Observation of the muon inner bremsstrahlung at LEP1

Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2 < Eγ ≤ 1 GeV and transverse momentum with respect to the parent muon pT < 40 MeV/c, and 1 < Eγ ≤ 10 GeV and pT < 80 MeV/c. A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z0 decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06 ± 0.12 ± 0.07 in the photon energy range 0.2 < Eγ ≤ 1 GeV and 1.04 ± 0.09 ± 0.12 in the photon energy range 1 < Eγ ≤ 10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP.