Measurement of Bd0-(B)over-bard0 mixing rate from the time evolution of dilepton events at the Υ(4S)

We report a determination of the B-d(n)-N-n(d) mixing parameter Deltam(d) based on the time evolution of dilepton yields in Y (4S) decays. The measurement is based on a 5.9 fb(-1) data sample collected by the Belie detector at KEKB. The proper-time difference distributions for same-sign and opposite-sign dilepton events are simultaneously fitted to an expression containing Deltam(d) as a free parameter. Using both muons and electrons, we obtain Deltam(d) = 0.463 +/- 0.008 (stat) +/- 0.016 (syst) ps(-1). This is the first determination of Deltam(d) from time evolution measurements at the Y (4S). We also place limits on possible CPT violations.