Evidence for the existence of the strange b-flavoured meson Bs0 in Z0 decays

We present evidence for the existence of the strange b-flavoured meson Bs0 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.) Ds−l+ pairs (or charge conjugate, l = e or l = μ) is found after background subtraction, where the Ds meson is detected in the øπ and K∗0 K final states. Interpreting this signal as coming from the semileptonic decay Bs0→Ds−l+vX and combining t he two decay modes of the Ds, we find f (b→Bs0)B(Bs0→Ds−l+vX)BDs−→φ π−) = [3.9±1.1 (stat.)±0.8(syst.)]×10−4, where f (b→Bs0) is the fraction of b quarks that results in a B0s meson in Z0 decays. This signal for the Bs0 is supported by our observation of an excess of 147 ± 48 inclusive Ds 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 Bs0→J/ψφ. Based on one candidate event the 90% confidence level upper limit is determined to be f (b → Bs0) B (Bs0 → J/ψφ) < 0.22%.