Study of B cð1PÞ + states in the B + c γ mass spectrum

The study of a wide peaking structure in the Bc+γ mass spectrum is reported using a data sample of proton-proton collisions collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9  fb-1. The observed structure is consistent with the lowest excited P-wave Bc+ states and exhibits a statistical significance exceeding 7 standard deviations relative to the background-only hypothesis. A two-peak model serves as an effective description of the data, with various theory-constrained models further explored to provide physical interpretation. Based on the predictions for the Bc(1P)+ spectrum, the relative production cross section of the overall Bc(1P)+ states with respect to the Bc+ ground state with the transverse momentum pT and rapidity y of Bc+ mesons in the regions pT<20  GeV/c and 2.0<y<4.5 at s=13  TeV is measured to be 0.20±0.03±0.02±0.03, where the uncertainty terms represent statistical, systematic, and uncertainties related to the choice of theoretical models, respectively. The results provide a test of theoretical models and deepen our understanding of quantum chromodynamics.