Visible narrow cusp structure in $\hat{I}{›}_c^{+}\to p{K}^{-}\ddot{I}{€}^{+}$ enhanced by triangle singularity

Abstract

A resonance-like structure as narrow as 10 MeV is observed in the $K^{-}p$ invariant mass distributions in $\hat{I}{›}_c^{+}\to p{K}^{-}\ddot{I}{€}^{+}$ 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 $\hat{I}›\hat{I}\cdot$ 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 $\hat{I}›$-$a_0(980{)}^{+}$ or $\hat{I}\cdot$-$\hat{I}\pounds (1660{)}^{+}$ rescatterings.

6 pages, 4 figures