Resonant structure and flavour tagging in the $B\pi^{\pm}$ system using fully reconstructed $B$ decays

Starting from a sample of four million hadronic $Z$ decays collected with the ALEPH detector at LEP, 404 charged and neutral $B$ mesons are fully reconstructed and used to look for resonant structure in the $B\pi$ system. An excess of events is observed above the expected background in the $B\pi$ mass spectrum at a mass $\approx 5.7$ GeV/c$^2$, consistent with the production and decay to $B^{(*)}\pi$ of the $B^{**}$ states predicted by Heavy Quark Symmetry (HQS). In the framework of HQS, it is found that the mass of the $B_2^*$ state is $(5739^{+\ 8}_{-11}(\mathrm{stat})^{+6}_{-4}(\mathrm{syst}))\,$MeV/c$^2$ and the relative production rate of the $B^{**}$ system is $BR(b \rightarrow B^{**} \to B^{(*)}\pi)/BR(b \rightarrow B_{u,d}) = (31\pm 9 \mathrm{(stat)}^{+6}_{-5}\mathrm{(syst))}\%$. In the same sample of $B$ mesons, significant $B\pi^{\pm}$ charge-flavour correlations are observed, which may prove important for tagging the initial $B$ state in future CP violation studies.