Photonic events with large missing energy in e+e− collisions at s=161 GeV

Photonic events with large missing energy have been observed with the OPAL detector at LEP using two search topologies. The first topology looks for events with significant missing energy and at least one photon in |cosθ| < 0.7 with large scaled energy, xγ > 0.2. The second topology looks for events with an acoplanar photon pair where both photons are in |cosθ| < 0.7 and have energy exceeding 1.75 GeV. In the data sample corresponding to an integrated luminosity of 10.0 pb−1, 11 events are selected for the first topology and a cross-section of 1.6 ± 0.5 pb is measured within the kinematic acceptance, consistent with that expected for the Standard Model process e+e− → ννγ(γ). No events are selected for the second topology giving a 95% CL cross-section upper limit of 0.41 pb within the kinematic acceptance. Upper limits on the cross-section times branching ratio for new particle production are also derived.