Observation of new excited $B_s^0$ states

Abstract

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 $m_1=6063.5\pm 1.2\text{ (stat)}\pm 0.8\text{ (syst) MeV},$ ${\Gamma }_1=26\pm 4\text{ (stat)}\pm 4\text{ (syst) MeV},$ $m_2=6114\pm 3\text{ (stat)}\pm 5\text{ (syst) MeV},$ ${\Gamma }_2=66\pm 18\text{ (stat)}\pm 21\text{ (syst) MeV}.$ Alternative values assuming a decay through $B^{\ast +}{K}^{-}$, with a missing photon from the $B^{\ast +}\to B^{+}\gamma$ decay, which are shifted by approximately 45 MeV are also determined. The possibility of a single state decaying in both channels is also considered. The ratio of the total production cross-section times branching fraction of the new states relative to the previously observed $B_{s2}^{\ast 0}$ state is determined to be $0.87\pm 0.15\text{ (stat)}\pm 0.19\text{ (syst)}$.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2020-026.html (LHCb public pages)