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

Abstract

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 − → ν ν ¯ γ γ ( γ ) . No evidence for new physics contributions to this final state is observed. Upper limits on σ ( e + e − → X X ) ⋅ BR 2 ( X → Y γ ) 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 = ν * , Y = ν ), to neutralino production ( X = χ ˜ 2 0 , Y = χ ˜ 1 0 ) and to supersymmetric models in which X = χ ˜ 1 0 and Y = G ˜ is a light gravitino.