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

TL;DR

This study analyzes photon events with missing energy in electron-positron collisions at 189 GeV, measuring neutrino counts and setting limits on new physics scenarios like excited neutrinos, neutralinos, and extra dimensions.

Contribution

It provides the first measurements of neutrino species and sets new upper limits on various beyond Standard Model processes at this energy.

Findings

Number of light neutrino species measured

No evidence for new physics observed

Upper limits set on exotic particle production

Abstract

Photonic events with large missing energy have been observed in e+e- collisions at a centre-of-mass energy of 189GeV 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- to nu nubar + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits are derived on sigma(e+e- to XY).BR(X to Y gamma) and sigma(e+e- to XX).BR**2(X to Y gamma) 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=neutralino_2, Y=neutralino_1) and to supersymmetric models in which X = neutralino_1 and Y = light gravitino. The case of macroscopic decay lengths of particle X is considered for e+e- to XX, X to Y gamma, when M_Y is of order zero. 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.