We estimate the QCD coupling constant from a lattice calculation of the bottomonium spectrum. The second order perturbative expansion of the plaquette expectation value is employed to determine alpha_S at a scale set by the 2S-1S and 1P-1S level splittings. The latter are computed in NRQCD in a dynamical gauge field background with two degenerate flavours of Wilson quarks at intermediate masses and extrapolated to the chiral limit. Combining the N_f=2 result with the quenched result at equal lattice spacing we extrapolate to the physical number of light flavours to find a value of alpha_{\bar MS}^{(5)}(m_Z) = 0.1118(17). The error quoted covers both statistical and systematic uncertainties in the scale determination. An additional 5% uncertainty comes from the choice of the underlying sea quark formulation and from truncation errors in perturbative expansions.
@article{arxiv.hep-lat/9906009,
title = {Alpha_S from Upsilon Spectroscopy with Dynamical Wilson Fermions},
author = {SESAM Collaboration and A. Spitz and H. Hoeber and N. Eicker and S. Guesken and Th. Lippert and K. Schilling and T. Struckmann and P. Ueberholz and J. Viehoff},
journal= {arXiv preprint arXiv:hep-lat/9906009},
year = {2009}
}