Search for B0_s - anti-B0_s oscillations in DELPHI using high-p_t leptons

Oscillations in the B0_s - anti-B0_s system were studied in events selected from about 4.3 million hadronic Z^0 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 B0_s oscillations was observed and limits on the mass difference between the physical B0_s states were obtained to be: \Delta m_s > 8.0 ps^{-1} at the 95% C.L. with a sensitivity of \Delta m_s = 9.1 ps^{-1} in the high p_t lepton analysis and \Delta m_s > 4.9 ps^{-1} at the 95% C.L. with a sensitivity of \Delta m_s = 8.6 ps^{-1} in the D_s-lepton analysis. Previously published results on these analyses are superseed. The combination of these results with those obtained in other independent analyses previously performed in DELPHI (D_s-hadron, exclusive B0_s, inclusive vertex) gives: \Delta m_s > 8.5 ps^{-1} at the 95% C.L. with a sensitivity of \Delta m_s = 12.0 ps^{-1}.