Preliminary measurement of Rs=Γs―s/Γhad using the DELPHI detector at LEP 100

Abstract

A measurement of the branching fraction R s =Γ ss̄ /Γ had of the Z 0 into strange quarks was made. For this measurement energetic φ 0 mesons decaying into K + K − and charged kaons were used to tag the presence of e + e − → Z 0 → ss̄ events. 1.4 million hadronic Z 0 events collected by the DELPHI detector in 1994 have been used for this measurement. The Ring Imaging Cherenkov detector (RICH) was used to identify the charged kaons. For φ 0 ’s with a fraction of momentum higher than 0.5 a ss̄ event purity of 77% was reached while for charged kaon analysis this purity was at the level of 40%. The ratio R s was measured to be R s = 0.233 ± 0.003(stat.) ± 0.024(syst.) in agreement with the Standard Model value and the already measured value of R_b.