Study of 𝐵_𝑐⁢(1⁢𝑃)⁺ states in the 𝐵⁺_𝑐⁢𝛾 mass spectrum

The study of a wide peaking structure in the B-c(+)gamma 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 B-c(+) 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 B-c(1P)(+) spectrum, the relative production cross section of the overall B-c(1P)(+) states with respect to the B-c(+) ground state with the transverse momentum p(T) and rapidity y of B-c(+) mesons in the regions p(T) < 20 GeV/c and 2.0 < y < 4.5 at root 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.