We report a measurement of the B0-B̄0 mixing parameter Δmd based on a 29.1 fb-1 sample of Y(4S) resonance decays collected by the Belle detector at the KEKB asymmetric e +e- collider. We use events with a partially reconstructed B0(B̄0)→D*mnplus;π ± candidate and where the flavor of the accompanying B meson is identified by the charge of the lepton from a B0(B̄ 0)→X∓ℓ±v decay. The proper-time difference between the two B mesons is determined from the distance between the two decay vertices. From a simultaneous fit to the proper-time distributions for the same-flavor (B0(B̄0), ℓ±) and opposite-flavor (B0(B̄ 0),ℓ∓) event samples, we measure the mass difference between the two mass eigenstates of the neutral B meson to be Δmd = (0.509 ± 0.017 (stat) ± 0.020 (syst)) ps-1.
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