Thesis

Search for new heavy quarks in multi-lepton topologies at ATLAS

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Experimental Particle Physics

• Supervisor:

  Nuno Castro

• Co-Supervisor:

• Institution:

  Universidade do Minho

• Host Institution:

  Laboratório de Instrumentação e Física Experimental de Partículas

• Abstract:

  The discovery of the Higgs boson during the first run of the Large Hadron Collider (LHC) was a crucial point for the consolidation of the Standard Model (SM) of particle physics. Nonetheless, the exact nature of the electroweak symmetry breaking mechanism is not yet determined. In particular, the SM violates the concept of naturalness when extrapolated to energies above the electroweak scale, since fine tuning is required to compensate the quadratic divergences of the fundamental scalar fields. Proposed models of physics beyond the SM typically address the naturalness problem by postulating a new symmetry, as for example, supersymmetry. Alternatively, the symmetry could be a spontaneously broken global symmetry of the extended theory. Examples of models implementing this idea are Little Higgs and Composite Higgs models, which predicts the existence vector-like quarks, defined as colour-triplet spin-1/2 fermions whose left- and right-handed chiral components have the same transformation properties under the weak isospin gauge group. Such quarks could mix with like-charge SM quarks, and the mixing of the SM top quark with a +2/3 vector-like quark could play a role in regulating the divergence of the Higgs mass-squared. The ATLAS and CMS collaborations have performed a comprehensive search program using the first run of the LHC with proton/proton collisions at a centre of mass energies of 7 and 8 TeV, collected in 2011 and 2012, respectively. Such searches showed no evidence for physics beyond the SM, but vector-like quarks with masses above ~800 GeV could not be probed yet. The increased centre of mass energy and luminosity at the run-II of the LHC will allow to continue the vector-like search program, and masses above 1 TeV are expected to be probed in both pair and single production modes. The present proposal foresees the integration of the applicant in the Portuguese ATLAS group, continuing the group work in the search for new vector-like quarks in multi-lepton topologies. A new analysis strategy will have to be developed in order to deal with the challenging pile-up conditions expected for the run-II 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.