Identification of a visible narrow cusp structure in Λc+→pK−π+

A resonance-like structure as narrow as 10 MeV is observed in the K−p invariant mass distributions in Λc+→pK−π+. This precise measurement is based on a data sample of about 1.5 million events, and the bin width of K−p invariant mass is only 1 MeV. The narrow peak precisely lies on the Λη threshold, because of which it is natural to identify it as a threshold cusp. Being different from the common two-body unitary cusp, we find that the narrowness of this cusp can be induced by a nearby triangle singularity of the Λa0(980) or ηΣ(1660) rescattering process.