A Search for Heavy Fermionic Top Quark Partners with Charge 5/3 Decaying to a Pair of Same-Sign Leptons wit the CMS Experiment
This event is part of the PhD Final Oral Exams.
Examining Committee: Tulika Bose, Kevin Black, Kenneth Lane, Shyam Erramilli, James Rohlf
In the millennia of recorded human knowledge, no model for describing the workings of Nature is as elegant or complete as the Standard Model of Particle Physics (SM). However, the SM has several open questions and there exist multiple phenomena that it cannot explain. A pressing question is related to the mass of the Higgs boson, whose value the SM has no natural way of explaining, relying instead on the fine tuning of parameters to one part in 10^(28). Many extensions of the SM propose new interactions and particles which solve this problem. A particularly common theme is that of new partners of the top quark, which in some models are fermionic and have vectorial couplings to the SM charged weak current. Such particles are referred to as vector-like quarks and represent a promising avenue of research.
A search is presented for a vector-like quark with an exotic 5/3 charge (in units of the charge of an electron), referred to as an X5/3 particle. These particles are predicted in Composite Higgs theories, which rely on the masses of the X5/3 to be not more than ~ 2 TeV in order to solve the unnaturalness inherent in the mass of the Higgs boson. The search uses data collected by the CMS experiment in both 2015 and 2016 consisting of proton-proton collisions at a center-of-mass energy of 13 TeV at the CERN LHC. No significant excess of events is seen above the predicted background and limits are placed on the mass of the new top quark partner at 95% confidence level, excluding masses less than 1200 (1160) GeV for X5/3 particles that decay with right-handed (left-handed) couplings to W bosons. These are the most stringent limits to date on the mass of the X5/3 particle in this final state.