Quantum vacuum structure of two Higgs doublet models


  • Call:

    IDPASC Portugal - PHD Programme 2016

  • Academic Year:

    2016 / 2017

  • Domains:

    Theoretical Particle Physics | Experimental Particle Physics

  • Supervisor:

    Pedro Ferreira

  • Co-Supervisor:

  • Institution:

    Faculdade de Ciências - Universidade de Lisboa

  • Host Institution:

    Faculdade de Ciências - Universidade de Lisboa

  • Abstract:

    The CERN's Large Hadron Collider (LHC) allowed for the discovery of the Higgs boson, the last remaining piece of the Standard Model (SM). The remarkable results which have been obtained permit us to probe the properties of the Higgs particle with great precision – through the production and decay of the Higgs boson, we have even a window into the quantum corrections to its interactions. Detailed analysis of these properties will allow us to probe whether the SM is complete, or instead detect hints at the presence of New Physics beyond the SM (BSM) – such as new particles not yet discovered. There are many BSM models proposed through the years, such as Supersymmetry, technicolour or one of the simplest extensions of the SM, the two-Higgs doublet model (2HDM). Proposed in 1973, it is a simple yet extremely rich model, allowing for possible explanations of the matter-antimatter asymmetry, dark matter candidates and a richer particle spectrum. The 2HDM has a rich vacuum structure – the state of minimum energy of the model is not unique, there can be states with different phenomenologies (some states break charge conservation and are unphysical; others break CP symmetry and could help explain the antimatter problem). It has been possible to show, in the past 10 years, that the stability of the model is almost ensured – when a minimum of a given type exists, the global minimum of the theory is of the same nature, and no tunneling to a deeper minimum of a different type may exist. But such analysis has been performed at the lowest level of perturbation theory. Recent one-loop calculations have shown that the results at tree-level may be completely changed by quantum corrections. The vacuum analysis needs therefore to be redone at one-loop, applying well-known methods of field theory: computation of the effective potential; minimization of the potential; calculation of the one-loop masses of all particles in the model. This will allow for more precise predictions of the 2HDM which can then be compared with the experimental data coming from LHC. The timing of this work is just right: currently, theorists have access to data which will allow us to prove, or disprove definitely, many of the proposed BSM models. This exciting time for particle physics requires, however, hard work on the theory side, and the precision measurements at the LHC require equally precise theoretical calculations. The project will involve a deep study and comprehension of the theory of the 2HDM and its effective potential; one-loop calculations in field theory to obtain the one-loop physical masses; a continuous comparison of theoretical predictions with experimental results – far from being an abstract thesis, the student will be required and encouraged to understand the connection to the phenomena being probed at the LHC. Travel to international conferences in particle physics, and collaboration with foreign experts of Higgs physics, will be expected and encouraged.