Search for Anomalous Production of Photonic Events with Missing Energy in e+e- Collisions at sqrt{s} = 130-172 GeV

TL;DR

This study searches for anomalous photon events with missing energy in e+e- collisions at energies 130-172 GeV, finding no evidence for new physics but setting limits on various hypothetical particles and models.

Contribution

It provides the first search for specific anomalous photonic events at these energies and sets new limits on excited neutrinos, neutralinos, and supersymmetric models.

Findings

No evidence for new physics observed.

Upper limits set on production cross-sections of hypothetical particles.

Lower bounds established on the neutralino mass in certain models.

Abstract

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- -> \nu \nu(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 = \nu^*, Y = \nu), to neutralino production (X = \chi^0_2, Y = \chi^0_1), and to supersymmetric models in which X = \chi^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.