The MARK J detector at PETRA was used in an experimental study of µ⁺µ⁻ and µ⁺µ⁻γ events. Data were collected at the center of mass energy range from 14 to 46 GeV. The standard model of electroweak interactions has been tested by the data with high statistics. Stringent limits on the extensions of the standard model have been derived.
The rate and angular distribution of muon pair events have been measured and are found to be in good agreement with the standard model. The R-value and charge asymmetry of the moon pairs are used to determine the electroweak parameters. For Mz = 93 GeV, we find, sin² θω = .20+.04-.025. Assuming e-µ universality, the lepton coupling constants are, g²A = .265 ± .032, g²V = 0.038 ± .043.Special emphasis has been placed on the understanding of the radiative effects by studying acollinear muon pairs and µ⁺µ⁻γ events.This provides a test of the electroweak theory to higher than leading order. Various distributions of µ⁺µ⁻γ events have been compared to the standard model predictions. The data and electroweak theoretical predictions are in excellent agreement.
Investigations have been carried out to constrain various non-standard processes and to search for new particles. Limits are set on the cutoff parameters, mass and coupling of excited muons, four fermion contact interaction, etc. Muons can be regarded as pointlike particles down to a distance of 8 x 10⁻¹⁷ cm. The pair production of excited muons is limited by the µ⁺µ⁻γγ event analysis, and the µ* mass is expected to be greater than 23 GeV at 95% confidence level. The fact that there is no excess in the production of µ⁺µ⁻γ events gives a tight constraint on the single production of excited muons.
Our results are compared to other experiments and the status of experimental tests of the standard model is summarized. Perspectives of testing tile standard model in the near future at LEP are discussed. Precision measurements of quantities such as the gauge boson masses, widths and various asymmetries will confront the standard model with more stringent constraints and/or reveal new physics.
