Measurement of the e+e−→Zγγ cross section and determination of quartic gauge boson couplings at LEP

A first measurement of the cross section of the process e+e−→Zγγ is reported using a total integrated luminosity of 231pb−1 collected with the L3 detector at centre-of-mass energies of 182.7GeV and 188.7GeV. By selecting hadronic events with two isolated photons the e+e−→Zγγ cross section is measured to be 0.49+0.20−0.17±0.04pb at 182.7GeV and 0.47±0.10±0.04pb at 188.7GeV. The measurements are consistent with Standard Model expectations. Limits on Quartic Gauge Boson Couplings a0/Λ2 and ac/Λ2 of −0.009GeV−2