syst.); are consistent with the Standard Model prediction of fSM = 1.0.