Observation of an Excited $B_c^\pm$ Meson State with the ATLAS Detector

A search for excited states of the $B_c^\pm$ meson is performed using 4.9 $\mathrm{fb}^{-1}$ of 7 TeV and 19.2 $\mathrm{fb}^{-1}$ of 8 TeV \emph{pp} collision data collected by the ATLAS experiment at the LHC. A new state is observed through its hadronic transition to the ground state, with the latter detected in the decay $B_c^\pm$ $\rightarrow$ $J/\psi\pi^\pm$. The state appears in the $m(B_c^\pm{\pi^+\pi^-}) - m(B_c^\pm) - 2m(\pi^\pm)$ mass difference distribution with a significance of 5.2 standard deviations. The mass of the observed state is 6842 $\pm$ 4 $\pm$ 5 MeV, where the first error is statistical and the second is systematic. The mass and decay of this state are consistent with expectations for the second \emph{S}-wave state of the $B_c^\pm$ meson, $B_c^\pm(2S)$.