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

Abstract

Using $pp$ collision data corresponding to an integrated luminosity of $8.5{\mathrm{fb}}^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of $\sqrt{s}=7$, $8$ and $13\mathrm{Te}\mathrm{V}$, 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 $6841.2\pm 0.6(\mathrm{stat})\pm 0.1(\mathrm{syst})\pm 0.8(B_c^{+})\mathrm{MeV}/c^2$, 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 $6872.1\pm 1.3(\mathrm{stat})\pm 0.1(\mathrm{syst})\pm 0.8(B_c^{+})\mathrm{MeV}/c^2$, and is consistent with the $B_c(2^1{S}_0{)}^{+}$ state. These mass measurements are the most precise to date.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2019-007.html

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