Measurement of the Branching Fractions and Forward-Backward Asymmetries of the Z^0 into Light Quarks

Abstract

Using approximately 4.3 million hadronic Z^0 decays collected with the OPAL detector at LEP between 1990 and 1995, we measure the branching fractions of the Z^0 into up-type and down-type light quarks, R_q, and the forward-backward asymmetries, A_FB(q), using high-momentum stable particles as a tag. Adopting a method that employs double tagged events to determine the flavour tagging efficiencies, and assuming the flavour independence of strong interactions and SU(2) isospin symmetry, we measure: R_{d,s}/(R_d + R_u + R_s) = 0.371 +/- 0.016(stat.) +/- 0.016(syst.) and A_FB(d,s) = 0.068 +/- 0.035(stat.) +/- 0.011(syst.), assuming the branching fractions and forward-backward asymmetries of down and strange quarks to be equal. The results are essentially free of assumptions based on hadronisation models. These results are in agreement with the Standard Model expectations and are used to infer the left and right handed couplings of strange/down quarks to the Z^0, yielding g_L^{d,s} = -0.44^{+0.13}_{-0.09} and g_R^{d,s} = +0.13^{+0.15}_{-0.17}. The results for the up quark, R_u/(R_d + R_u + R_s) = 0.258 +/- 0.031(stat.) +/- 0.032(syst.) and A_FB(u)= 0.040 +/- 0.067(stat.) +/- 0.028(syst.), are fully negatively correlated and almost completely positively correlated, respectively, with the corresponding down-type results.