Observation of a threshold cusp at the Λη threshold in the pK- mass spectrum with Λc+ RIGHTWARDS ARROW pK- π+ decays
Abstract
We observe a narrow peaking structure in the pK(-) invariant-mass spectrum near the Lambda eta threshold. The peak is clearly seen in 1.5 million events of Lambda(+)(c) -> pK(-)pi(+) decay using the 980 fb(-1) data sample collected by the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider.
We try two approaches to explain this structure: as a new resonance and as a cusp at the Lambda eta threshold. The best fit is obtained with a coherent sum of a Flatte function and a constant background amplitude with the reduced chi(2) of 257= 243 (p = 0.25), while the fits to Breit-Wigner functions are unfavored by more than 7 sigma.
The best fit explains the structure as a cusp at the.. threshold, and the obtained parameters are consistent with the known properties of Lambda(1670). The observation gives the first identification of a threshold cusp in hadrons from the spectrum shape.