The role of the triangle singularity in $\Lambda(1405)$ production in the $\pi^-p\rightarrow K^0\pi\Sigma$ and $pp\rightarrow pK^+\pi\Sigma$ processes

TL;DR

This paper investigates how a triangle singularity mechanism influences the production of the $ ext{Lambda}(1405)$ resonance in specific reactions, revealing a peak around 2100 MeV due to a resonance effect, which explains experimental observations.

Contribution

It introduces a detailed analysis of the triangle singularity mechanism in $ ext{Lambda}(1405)$ production, highlighting the role of a $N^*$ resonance in shifting the peak position.

Findings

A clear peak appears around 2100 MeV in the $K ext{Lambda}(1405)$ invariant mass.

The peak is about 40 MeV lower than initial expectations due to the $N^*$ resonance.

The mechanism explains the $ ext{Lambda}(1405)$ peak observed below 1400 MeV in experiments.

Abstract

We have investigated the cross section for the $\pi^-p\rightarrow K^0\pi\Sigma$ and $pp\rightarrow pK^+\pi\Sigma$ reactions paying attention to a mechanism that develops a triangle singularity. The triangle diagram is realized by the decay of a $N^*$ to $K^*\Sigma$ and the $K^*$ decay into $\pi K$, and the $\pi\Sigma$ finally merges into $\Lambda(1405)$. The mechanism is expected to produce a peak around $2140$ MeV in the $K\Lambda(1405)$ invariant mass. We found that a clear peak appears around $2100$ MeV in the $K\Lambda(1405)$ invariant mass which is about $40$ MeV lower than the expectation, and that is due to the resonance peak of a $N^*$ resonance which plays a crucial role in the $K^*\Sigma$ production. The mechanism studied produces the peak of the $\Lambda(1405)$ around or below 1400 MeV, as is seen in the $pp\rightarrow pK^+\pi\Sigma$ HADES experiment.