Production asymmetry of strange quarks in electron-positron annihilation at LEP

Sammanfattning: The forward-backward asymmetry in the production of strange quarks in electron-positron annihilation has been measured at a centre-of-mass energy corresponding to the mass of the Z0 particle. The asymmetry at this energy is highly sensitive to the ratio between the vector and axial-vector couplings of the quark to the Z0 and to the value of the electroweak mixing angle sin2 θw. Together with the previously measured value of the forward-backward asymmetry of the bottom quark, the measurement provides a test of the important theoretical prediction that the couplings to the Z0 are universal for all down-type quarks.The analysis is based on 1.4 million hadronic Z0 decays recorded in 1994 with the DELPHI detector system at the LEP collider at CERN. The strange quarks are selected using high-momentum charged kaons identified by the Ring Imaging CHerenkov (RICH) detectors. The charged kaon momentum is used to evaluate the probability for an event to belong to one of six possible event classes. Five classes are determined by the flavour of the primary quark (up, down, charm, strange and bottom) and the sixth class contains misidentified particles. The forward-backward asymmetry is determined using a maximum-likelihood method. At the Z0 pole, the asymmetry is found to beAFB0, s = 0.105 ± 0,016(stat.) ± 0.0006(syst.)which is the most precise measurement available for the strange quark asymmetry. The result is consistent with the previously measured value of the asymmetry of, the bottom quark, and thus with the predicted universality of the down-type quark couplings to the Z0. The obtained asymmetry corresponds to an electroweak mixing angle sin2 θw = 0.2229 ± 0.0030.