Observation of orbitally excited $B$ mesons in $\mathit{p}\overline{p}$ collisions at $\sqrt{\mathit{s}}=1.8\mathrm{}\mathbf{\hspace{0.5em}}\mathbf{TeV}$

We measure the relative rate of production of orbitally excited $\mathrm{}(L=1\mathrm{})\mathrm{}$ states of B mesons ${(B}^{**})$ by observing their decays into $B{\pi }^{\pm }.$ We reconstruct B mesons through semileptonic decay channels using data collected in $p\overline{p}$ collisions at $\sqrt{s}=1.8\mathrm{}\mathrm{TeV}.$ The fraction of light B mesons that are produced as ${L=1B}^{**}$ states is measured to be $0.28\pm 0.06\mathrm{}\left(\mathrm{stat}\right)\mathrm{}\pm 0.03\mathrm{}\left(\mathrm{syst}\right)\mathrm{}.$ We also measure the collective mass of the $B^{**}$ states, and quantify the result by quoting the (model-dependent) mass of the lowest $B^{**}$ state to be ${m(B}_1)=5.71\mathrm{}\pm 0.02\mathrm{GeV}{/c}^2.$