Probing the structure of the top quark
IDPASC Portugal - PHD Programme 2016
2016 / 2017
Theoretical Particle Physics | Experimental Particle Physics
Universidade do Minho
FCUL and LIP
The CERN's Large Hadron Collider (LHC) is a veritable top quark factory: several million top quarks are produced each year, which allows for very detailed studies of the properties of this particle. Since the top quark is the heaviest elementary particle known so far, its study carries great significance, since its heavy mass makes it an ideal “laboratory” to detect deviations from the behavior predicted by the Standard Model (SM). Such deviations, if they exist, would correspond to clear signals of new physics beyond the SM. Such deviations could be caused by new particles not yet discovered (for instance, Supersymetric particles) which might alter the couplings of the top quark. Another possibility is that the top is not, indeed, an elementary particle. Many theories of fermion substructure have been proposed, and tested, over the years, but if signs of compositness are possible, they will likely come from the top quark, due to its large mass. How would a non-elementary top particle manifest itself? In principle, the interactions of the top to other particles (such as the photon, the gluon, even the W bosons) would be subtly changed, and those changes would have consequences at observables measured at the LHC. It becomes therefore possible to use the current (run-I) LHC results to set contraints on an eventual compositness of the quark – for instance, to obtain a limit on the radius of the top particle. Further, the current (run-II) acquisition of data by the LHC will provide, over the next years, the greatest wealth of data available on the physics of the top quark –the timing for these studies is therefore perfect. This work proposal involves: (a) a detailed analysis of the existing theories of quark structure; (b) the development of a working simple model of top compositness, based on analogous “bag models” developed for nuclear structure; (c) the calculation of the effects that top structure would have on LHC observables, such as top production cross sections and decays; (d) analysis of LHC data to obtain state-of-the-art constraints on the structure parameters of the top quark. The work will involve a close collaboration with ATLAS Collaboration experimental physicists, so that the expertise that group has on top physics can benefit the student. Travel to international conferences in particle physics, and collaboration with foreign experts of top quark physics, will be expected and encouraged.