Observation of an Excited B_c^+ 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 sqrt[s]=7, 8, and 13 TeV, the observation of an excited B_c^+ state in the B_c^+π^+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/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^*(2^3S_1)^+ state reconstructed without the low-energy photon from the B_c^*(1^3S_1)^+→B_c^+γ decay following B_c^*(2^3S_1)^+→B_c^*(1^3S_1)^+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/c^2, and is consistent with the B_c(2^1S_0)^+ state. These mass measurements are the most precise to date