# Visible narrow cusp structure in $\Lambda_c^+\to p K^- \pi^+$ enhanced by triangle singularity

## TL;DR

This paper reports a narrow resonance-like structure in the $ ext{K}^- p$ invariant mass spectrum of $ ext{Lambda}_c^+ o p K^- ext{pi}^+$ decays, which is explained as a threshold cusp enhanced by a triangle singularity rather than a new particle.

## Contribution

The study provides a novel interpretation of a narrow structure as a threshold cusp amplified by triangle singularity effects, challenging the quark model state hypothesis.

## Key findings

- Narrow peak at the $ ext{Lambda} ext{eta}$ threshold observed with high precision.
- The structure is explained by a threshold cusp enhanced by triangle singularity effects.
- No evidence found for a quark model resonance at this mass.

## Abstract

A resonance-like structure as narrow as 10 MeV is observed in the $K^-p$ invariant mass distributions in $\Lambda_c^+\to p K^- \pi^+$ at Belle. Based on the large data sample of about 1.5 million events and the small bin width of just 1 MeV for the $K^-p$ invariant mass spectrum, the narrow peak is found precisely lying at the $\Lambda\eta$ threshold. While lacking evidence for a quark model state with such a narrow width at this mass region, we find that this narrow structure can be naturally identified as a threshold cusp but enhanced by the nearby triangle singularity via the $\Lambda$-$a_0(980)^+$ or $\eta$-$\Sigma(1660)^+$ rescatterings.