Photonic events with missing energy in e+e- collisions at √s=189 GeV

Photonic events with large missing energy have been observed in e(+)e(-) collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process e(+)e(-) --> nu<()over bar> + photon(s). 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 = <()over tilde>(0)(2),Y = <()over tilde>(0)(1)) and to supersymmetric models in which X = <()over tilde>(0)(1) and Y = (G) over tilde is a Light gravitino. The case of macroscopic decay lengths of particle X is considered for e(+)e(-) --> XX, X --> Y gamma, when M-Y approximate to 0. The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.