Resonant Structure and Flavor-tagging in the $B\pi^{\pm}$ system using fully reconstructed $B$ decays

Exclusive reconstruction of $B^{**}\rightarrow B\pi^{\pm}$, with a mass resolution of only a few $MeV/c^2$, is an important tool for the study of $B^{**}$ spectroscopy. Therefore, with the \Aleph\ detector at LEP, we have looked for resonant structure in the $B\pi$ system with a data sample of almost 500 fully-reconstructed charged and neutral $B$ mesons. We observe a narrow structure above the expected background with a mass $m_{narrow}=5703\pm 14\, MeV/c^2$, and a width $\sigma_{narrow}=28^{+18}_{-14}\, MeV/c^2$. There is also a hint of a broader structure at lower mass. The total production rate is estimated to be $BR(b\rightarrow B^{**}\rightarrow B^{(*)}\pi)/BR(b\rightarrow B_{u,d})=(30\pm 8) \%$. In the same data sample, we have measured the efficiency and mistag rate for a $B$ flavor-tag based on the $\pi^{\pm}$ with the highest longitudinal momentum relative to the $B$ direction. We find a tag efficiency of $85\pm3\,\%$ and a mistag rate of $29\pm3\,\%$ and show that this tag reproduces the expected results in $B-\bar{B}$ mixing. This tag may well supplement, and cross-check other tags in CP violation and mixing studies in the $B$ system.