A search for excited states of the $B_{c}^{\pm }$ 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 $B_{c}^{\pm } \rightarrow J/\psi \pi ^{\pm }$. The state appears in the $m\left ( B_{c}^{\pm }\pi ^{+}\pi ^{-} \right )-m\left ( B_{c}^{\pm } \right )-2\left ( \pi ^{\pm } \right )$ 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-wave state of the $B_{c}^{\pm }$ meson, $B_{c}^{\pm } \left ( 2S \right )$.
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Wydział Fizyki, Astronomii i Informatyki Stosowanej : Instytut Fizyki im. Mariana Smoluchowskiego