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 Lambda(+)(c)-> pK(-)pi(+) at Belie. 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.