Observation of Orbitally Excited $B_c^+$ States

The observation of a wide peaking structure in the B c + γ mass spectrum is reported using proton-proton collision data collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to a total integrated luminosity of 9 fb − 1 . The statistical significance over the background-only hypothesis exceeds seven standard deviations. The width of the observed structure is larger than the expectation from a single-peak hypothesis, and is well described by an effective minimal model consisting of two narrow peaks located at 6704.8 ± 5.5 ± 2.8 ± 0.3 MeV / c 2 and 6752.4 ± 9.5 ± 3.1 ± 0.3 MeV / c 2 . The uncertainty terms are statistical, systematic, and associated to the knowledge of the B c + mass, respectively. The measured peak locations are in line with theoretical predictions for lowest excited P -wave B c + states, marking the first observation of orbitally excited beauty-charm mesons and providing important insights into the internal dynamics of hadrons containing two heavy quarks.