ASSOCIATED PRODUCTION OF A SINGLE TOP QUARK AND A Z BOSON IN p s = 13 TEV pp COLLISIONS USING THE ATLAS DETECTOR
This event is part of the PhD Final Oral Exams.
Examining Committee: Kevin Black (advisor) John Butler Ken Lane Karl Ludwig Ed Kearns
The production of an electroweak single top quark produced in association with a Z boson is studied using 140fb−1 of proton-proton collison data collected by the ATLAS experiment at the LHC over the period from 2015 to 2018, at a center-of-mass energy of √s = 13 TeV. This search is performed in the three lepton channel in which the Z boson decays leptonically and top quark decays semi-leptonically. Events are analyzed from a final state that contains three high-pT electrons and/or muons, two or three jets (one identified as coming from a b-quark), and substantial missing transverse energy. Major irreducible backgrounds come from W Z diboson and tt¯V + tt¯H + tW Z , with irreducible backgrounds from tt¯ and Z + jets events. A gradient boosting algorithm is trained and applied in order to separate signal and background distributions, with a binned likelihood fit applied to the discriminant produced by the algorithm. From this fit a null-hypothesis significance and cross-section measurement are extracted. The observed(expected) significance is 9.0(9.1)σ with an extracted cross-section of σtZ q = 770 ± 108 (statistical) ± 53 (systematic) fb. This cross-section is compatible with prediction and therefore this measurement clears the threshold to