Observation of a threshold cusp at the Λη threshold in the pK^- mass spectrum with Λ_c⁺ → pK^- π⁺ decays

We observe a narrow peaking structure in the pK- invariant-mass spectrum near the Λη threshold. The peak is clearly seen in 1.5 million events of Λc+→pK-π+ 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 Λη threshold. The best fit is obtained with a coherent sum of a Flatté function and a constant background amplitude with the reduced χ2 of 257/243 (p=0.25), while the fits to Breit-Wigner functions are unfavored by more than 7σ. The best fit explains the structure as a cusp at the Λη threshold, and the obtained parameters are consistent with the known properties of Λ(1670). The observation gives the first identification of a threshold cusp in hadrons from the spectrum shape.