Determination of the b quark mass at the Z mass scale

In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using O(alpha (2)(S)) calculations for massive quarks, this difference was used to determine the b quark mass in the YI-S renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be (m) over bar (b) (m(Z)) = (2.67 +/-0.03(stat.)(-0.37)(+0.29)(syst.)+/-0.19(theo.)) GeV. Evolving this value to the b quark mass scale itself yields (m) over bar (b)((m) over bar (b)) = (3.95(-0.62)(+0.52))GeV, consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations.