Observation of New States Decaying into Λ_c^+ K^-π^+ and Λ_c^+ K⁰_Sπ^-

We report the first observation of two charmed strange baryons that decay into $\Lambda_c^+ K^-\pi^+$. The broader of the two states is measured to have a mass of $2978.5\pm 2.1\pm 2.0$ MeV/$c^2$ and a width of $43.5\pm 7.5\pm 7.0$ MeV/$c^2$. The mass and width of the narrow state are measured to be $3076.7\pm 0.9\pm 0.5$ MeV/$c^2$ and $6.2\pm 1.2\pm 0.8$ MeV/$c^2$, respectively. We also perform a search for the isospin partner states that decay into $\Lambda_c^+ K_S^0\pi^-$ and observe a significant signal at the mass of $3082.8\pm 1.8\pm 1.5$ MeV/$c^2$. The data used for this analysis was accumulated at or near the $\Upsilon(4S)$ resonance, using the Belle detector at the $e^+ e^-$ asymmetric-energy collider KEKB. The integrated luminosity of the data sample used is $461.5 ~\mathrm{fb}^{-1}$.