MEASURING CHARGE ASYMMETRIES IN B-HADRON DECAYS USING TOP QUARK EVENTS WITH THE ATLAS EXPERIMENT

Abstract

The large number of top-antitop pair events produced in pp collisions at the Large Hadron Collider (LHC) provides a unique source of b-quarks that can be used to probe the CP violation in heavy-flavour mixing and decay. The measurement presented in this thesis focuses on semi-leptonic top-antitop events where one of the W-bosons decays hadronically and the other one decays leptonically. The charge of the lepton (electron or muon) from the W-boson tags the charge of the b-quark at production. In events where a muon is associated to the semi-leptonic decay of the b-quark (either directly or after a b ! c hadronic transition), two charge asymmetries (CA) and several CP asymmetries, based on the charges of the lepton from the W-boson and this muon, can be measured. My contribution to the first measurement of the CA and CP asymmetries in heavy flavour b- or c-decays from top-antitop lepton+jets events is presented, using the data collected with the ATLAS detector during Run 1 of the LHC (20.3 fb 1) at a centre of mass energy of p s = 8 TeV. However, most of the thesis focuses on my (larger) contribution to this same measurement, performed with data collected by the ATLAS experiment during Run 2, 2015-16 data taking periods, corresponding to an integrated luminosity of 36.1 fb 1 at a centre of mass energy of p s = 13 TeV. Four CP asymmetries (one mixing and three direct) were measured, using Run 1 data, and they are found to be consistent with the Standard Model. The Run 2 analysis results in the measurement of two charge asymmetries, both compatible with zero and consistent with the Standard Model expectations. The overall uncertainty on the Run 2 measurement is halved with respect to the Run 1 result.