Measurement of the ttZ Production Cross Section in the Final State with Three Charged Leptons using 36.1 fb^-1 of pp Collisions at 13 TeV at the ATLAS Detector
by Nils-Arne Rosien
Date of Examination:2018-01-22
Date of issue:2018-02-14
Advisor:Prof. Dr. Arnulf Quadt
Referee:Prof. Dr. Arnulf Quadt
Referee:Prof. Dr. Stan Lai
Referee:Prof. Dr. Ariane Frey
Referee:Prof. Dr. Baida Achkar
Referee:Prof. Dr. Laura Covi
Referee:Prof. Dr. Steffen Schumann
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Abstract
English
A measurement of the production cross section for a top quark pair in association with a Z boson (ttZ) is presented in this PhD thesis. Final states with exactly three charged leptons (electrons or muons) are used, taking into account the decay of the top quark pair in the lepton+jets channel and the decay of the Z boson into two charged leptons. The dataset used for this analysis corresponds to 36.1 fb^-1 of proton-proton collisions at a centre-of-mass energy of 13 TeV, recorded during 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The result of a profile likelihood fit to the event yields in four signal enriched regions and two background enriched regions is sigma(ttZ)=966+114-102(stat.)+115-114(syst.) fb. The observed (expected) significance is 7.2 (6.4) standard deviations from the background-only hypothesis. Within the experimental uncertainties, the result is in good agreement with the Standard Model prediction. This result is compared with two other ttZ analysis channels, using the same dataset but different lepton multiplicities. The analysis presented here is found to be the most sensitive one in terms of observed significance. The result of a combined fit of all three analysis channels is discussed. Two feasibility studies of possible future ttZ analysis techniques are demonstrated.
Keywords: top quark; ATLAS; LHC; particle physics; Z boson; ttZ