Measurement of the $\rm B^+$ and $\rm B^0$ lifetimes and search for CP(T) violation using reconstructed secondary vertices

Abstract

The lifetimes of the $\rm B^+$ and $\rm B^0$ mesons, and their ratio, have been measured in the OPAL experiment using 2.4 million hadronic $\rm Z^0$ decays recorded at LEP. $\rm Z^0 \rightarrow \rm b\overline{b}$ decays were tagged using displaced secondary vertices and high momentum electrons and muons. The lifetimes were then measured using well-reconstructed charged and neutral secondary vertices selected in this tagged data sample. The results are \begin{eqnarray*} \tau_{B^+} & = & 1.643 \pm 0.037\pm 0.025 \rm\,ps \tau_{B^0} & = & 1.523 \pm 0.057 \pm 0.053 \rm\,ps \tau_{B^+}/ \tau_{B^0} & = & 1.079 \pm 0.064 \pm 0.041 \,, \end{eqnarray*} where in each case the first error is statistical and the second systematic. A larger data sample of 3.1 million hadronic $\rm Z^0$ decays has been used to search for CP and CPT violating effects by comparison of inclusive b and $\rm\overline{b}$ hadron decays. No evidence for such effects is seen. The CP violation parameter $\rm Re(\epsilon_B)$ is measured to be \[ \rm Re(\epsilon_B) = 0.001 \pm 0.014 \pm 0.003 \] and the fractional difference between b and $\rm\overline{b}$ hadron lifetimes is measured to be \[ \left(\frac{\Delta\tau}{\tau}\right)_{\rm b} \equiv \frac{\rm\tau (b \mbox{hadron}) - \tau \mbox{(\rm\overline{b} hadron)}} {\rm\tau (average)} = -0.001\pm 0.012\pm 0.008 \,. \]