Thesis

Search for heavy fermions with LHC data

• Call:

  IDPASC Portugal - PHD Programme 2017

• Academic Year:

  2017 / 2018

• Domain:

  Experimental Particle Physics

• Supervisor:

  Nuno Castro

• Co-Supervisor:

  Patricia Conde Muino

• Institution:

  Universidade do Minho

• Host Institution:

  LIP, UMinho and ULisboa

• Abstract:

  A cornerstone of the Standard Model (SM) of Particle Physics is the formulation of the electroweak interactions as arising from a spontaneously broken gauge symmetry. Experiments over the past four decades have confirmed this hypothesis with precision, most notably the LEP and SLC collider programs. The ATLAS and CMS collaborations reported in 2012 observations of a new particle produced at the CERN Large Hadron Collider (LHC) possessing properties so far consistent with those predicted for the SM Higgs boson. It should be noted, nonetheless, that the exact nature of the symmetry-breaking mechanism is not yet determined. Furthermore, regardless of the many experimental validations of the SM, it is known that it cannot be regarded as a final theory, given the many problems it fails to solve, namely the hierarchy problem, the lack of a dark matter candidate, matter-antimatter asymmetry, and others. Attempting to solve these problems, many beyond the SM theories were built. Vector-like quarks (VLQ) are featured in some of these models, as spin 1/2 fermions, color triplets with the same left and right quantum numbers, mixing with SM quarks. The current proposal foresees the development and implementation of a search for vector-like quarks decaying through a Z boson and a third generation quark, using data collected by the ATLAS experiment during the run-2 of the LHC. This search will take into account the different topologies and production mechanisms. We propose to define a strategy in which signal and control regions are built and optimized, systematic uncertainties are studied and all the steps necessary to set limits are done, including the definition of discriminant variables that allow to distinguish signal from background, so that an evidence of signal can be searched, or, in case none is found, to significantly improve the current exclusion limits. The phenomenological consequences of the experimental results will be also be studied, making the most of the LHC data. Given the multitude of final states that vector-like quark production can have, it is important to branch out the searches, profiting from the distinctive kinematics that these quarks are expected to have, and improving the final results with a combination of these multiple topologies and their respective sensitivities. In this project, the focus will be set on VLQ decays trough a Z boson and a third generation quark. More specifically, the applicant will be keying in the dileptonic channel, searching for events with a pair of opposite-sign, same flavor leptons that may serve as a Z boson reconstruction, and a number of b-tagged jets. A selection based on the kinematics of these objects will be laid out, taking advantage of the high mass of the VLQ and how their decays are expected to be more boosted than objects from background processes. In a second phase, deep learning techniques will be used, allowing to explore low-level information in order to properly consider the distinguishable properties of the many possible final states in the pair production of top-like and bottom-like VLQs at the LHC. A detailed study of the impact of systematic sources of uncertainties during the training phase will be done. The use of such analysis techniques can be particularly relevant in the context of alternative production mechanisms for VLQs, such as Z' or heavy gluons. In order to fully explore the physics implications of the obtained results a close collaboration with the phenomenological community will be pursued. The proposed working plan foresees the integration of the PhD candidate in the Portuguese Group of the ATLAS Collaboration. Being a member of the ATLAS collaboration, the candidate will be expected to contribute to the operation of the detector and to perform technical tasks, namely to the development and study of dedicated high-level trigger chains which combine information from standard jet triggers with triggers from the ATLAS Forward Proton (AFP) detector. The proposed plan is ambitious and aims at providing a suitable and comprehensive training in High Energy Physics, covering different expertises at the detector, analysis and phenomenology levels. Either a discovery or the most stringent limits on the proposed searches are expected to be obtained.