Observation of orbitally excited $B_c^{+}$ states

The observation of a wide peaking structure in the $B_c^{+}\gamma$ mass spectrum is reported using proton-proton collision data collected by the LHCb detector at center-of-mass energies of $7$, $8$, and $13\text{ TeV}$, corresponding to a total integrated luminosity of $9{\text{fb}}^{-1}$. 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 $6704.8\pm 5.5\pm 2.8\pm 0.3\mathrm{M}\mathrm{e}\mathrm{V}\text{\hspace{0.17em}⁣}\mathrm{/}{c}^2$ and $6752.4\pm 9.5\pm 3.1\pm 0.3\mathrm{M}\mathrm{e}\mathrm{V}\text{\hspace{0.17em}⁣}\mathrm{/}{c}^2$. 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.