Study of final state photons in hadronicZ ⁰ decay and limits on new phenomena

Abstract

The differential cross section for final state radiation from primary quarks is obtained from a study of isolated energetic photons produced in the reactione ⁺ e ⁻→Z ⁰→hadrons+γ, as measured in the DELPHI detector at the CERN LEP collider. When combined with the measurement of the total hadronic width of theZ ⁰, the observed rate determines the electroweak coupling constants of up and down type quarks, i.e.,

$$v_{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}}^2 + a_{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}}^2 = 1.13 \pm 0.29 and v_{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}}^2 + a_{{2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}}^2 = 1.65 \pm 043.$$

No evidence is seen for additional photon production from anomalous decays of theZ ⁰ or from decays of new particles. This measurement leads to upper limits on the production cross section times branching fraction of (a) the Higgs boson in the reactione ⁺ e ⁻→Z ⁰→H +γ,H→hadrons, (b) an excited quark,q ^*→q+γ, and (c) the contribution of an anomalous decay of theZ ⁰ into a photon and hadrons. These limits, all at the 95% confidence level, vary from 3 to 10pb as the mass of the intermediate state (H,q ^* orZ ^*) varies from 10 GeV/c² to 80 GeV/c².