Evidence for an excited hyperon state in pp --> pK+Y0*.

Indications for the production of a neutral excited hyperon in the reaction pp --> pK+Y0* are observed in an experiment performed with the ANKE spectrometer at COSY-Jülich at p(beam) = 3.65 GeV/c. Two final states were investigated simultaneously, viz. Y0* --> pi+X- and pi-X+, and consistent results were obtained in spite of the quite different experimental conditions. The parameters of the hyperon state are M(Y0*) = (1480 +/-15) MeV/c2 and gamma(Y0*) = (60 +/- 15) MeV/c2. The production cross section for Y0* decaying through these channels is of the order of few hundred nanobarns. Since the isospin of the Y0* 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 Y0* may be of exotic nature.