Excited hyperons produced in proton-proton collisions with ANKE at COSY

Excited neutral hyperons Y^0* produced in the pp → pK⁺Y^0* reaction with a COSY beam momentum of 3.65 MeV/c have masses below 1540 MeV/c². The ANKE spectrometer allows the simultaneous observation of different decay modes: Y^0* → π⁰Σ⁰, π^∓Σ^±, π⁰Λ, K^-p by measuring kaons and pions of either charge in coincidence with protons. We have found indications for a neutral excited hyperon resonance Y^0* with a mass of M(Y^0*) = (1480 ± 15) MeV/c² and a width of Γ(Y^0*) = (60±15) MeV/c². The cross section for Y^0* is of the order of few hundred nanobarns. It can be either a Σ⁰ or a Λ hyperon and on the basis of existing data no conclusion could be made whether it is a three–quark baryon or an exotic state. Missing- and invariant–mass techniques have been used to identify the Λ(1405) resonance decaying via Σ0π⁰. The cross section for Λ(1405) production is equal to (4.5 ± 0.9_stat ± 1.8_syst) µb. The shape and position of the Λ(1405) distribution are similar to those found from other decay modes, so no support is given to the two-pole model. (author)