  A search for excited states of the $B^{\pm}_c$ meson is performed using 4.9
fb$^{-1}$ of 7 TeV and 19.2 fb$^{-1}$ of 8 TeV $pp$ 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
$B_c^{\pm} \to J/\psi \pi^{\pm}$. 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 $S\mbox{-}$wave state of the $B^{\pm}_c$
meson, $B^{\pm}_c(2S)$.
