Photonic events with missing energy
e+e- collisions at
= 189 GeV
The OPAL Collaboration
G.Abbiendi - et al.
2 Dipartimento di Fisica dell' Università di Bologna and INFN,
40126 Bologna, Italy
Received: 4 April 2000 / Published online: 27 November 2000 - © Springer-Verlag 2000
Photonic events with large missing energy have been observed in 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 + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on and are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos ( ), to neutralino production ( ) and to supersymmetric models in which and is a light gravitino. The case of macroscopic decay lengths of particle X is considered for , , when . 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.
Copyright Società Italiana di Fisica, Springer-Verlag 2000