Search for Pentaquarks in 920 GEV Proton-Nucleus Collisions with Hera-B

HERA-B has searched in vain for evidence of the production of two recently reported states which have been identified as possible pentaquarks: the Θ⁺(1540), decaying into pK_s⁰, and the Xi_{3/2}^{-}$, decaying into Ξ^-π^-. Upper limits on production cross sections at mid-rapidity and on ratios of the production cross sections to those of well-known resonances in 920 GeV proton-nucleus interactions are reported.