Search for Pentaquarks in 920 GeV Proton-Nucleus Collisions

Evidence for the observation of the narrow Θ+(1540) resonance, tentatively identified as a 1/2+ pentaquark state of strangeness +1, has been recently reported by several collaborations by using incident beams of photons, kaons, neutrinos, and electrons. The NA49 Collaboration has furthermore reported the observation of narrow resonances at 1862 GeV/c2 decaying via exotic charge combinations of (Ξ,π) which could be identified as doubly-strange members of a pentaquark anti-decuplet. We report on our search for these resonances at HERA-B which is a fixed target experiment installed on the 920 GeV/c2 HERA proton beam at DESY with acceptance near xF=0. The data sample consists of 200 million minimum bias events taken during the 2002/2003 HERA run. We discuss details of our analysis of (p,KS0) and (Ξ,π) mass spectra, and present results including a cross section limit for Θ+ production on carbon and tungsten targets, and limits for particle ratios such as Θ+/Λ(1520) and Ξ−−/Ξ0(1530).