DOI 10.1007/s100520000522
Photonic events with missing energy
in
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
Abstract
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
