Search for anomalous Z→γγγ events at LEP

Abstract

We have searched for anomalous Z → γγγ events with the L3 detector at LEP. No significant deviations from the expected QED e⁺e⁻ → γγγ events are observed. The branching ratio upper limit for a compoite Z decaying directly into three photons is found to be 1.0 × 10⁻⁵ at 95% C.L. The branching ratio upper limits for the process Z → γX, X → γγ are in the range of 0.4 to 1.3 × 10⁻⁵, depending on the mass and width of the scalar particle X. In the context of a model with magnetic monopoles coupling to the Z, we find BR(Z → γγγ) < 0.8 × 10⁻⁵ at 95% C.L.; this results in a lower mass limit of 510 GeV for a magnetic monopole.