The LHCb pentaquark as a $\bar{D}^*Σ_c-\bar{D}^*Σ_c^*$ molecular state

We perform a theoretical analysis of the $Λ_b \to J/ψK^- p$ reaction from where a recent LHCb experiment extracts a $Λ(1405)$ contribution in the $K^- p$ spectrum close to threshold and two baryon states of hidden charm in the $J/ψ\,p$ spectrum. We recall that baryon states of this type have been theoretically predicted matching the mass, width and $J^P$ of the experiment, concretely some states built up from the $J/ψ\, N$, $\bar D^* Λ_c$, $\bar D^* Σ_c$, $\bar D Σ^*_c$ and $\bar D^* Σ^*_c$ coupled channels. We assume that the observed narrow state around 4450 MeV has this nature and we are able to describe simultaneously the shapes and relative strength of the the $K^- p$ mass distribution close to threshold and the peak of the $J/ψ\,p$ distribution, with values of the $J/ψ\, p$ coupling to the resonance in line with the theoretical ones. The non trivial matching of many properties gives support to a $J^P=3/2^-$ assignment to this state and to its nature as a molecular state mostly made of $\bar D^* Σ_c$ and $\bar D^* Σ^*_c$.

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