b quark mass effects at LEP1 with DELPHI detector

A sample of 2.8 million hadronic Z/sup 0/ decays collected by the DELPHI experiment at LEP during the years 1992, 1993 and 1994 have been used to measure the running mass of the b quark, m/sub b/, defined in the MS renormalization scheme. The value obtained is: m /sub b/(M/sub Z/)=2.67+or-0.25 (stat.)+or-0.34 (frag.)+or-0.27 (theo.) GeV/c/sup 2/. The next-to-leading order QCD corrections to the cross section for e/sup +/e/sup -/ annihilation at energies square root s=M/sub Z/ into three jets for massive quarks have been considered in this analysis. The net change in the running mass from the bb production threshold to LEP scales is: m/sup b/(M Upsilon /2)- m/sub b/(M/sub Z/)=+1.49+or-0.52 GeV/c/sup 2/, which is in good agreement with the predicted QCD evolution and provides the first experimental evidence of the running of the quark masses. The result also allows to test the flavour independence of the strong coupling constant for b and light quarks, that is verified within 1% accuracy. (12 refs).