, Volume 16, Issue 4, pp 555-578

Measurement of the ${\mathrm{B^0_s}}$ lifetime and study of ${\mathrm{B^0_s}}$ - ${\overline{\mathrm{B^0_s}}}$ oscillations using ${{\mathrm{ D_s}}\ell}$ events

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Lifetime and oscillations of ${\rm B^0_s}$ mesons have been studied in events with a large transverse momentum lepton and a ${\rm D_s}$ of opposite electric charge in the same hemisphere, selected from about 3.6 million hadronic ${\rm Z^0}$ decays accumulated by DELPHI between 1992 and 1995. The ${\rm B^0_s}$ lifetime and the fractional width difference between the two physical ${\rm B^0_s}$ states have been found to be:

\[ \tau_{{\rm B^0_s}} = (1.42^{+0.14}_{-0.13}(stat.)\pm 0.03(syst.)) ps \] \[ \Delta \Gamma_{\rm B^0_s}/\Gamma_{\rm B^0_s} < 0.46 \mbox{at the 95\% C.L.} \]

In the latter result it has been assumed that $\tau_{{\rm B^0_s}}=\tau_{\rm B_d^}$ . Using the same sample, a limit on the mass difference between the physical $\mbox{B}^0_{\mathrm s}$ states has been set:

\begin{eqnarray*} \begin{array}{ll} \Delta m_{\rm B^0_s} > 7.4 \mbox{ps}^{-1} \mbox{at the 95\% C.L.} & \end{array} \end{eqnarray*}

with a corresponding sensitivity equal to $8.1 ps^{-1}$ .

Received: 7 February 2000 / Revised version: 23 March 2000 / Published online: 8 June 2000