Measurement of the B0-B0 mixing parameter Δmd using semileptonic B0 decays

We present a measurement of the B 0 -B 0 mixing parameter Δm d using neutral B meson pairs in a 29.1 fb - 1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e + e - collider.We exclusively reconstruct one neutral B meson in the semileptonic B 0 → D* - + v decay mode and identify the flavor of the accompanying B meson from its decay products. From the distribution of the time intervals between the two flavor-tagged B meson decay points, we obtain Δm d = (0.494 ′ 0.012 ′ 0.015) ps - 1 , where the first error is statistical and the second error is systematic.