Search for the Theta+ pentaquark in antiproton He-4 annihilation at rest

Abstract A research has been made to look for a signature of the positive-strangeness baryon resonance Θ + ( 1530 ) , which is expected to decay into p K 0 . A peak is observed in the p π + π − invariant mass distribution in antiproton– 4 He annihilation reactions at rest with production of two π + π − pairs with invariant mass close to the K S 0 mass. The annihilation reactions on 4 He 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.