Search for anomalous production of photonic events with missing energy in e+e- collisions at √s=130-172 GeV

Photonic events with large missing energy have been observed in e(+)e(-) collisions at centre-of-mass energies of 130, 136, 161 and 172 GeV using the OPAL detector at LEP. Results are presented based on search topologies designed to select events with a single photon and missing transverse energy or events with a pair of acoplanar photons. In both search topologies, cross-section measurements are performed within the kinematic acceptance of the selection. These results are compared with the expectations from the Standard Model processes e(+)e(-) --> nu<(nu)over bar>gamma(gamma) (single-photon) and e(+)e(-) --> nu<(nu)over bar>gamma gamma(gamma) (acoplanar-photons). No evidence is observed for new physics contributions to these final states. Upper limits on sigma(e(+)e(-) --> XY).BR(X --> Y gamma) and sigma(e(+)e(-) --> XX).BR2(X --> Y gamma) 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)over tilde>(0)(2),Y = <(chi)over tilde>(0)(1)), and to supersymmetric models in which X = <(chi)over tilde>(0)(1) and Y = (G) over tilde is a light gravitino. For the latter scenario, the results of the acoplanar-photons search are used to provide model-dependent lower limits on the mass of the lightest neutralino.