2020 Impact factor 4.590
Particles and Fields
Eur. Phys. J. C 18, 253-272
DOI 10.1007/s100520000522

Photonic events with missing energy
in e+e- collisions at $\sqrt{s}$ = 189 GeV

The OPAL Collaboration

G.Abbiendi$^{\rm 2}$ - 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 $\rm e^+e^-$ 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 $
\mathrm{e}^+\mathrm{e}^-
\to \nu\overline{\nu}$ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma(
\mathrm{e}^+\mathrm{e}^-
\to \mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(
\mathrm{X}
\to \mathrm{Y}\gamma
)$ and $
\sigma(\mathrm{e}^+\mathrm{e}^-\to \mathrm{XX})\cdot\mathrm{BR}^2
(\mathrm{X}\to\mathrm{Y}\gamma)
$ are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos ( $\mathrm{X} = \nu^*, \mathrm{Y} =
\nu$), to neutralino production ( $\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}$) and to supersymmetric models in which $\mathrm{X} = {{{\tilde{\chi}}^{0}}_{1}}$ and $\mathrm{Y}=\tilde{\mathrm{G}}$ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for $
\mathrm{e}^+\mathrm{e}^-
\to \mathrm{XX}$, $\rm X \to Y \gamma$, when $M_{{\mathrm{Y}}}\approx 0$. 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