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

Photonic events with large missing energy have been observed in \(\rm e^+e^-\) 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 \(\mathrme^+\mathrme^-\to \nu\overline{\nu}\) + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on \(\sigma(\mathrme^+\mathrme^-\to\mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(\mathrm{X}\to\mathrm{Y}\gamma)\) and \(\sigma(\mathrme^+\mathrme^-\to\mathrm{XX})\cdot\mathrm{BR}^2(\mathrm{X}\to\mathrm{Y}\gamma)\) are derived for the case of stable and invisible \(\mathrm{Y}\). These limits apply to single and pair production of excited neutrinos (\(\mathrm{X} = \nu^*, \mathrm{Y} = \nu\)), to neutralino production (\(\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}\)) and to supersymmetric models in which \(\mathrm{X} ={{{\tilde{\chi}}^{0}}_{1}}\) and \(\mathrm{Y}={\tilde{\mathrm{G}}}\) is a light gravitino. The case of macroscopic decay lengths of particle X is considered for \(\mathrme^+\mathrme^- \to \mathrm{XX}\), \(\rm X \to Y \gamma\), when \(M_{\mathrm Y}\approx 0\). 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.