2026/05/15 by ATLAS Collaboration · 1 voice · 1 citation
#hep-ex
The first observation of a new meson state decaying into Bc+ and a photon is presented using a dataset of pp collisions at a centre-of-mass energy √(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.)+1.0-1.4(syst.) MeV. This corresponds to a mass for the observed state of 6339.0 ± 1.4(stat.)+1.0-1.4(syst.)± 0.3(mBc+) 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 bc quarkonium system, the Bc*+ meson.