Observation of an Excited Bc+ State

Abstract

Using pp collision data corresponding to an integrated luminosity of 8.5 fb−1 recorded by the LHCb experiment at center-of-mass energies of √s=7, 8, and 13 TeV, the observation of an excited B+c state in the B+cπ+π− invariant-mass spectrum is reported. The observed peak has a mass of 6841.2±0.6(stat)±0.1(syst)±0.8(B+c) MeV/c2, where the last uncertainty is due to the limited knowledge of the B+c mass. It is consistent with expectations of the B∗c(23S1)+ state reconstructed without the low-energy photon from the B∗c(13S1)+→B+cγ decay following B∗c(23S1)+→B∗c(13S1)+π+π−. A second state is seen with a global (local) statistical significance of 2.2σ (3.2σ) and a mass of 6872.1±1.3(stat)±0.1(syst)±0.8(B+c) MeV/c2, and is consistent with the Bc(21S0)+ state. These mass measurements are the most precise to date.