Mulit-Photon Events with Large Missing energy in e+e- collisions at sqrt(s)=192-209 GeV

TL;DR

This study analyzed events with multiple photons and large missing energy in electron-positron collisions at 192-209 GeV, finding no evidence of new physics and setting upper limits on certain hypothetical particle production cross-sections.

Contribution

The paper provides the first comprehensive search for new physics in multi-photon plus missing energy events at LEP energies, establishing upper limits on various beyond Standard Model scenarios.

Findings

No evidence for new physics observed.

Upper limits on cross-sections range from 10 fb to 60 fb.

Results constrain models with excited neutrinos and supersymmetric particles.

Abstract

Events with a final state consisting of two or more photons and large missing transverse energy have been observed in e+e- collisions at centre-of-mass energies in the range 192 - 209 GeV using the OPAL detector at LEP. Cross-section measurements are performed within the kinematic acceptance of the selection and compared with the expectation from the Standard Model process e+e- -> nu nu gamma gamma(gamma). No evidence for new physics contributions to this final state is observed. Upper limits on sigma (e+e- -> XX) BR^2(X -> Ygamma) are derived for the case of stable and invisible Y. In the case of massive Y the combined limits obtained from all the data range from 10 fb to 60 fb, while for the special case of massless Y the range is 20 fb to 40 fb. The limits apply to pair production of excited neutrions (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=~G is a light gravitino.