Starting with the example of the quantum mechanical harmonic oscillator, we develop the concept of euclidean lattice field theory. After describing Wilson's formulation of quantum chromodynamics on the lattice, we will introduce modern lattice QCD actions which greatly reduce lattice artefacts or are even chiral invariant. The substantial algorithmic improvements of the last couple of years will be shown which led to a real breakthrough for dynamical Wilson fermion simulations. Finally, we will present some results of present simulations with dynamical quarks and demonstrate that nowadays even at small values of the quark mass high precision simulation results for physical quantities can be obtained.
The Luttinger–Ward functional was proposed more than five decades ago and has been used to formally justify most practically used Green’s function methods for quantum many-body systems. Nonetheless, the very existence of the Luttinger–Ward functional has been challenged by recent theoretical and numerical evidence. We provide a rigorously justified Luttinger–Ward formalism, in the context of Euclidean lattice field theory. Using the Luttinger–Ward functional, the free energy can be variationally minimized with respect to Green’s functions in its domain. We then derive the widely used bold diagrammatic expansion rigorously, without relying on formal arguments such as partial resummation of bare diagrams to infinite order.
Variational structure of Luttinger–Ward formalism and bold diagrammatic expansion for Euclidean lattice field theory
