Evidence for the existence of the strange b-flavoured meson B_s ⁰ in Z⁰ decays

We present evidence for the existence of the strange b-flavoured meson B_s ⁰ in a data sample of 470 628 hadronic Z⁰ decays recorded with the OPAL detector at LEP. A signal of 18.3±5.2 (stat.)±0.9(syst.) D_s ^-l⁺ pairs (or charge conjugate, l = e or l = μ) is found after background subtraction, where the D_s meson is detected in the øπ and K^*0 K final states. Interpreting this signal as coming from the semileptonic decay B_s ⁰→D_s ^-l⁺vX and combining t he two decay modes of the D_s, we find f (b→B_s ⁰)B(B_s ⁰→D _s ^-l⁺vX)BD_s ^-→φ π^-) = [3.9±1.1 (stat.)±0.8(syst.)]×10^-4, where f (b→B_s ⁰) is the fraction of b quarks that results in a B⁰ _s meson in Z⁰ decays. This signal for the B_s ⁰ is supported by our observation of an excess of 147 ± 48 inclusive D_s mesons in the φπ and K^*0K modes above the number expected from B⁰ and B⁺ decays and from the fragmentation of primary c quarks. In addition, a search is made for the exclusive decay B_s ⁰→J/ψφ. Based on one candidate event the 90% confidence level upper limit is determined to be f (b → B_s ⁰) B (B_s ⁰ → J/ψφ) < 0.22%.