Observation of a threshold cusp at the threshold in the mass spectrum with decays

Yang, S.*; 谷田 聖; Belle Collaboration*; 他196名*

Physical Review D, 108(3), p.L031104_1 - L031104_6, 2023/08

https://doi.org/10.1103/PhysRevD.108.L031104

We observe a narrow peaking structure in the invariant-mass spectrum near the threshold. The peak is clearly seen in 1.5 million events of decay using the data sample collected by the Belle detector at the KEKB asymmetric-energy collider. We try two approaches to explain this structure: as a new resonance and as a cusp at the threshold. The best fit is obtained with a coherent sum of a Flatt function and a constant background amplitude with the (), while the fits to Breit-Wigner functions are unfavored by more than 7. The best fit explains the structure as a cusp at the threshold, and the obtained parameters are consistent with the known properties of . The observation gives the first identification of a threshold cusp in hadrons from the spectrum shape.