First observation of the decay ${\mathrm{\Lambda }}_b^0\to {\eta }_c(1S)p{K}^{-}$

The decay ${\mathrm{\Lambda }}_b^0\to {\eta }_c(1S)p{K}^{-}$ is observed for the first time using a data sample of proton-proton collisions, corresponding to an integrated luminosity of $5.5{\mathrm{fb}}^{-1}$, collected with the LHCb experiment at a center-of-mass energy of 13 TeV. The branching fraction of the decay is measured, using the ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ decay as a normalization mode, to be $\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\eta }_c(1S)p{K}^{-})=(1.06\pm 0.16\pm 0.06_{-0.19}^{+0.22})\times {10}^{-4}$, where the quoted uncertainties are statistical, systematic and due to external inputs, respectively. A study of the ${\eta }_c(1S)p$ mass spectrum is performed to search for the $P_c(4312{)}^{+}$ pentaquark state. No evidence is observed and an upper limit of $\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to P_c(4312{)}^{+}{K}^{-})\times \mathcal{B}(P_c(4312{)}^{+}\to {\eta }_c(1S)p)}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\eta }_c(1S)p{K}^{-})}<0.24$ is obtained at the 95% confidence level.

Figure 1

Distributions of (a) $m(p\overline{p}p{K}^{-})$ and (b) $m(p\overline{p})$ of the selected candidates. The data are shown as black circles, while the blue solid line shows the fit result. Individual components are given in the legend.

Figure 2

The invariant-mass spectra of (a) the ${\eta }_{c}p$ system of the ${\mathrm{\Lambda }}_b^0\to {\eta }_{c}p{K}^{-}$ decays and (b) the $J/\psi p$ system of the ${\mathrm{\Lambda }}_b^0\to J/\psi p{K}^{-}$ decays. The black points represent the background-subtracted data and the red points correspond to the expectation from a simulation generated according to a uniform phase-space model. The blue solid line in panel (a) shows the fit projection of the ${\eta }_{c}p$ mass spectrum including the contribution from a $P_c(4312{)}^{+}$ resonance in the mass range $[4000,4400]\mathrm{MeV}/c^2$.