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 $13~\text{ TeV}$, corresponding to an integrated luminosity of $9~\text{ fb}^{ -1}$. The observed structure is consistent with the lowest excited $P$-wave $B_{ c}^{ +}$ states and exhibits a statistical significance exceeding seven 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 $p_{ \text{ T}}$ and rapidity $y$ of $B_{ c}^{ +}$ mesons in the regions $p_{ \text{ T}}<20~\mathrm{ Ge\kern -0.1em V\!/}c$ and $2.0<y<4.5$ at $\sqrt{ s}=13~\text{ TeV}$ is measured to be $0.20 \pm 0.03 \pm 0.02 \pm 0.03$, 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.
Categories: hep-ex