Würzburg 2018 – scientific programme

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T: Fachverband Teilchenphysik

T 73: Top-Quarks: Eigenschaften und Zerfälle III

T 73.5: Talk

Thursday, March 22, 2018, 17:35–17:50, Philo-HS1

Measurement of the ttZ cross section in the 4ℓ channel with the ATLAS experiment at 13 TeV — Julien Caudron¹, Markus Cristinziani¹, Mazuza Ghneimat¹, Carlo A. Gottardo¹, •Sebastian Heer¹, Vadim Kostyukhin¹, Ö. Oğul Öncel^1,2, Arshia Ruina¹, and Andrea Sciandra¹ — ¹Physikalisches Institut, Universität Bonn — ²Institut für Kernphysik, Universität zu Köln

In the Standard Model the tZ coupling is predicted via the weak interaction and can be tested by measuring the cross section of the ttZ process. The decay channel with 4 leptons in the final state has the smallest branching ratio (0.5% of all ttZ decays) but is the region with the highest purity. In the Standard Model, the available processes that also have 4 leptons in the final state are few, with the ZZ, the tWZ and the ttH processes being the dominant backgrounds.

This talk will present the latest results, using the full 2015 and 2016 dataset of 36.1 fb⁻¹ taken by the ATLAS detector at a centre-of-mass energy of 13 TeV, using proton–proton collisions. In total 48 events have been observed. Performing a profile likelihood fit in the 4ℓ-channel alone yields a signal strength compatible with the Standard Model prediction with an expected significance of 5.0σ and an observed significance of 5.3σ. The uncertainties in this analysis are driven by the available statistics, while the dominant systematic uncertainties are related to flavour tagging and signal modelling.