Research field and interests

My research field is Hadronic Physics, the study of strongly-interacting matter with the aim to understand its  properties and interactions in terms of the underlying fundamental theory, i.e. Quantum Chromodynamics. I am interested in the investigation of the two emergent phenomena of the strong interaction: confinement – the fact that single quarks are not observed in isolation – and dynamical chiral-symmetry breaking – the origin of most of the mass of ordinary matter in the Universe. These phenomena dominate the observed properties of hadrons measured in experiments at LHC, JLab, FAIR-GSI, and the BES Collaboration, and they can be studied theoretically through dynamical quark models describing hadrons.                                                                                                                                                       Figures: credits to Joshua Rubin, Argonne


Covariant spectator theory

For the phenomenological investigation of confinement and dynamical chiral-symmetry breaking we apply non-perturbative, covariant methods based on quantum field theory, such as formulations of field-theoretic amplitudes in terms of integral equations that effectively sum an infinite set of diagrams. We use the covariant spectator theory (CST), which works in Minkowski space, to develop a dynamical quark model that can describe the structure and the mass spectrum of both, heavy and light quark systems.

The CST-Dyson integral equation describes the dynamical generation of a dressed quark mass due to the self-interactions of the quarks. The thick red line is the dressed quark propagator, the thin black line is the bare quark propagator and the orange zig-zag line is the interaction kernel. A red (white) cross on a quark line indicates that the corresponding quark is on mass shell with positive (negative) energy.

The CST-Bethe-Salpeter integral equation describes quark-antiquark bound states, i.e. quark-antiquark mesons such as the pion. The green triangle is the meson vertex function and the solid colored lines are dressed quark propagators. A colored (white) cross on a quark line indicates that the corresponding quark is on mass shell with positive (negative) energy.

Franz Gross (JLab, USA)
Sofia Leitão (CFTP-IST, Portugal)
Teresa Peña (CFTP-IST,
Emílio Ribeiro (CeFMA-IST
, Portugal)
Alfred Stadler (U. Évora
, Portugal)

Point-form Hamiltonian dynamics

Another covariant approach to study relativistic quantum systems in hadronic physics is point-form Hamiltonian dynamics. It can be applied to both, relativistic quantum mechanics and quantum field theory, and it has the advantage that only the space-time translations of the Poincaré transformations are interaction dependent, and Lorentz boosts and spatial rotations are free of interactions.The point form is characterized by a space-time hyperboloid in Minkowski space that is left invariant under the Lorentz group.
The forward hyperboloid x²=const., the quantization surface in the point form.

William Klink (U. Iowa, USA)
Wolfgang Schweiger (U. Graz, Austria)