Improved measurement of the $\overline{B}^{0}$ and B$^{-}$ meson lifetimes

The lifetimes of the \b0 \ and \bp\ mesons have been measured with the \aleph\ detector at LEP, using approximately 3 million hadronic Z decays collected in the period 1991--1994. In the first of three methods, semileptonic decays of \b0 \ and \bp\ mesons were partially reconstructed by identifying events containing a lepton with an associated \ds\ or \d0 \ meson. The second method used fully reconstructed \bz\ and \bp\ mesons. The third method, used to measure the \bz\ lifetime, employed a partial reconstruction technique to identify \bz\to D^{*+}\pi^- X decays. The combined results are \begin{eqnarray*} \t0 & = & 1.55\pm 0.06\pm 0.03 \mathrm{\ ps}, \\ \tp & = & 1.58\pm 0.09\pm 0.03 \mathrm{\ ps}, \\ \trat & = & 1.03\pm 0.08\pm 0.02.