A direct determination of the number of light neutrino families from e +e -→ν overlineνγ at LEP

The L3 detector at LEP has been used to determine the number of light neutrino families by measuring the cross section of single photon even in e ⁺e ^- collisions at energies near the Z ⁰ resonance. We have observed 61 single photon candidates with more than 1.5 GeV of deposited energy in the barrel electromagnetic calorimeter, for a total integrated luminosity of 3.0 pb ^-1. From a likelihood fir to the single photon cross sections, we determin N_ν=3.24 ± 0.46 ( statistical) ±0.22 ( systematic).