QED processes up to O(α4) have been studied at center-of-mass energies between 50 and 57 GeV using the AMY detector at TRISTAN. A possible substructure of the electron and muon is investigated in terms of chiral invariant contact interactions and also in terms of excited lepton production. The differential cross section data for e+e-→e+e- are consistent with the standard model at a confidence level of only 3.5%. From the absence of excited lepton production, we establish new upper limits on the coupling to excited electrons of mass up to 56 GeV/c2 and on the coupling to excited muons of mass up to 52 GeV/c2.
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We thank the TRISTAN staff for the rapid com-missioninga nde xcellenot perationo f the storageri ng. In addition we acknowledgteh e strong support and enthusiastiacs sistancep rovidedb y the staffs of our home institutionsT. his work has been supportedb y the Japan Ministry of Education, Science and Culture (Monbusho), US Departmento f Energy and NationalS cienceF oundationt,h eKorean Sciencea nd EngineeringF oundationa nd Ministry of Education, and the AcademiaS inicaof the People'sR epublicof China.