Measurement of spin correlation in dileptonic top quark pair decays with the CMS experiment

Résumé fourni par la source

The Large Hadron Collider close to Geneva is the world must powerful particle accelerator with a center of mass energy of \sqrt{s} = 7 TeV and providing instantaneous luminosityof up to L = 4.07·1033 cm^−2 s^−1 during 2011. The CMS experiment recorded during this run period a dataset of an integrated luminosity of Lint= 5.55 fb^−1 corresponding to 850,000 top quark pair events. These are predominantly produced by gluon-gluon fusion preferring top quark pairs with same spin. Therefore, the spin directions of top quarkpairs are correlated and the asymmetry between the number of top quark pairs with same and not same spin is not vanishing. The top quarks decay before hadronization takes place due to their high mass. Hence, the kinematics of their decay products are influenced by their helicity and the initial spin asymmetry is accessible via their decayproducts. The Standard Model predicts an asymmetry of A= 0.31 for proton-proton collisions at \sqrt{s} = 7TeV. The difference of the azimuth, ∆φ, of the two leptons in dileptonic top quark pair decays is sensitive to spin correlations and provides access to the asymmetry. This thesis presents a measurement of the asymmetry A and tests its Standard Model prediction. The 2011 dataset of proton-proton collisions is investigated. The data contains validated collisions corresponding to an integrated luminosity of 5.0 fb^−1. The asymmetry is extracted from top quark pair events, which decay electroweak into electrons and muons as signal signature. The signal event yield is enriched by an optimized selection with a final set of 11,638 candidate signatures. Event yields and differential distributions of background processes are modeled by Monte Carlo methods and normalized by data-driven methods. The obtained ∆φ distribution of the selected events is investigated by a template fit, which quantifies the similarity to Standard Model expectation and a model of vanishing spin correlation. Finally, the influence of several systematic uncertainties on the measurement is studied. An asymmetry ofA = 0.24 ± 0.03(stat.) ± 0.07(sys.) is measured in data, which is compatible with the Standard Model expectation within the given uncertainties.

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