A precise determination of the forward-backward asymmetry of b quarks: The final DELPHI result for A(FB)b using inclusive charge reconstruction and lifetime tagging

This thesis concerns a novel high precision measurement of the b quark forwardbackward asymmetry at the Z pole, which has been performed on a sample of 3,560,890 hadronic events collected with the DELPHI detector at CERN’s Large Electron Positron Collider in 1992 to 2000. As part of the analysis technique, an enhanced impact parameter tag provides a high purity b sample. For event hemispheres with a reconstructed secondary vertex the charge of the corresponding quark or anti-quark is determined using a neural network tag which combines in an optimal way the full available charge information from the vertex charge, the jet charge and from identified leptons and hadrons. The probability of correctly identifying b quarks and anti-quarks is measured on the data themselves comparing the rates of double hemisphere tagged like-sign and unlike-sign events. The b quark forward-backward asymmetry is determined from the differential asymmetry, taking small corrections due to hemisphere correlations and background contributions into account. The results for different centre-of-mass energies are: AbFB (89.449 GeV) = 0.0637 ± 0.0143(stat.) ± 0.0017(syst.) AbFB (91.231 GeV) = 0.0958 ± 0.0032(stat.) ± 0.0014(syst.) AbFB (92.990 GeV) = 0.1041 ± 0.0115(stat.) ± 0.0024(syst.) They correspond to an effective electroweak mixing angle of sin2θ leff = 0.23230 ± 0.00054 . Statistical correlations with other determinations of AbFB on the common data-sets are evaluated, and a combined DELPHI result: A0,bFB = 0.0990 ± 0.0029 obtained. It agrees with the results from other LEP experiments, and the combined value is interpreted in the framework of the Standard Model and beyond, taking into account also measurements at LEP2 energies above the Z pole.