Measurement of the cross section of top quark pairs in association with a photon in lepton+jets events at √s =13 TeV
The Large Hadron Collider (LHC) at CERN is the world’s biggest and most powerful accelerator. Colliding protons at the highest centre of mass energy yet, data are being collected by the detectors at an unprecendented rate, making it possible to search for new particles as well as make precise measurements. With the increase in statistics and energy of the data collected, it is now possible to make precise measurements with events that have a relatively low probability of occuring. In this thesis, the production cross section of a top quark pair plus a radiated photon, tt¯+γ, is measured in proton-proton collisions at the center of mass energy of 13 TeV. The data was recorded by the CMS experiment at the LHC during 2016, and correspond to an integrated luminosity of 35.9 fb−1. This measurement is performed in events with a well isolated, high pT lepton (electron and muon), at least four jets from the hadronization of quarks, and an isolated photon. The photons may be emitted from initial state radiation, top quarks as well as from decay products of the top quarks. The analysis makes use of simultaneous likelihood fits in several control regions to distinguish tt¯+γ signal from background. The measured inclusive cross sections are 6.625 ± 0.19(stat) ± 0.41(syst) pb (e+jets), 6.67 ± 0.15(stat) ± 0.42(syst) pb (µ+jets), and 6.41 ± 0.14(stat) ± 0.33(syst) pb (combined). The results are in agreement with the 6.507 pb standard model next-to-leading order prediction.