The Effective Fine Structure Constant at TESLA Energies

We present a new estimate of the hadronic contribution to the shift in the fine structure constant at LEP and TESLA energies and calculate the effective fine structure constant. Substantial progress in a precise determination of this important parameter is a consequence of substantially improved total cross section measurements by the BES II collaboration and an improved theoretical understanding. In the standard approach which relies to a large extend on experimental data we find �� (5) (M 2 Z ) = 0.027896 ± 0.000395 which yields � −1 (M 2