PhD thesis defense
"Our colleague Oleksii Toldaiev successfully defended his PhD thesis "Top quark physics and search for physics beyond the Standard Model at the Large Hadron Collider". The research work was developed at LIP and the thesis was (remotely) defended at IST"
On October 29, 2020, Oleksii Toldaiev successfully defended his PhD thesis. The thesis was approved with the highest mark, "distinção e louvor”. The thesis, entitled "Top quark physics and search for physics beyond the Standard Model at the Large Hadron Collider”, was developed within the LIP group involved in the CMS experiment at CERN’s LHC. The work of Oleksii focused on the measurement of the top quark pair cross section in the dilepton final state including tau leptons. This is an important measurement that was performed with the data collected by CMS in proton-proton collisions. In particular, the inclusion of the final state with tau leptons is of pivotal importance to determine the possible presence of New Physics. Due to its large mass, the tau may couple to the Higgs (or any new particle) and an abnormal rate in this final state may signal New Physics in the data.
The thesis contains original work that was published in an International peer-reviewed journal, the Journal of High Energy Physics, JHEP 02 (2020) 191. Oleksii was the main author of the publication, from the beginning until the end, following up with responses to internal and external referees, until the publication. He devised the analysis methods, event selection and developed dedicated studies to constrain the backgrounds and evaluate the systematic uncertainties in the measurement. Furthermore, Oleksii has been actively involved in the development of the reconstruction code for the tau leptons within the CMS collaboration. Three measurements are presented in the publication: the measurement of the cross section, the ratio of the partial width to the total decay width, and the ratio of the cross section with respect to the light dilepton final state final. This is the first measurement of the top quark pair production cross-section in proton-proton collisions at 13 TeV that explicitly includes tau leptons, and it is the most accurate to date in this final state. The higher precision is achieved through a dedicated study developed by Oleksii. The ratio of the partial width to the total width improves the precision over the previous best measurements. The ratio of the cross sections yields a model-independent value that is consistent with lepton universality.
Congratulations, Oleksii!
