Measurements of jet mass, top quark mass and top tagging efficiencies in decays of boosted top quarks at CMS

Abstract

Top quarks with high momenta are abundantly produced at the Large Hadron Collider (LHC). They are of relevance for both searches for new physics and measurements of fundamental parameters of the Standard Model of particle physics (SM). At high momenta, top quarks are boosted with high Lorentz factors, such that hadronic decay products collimate, merge into a single jet, and jet substructure techniques become crucial for further analysis. In this thesis, two measurements studying top quarks at high momenta are presented. They make use of proton-proton collision data recorded during the Run 2 data taking period with the Compact Muon Solenoid (CMS) experiment at the LHC, operating at a center-of-mass energy of 13 TeV. Both analyses target the lepton+jets decay channel of top quark pair production (tt) and use the leptonic leg t -> b W -> b l nu to distinguish signal events from background, while the measurement is performed using the hadronic decay t -> b W -> b q q.

The first analysis is a measurement of the differential tt production cross section as a function of the jet mass. The measurement is carried out using data recorded in 2016 corresponding to an integrated luminosity of 35.9 fb^-1. Using information from simulated events, the data are unfolded to the particle level. In addition to the measurement of the jet mass itself, the top quark mass is extracted and measured to be mtop = 172.6 +- 2.5 GeV. This presents a complementary measurement of the top quark mass in a topology very different than threshold production. In addition, the jet mass distribution can be calculated analytically at high top quark momenta, which allows for a direct comparison with data and can help to resolve ambiguities in the definition of the top quark mass scheme.

In the second analysis, the identification of jets that originate from hadronic decays of boosted top quarks, referred to as top tagging, is studied, and the corresponding efficiencies are measured in data and simulation. The measurements are carried out using all three data taking periods in 2016, 2017, and 2018, which correspond to integrated luminosities of 35.9 fb^-1, 41.5 fb^-1, and 59.7 fb^-1, respectively. Top tagging techniques using jet substructure are validated, and the efficiencies are extracted using template fits to data. This enables the efficiency measurement of individual contributions from fully merged, semimerged, and unmerged events, depending on the containment of the top quark decay products in a single jet. As a result, correction factors are derived that account for differences between data and jet substructure modeling in simulation. The correction factors obtained in this measurement are endorsed by the CMS Collaboration for the use in analyses.