The bb forward-backward asymmetry has been determined from the average char
ge how measured in a sample of 3,500,000 hadronic Z decays collected with t
he DELPHI detector in 1992 - 1995. The measurement is performed in an enric
hed b (b) over bar sample selected using an impact parameter tag and result
s in the following values for the bl; forward-backward asymmetry:
A(FB)(b (b) over bar) (89.55 GeV) = 0.068 +/- 0.016 (stat.) +/- 0.0013(syst
A(FB)(b (b) over bar)(91.26 GeV) = 0.0982 +/- 0.0047(stat.) +/- 0.0016(syst
A(FB)(b (b) over bar)(92.94 GeV) = 0.123 +/- 0.016 (stat.) +/- 0.0027(syst.
The b (b) over bar charge separation required for this analysis is directly
measured in the b tagged sample, while the other charge separations are ob
tained from a fragmentation model precisely calibrated to data. The effecti
ve weak mixing angle is deduced from the measurement to be:
sin(2)theta(eff)1 = 0.23186 +/- 0.00063.