Anomalous enhancement of the isospin-violating $\Lambda (1405)$ production by a triangle singularity in ${\Lambda }_c\to {\pi }^{+}{\pi }^0{\pi }^0{\Sigma }^0$

Abstract

The decay of ${\Lambda }_c^{+}$ into ${\pi }^{+}{\pi }^0\Lambda (1405)$ with the $\Lambda (1405)$ decay into ${\pi }^0{\Sigma }^0$ through a triangle diagram is studied. This process is initiated by ${\Lambda }_c^{+}\to {\pi }^{+}{\bar{K}}^{\ast }N$, then the ${\bar{K}}^{\ast }$ decays into $\bar{K}\pi$ and $\bar{K}N$ produce the $\Lambda (1405)$ through a triangle loop containing ${\bar{K}}^{\ast }N\bar{K}$ which develops a singularity around $1890$ MeV. This process is prohibited by the isospin symmetry, but the decay into this channel is enhanced by the contribution of the triangle diagram, which is sensitive to the mass of the internal particles. We find a narrow peak in the ${\pi }^0{\Sigma }^0$ invariant mass distribution, which originates from the $\Lambda (1405)$ amplitude, but is tied to the mass differences between the charged and neutral $\bar{K}$ or $N$ states. The observation of the unavoidable peak of the triangle singularity in the isospin-violating $\Lambda (1405)$ production would provide further support for hadronic molecular picture of the $\Lambda (1405)$ and further information on the $\bar{K}N$ interaction.

Comments: 18 pages, 6 figures