Observation of $B_c^{\mathbf{+}}\to J/\psi D_s^{\mathbf{+}}$ and $B_c^{\mathbf{+}}\to J/\psi D_s^{*\mathbf{+}}$ decays

The decays $B_c^{+}\to J/\psi D_s^{+}$ and $B_c^{+}\to J/\psi D_s^{*+}$ are observed for the first time using a dataset, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$, collected by the LHCb experiment in proton-proton collisions at center-of-mass energies of $\sqrt{s}=7$ and 8 TeV. The statistical significance for both signals is in excess of 9 standard deviations. The following ratios of branching fractions are measured to be $\frac{\mathcal{B}(B_c^{+}\to J/\psi D_s^{+})}{\mathcal{B}(B_c^{+}\to J/\psi {\pi }^{+})}=2.90\pm 0.57\pm 0.24$, $\frac{\mathcal{B}(B_c^{+}\to J/\psi D_s^{*+})}{\mathcal{B}(B_c^{+}\to J/\psi D_s^{+})}=2.37\pm 0.56\pm 0.10$, where the first uncertainties are statistical and the second systematic. The mass of the $B_c^{+}$ meson is measured to be $m_{B_c^{+}}=6276.28\pm 1.44(\mathrm{stat})\pm 0.36(\mathrm{syst})\mathrm{MeV}/c^2$, using the $B_c^{+}\to J/\psi D_s^{+}$ decay mode.

Figure 1

Feynman diagrams for $B_c^{+}\to J/\psi D_s^{+}$ decays: (a) spectator, (b) color-suppressed spectator and (c) annihilation topology.

Figure 2

Mass distributions for selected $J/\psi D_s^{+}$ pairs. The solid curve represents the result of a fit to the model described in the text. The contribution from the $B_c^{+}\to J/\psi D_s^{*+}$ decay is shown with thin green dotted and thin yellow dash-dotted lines for the ${\mathcal{A}}_{\pm \pm }$ and ${\mathcal{A}}_{00}$ amplitudes, respectively. The inset shows a zoom of the $B_c^{+}$ mass region.

Figure 3

Mass distribution for selected $B_c^{+}\to J/\psi {\pi }^{+}$ candidates. The results of a fit to the model described in the text are superimposed (solid line) together with the background component (dotted line).

Figure 4

Distributions of (a) ${\chi }_{\mathrm{fit}}^2(B_c^{+})$ and (b) ${\chi }_{\mathrm{IP}}^2(B_c^{+})$ for $B_c^{+}\to J/\psi {\pi }^{+}$ events: background subtracted data (red points with error bars) and simulation (blue histogram).