Pentaquarks from intrinsic charms in ${\Lambda }_b$ decays

Abstract

We study the three-body ${\Lambda }_b$ decays of ${\Lambda }_b\to J/\psi pM$ with $M=K^{-}$ and ${\pi }^{-}$. The two new states ${\mathcal{P}}_{c1}\equiv {\mathcal{P}}_c(4380{)}^{+}$ and ${\mathcal{P}}_{c2}\equiv {\mathcal{P}}_c(4450{)}^{+}$ observed recently as the resonances in the $J/\psi p$ invariant mass spectrum of ${\Lambda }_b\to J/\psi p{K}^{-}$ can be identified to consist of five quarks, $uudc\bar{c}$, being consistent with the existence of the pentaquark states. We argue that, in the doubly charmful ${\Lambda }_b$ decays of ${\Lambda }_b\to J/\psi p{K}^{-}$ through $b\to c\bar{c}s$, apart from those through the non-resonant ${\Lambda }_b\to p{K}^{-}$ and resonant ${\Lambda }_b\to {\Lambda }^{\ast }\to p{K}^{-}$ transitions, the third contribution with the non-factorizable effects is not the dominant part for the resonant ${\Lambda }_b\to K^{-}{\mathcal{P}}_{c1,c2},{\mathcal{P}}_{c1,c2}\to J/\psi p$ processes, such that we propose that the ${\mathcal{P}}_{c1,c2}$ productions are mainly from the charmless ${\Lambda }_b$ decays through $b\to \bar{u}us$, in which the $c\bar{c}$ content in ${\mathcal{P}}_{c1,c2}$ arises from the intrinsic charms within the ${\Lambda }_b$ baryon. We hence predict the observables related to the branching ratios and the direct CP violating asymmetries to be $\mathcal{B}({\Lambda }_b\to {\pi }^{-}({\mathcal{P}}_{c1,c2}\to )J/\psi p)/\mathcal{B}({\Lambda }_b\to K^{-}({\mathcal{P}}_{c1,c2}\to )J/\psi p)=0.8\pm 0.1$, ${\mathcal{A}}_{CP}({\Lambda }_b\to {\pi }^{-}({\mathcal{P}}_{c1,c2}\to )J/\psi p)=(-3.9\pm 0.2)\%$, and ${\mathcal{A}}_{CP}({\Lambda }_b\to K^{-}({\mathcal{P}}_{c1,c2}\to )J/\psi p)=(5.8\pm 0.2)\%$, which can alleviate the inconsistency between the theoretical expectations from the three contributions in the doubly charmful ${\Lambda }_b$ decays and the observed data.

Comments: 10 pages, 2 figures, revised version accepted by PLB