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

Photonic events with large missing energyhave been observed in \(\mathrm{e^+e^-}\) 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 acoplanarphotons. 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 \({\rm e^+e^-}\to \nu\ overline\nu \gamma (\gamma )\) (single-photon) and \({\rm e^+e^-}\to \nu\ overline\nu \gamma \gamma (\gamma )\) ( acoplanar-photons). No evidence is observed for new physics contributions to these final states. Upper limits on \(\sigma({\rm e^+e^- \to XY)\cdot BR(X\to Y\gamma })\) and \(\sigma({\rm e^+e^- \to XX)\cdot BR^2(X\to Y\gamma })\) are derived for the case of stable and invisible Y. These limits apply to single and pair productionof excited neutrinos (\({\rm X}= \nu^*,{\rm Y}= \nu\)), to neutralinoproduction (\({\rm X}=\ tilde\chi^0_2, {\rm Y}=\ tilde\chi^0_1\)), and to supersymmetric models in which \({\rm X}= \ tilde\chi^0_1\) and \({\rm Y}=\ tilde{\rm G}\) is a light gravitino. For the latter scenario, the results of the acoplanar-photons search are used to provide mode l-dependent lower limits on the mass of the lightest neutralino.