Thesis

Measurement of the tZ production at the ATLAS experiment and search for flavour changing neutral currents

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domains:

  Theoretical Particle Physics | Experimental Particle Physics

• Supervisor:

  Nuno Castro

• Co-Supervisor:

  Rui Santos

• Institution:

  Universidade do Minho

• Host Institution:

  LIP and CFTC

• Abstract:

  The top quark is the heaviest elementary particle discovered so far and the study of its properties provides not only an important test of the Standard Model (SM) of particle physics, but also a window to physics beyond it. At the run-1 of CERN's Large Hadron Collider (LHC), both the ATLAS and CMS collaborations developed an extensive programme devoted to the measurement of top quark properties. In fact, the LHC is a true top-factory with several million top quarks being produced every year. Top-quark physics is thus precision physics at the LHC and the top-quark will be studied like never before. It will certainly be the best place to indirectly look for physics beyond the SM. With so many top-quarks being produced, a precise measurement of the top quark couplings will be performed at next LHC run. The expected luminosity and centre of mass energy, at the LHC run-2, will allow to further pursue top-quark studies, namely on the flavour changing neutral (FCN) couplings to the SM gauge bosons. In the SM the top quark decays via flavour changing neutral currents (FCNC) have extremely small branching ratios but some of its extensions, such as supersymmetry, technicolor or multi-Higgs doublet models predict a significant enhancement of the probability for such decays. An important way of probing the FCN coupling tqZ (with q being a u or c- quark) is the search of tZ production via FCNC. A similar search would also allow to probe the tqg FCN coupling (where g is a gluon). Furthermore, the study of the tZ production in the context of the SM is an important measurement, providing relevant information to many other important results (as background), such as the measurement of the ttZ and ttH cross-sections. The project has a theoretical component related to the interpretation of the results in the context of an effective theory. This effective theory will be presented in the form of a dimension six Lagrangian allowing to parametrize new physics that could be discovered at the LHC in a somehow model independent way. The present proposal foresees the integration of the applicant in the Portuguese ATLAS group, profiting from the expertise of the group in multi-lepton topologies. An analysis strategy will have to be developed, considering the challenging pile-up conditions expected for the run-2 data. The evaluation of systematics uncertainties, testing new strategies to control them, and a close collaboration with the phenomenological community in the interpretation of the experimental results will be crucial to fully exploit the potential of the new LHC data.