Acciarri, M (reprint author), Ist Nazl Fis Nucl, Sez Milano, Via Celoria 16, I-20133 Milan, Italy.
A determination of the number of light neutrino families performed by measuring the cross section of single photon production in e(+)e(-) collision near the Z resonance is reported. From an integrated luminosity of 100 pb(-1), collected during the years 1991-94, we have observed 2091 single photon candidates with an energy above 1 GeV in the polar angular region 45 degrees < theta(gamma) < 135 degrees. From a maximum likelihood fit to the single photon cross section, the Z decay width into invisible particles is measured to be Gamma(inv) = 498 +/- 12(stat) +/- 12(sys)MeV. Using the Standard Model couplings of neutrinos to the Z, the number of light neutrino species is determined to be N-v = 2.98 +/- 0.07(stat) +/- 0.07(sys). (C) 1998 Published by Elsevier Science B.V. All rights reserved.