Search for resonances decaying to e+-jet in e+p interactions at HERA

The $e^+$ -jet invariant mass spectrum produced in the reaction $e^+ p \rightarrow e^+ X$ has been studied at a center-of-mass energy of 300 GeV. The data were collected using the ZEUS detector operating at the HERA collider, and correspond to an integrated luminosity of $47.7$ pb $^{-1}$ . The observed mass spectrum is in good agreement with Standard Model expectations up to an $e^+$ -jet mass of $210$ GeV. Above this mass, some excess is seen. The angular distribution of these events is typical of high- $Q^2$ neutral current events and does not give convincing evidence for the presence of a narrow scalar or vector state. Limits are presented on the product of cross section and branching ratio for such a state and are interpreted as limits on leptoquark or R-parity-violating squark production. Specific leptoquark types are ruled out at 95% confidence level for coupling strength $\lambda=0.3$ for masses between 150 and 280 GeV.