  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 $\Lambda_{c}^{+} \rightarrow p K^{-} \pi^{+}$ 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 $\Lambda\eta$ threshold. The
best fit is obtained with a coherent sum of a Flatt$\rm{\acute{e}}$ function
and a constant background amplitude with the $\chi^2/{\rm n.d.f}=257/243$ ($p =
0.25$), 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 $\Lambda\eta$
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.

High Energy Physics - Experiment