First study of the interference between initial and final state radiation at the Z resonance

The interference between initial and final state radiation in the processe + e −→μ + μ − at √s≈MZ has been studied by measuring the forward-backward asymmetry as a function of the acoplanarity angle between the final state muons. The interference is expected to be sensitive to the space-time separation of the initial and final state radiation. The measured asymmetry distribution has been compared to theoretical predictions using thekoralz generator, with and without\({\mathcal{O}}(\alpha )\) interference. The magnitude of the interference between initial and final state radiation was found to be of the order predicted and to follow the expected distribution. Using the theoretical predictions, a value of $$\Gamma _{\rm Z} = 2.50 \pm 0.21 (stat.) \pm 0.06 (syst.) GeV.$$ has been extracted. The interpretation of this result is discussed. There is an additional uncertainty in the estimate ofΓ Z from as yet uncalculated higher order interference terms. By assuming a value ofΓ Z consistent with the world average, the data were used to estimate the size of these uncalculated corrections.