Visible narrow cusp structure in Λc+→p K-π+ enhanced by triangle singularity

A resonance-like structure as narrow as 10 MeV is observed in the K^-p invariant mass distributions in Λ_c⁺→p K^-π⁺ 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 Λ η 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 Λ -a₀(980 )⁺ or η -Σ (1660 )⁺ rescatterings.