Observation of an excited $B_c^{+}$ state

Using $pp$ collision data corresponding to an integrated luminosity of $8.5{\mathrm{f}\mathrm{b}}^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of $\sqrt{s}=7$, $8$ and $13\mathrm{T}\mathrm{e}\mathrm{V}$, the observation of an excited $B_c^{+}$ state in the $B_c^{+}{\pi }^{+}{\pi }^{-}$ invariant-mass spectrum is reported. The observed peak has a mass of $6841.2 \pm 0.6 {,\rm (stat),} \pm 0.1 {,\rm (syst),} \pm 0.8,(B_c^+) $\mathrm{M}\mathrm{e}\mathrm{V}$/c^2$, where the last uncertainty is due to the limited knowledge of the $B_c^{+}$ mass. It is consistent with expectations of the $B_c^{\ast }(2^3{S}_1{)}^{+}$ state reconstructed without the low-energy photon from the $B_c^{\ast }(1^3{S}_1{)}^{+}\to B_c^{+}\gamma$ decay following $B_c^{\ast }(2^3{S}_1{)}^{+}\to B_c^{\ast }(1^3{S}_1{)}^{+}{\pi }^{+}{\pi }^{-}$. A second state is seen with a global (local) statistical significance of $2.2\sigma$ ($3.2\sigma$) and a mass of $6872.1 \pm 1.3 {,\rm (stat),} \pm 0.1 {,\rm (syst),} \pm 0.8,(B_c^+) $\mathrm{M}\mathrm{e}\mathrm{V}$/c^2$, and is consistent with the $B_c(2^1{S}_0{)}^{+}$ state. These mass measurements are the most precise to date.