  A study of the antineutrino-jet mass spectrum in e^+p -> antineutrino+X
events at center-of-mass energy 300 GeV has been performed with the ZEUS
detector at HERA using an integrated luminosity of 47.7 pb^-1. The mass
spectrum is in good agreement with that expected from Standard Model processes
over the antineutrino -jet mass range studied. No significant excess
attributable to the decay of a narrow resonance is observed. By using both e^+p
-> e^+ X and e^+p -> antineutrino X data, mass-dependent limits are set on the
s-channel production of scalar and vector resonant states. Couplings to
first-generation quarks are considered and limits are presented as a function
of the e^+q and antineutrino-q branching ratios. These limits are used to
constrain the production of leptoquarks and R-parity violating squarks.
