Observation of a B_{c}^{*+} Meson with the ATLAS Detector

The first observation of a new meson state decaying into B_{c}^{+} and a photon is presented using a dataset of pp collisions at a center-of-mass energy sqrt[s]=13  TeV collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140  fb^{-1}. The B_{c}^{+} mesons are reconstructed from a three-muon final state from B_{c}^{+}→J/ψ(μ^{+}μ^{-})μ^{+}ν_{μ}X decays, along with photons reconstructed via conversions to electron-positron pairs in the detector material. The new state is observed with a significance that exceeds 8 standard deviations. The mass difference between the new meson state and the ground-state B_{c}^{+} meson is measured to be 64.5±1.4(stat) _{-1.4}^{+1.0}(syst)  MeV. This corresponds to a mass for the observed state of 6339.0±1.4(stat) _{-1.4}^{+1.0}(syst)±0.3(m_{B_{c}^{+}})   MeV, where the last uncertainty is due to the precision of the best knowledge of the B_{c}^{+} meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the b[over ¯]c system, the B_{c}^{*+} meson.