Computation of the strong coupling in N_f =4 QCD
Lattice discretisation and numerical simulations are well established tools for the computation of input parameters of perturbative QCD in terms of low energy hadronic observables. Using the Schroedinger functional strategy for these computations enables us to overcome the multi-scale problem in QCD and keep full control over the systematic errors. In this talk I will present a non-perturbative computation of the running of Nf=4 QCD coupling with four flavours of O(a) improved Wilson fermions and Wilson plaquette action. We check that cutoff effects in the step scaling function are weak by taking one more resolution compared to a previous computation. Finally, we show an improved result of the Lambda parameter in four flavour QCD in the units of hadronic scale Lmax.