| Searches for strange pentaquarks, including theta +, which decays to K0sp , and X--3/2 , which decays to X-p- have been performed with the ZEUS detector at HERA. The data sample consists of deep inelastic scattering events at center-of-mass energies of 300 and 318 GeV, and corresponds to 121 pb-1 of integrated luminosity.; ZEUS has recently published evidence for excess events in the K0sp invariant mass spectrum around 1520 MeV, which may be a theta + pentaquark signal. In this analysis, a better proton identification method has been developed, with the hope to improving the signal significance. The new proton identification method uses both tracking and calorimetry information, whereas the traditional method uses tracking information only. A neural network was constructed to do the pattern recognition based on the multi-variables from both tracking and calorimetry. The efficiency of the neural network was demonstrated for a well established state, the Λ(1520), which decays into charged kaons and protons. However, the new proton identification method failed to improve the significance of the theta+ candidate. In face, the significance was reduced with the purer proton selection. This weakens the hypothesis that the published excess is a pentaquark candidate.; No signal was observed for the X--3/2 in the X-p- invariant mass spectrum. The sensitivity of the search was demonstrated with the observation of a clear signal for X0 (1530) in a similar decay channel. Upper limits on the ratio of the number of possible X--3/2 events to the number of the X0 (1530) events were set in a mass range of 1650--2350 MeV. |
