Photonic events with missing energy in e(+)e(-) collisions at root s=189 GeV

Photonic events with large missing energy have been observed in e(+)e(-) co llisions at a centre-of-mass energy of 189 GeV using the OPAL detector at L EP. Results are presented for event topologies consistent with a single pho ton or with an acoplanar photon pair. Cross-section measurements are perfor med within the kinematic acceptance of each selection, and the number of li ght neutrino species is measured. Cross-section results are compared with t he expectations from the Standard Model process e(+)e(-) --> nu<(<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-ga mma) and sigma (e(+)e(-) --> XX) . BR2(X --> Y gamma) are derived for the c ase of stable and invisible Y. These limits apply to single and pair produc tion of excited neutrinos (X = nu*,Y = nu), to neutralino production (X = < (<chi>)over tilde>(0)(2),Y = <(<chi>)over tilde>(0)(1)) and to supersymmetr ic models in which X = <(<chi>)over tilde>(0)(1) and Y = (G) over tilde is a Light gravitino. The case of macroscopic decay lengths of particle X is c onsidered for e(+)e(-) --> XX, X --> Y gamma, when M-Y approximate to 0. Th e single-photon results are also used to place upper limits on superlight g ravitino pair production as well as graviton-photon production in the conte xt of theories with additional space dimensions.