MEASUREMENT OF THE RATIO OF TOP QUARK BRANCHING FRACTIONS B(T→WB)/B(T→WQ) IN PROTON-PROTON COLLISIONS AT 13 TEV USING THE ATLAS DETECTOR

Abstract

This thesis presents the measurement of Rb performed using the data collected by the AT LAS experiment at the LHC in 2016 and 2017, corresponding to an integrated luminosity of 78f b−1 of proton-proton collisions at center-of-mass energy of 13 TeV. Events with two isolated charged leptons, of which one electron and one muon of opposite electric charge, and two jets have been considered. A kinematical reconstruction has been performed, with the purpose of associating the final state objects to the decay of two top quarks in the event. Based on this, events have been classified into four statistical independent regions: one enriched in events having two jets most likely coming from the top decay, three regions enriched in events having at least one jet coming from ISR and/or FSR activity. The control regions have been used to correct the predicted jet-to-top assignment fractions using collision data. The parameter of interest has been extracted performing a profile like lihood fit on the b-jet multiplicity distribution in the signal region. The result is: Rb = 1.022±0.031, (0.1) which is compatible with the Rb = 1 hypothesis and represents the most precise single mea surement of this quantity. The leading source of uncertainty is the one associated to the b-tagging algorithm perfor mance, contributing with a 2.5% to the total uncertainty. A dedicated calibration of the b-tagging efficiency of b-jets has been performed in this work. This calibration relies on the p rel T method, applied on a sample of multi-jets events with at least on jet spatially matched to a muon. The final uncertainty on the b-tagging efficiency has been strongly reduced by a new esti mation of the semi-leptonic to inclusive extrapolation uncertainty of the scale factors. This uncertainty was, in the Run 1 calibration, 4% in the whole jet kinematical range: it has been lowered to the 1% thanks to a new estimation performed in bins of jet pT, using the larger data set of the Run 2.