Strange hidden-charm pentaquark poles from $B^{-}\to J\mathrm{/}\psi \mathrm{\Lambda }\bar{p}$

Recent LHCb data for $B^{-}\to J\mathrm{/}\psi \mathrm{\Lambda }\bar{p}$ show a clear peak structure at the ${\mathrm{\Xi }}_c\bar{D}$ threshold in the $J\mathrm{/}\psi \mathrm{\Lambda }$ invariant mass ($M_{J\mathrm{/}\psi \mathrm{\Lambda }}$) distribution. The LHCb’s amplitude analysis identified the peak with the first hidden-charm pentaquark with strangeness $P_{\psi s}^{\mathrm{\Lambda }}(4338)$. We conduct a coupled-channel amplitude analysis of the LHCb data by simultaneously fitting the $M_{J\mathrm{/}\psi \mathrm{\Lambda }}$, $M_{J\mathrm{/}\psi \bar{p}}$, $M_{\mathrm{\Lambda }\bar{p}}$, and $\cos {\theta }_{K^{\ast }}$ distributions. Rather than the Breit-Wigner fit employed in the LHCb analysis, we consider relevant threshold effects and a unitary ${\mathrm{\Xi }}_c\bar{D}$-${\mathrm{\Lambda }}_c{\bar{D}}_s$ coupled-channel scattering amplitude from which $P_{\psi s}^{\mathrm{\Lambda }}$ poles are extracted for the first time. In our default fit, the $P_{\psi s}^{\mathrm{\Lambda }}(4338)$ pole is almost a ${\mathrm{\Xi }}_c\bar{D}$ bound state at $(4338.2\pm 1.4)-(1.9\pm 0.5)i$ MeV. Our default model also fits a large fluctuation at the ${\mathrm{\Lambda }}_c{\bar{D}}_s$ threshold, giving a ${\mathrm{\Lambda }}_c{\bar{D}}_s$ virtual state, $P_{\psi s}^{\mathrm{\Lambda }}(4255)$, at $4254.7\pm 0.4$ MeV. We also found that the $P_{\psi s}^{\mathrm{\Lambda }}(4338)$ peak cannot solely be a kinematical effect, and a nearby pole is needed.