EPJ

2024 Impact factor 4.8
EPJ C - Particles and Fields
Particles and Fields
Eur. Phys. J. C 2, 607-625
DOI 10.1007/s100529800865

Search for anomalous production of photonic events
with missing energy in e+e- collisions at $\sqrt{s}= 130\mbox{--}172\,\mathrm{GeV}$

The OPAL Collaboration, K.Ackerstaff8 et al.

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 ${\mathrm{e^+e^-}}\to \nu\overline{\nu}\gamma(\gamma)$ (single-photon) and ${\mathrm{e^+e^-}}\to \nu\overline{\nu}\gamma\gamma(\gamma)$ (acoplanar-photons). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma({\mathrm{e^+e^-}}\to {\mathrm X}{\mathrm{Y}})\cdot{\mathrm{BR}}({\mathrm X}\to {\mathrm{Y}}\gamma)$ and $\sigma({\mathrm{e^+e^-}}\to {\mathrm X}{\mathrm X})\cdot{\mathrm{BR}}^2({\mathrm X}\to {\mathrm{Y}}\gamma)$ are derived for the case of stable and invisible ${\mathrm{Y}}$. 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. For the latter scenario, the results of the acoplanar-photons search are used to provide model-dependent lower limits on the mass of the lightest neutralino.

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