Observation of new excited ${B} ^0_{s} $ states

A structure is observed in the $B^+K^-$ mass spectrum in a sample of proton--proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of 9 fb${}^-1$. The structure is interpreted as the result of overlapping excited $B_s^0$ states. With high significance, a two-peak hypothesis provides a better description of the data than a single resonance. Under this hypothesis the masses and widths of the two states, assuming they decay directly to $B^+K^-$, are determined to be \begin{align*} m_1 &= 6063.5 \pm 1.2 \text{ (stat)} \pm 0.8\text{ (syst) MeV}, \\ \Gamma_1 &= 26 \pm 4 \text{ (stat)} \pm 4\text{ (syst) MeV}, \\ m_2 &= 6114 \pm 3 \text{ (stat)} \pm 5\text{ (syst) MeV}, \\ \Gamma_2 &= 66 \pm 18 \text{ (stat)} \pm 21\text{ (syst) MeV}. \end{align*} Alternative values assuming a decay through $B^{*+}K^-$, with a missing photon from the $B^{*+} \rightarrow B^+\gamma$ decay, are also determined. The ratio of the total production cross-section times branching fraction of the new states relative to the previously observed $B_{s2}^{*0}$ state is determined to be $0.87 \pm 0.15 \text{ (stat)} \pm 0.19 \text{ (syst)}$.