A measurement of the Z0 invisible width by single photon counting

The OPAL detector at LEP is used to measure the partial decay width of the Z0 into invisible particles by measuring the cross section of single photon events in e+e- collisions at centre-of-mass energies near the Z0 resonance. In a data sample of 5.3 pb-1, 73 events were observed with single photons depositing more than 1.5 GeV in the electromagnetic calorimeter, with an expected background of 8±2 events. With this data the Z0 invisible width was determined to be 0.50±0.07±0.03 GeV, where the first error is statistical and the second systematic. This corresponds to 3.0±0.4±0.2 light neutrino generations in the Standard Model. (author) 10 refs., 7 figs., 2 tabs