Observation of Two Excited B$^+_\mathrm{c}$ States and Measurement of the B$^+_\mathrm{c}$(2S) Mass in pp Collisions at $\sqrt{s} =$ 13 TeV
Phys.Rev.Lett. 122 (2019) 132001
The collaboration

Abstract (data abstract)
CERN-LHC. Signals consistent with the $B_{c}^{+}(2S)$ and $B_{c}^{+*}(2S)$ states are observed in proton-proton collisions at $\sqrt{s} = 13$ TeV, in an event sample corresponding to an integrated luminosity of $143$ fb$^{-1}$, collected by the CMS experiment during the 2015--2018 LHC running periods. These excited $\bar{b}c$ states are observed in the $B_{c}^{+}\pi\pi$ invariant mass spectrum, with the ground state $B_{c}^{+}$ reconstructed through its decay to $J/\psi\pi$. The two states are reconstructed as two well-resolved peaks, separated in mass by $29.1 \pm 1.5(stat) \pm 0.7(syst)$ MeV, and are observed with a significance exceeding five standard deviations. The mass of the $B_{c}^{+}(2S)$ meson is measured to be $6871.0 \pm 1.2(stat) \pm 0.8(syst) \pm 0.8 \, (B_{c})$ MeV, where the last term corresponds to the uncertainty in the world-average $B_{c}^{+}$ mass.