Observation of $B_c^{+}\to D^0{K}^{+}$ Decays

Using proton-proton collision data corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, recorded by the LHCb detector at center-of-mass energies of 7 and 8 TeV, the $B_c^{+}\to D^0{K}^{+}$ decay is observed with a statistical significance of 5.1 standard deviations. By normalizing to $B^{+}\to {\overline{D}}^0{\pi }^{+}$ decays, a measurement of the branching fraction multiplied by the production rates for $B_c^{+}$ relative to $B^{+}$ mesons in the LHCb acceptance is obtained, $R_{D^{0}K}=(f_c/f_u)\times \mathcal{B}(B_c^{+}\to D^0{K}^{+})=(9.3_{-2.5}^{+2.8}\pm 0.6)\times {10}^{-7}$, where the first uncertainty is statistical and the second is systematic. This decay is expected to proceed predominantly through weak annihilation and penguin amplitudes, and is the first $B_c^{+}$ decay of this nature to be observed.

Figure 1

Tree (a), penguin (b), and weak annihilation (c) diagrams for the decays studied. In each case, the meson appearing before the comma denotes the favored decay.

Figure 2

Results of the simultaneous fit to the $D^0{K}^{+}$ (top plot) and $D^0{\pi }^{+}$ (bottom plot) invariant mass distributions in the $B_c^{+}$ mass region, including the $D^0\to K^{-}{\pi }^{+}$ and $D^0\to K^{-}{\pi }^{+}{\pi }^{-}{\pi }^{+}$ final states. Inclusion of the charge conjugate decays is implied. The red solid curve illustrates $B_c^{+}\to D^0{K}^{+}$ decays, the red dashed curve illustrates $B_c^{+}\to D^{*0}{K}^{+}$ decays, the green dashed curve represents $B_c^{+}\to D^{*0}{\pi }^{+}$ decays, the gray shaded region represents partially reconstructed background decays, the cyan dashed line represents the combinatorial background, and the total PDF is displayed as a blue solid line. The small drop visible in the total $B_c^{+}\to D^{(*)0}{\pi }^{+}$ PDF around the $B_c^{+}$ mass arises from the fact that the fit finds a small negative value for the $B_c^{+}\to D^0{\pi }^{+}$ yield.

Figure 3

${\mathrm{CL}}_{\mathrm{s}}$ $p$-value distributions for the $R_{D^{0}h}$ observables. The dashed line represents the expected ${\mathrm{CL}}_{\mathrm{s}}$ values, where the $1\sigma$ and $2\sigma$ contours are indicated by the green (dark) and yellow (light) shaded regions, respectively. Upper limits are determined by the points at which the observed ${\mathrm{CL}}_{\mathrm{s}+\mathrm{b}}$ $p$-values (black points connected by straight lines) fall below 5% (red solid line). Also displayed are the corresponding ${\mathrm{CL}}_{\mathrm{s}}={\mathrm{CL}}_{\mathrm{s}+\mathrm{b}}/{\mathrm{CL}}_{\mathrm{b}}$ values (blue points connected by straight dotted lines).