Measurement of the Forward-backward Asymmetry of E(+)e(-)-]z-]b(b)over-bar Using Prompt Leptons and A Lifetime Tag

The forward-backward asymmetry of the process e(+)e(-) --> Z --> b ($) over bar b has been measured using events collected ee by the DELPHI experiment during the 1991 and 1992 LEP runs. This data sample corresponded to 884 000 hadronic Z decays at a centre-of-mass energy root s similar to M(Z). The tagging of b-quark events was performed using two approaches; the first was based on the semileptonic decay channels b --> X + mu and b --> X + e, the second used a lifetime tag with jet-charge reconstruction. The results of these two methods were combined to give A(FB)(b ($) over bar b) = 0.107 +/- 0.011(stat. + syst. + mixing). With the semileptonic sample, the forward-backward asymmetry of the process e(+)e(-) --> Z --> c ($) over bar c was also measured to be A(FB)(c ($) over bar c) = 0.083 +/- 0.022(stat.) +/- 0.016(syst.). The effective value of the Weinberg mixing angle derived from these measurements was sin(2) theta(eff)(lep) = 0.2294 +/- 0.0021.