Low-energy particle physics and chiral extrapolations

In this review I discuss the role of chiral extrapolations for the determination of several phenomenologically relevant quantities, including light quark masses, meson decay constants and the axial charge of the nucleon. In particular, I investigate whether chiral extrapolations are sufficiently controlled in order to rightfully claim the accuracy which is quoted in recent compilations of these quantities. While this is the case for the masses of the light quarks and the ratio fK/fpi of decay constants, small inconsistencies in the chiral and continuum behaviour of individual decay constants fK and fpi, as well as the hadronic radii r0, r1 remain and must be clarified. In the case of the nucleon axial charge, gA, the chiral behaviour is still poorly understood due to the presence of other systematic effects.Comment: 14 pages, plenary talk presented at the XXIX International Symposium on Lattice Field Theory - Lattice 2011 July 10-16, 2011, Squaw Valley, Lake Tahoe, Californi

Towards leading isospin breaking effects in mesonic masses with O(a)O(a) improved Wilson fermions

We present an exploratory study of leading isospin breaking effects in mesonic masses using $O(a)$ improved Wilson fermions. Isospin symmetry is explicitly broken by distinct masses and electric charges of the up and down quarks. In order to be able to make use of existing isosymmetric QCD gauge ensembles we apply reweighting techniques. The path integral describing QCD+QED is expanded perturbatively in powers of the light quarks' mass deviations and the electromagnetic coupling. We employ QED$_{\mathrm{L}}$ as a finite volume formulation of QED.Comment: 8 pages, 1 figure, proceedings to poster presented at 35th annual International Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai

String Breaking in SU(2) Yang Mills Theory with Adjoint Sources

We compute the static potential in three-dimensional SU(2) Yang Mills Theory with adjoint sources using numerical simulations. By employing a variational approach involving string and gluelump operators, we obtain clear evidence for string breaking in the adjoint potential. The breaking scale r_b is computed and extrapolated to the continuum limit. The result in units of the scalar glueball mass is r_b*m_G = 10.3 +/- 1.5. We also resolve the structure of higher excitations of the flux-tube and gluelumps. Furthermore we discuss the implications of our findings for the case of the four-dimensional theory.Comment: 16 pages, 5 postscript figures, LaTe

Fundamental parameters of QCD

The theory of strong interactions, QCD, is described in terms of a few parameters, namely the strong coupling constant alpha_s and the quark masses. We show how these parameters can be determined reliably using computer simulations of QCD on a space-time lattice, and by employing a finite-size scaling method, which allows to trace the energy dependence of alpha_s and quark masses over several orders of magnitude. We also discuss methods designed to reduce the effects of finite lattice spacing and address the issue of computer resources required.Comment: Contribution to proceedings of NIC Symposium 2001, 13 pages, 7 figures, uses nic-series.cl