A direct search for neutralino production at LEP

A search has been performed for the production of neutralinos (χ, χ′) in e+e− annihilation at energies near the Z0 pole. No evidence for these particles was found either in searches for events with two acoplanar jets, low visible energy, and missing pt (sensitive to Z0→χχ′→χχff) or in searches for single-photon events (sensitive to Z0→χχ′→χχγ). Model independent upper limits (at the 95% CL) on the branching ratio for the decay mode Z0 →χχ′ of a few 10−4 are obtained for most of the range of neutralino masses that is kinematically accessible at LEP energies. Upper limits on the mixing factor of neutralinos are also placed as a function of the neutralino masses.