Search

We compute the fermionic contribution to the strong coupling αqq extracted from the static force in Lattice QCD up to order g4 in perturbation theory. This allows us to subtract the leading fermionic lattice artifacts from recent determinations of αqq produced in simulations of two dynamical charm quarks. Moreover, by using a suitable parametrization of the βqq-function, we can evaluate the charm loop effects on αqq in the continuum limit.

A high precision determination of the strong coupling constant in the MS scheme at the Z-mass scale, using low energy quantities, namely pion/kaon decay constants and masses, as experimental input is presented. The computation employs two different massless finite volume renormalization schemes to non-perturbatively trace the scale dependence of the respective running couplings from a scale of about 200 MeV to 100 GeV. At the largest energies perturbation theory is reliable. At high energies the Schrödinger-Functional scheme is used, while the running at low and intermediate energies is computed in a novel renormalization scheme based on an improved gradient flow. Large volume Nf = 2 + 1 QCD simulations by CLS are used to set the overall scale. The result is compared to world averages by FLAG and the PDG.

We study the effective theory of decoupling of a charm quark at low energies. We do this by simulating a model, QCD with two mass-degenerate charm quarks. At leading order the effective theory is a pure gauge theory. By computing ratios of hadronic scales we have direct access to the power corrections in the effective theory. We show that these corrections follow the expected leading behavior, which is quadratic in the inverse charm quark mass.