Measurement ofBd0−B¯d0Mixing Rate from the Time Evolution of Dilepton Events at theϒ(4S)

We report a determination of the ${B}_{d}^{0}\ensuremath{-}{\overline{B}}_{d}^{0}$ mixing parameter $\ensuremath{\Delta}{m}_{d}$ based on the time evolution of dilepton yields in $\ensuremath{\Upsilon}(4S)$ decays. The measurement is based on a $5.9{\mathrm{fb}}^{\ensuremath{-}1}$ data sample collected by the Belle detector at KEKB. The proper-time difference distributions for same-sign and opposite-sign dilepton events are simultaneously fitted to an expression containing $\ensuremath{\Delta}{m}_{d}$ as a free parameter. Using both muons and electrons, we obtain $\ensuremath{\Delta}{m}_{d}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.463\ifmmode\pm\else\textpm\fi{}0.008(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.016(\mathrm{syst}){\mathrm{ps}}^{\ensuremath{-}1}$. This is the first determination of $\ensuremath{\Delta}{m}_{d}$ from time evolution measurements at the $\ensuremath{\Upsilon}(4S)$. We also place limits on possible $\mathrm{CPT}$ violations.