Search for new phenomena using single photon events at LEP1

Abstract

Data are presented on the reaction e+e --> gamma + no other detected particle at center-of-mass energies, ps = 89.48 GeV, 91.26 GeV and 93.08 GeV. The cross section for this reaction is related directly to the number of light neutrino generations which couple to the Z0 boson, and to several other phenomena such as excited neutrinos, the production of an invisible 'X' particle, a possible magnetic moment of the tau neutrino, and neutral monojets. Based on the observed number of single photon events, the number of light neutrinos which couple to the Z0 is measured to be N = 3.15 +/- 0.34. No evidence is found for anomalous production of energetic single photons, and upper limits at the 95% condence level are determined for excited neutrino production (BR < 4 - 9 x 10-6), production of an invisible 'X' particle ( < 0.1 pb), and the magneticmoment of the tau neutrino (< 5.2 x 10-6 µB). No event with the topology of a neutral monojet is found, and this corresponds to the limit < 0.044= pb at the 95% condence level, where is the unknown overall monojet detection effciency.