Multi-photon events with large missing energy in e⁺e⁻ collisions at √s = 192-209 GeV

Events with a final state consisting of two or more photons and large missing transverse energy have been observed in e(+)e(-) collisions at centre-of-mass energies in the range 192-209 GeV using the OPAL detector at LEP. Cross-section measurements are performed within the kinematic acceptance of the selection and compared with the expectations from the Standard Model process e(+)e(-) --> v (v) over bar gammagamma(gamma). No evidence for new physics contributions to this final state is observed. Upper limits on or (e(+)e(-) --> XX) (.) BR2 (X --> Ygamma) are derived for the case of stable and invisible Y. In the case of massive Y the combined limits obtained from all the data range from 10 to 60 fb, while for the special case of massless Y the range is 20 to 40 fb. The limits apply to pair production of excited neutrinos (X = v*, Y = v), to neutralino production (X = (x) over bar (0)(2), Y = (x) over bar (0)(1)) and to 2 1 supersymmetric models in which X = (x) over bar (0)(1) and Y = (G) over bar is a light gravitino. (C) 2004 Published by Elsevier B.V.