Search for the Θ+(1530) pentaquark in antiproton 4He annihilation at rest

A research has been made to look for a signature of the positive-strangeness baryon resonance Θ+(1530), which is expected to decay into pK0. A peak is observed in the pπ+π− invariant mass distribution in antiproton– 4He annihilation reactions at rest with production of two π+π− pairs with invariant mass close to the mass. The annihilation reactions on 4He can involve two to four nucleons producing an energy blob characterized by high strangeness content. The peak is centred around 1560.0±3.7 MeV and its width is very narrow. The statistical significance is 2.7; this value is considered of not sufficient significance to claim support of the Θ+. The data were collected with the Obelix spectrometer at the LEAR machine of CERN.