MEASUREMENT OF SINGLE-PHOTON PRODUCTION IN E(- COLLISIONS NEAR THE Z(0) RESONANCE()E)

A measurement of the single photon production cross-section is present ed based on a data-sample of 40.5 pb(-1) collected with the OPAL detec tor at centre-of-mass energies within 3 GeV of the Z(0) mass. Single p hoton events arise from initial state radiation and the production of an ''invisible'' final state consisting of neutrinos or possibly parti cles such as sneutrinos or photinos. The single photon topology is als o sensitive to new Z(0) decays such as Z(0) --> <(nu)over bar>nu --> <(nu)over bar nu gamma or Z(0) --> gamma X, X --> invisible particles. A total of 447 single photon candidates were observed with energy exc eeding 1.75 GeV in the polar angle region \cos theta\ < 0.7. The estim ated background from processes with visible reaction products, mainly e(+)e(-) --> e(+)e(-)gamma, is 37 +/- 6 events. Interpreting the cross -sections as being solely due to Z(0) decay to invisible particles and the expected W-contributions, the Z(0) invisible width is determined to be 539 +/- 26 +/- 17 MeV corresponding to N-nu = 3.23 +/- 0.16 +/- 0.10 light neutrino generations. The differential cross-section with p hoton energy is presented. Upper limits are set on additional invisibl e contributions to the Z(0) width, on possible non-resonant processes, and on Z(0) decays to single photons. The energy spectra are used to constrain exotic sources of high energy single photons. In particular, the radiative two-body decay of the Z(0) to a new particle X, with ma ss below 64 GeV and an invisible signature, has a Z(0) branching ratio of less than 4.3 x 10(-6) at 95 % confidence level.