Observation of a Threshold Cusp at the $\Lambda \eta$ Threshold in the $p{K}^{-}$ Mass Spectrum with ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ Decays

Abstract

We observe a narrow peaking structure in the $p{K}^{-}$ invariant-mass spectrum near the $\Lambda \eta$ threshold. The peak is clearly seen in 1.5 million events of ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ decay using the $980\mathrm{f}{\mathrm{b}}^{-1}$ 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$\acute{\mathrm{e}}$ function and a constant background amplitude with the ${\chi }^2/n.d.f=257/243$ ($p=0.25$), 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.

12 pages, 5 figures