Search for Anomalous Production of Photonic Events with Missing Energy in e+e- Collisions at sqrt{s} = 130-172 GeV
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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(bar) gamma (gamma) (single-photon) and e+e- -> νν(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)*Br**2(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 = ν^*, Y = ν), to neutralino production (X = χ^0_2, Y = χ^0_1), and to supersymmetric models in which X = χ^0_1 and Y = \tilde{G} 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.
41 pages, LaTeX, uses a4p and pennames.sty, 19 eps figures included (4 bitmapped), submitted to Z. Phys. C
41 pages, LaTeX, uses a4p and pennames.sty, 19 eps figures included (4 bitmapped), submitted to Z. Phys. C