1 Light hadron spectroscopy with O(a) improved dynamical fermions.

Light hadron spectroscopy with O(a) improved dynamical fermions. We present results for the hadron spectrum and static quark potential from a simulation with two flavours of O(a) improved dynamical Wilson fermions at β = 5.2. We address the issues of sea quark dependence of observables and finite-size effects. Quenching is the only non-systematically im-provable approximation in lattice QCD. Moreover , sea-quark effects are to be taken into account if we are to make theoretical predictions on such phenomena as η ′ − π splitting and string breaking. Dynamical simulations are computa-tionally very expensive and control of systematic effects is thus of the utmost importance. The improvement counterterm c SW (g 2 0) for the SW-improved action has been computed non-perturbatively, thus yielding full O(a) improvement with N f = 2 dynamical fermions [1]. The simulation parameters are summarized in tab. 1. For each volume and each value of κ sea we have simulated valence quarks with an appropriate set of κ's to exploit the full (κ sea , κ val) plane for " strange " physics [2]. At β = 5.2 the PS/V mass ratios for the relevant κ's are shown in tab. 2. The value of c SW used differs from the final estimate of the Alpha Collaboration [1], but since a notable part of the simulation had already been carried out with this improvement coefficient, UKQCD have decided to continue the simulation, concentrating on finite-size effects and looking for signs of sea-quark effects in both the static quark potential and the light hadron spectrum. The complete removal of O(a) artefacts is thus not the highest priority in this work. Details of the simulation and results can be found in [2], while we refer the reader to [3] for an up-to-date review on spectroscopy.