The European Physical Journal C

ISSN: 1434-6044 (printed version)
ISSN: 1434-6052 (electronic version)

Table of Contents

Abstract Volume 2 Issue 4 (1998) pp 607-625
DOI 10.1007/s100529800865

experimental physics:
Search for anomalous production of photonic events with missing energy in ${\rm e^+e^-}$ collisions at $\sqrt{s} = 130$-172 GeV

The OPAL Collaboration , K. Ackerstaff et al. (8)

(8) CERN, European Organisation for Particle Physics, CH-1211 Geneva 23, Switzerland

Received: 6 October 1997 / Published online: 10 March 1998

Abstract. Photonic events with large missing energy have been observed in $\mathrm{e^+e^-}$ collisions at centre-of-mass energies of 130, 136, 161 and 172 GeV using the OPAL detector at LEP. Results are presented based on search topologies designed to select events with a single photon and missing transverse energy or events with a pair of acoplanar photons. In both search topologies, cross-section measurements are performed within the kinematic acceptance of the selection. These results are compared with the expectations from the Standard Model processes ${\rm e^+e^-}\to \nu\overline\nu \gamma (\gamma )$ (single-photon) and ${\rm e^+e^-}\to \nu\overline\nu \gamma \gamma (\gamma )$ (acoplanar-photons). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma({\rm e^+e^- \to XY)\cdot BR(X\to Y\gamma })$ and $\sigma({\rm e^+e^- \to XX)\cdot BR^2(X\to Y\gamma })$ are derived for the case of stable and invisible Y. These limits apply to single and pair production of excited neutrinos (${\rm X}= \nu^*,{\rm Y}= \nu$), to neutralino production (${\rm X}=\tilde\chi^0_2, {\rm Y}=\tilde\chi^0_1$), and to supersymmetric models in which ${\rm X}= \tilde\chi^0_1$ and ${\rm Y}=\tilde{\rm G}$ is a light gravitino. For the latter scenario, the results of the acoplanar-photons search are used to provide mode l-dependent lower limits on the mass of the lightest neutralino.

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Online publication: May 25, 1998
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