The relation between the (MS)over-bar and the on-shell quark mass at order αs3

The relation between the on-shell and <(MS)over bar> mass can be expressed through scalar and vector part of the quark propagator. In principle these two-point functions have to be evaluated on-shell which is a non-trivial task at three-loop order. Instead, we evaluate the quark self-energy in the limit of large and small external momentum and use conformal mapping in combination with Pade improvement in order to construct a numerical approximation for the relation. The errors of our final result are conservatively estimated to be below 3%. The numerical implications of the results are discussed in particular in view of top and bottom quark production near threshold. We show that the knowledge of new partial derivative(alpha(s)(3)) correction leads to a significant reduction of the theoretical uncertainty in the determination of the quark masses, (C) 2000 Elsevier Science B.V. All rights reserved.