Observation of a Threshold Cusp at the $Λη$ Threshold in the $pK^{-}$ Mass Spectrum with $Λ_{c}^{+} \rightarrow p K^{-} π^{+}$ 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 $Λ_{c}^{+} \rightarrow p K^{-} π^{+}$ decay using the $980~{\rm fb^{-1}}$ 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 Flatt$\rm{\acute{e}}$ function and a constant background amplitude with the $χ^2/{\rm n.d.f}=257/243$ ($p = 0.25$), while the fits to Breit-Wigner functions are unfavored by more than 7$σ$. 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.

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