Measuring CP-Violation by Tagging Soft Muons in ttbar Events Using the ATLAS Detector at the Large Hadron Collider

A measurement of CP-violation is presented, making use of an event selection whereby soft muons from the semileptonic decays of b-hadrons and c-hadrons are tagged in lepton+jets ttbar events originating from proton-proton collisions in the ATLAS detector at the Large Hadron Collider. Four methods are considered for the process of same-top/different-top assignment, with the kinematic likelihood fitting method found to marginally perform best when assessed in terms of the total uncertainty on the final results. The feasibility of extending the measurement by complementing the existing time-integrated asymmetries with a time-dependent asymmetry is explored and is found to be promising. The combined charge count distribution Nlμ of the soft muon and prompt lepton is presented for Monte Carlo simulation and for data corresponding to an integrated luminosity of 139 fb−1 and a centre-of-mass energy of √s = 13 TeV. In addition, the Monte Carlo distribution is unfolded to a fiducial volume and combined with a set of measured decay chain fractions to extract observable charge asymmetries and a set of underlying CP-violation asymmetries, which are presented along with their associated statistical and systematic uncertainties. These asymmetries and their uncertainties are discussed, along with the ramifications for the measurement of these asymmetries in the experimental data and the impact of such a measurement on searches for sources of CP-violation not described by the Standard Model.