Cutoff-effects in the spectrum of dynamical Wilson fermions

We investigate the low-lying eigenvalues of the improved Wilson-Dirac operator in the Schroedinger functional with two dynamical quark flavors. At a lattice spacing of approximately 0.1 fm we find more very small eigenvalues than in the quenched case. These cause problems with HMC-type algorithms and in the evaluation of fermionic correlation functions. Through a simulation at a finer lattice spacing we are able to establish their nature as cutoff-effectsComment: Lattice2004(machines), 3 pages, 3 figures, talk by R.

Glueball masses from ratios of path integrals

By generalizing our previous work on the parity symmetry, the partition function of a Yang-Mills theory is decomposed into a sum of path integrals each giving the contribution from multiplets of states with fixed quantum numbers associated to parity, charge conjugation, translations, rotations and central conjugations. Ratios of path integrals and correlation functions can then be computed with a multi-level Monte Carlo integration scheme whose numerical cost, at a fixed statistical precision and at asymptotically large times, increases power-like with the time extent of the lattice. The strategy is implemented for the SU(3) Yang-Mills theory, and a full-fledged computation of the mass and multiplicity of the lightest glueball with vacuum quantum numbers is carried out at two values of the lattice spacing (0.17 and 0.12 fm).Comment: Poster contribution to Lattice 2011. 7 pages, 2 tables, 2 figure

A non-perturbative study of massive gauge theories

We consider a non-perturbative formulation of an SU(2) massive gauge theory on a space-time lattice, which is also a discretised gauged non-linear chiral model. The lattice model is shown to have an exactly conserved global SU(2) symmetry. If a scaling region for the lattice model exists and the lightest degrees of freedom are spin one vector particles with the same quantum numbers as the conserved current, we argue that the most general effective theory describing their low-energy dynamics must be a massive gauge theory. We present results of a exploratory numerical simulation of the model and find indications for the presence of a scaling region where both a triplet vector and a scalar remain light.Comment: 1+22 pages, 8 figures, 1 table and 1 appendix. Few typos corrected and references added. Conclusions unchanged. Version accepted for publication in JHE

Wilson-like fermions and the static B_B parameter with no chirality breaking mixings

I consider the recent proposal by R. Frezzotti and G. Rossi to chirally improve Wilson fermions in such a way that mixings among operators of different chirality can be excluded. The method, which is based on the use of twisted mass QCD with several replica of valence quarks, is extended here to static-light systems. The operators relevant for the computation of the B_B parameter (in the static approximation) are discussed. In this case the same renormalization pattern as for Ginsparg-Wilson fermions is obtained by a simple modification of the discretization of the action for valence quarks.Comment: 3 pages, talk given at Lattice2004(heavy), Fermilab, June 21-26, 200

A lattice approach to QCD in the chiral regime

Non-perturbative lattice studies of QCD in the chiral thermodynamic regime, where chiral symmetry is spontaneously broken, require to deal with almost quark zero modes in a theoretically clean and computationally efficient way. We discuss the basic features and some realistic tests of a formulation, known as lattice tmQCD, that fulfills these requirements. Based on a talk given by R. Frezzotti at the conference HEP2001 (Budapest, July 2001)Comment: Contribution to the Proceedings of the conference HEP2001 (Budapest, July 2001), to appear on JHEP-Proceedings: 11 pages, 4 figures, 1 tabl

Towards precision heavy flavour physics from lattice QCD

I convey an idea of the significant recent progress, which opens up good perspectives for high-precision ab-initio computations in heavy flavour physics based on lattice QCD. This report focuses on the strategy and the challenges of fully non-perturbative investigations in the B-meson sector, where the b-quark is treated within an effective theory, as followed by the ALPHA Collaboration. As an application, I outline its use to determine the b-quark mass and summarize the status of our ongoing project in the two dynamical flavour theory.Comment: Invited talk at the Third Workshop on Theory, Phenomenology and Experiments in Heavy Flavour Physics, 5-7 July 2010, Capri, Italy; 8 pages including figures, latex2e, uses espcrc2.st