Search for $\mbox{B}^0_s$- $\overline{\mbox{B}^0_s}$ oscillations in DELPHI using high-p _t leptons

Abstract.

Oscillations in the B ⁰ _s - $\overline{{B}^0_s}$ system were studied in events selected from about 4.3 million hadronic Z⁰ decays registered by DELPHI between 1992 and 2000. This paper presents updates of two published analyses ([11, 12]). The first analysis, which utilizes leptons emitted with large momentum transverse to a jet, was improved by means of a better algorithm for the vertex reconstuction and a new algorithm for flavour-tagging at production time. The second analysis, which utilizes D _s -lepton events, was improved by optimizing the treatment of proper time resolution. No signal of B ⁰ _s oscillations was observed and limits on the mass difference between the physical B ⁰ _s states were obtained to be: $\begin{array}{@{}l@{}} \Delta m_s > 8.0\, {\mathrm{{ps}^{-1}~at~the~95\%~C.L.}}\\ {\mathrm{with~a~sensitivity~of}}~~\Delta m_s = 9.1\,{\mathrm{ps}^{-1}} \end{array}$ in the high p _t lepton analysis and $\begin{array}{@{}l@{}} \Delta m_s > 4.9\, {\mathrm{{ps}^{-1}~at~the~95\%~C.L.}}\\ {\mathrm{with~a~sensitivity of}}~~\Delta m_s = 8.6\,{\mathrm{ps}}^{-1} \end{array}$ in the D _s -lepton analysis. Previously published results on these analyses are superseded. The combination of these results with those obtained in other independent analyses previously performed in DELPHI (D _s -hadron, exclusive B _s ⁰, inclusive vertex) gives: $\begin{array}{@{}l@{}} \Delta m_s > 8.5\,{\mathrm{{ps}^{-1}~at~the~95\%~C.L.}} \\ {\mathrm{with~a~sensitivity~of}}~~ \Delta m_s = 12.0\,{\mathrm{ps}}^{-1}. \end{array}$