HUN-REN-ELTE Theoretical Physics Research Group

The Standard Model of elementary particles successfully describes the high energy experiments. The Higgs boson was discovered at the Large Hadron Collider (LHC) at CERN. The mass of the Higgs is 125 GeV, which implies that the Standard Model could be valid up to the Planck scale, where gravity must be taken into account. Observations in astrophysics and cosmology suggest new fields or particles, as nearly seventy percent of the Universe is dark energy and approximately one quarter is non-baryonic dark matter. Particle theorists proposed a large number of dark matter models. The hypothetical new particles generally couple to the newly discovered Higgs boson, this way the Higgs opens a portal to new physics. The study of simplified models coupled to the Standard Model in field theory and phenomenology is an important and active research area. These help us to learn more about the viability, scales and parameters of complex models applying the constraints of the LHC or recent experiments devoted to dark matter, such as XENON, LUX, PandaX.

Effective field theories are important in beyond the standard model particle physics, built on the relevant degrees of freedom. Even the Standard Model of Electroweak Interactions is generally accepted to be an effective theory. Effective field theories have a well defined range of validity, most easily taken into account by a cutoff. The naive momentum cutoff however breaks the space-time and gauge symmetries in the theory. On the other hand in the presence of gravity or supersymmetry calculations in four dimensions are preferred, as in these cases the most widely used dimensional regularization faces problems. We worked out a regularization method proposed in four dimensions, which defines a cutoff respecting the symmetries of the models. As applications of the improved symmetry preserving cutoff we study the effects of one-loop corrections in effective and non-renormalizable models, even in the coupled Einstein-Maxwell theory.

Quantum chromodynamics (QCD), the theory of strong interaction between fundamental degrees of freedom (quarks and gluons) making up composite hadrons, is highly nonperturbative and can be solved in a reliable and predictive way only with Monte Carlo based lattice field theoretical techniques. Low energy effective models of QCD containing mesons besides fermionic effective degrees of freedom are constructed based on the chiral symmetry of QCD which is broken spontaneously by the presence of the quark-antiquark condensate and explicitly by the finite mass of the light current quarks. We study these effective models with various functional methods, such as resummed perturbation theory, higher effective actions, functional renormalization group (FRG), Dyson-Schwinger equations. Within these effective models we determine the phase diagram of the strongly interacting matter related to the restoration of chiral symmetry at finite temperature and baryon density. Special attention is devoted to the existence and prediction of the location of the critical end point of a first order transition line in the temperature - baryon density plane which is planed to be explored in forthcoming heavy-ion collision experiments at FAIR-GSI and NICA. Based on the equation of state of the strongly interacting matter, which we plan to determine at low temperatures and high densities, we would like to describe neutron star properties and explore its possible constituent form of matter compatible with current astrophysical observations.