Forward-backward $b$-quark asymmetry at the Z pole: QCD uncertainties redux

The forward-backward asymmetry of b-quarks measured at LEP in e + e − collisionsat the Z pole, A 0,b fb | exp = 0.0992 ± 0.0016, remains today the electroweakprecision observable with the largest disagreement (2.8σ) with the StandardModel theoretical prediction, A 0,b fb | th = 0.1037 ± 0.0008. The dominantsystematic uncertainties are due to QCD effects — b, c-quark showering andfragmentation, and B, D meson decay models — which have not been revisited inthe last 20 years. We reassess the QCD uncertainties of the eight original LEPmeasurements of A 0,b fb , using modern parton shower simulations based onpythia 8 and pythia 8 + vincia with different tunes of soft and collinearradiation as well as of hadronization. Our analysis indicates QCD uncertainties,of order ±0.4% and ±1% for the jet-charge and lepton-charge based analyses, thatare overall slightly smaller but still consistent with the original ones. Usingthe updated QCD systematic uncertainties, we obtain A 0,b fb = 0.0996 ± 0.0016.