Evidence for an Excited Hyperon State in pp{yields}pK{sup +}Y{sup 0}

Indications for the production of a neutral excited hyperon in the reaction pp{yields}pK{sup +}Y{sup 0}* are observed in an experiment performed with the ANKE spectrometer at COSY-Juelich at p{sub beam}=3.65 GeV/c. Two final states were investigated simultaneously, viz. Y{sup 0}*{yields}{pi}{sup +}X{sup -} and {pi}{sup -}X{sup +}, and consistent results were obtained in spite of the quite different experimental conditions. The parameters of the hyperon state are M(Y{sup 0}*)=(1480{+-}15) MeV/c{sup 2} and {gamma}(Y{sup 0}*)=(60{+-}15) MeV/c{sup 2}. The production cross section for Y{sup 0}* decaying through these channels is of the order of few hundred nanobarns. Since the isospin of the Y{sup 0}* has not been determined here, it could either be an observation of the {sigma}(1480), a one-star resonance of the Particle Data Group tables, or, alternatively, a {lambda} hyperon. Relativistic quark models for the baryon spectrum do not predict any excited hyperon in this mass range and so the Y{sup 0}* may be of exotic nature.