Access Type

Open Access Dissertation

Date of Award

January 2021

Degree Type


Degree Name



Physics and Astronomy

First Advisor

Robert Harr


The inclusive production cross section of top quark pairs in association with a photon is measured in proton-proton collisions at the LHC with 13 TeV energy using the full RunII data collected by CMS in 2016, 2017, and 2018 with a total corresponding integrated luminosity of 137 fb −1 . The relative fraction of ttγ events normalized to inclusive tt production is measured. The cross section measurement provides important information about the electromagnetic coupling of the standard model top quark and is sensitive to physics beyond the standard model. The analysis is carried out in the in semileptonic decay channel with a well isolated high P T lepton (electron and muon), at least four jets out of which at least one must be b-tagged, and an isolated photon. Photons may be emitted from initial state radiation, top quarks, and decay products of top quarks. A simultaneous likelihood fit of control regions with the signal region is done to constraint the backgrounds and to extract the ttγ cross section. The measurement of the ratio of ttγ to tt is 0.02055 ± 0.00099 (syst.) ± 0.00099 (stat.) in the e + jets channel, 0.02156 ± 0.00068 (syst.) ± 0.00068 (stat.) in µ + jets channel, and 0.02203 ± 0.00064 (syst.) ± 0.00064 (stat.) in the l + jets channel. The measured inclusive cross section is 3.81 ± 0.15 (syst.) ± 0.10 (stat.) pb in e + jets channel, 3.87 ± 0.11 (syst.) ± 0.07 (stat.) pb in µ + jets channel, and 3.96 ± 0.10 (syst.) ± 0.06 (stat.) pb in l + jets channel for full RunII data. The results are in agreement with the standard model next to leading order prediction.