Observation of the suppressed ${\mathrm{\Lambda }}_b^0\to Dp{K}^{-}$ decay with $D\to K^{+}{\pi }^{-}$ and measurement of its $CP$ asymmetry

A study of ${\mathrm{\Lambda }}_b^0$ baryon decays to the $Dp{K}^{-}$ final state is presented based on a proton-proton collision data sample corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ collected with the LHCb detector. Two ${\mathrm{\Lambda }}_b^0$ decays are considered, ${\mathrm{\Lambda }}_b^0\to Dp{K}^{-}$ with $D\to K^{-}{\pi }^{+}$ and $D\to K^{+}{\pi }^{-}$, where $D$ represents a superposition of $D^0$ and ${\overline{D}}^0$ states. The latter process is expected to be suppressed relative to the former, and is observed for the first time. The ratio of branching fractions of the two decays is measured, and the $CP$ asymmetry of the suppressed mode, which is sensitive to the Cabibbo-Kobayashi-Maskawa angle $\gamma$, is also reported.

Figure 1

Distributions of the invariant mass for selected (left) ${\mathrm{\Lambda }}_b^0\to [K^{-}{\pi }^{+}{]}_{D}p{K}^{-}$ and (right) ${\mathrm{\Lambda }}_b^0\to [K^{+}{\pi }^{-}{]}_{D}p{K}^{-}$ candidates in the full phase space (black points) corresponding to the favored and suppressed decays, respectively. The total fit model, as described in the text, is indicated by the solid blue line, and individual components are indicated.

Figure 2

Distributions of the invariant mass for (left) ${\mathrm{\Lambda }}_b^0\to [K^{-}{\pi }^{+}{]}_{D}p{K}^{-}$ and (right) ${\mathrm{\Lambda }}_b^0\to [K^{+}{\pi }^{-}{]}_{D}p{K}^{-}$ candidates, corresponding to the favored and suppressed decays, respectively, in the restricted phase-space region $M^2(p{K}^{-})<5{\mathrm{GeV}}^2/c^4$. The fit, as described in the text, is overlaid.

Figure 3

Distributions of the invariant mass for (left) ${\mathrm{\Lambda }}_b^0\to [K^{+}{\pi }^{-}{]}_{D}p{K}^{-}$ and (right) $\overline{{\mathrm{\Lambda }}_b^0}\to [K^{-}{\pi }^{+}{]}_D\overline{p}{K}^{+}$ candidates in the full phase space (black points), corresponding to separation of the suppressed decay sample by ${\mathrm{\Lambda }}_b^0$ flavour. The fit, as described in the text, is indicated by the solid blue line, and individual components are indicated.

Figure 4

Distributions of the invariant mass for (left) ${\mathrm{\Lambda }}_b^0\to [K^{+}{\pi }^{-}{]}_{D}p{K}^{-}$ and (right) $\overline{{\mathrm{\Lambda }}_b^0}\to [K^{-}{\pi }^{+}{]}_D\overline{p}{K}^{+}$ candidates in the restricted phase-space region (black points), where the separation is made according to the ${\mathrm{\Lambda }}_b^0$ flavor. The fit, as described in the text, is indicated by the solid blue line, and individual components are indicated.

Figure 5

Phase space of the (left) ${\mathrm{\Lambda }}_b^0\to [K^{-}{\pi }^{+}{]}_{D}p{K}^{-}$ and (right) ${\mathrm{\Lambda }}_b^0\to [K^{+}{\pi }^{-}{]}_{D}p{K}^{-}$ candidates having a mass between 5600 and $5640\mathrm{MeV}/c^2$. No background subtraction is applied.

Figure 6

Invariant-mass projections of the ${\mathrm{\Lambda }}_b^0$ phase space in (left) $M(Dp)$, (middle) $M(p{K}^{-})$, and (right) $M(D{K}^{-})$ for the ${\mathrm{\Lambda }}_b^0\to [K^{-}{\pi }^{+}{]}_{D}p{K}^{-}$ candidates.

Figure 7

Invariant-mass projections of the ${\mathrm{\Lambda }}_b^0$ phase space in (left) $M(Dp)$, (middle) $M(p{K}^{-})$, and (right) $M(D{K}^{-})$ for the ${\mathrm{\Lambda }}_b^0\to [K^{+}{\pi }^{-}{]}_{D}p{K}^{-}$ candidates.

Figure 8

Relative efficiency correction as a function of position within the ${\mathrm{\Lambda }}_b^0$ phase space, obtained from simulated samples.