Search for anomalous photonic events with missing energy in

Photonic events with large missing energy have been observed in e+e-collisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 1997using the OPAL detector at LEP. Results are presented for event topologies witha single photon and missing transverse energy or with an acoplanar photon pair.Cross-section measurements are performed within the kinematic acceptance ofeach selection. These results are compared with the expectations from theStandard Model process e+e- to nunubar + photon(s). No evidence is observed fornew physics contributions to these final states. Using the data at Ecm = 183GeV, upper limits on sigma(e+e- to X.Y)*BR(X to Y gamma) and sigma(e+e- toX.X)*BR(X to Y gamma)**2 are derived for the case of stable and invisible Y.These limits apply to single and pair production of excited neutrinos (X = nu*,Y = nu), to neutralino production (X = Chi_2^0, Y = Chi_1^0) and tosupersymmetric models in which X = chi_1^0 and Y is a light gravitino.Comment: 36 pages, LaTeX, including 15 ps figures, submitted to European Physical Journal