Measurement of the B^0 -\bar{B}^0 mixing rate with B^0(\bar{B}^0) \to D^{*\mp} π^{\pm} partial reconstruction
Abstract
We report a measurement of the B^0-\bar{B}^0 mixing parameter \Delta m_d based on a \rm 29.1~fb^{-1} sample of \Upsilon (4S) resonance decays collected by the Belle detector at the KEKB asymmetric e^+ e^- collider. We use events with a partially reconstructed B^0(\bar{B}^0) \to D^{*\mp} \pi^{\pm} candidate and where the flavor of the accompanying B meson is identified by the charge of the lepton from a B^0(\bar{B}^0) \to X^{\mp} \ell^{\pm} \nu 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 (B^0(\bar{B}^0), \ell^{\pm}) and opposite-flavor (B^0(\bar{B}^0), \ell^{\mp}) event samples, we measure the mass difference between the two mass eigenstates of the neutral B meson to be \Delta m_d= (0.509 \pm 0.017~(\rm stat) \pm 0.020~(\rm syst))~ps^{-1}.