Simultaneous Measurement of the B0 Meson Lifetime and Mixing Frequency with B0 --> D*- l+ nu Decays

We measure the B0 lifetime \tau_{B0} and the B0-B0bar oscillation frequency \Delta m_d with a sample of approximately 14,000 exclusively reconstructed B0 --> D*- l+ nu signal events, selected from 23 million BBbar pairs recorded at the \Upsilon(4S) resonance with the BABAR detector at the Stanford Linear Accelerator Center. The decay position of the other B is determined with the remaining tracks in the event, and its b-quark flavor at the time of decay is determined with a tagging algorithm that exploits the correlation between the flavor of the b-quark and the charges of its decay products. The lifetime and oscillation frequency are measured simultaneously with an unbinned maximum-likelihood fit that uses, for each event, the measured difference in decay times of the two B mesons (\Delta t), the calculated uncertainty on \Delta t, the signal and background probabilities, and b-quark tagging information for the other B. The results are \tau_{B^0} = (1.523^{+0.024}_{-0.023} \pm 0.022) \rm{ps} and \Delta m_d = (0.492 \pm 0.018 \pm 0.013) \rm{ps}^{-1}. The statistical correlation coefficient between \tau_{B0} and \Delta m_d is -0.22.