The LHCb state $P_{\psi s}^{\mathrm{\Lambda }}(4338)$ as a triangle singularity

We present a model for the $J\mathrm{/}\psi \mathrm{\Lambda }$ spectrum in $B^{-}\to J\mathrm{/}\psi \mathrm{\Lambda }\bar{p}$ decays, including the $P_{\psi s}^{\mathrm{\Lambda }}(4338)$ baryon recently observed by the LHCb collaboration. We assume production via triangle diagrams which couple to the final state via non-perturbative interactions which are constrained by heavy-quark and $S{U}_3$-flavor symmetry. The bulk of the distribution is described by a triangle diagram with a color-favored electroweak vertex, while the sharp $P_{\psi s}^{\mathrm{\Lambda }}(4338)$ enhancement is due to the triangle singularity in another diagram featuring a $1\mathrm{/}{2}^{-}$ baryon consistent with ${\mathrm{\Sigma }}_c(2800)$. We predict a comparable $P_{\psi s}^{\mathrm{\Lambda }}(4338)$ signal in ${\eta }_c\mathrm{\Lambda }$, and anticipate possible large isospin mixing effects through decays to $J\mathrm{/}\psi {\mathrm{\Sigma }}^0$ and ${\eta }_c{\mathrm{\Sigma }}^0$