Visible narrow cusp structure in Λc+→pK−π+ enhanced by triangle singularity

A resonance-like structure as narrow as 10 MeV is observed in the K−p invariant mass distributions in Λc+→pK−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 Λη 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.