Observation of a B c * + Meson with the ATLAS Detector

The first observation of a new meson state decaying into Bc⁺ 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 Bc⁺ mesons are reconstructed from a three-muon final state from Bc⁺→J/ψ(μ⁺μ⁻)μ⁺νμX 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 Bc⁺ 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_{Bc⁺}) MeV, where the last uncertainty is due to the precision of the best knowledge of the Bc⁺ meson mass. The low mass difference value matches the theory expectations for the lowest vector state of the b[over ¯]c system, the Bc*+ meson.