Measurement of single photon production in e⁺e⁻ collisions near the Z⁰ resonance

Abstract

A measurement of the single photon production cross-section is presented based on a data-sample of 40.5 pb⁻¹ collected with the OPAL detector at centre-of-mass energies within 3 GeV of the Z⁰ mass. Single photon events arise from initial state radiation and the production of an “invisible” final state consisting of neutrinos or possibly particles such as sneutrinos or photinos. The single photon topology is also sensitive to new Z⁰ decays such as$Z^0 \to \bar vv* \to \bar vv\gamma$ orZ ⁰→γX, X→invisible particles. A total of 447 single photon candidates were observed with energy exceeding 1.75 GeV in the polar angle region |cosθ|<0.7. The estimated background from processes with visible reaction products, mainly e⁺e⁻→e⁺e⁻γ, is 37±6 events. Interpreting the cross-sections as being solely due to Z⁰ decay to invisible particles and the expected W-contributions, the Z⁰ invisible with is determined to be 539±26±17 MeV corresponding toN _v=3.23±0.16±0.10 light neutrino generations. The differential cross-section with photon energy is presented. Upper limits are set on additional invisible contributions to the Z⁰ width, on possible non-resconant processes, and on Z⁰ decays to single photons. The energy spectra are used to constrain exotic sources of high energy single photons. In particular, the radiative twobody decay of the Z⁰ to a new particle X, with mass below 64 GeV and an invisible signature, has a Z⁰ branching ratio of less than 4.3×10⁻⁶ at 95% confidence level.