Mainz 2014 – scientific programme

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T: Fachverband Teilchenphysik

T 112: Top-Quarks: Assoziierte Produktion

T 112.8: Talk

Thursday, March 27, 2014, 18:30–18:45, P102

Measurement of associated production of vector bosons and tt in the dilepton channel within the ATLAS experiment at √s=8 TeV — Kevin Kröninger, Arnulf Quadt, Elizaveta Shabalina, and •Tamara Vázquez Schröder — II. Physikalisches Institut, Georg-August-Universität, Göttingen, Germany

Although the top quark was discovered in 1995, most of its couplings have never been directly measured. In particular the top quark NC weak coupling was so far not accessible. Extensions of the Standard Model could alter the top-quark couplings. A measurement of the production of a top-quark pair in association with vector bosons is a key test of the validity of the SM at the TeV scale. The measurement of tt+V, in particular tt+Z, production cross section is performed in the dilepton channel. The two opposite-sign leptons can originate either from the top quark or from the associated vector boson. Two orthogonal event selections define the signal regions: the first one contains mainly processes where the tt pair decays dileptonically and the vector boson decays hadronically and the second one contains mainly processes where the tt pair decays hadronically and the vector boson (mainly Z boson) leptonically. The event selection and object definitions are chosen as loose as possible in order to increase the signal acceptance. Given the high background fraction in this channel, a multivariate analysis is performed, building a Neural Network response with a minimum set of best discriminating and well-modeled variables.