Observation of the Decay $B_c^{\mathbf{+}}\mathbf{\to }{B}_s^0{\pi }^{\mathbf{+}}$

The result of a search for the decay $B_c^{+}\to B_s^0{\pi }^{+}$ is presented, using the $B_s^0\to D_s^{-}{\pi }^{+}$ and $B_s^0\to J/\psi \varphi$ channels. The analysis is based on a data sample of $pp$ collisions collected with the LHCb detector, corresponding to an integrated luminosity of $1{\mathrm{fb}}^{-1}$ taken at a center-of-mass energy of 7 TeV, and $2{\mathrm{fb}}^{-1}$ taken at 8 TeV. The decay $B_c^{+}\to B_s^0{\pi }^{+}$ is observed with significance in excess of 5 standard deviations independently in both decay channels. The measured product of the ratio of cross sections and branching fraction is $[\sigma (B_c^{+})/\sigma (B_s^0)]\times \mathcal{B}(B_c^{+}\to B_s^0{\pi }^{+})=[2.37\pm 0.31(\mathrm{stat})\pm 0.11(\mathrm{syst}{)}_{-0.13}^{+0.17}({\tau }_{B_c^{+}})]\times {10}^{-3}$, in the pseudorapidity range $2<\eta (B)<5$, where the first uncertainty is statistical, the second is systematic, and the third is due to the uncertainty on the $B_c^{+}$ lifetime. This is the first observation of a $B$ meson decaying to another $B$ meson via the weak interaction.

Figure 1

Leading-order Feynman diagram of the decay $B_c^{+}\to B_s^0{\pi }^{+}$.

Figure 2

Invariant mass distributions of (a) $B_s^0\to D_s^{-}{\pi }^{+}$ and (b) $B_s^0\to J/\psi \varphi$ candidates. The different components are defined in the legend.

Figure 3

$B_c^{+}$ mass fits for the combined 2011 and 2012 data sets for (a) $B_c^{+}\to B_s^0(\to D_s^{-}{\pi }^{+}){\pi }^{+}$ and (b) $B_c^{+}\to B_s^0(\to J/\psi \varphi ){\pi }^{+}$ candidates. The different components are indicated in the legends.