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 centre-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 $B_c^+\to J/\psi(\mu^+\mu^-)\mu^+\nu_\mu X$ 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 $64.5 \pm 1.4\text{(stat.)}^{+1.0}_{-1.4}\text{(syst.)}$ MeV. This corresponds to a mass for the observed state of $6339.0 \pm 1.4\text{(stat.)}^{+1.0}_{-1.4}\text{(syst.)}\pm 0.3(m_{B_c^+})$ 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 $\bar{b}c$ quarkonium system, the $B_c^{*+}$ meson.