[工程科技]Photonic events with missing energy in e+e- collisions at sqrts=189 GeV.pdf

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN-EP-2000-050 06 Apr 2000 Photonic Events with Missing Energy + − √s = 189 GeV in e e Collisions at 0 0 0 2 y The OPAL Collaboration a M 3 Abstract 2 v 2 + − Photonic events with large missing energy have been observed in e e collisions at a centre-of- 0 mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies 0 5 consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are 0 performed within the kinematic acceptance of each selection, and the number of light neutrino 0 species is measured. Cross-section results are compared with the expectations from the Standard 0 + − / Model process e e → νν + photon(s). No evidence is observed for new physics contributions to + − + − 2 x these final states. Upper limits on σ (e e → XY) BR(X → Yγ ) and σ (e e → XX) BR (X → e Yγ ) are derived for the case of stable and invisible Y. These limits apply to single and pair - ∗ 0 0 production of excited neutrinos (X = ν , Y = ν), to neutralino production (X = χ˜ , Y = χ˜ ) and to p 2 1