We review a number of topics related to block variable renormalisation group transformations of quantum fields on the lattice, and to the emerging perfect lattice actions. We first illustrate this procedure by considering scalar fields. Then we proceed to lattice fermions, where we discuss perfect actions for free fields, for the Gross-Neveu model and for a supersymmetric spin model. We also consider the extension to perfect lattice perturbation theory, in particular regarding the axial anomaly and the quark gluon vertex function. Next we deal with properties and applications of truncated perfect fermions, and their chiral correction by means of the overlap formula. This yields a formulation of lattice fermions, which combines exact chiral symmetry with an optimisation of further essential properties. We summarise simulation results for these so-called overlap-hypercube fermions in the two-flavour Schwinger model and in quenched QCD. In the latter framework we establish a link to Chiral Perturbation Theory, both, in the p-regime and in the -regime. In particular we present an evaluation of the leading Low Energy Constants of the chiral Lagrangian -the chiral condensate and the pion decay constant -from QCD simulations with extremely light quarks.
Motivation and overviewOver the recent decades quantum field theory has been established as the appropriate formalism for particle physics, as far as it is explored experimentally. Its treatment by perturbation theory led to successful results, for instance in Quantum Electrodynamics (QED), in the electroweak sector of the Standard Model and in Quantum Chromodynamics (QCD) at high energy. However, there are still many open questions, which require results at finite coupling strength -beyond the range of perturbation theory -such as numerous aspects of QCD at low and moderate energy.A method is known which has the potential to provide fully non-perturbative results for a number of field theoretic questions. This method applies Monte Carlo simulations to lattice regularised quantum field theories. The generic uncertainty of perturbation theory -uncontrolled contributions beyond the calculated order -disappears in this approach. However, one has to deal with statistical errors, as well as ambiguities in the extrapolation to the continuum and to a large volume.Simulation results are obtained at finite lattice spacing, which causes systematic artifacts in the numerically measured observables. The stability of dimensionless ratios of observables under the variation of the lattice spacing is denoted as the scaling behaviour. Its quality, which is vital for the reliability of the continuum extrapolation, depends on the way in which the lattice regularisation is implemented. This work deals * www.fp-journal.org Fortschr. Phys. 56, No. 2 (2008)
109For practical applications, i.e. for the applicability in simulations, the couplings have to be truncated. In Sect. 6 we describe our truncation scheme for the perfect fermion to a so-called hypercube fermion, which has been simulate...