Observation of the decay $B_c^{\mathbf{+}}\to \psi \mathbf{(}2S\mathbf{)}{\pi }^{\mathbf{+}}$

The decay $B_c^{+}\to \psi (2S){\pi }^{+}$ with $\psi (2S)\to {\mu }^{+}{\mu }^{-}$ is observed with a significance of $5.2\sigma$ using $pp$ collision data corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$ collected by the LHCb experiment. The branching fraction of $B_c^{+}\to \psi (2S){\pi }^{+}$ decays relative to that of the $B_c^{+}\to J/\psi {\pi }^{+}$ mode is measured to be $\frac{\mathcal{B}(B_c^{+}\to \psi (2S){\pi }^{+})}{\mathcal{B}(B_c^{+}\to J/\psi {\pi }^{+})}=0.250\pm 0.068(\mathrm{stat})\pm 0.014(\mathrm{syst})\pm 0.006(\mathcal{B})$. The last term is the uncertainty on the ratio $\mathcal{B}(\psi (2S)\to {\mu }^{+}{\mu }^{-})/\mathcal{B}(J/\psi \to {\mu }^{+}{\mu }^{-})$.

Figure 1

Invariant mass distributions of $B_c^{+}$ candidates reconstructed as (a) $B_c^{+}\to J/\psi {\pi }^{+}$ and (b) $B_c^{+}\to \psi (2S){\pi }^{+}$. Points with error bars (black) show the data, the thick solid line (blue) represents the fit of the data, the dashed line (red) the signal distribution, the dotted line (green) the combinatorial background, the dot-dashed line (purple) the partially reconstructed background, and the thin solid line (light blue) the background from the $B_c^{+}\to J/\psi K^{+}$ channel.