B0B¯0mixing at theΥ(4S)

We have measured $B^0$B${¯}^0$ mixing by observing like-sign dilepton events in Υ(4S) decay. Assuming that the semileptonic branching fractions of the charged and neutral B mesons are equal and that the Υ(4S) decays to $B^{+}$ $B^{\mathrm{-}}$ 55% of the time and to $B^0$B${¯}^0$ 45% of the time, we measure the mixing parameter r to be 0.19±0.06±0.06, where the first error is statistical and the second is systematic.