VOLUME 43, NUMBER 26 P HYSICAI, REVIEW I. ETTERS 24 DECEMBER 1979 Test of Universality of Charged Leptons D. P. . Barber, U. Becker, ' H. Benda, A. Boehm, J. G. Branson, J. Bron, D. Buikman, J. Burger, C. C. Chang, H. S. Chen, M. Chen, C. P. Cheng, Y. S. Chu, R. Clare, P. Duinker, G. Y. Fang, H. Fesefeldt, D. Fong, M. Fukushima, J. C. Guo, A. Hariri, G. Herten, M. C. Ho, H. K. Hsu, T. T. Hsu, R. W. Kadel, W. Krenz, J. Li, Q. Z. Li, M. Lu, D. Luckey, D. A. Ma, C. M. Ma, G. G. G. Massaro, T. Matsuda, H. Newman, J. Paradiso, F. P. Poschmann, J. P. Revol, M. Rohde, H. Rykaczewski, K. Sinram, H. W. Tang, L. G. Tang, Samoel C. C. Ting, K. L. Tung, F. Vannucci, ' X. R. Wang, P. S. Wei, M. White, G. H. Wu, T. W. Wu, J. P. Xi, P. C. Yang, X. H. Yu, N. L. Zhang, and R. Y. Zhu III Physikalisches Institut Technische Hochschule, Aachen, West Germany, and Deutsches Elektxonen-Synchrotron (DES'), Hamburg, West Germany, and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts OZIS9, and Sectie H, Nationaal Instituut ooor Kernfysica en Hoge Energi-efysica, (NIKHEF), Amsterdam, The Netherlands, and Institute of High Energy Physics, Chinese Academy of Science, Peking, People's Republic of China (Received 22 August 1979) Measurements of the reactions e++e e++e, p'+p, and 7 + T at PETBA ener- gies (s = 13, 17, 27.4, 30, and 31.6 GeV) are reported. The results show that these reactions agree well with the predictions of quantum electrodynamics thus determining that all the known charged leptons are pointlike particles to a distance & -10 ~8 cm. There have been many experiments testing quantum electrodynamics with electrons and mu- ons at electron-positron storage rings. The most notable experiments were done by Alles-Borelli et a/. , ' Newman et al. , ' Augustin et al. , ' O' Neill et al. , ' and by our group at PETRA' up to a cen- ter-of-mass energy of 17 GeV. For a good re- view of QED work, see Brodsky and Drell. ' Much has been learned about the properties of the heavy lepton ~ since the original search began at ADQNE on e '+ e - pe +. . . . ' The discovery of the 7 lepton at Standord Linear Accelerator Cen- ter' and its subsequent confirmation at DESY' have inspired further studies. We know it is a spin-z particle which decays weakly" and whose properties are very similar to the muon. In this experiment we study the reactions e'+ e - l'+l (l =e, ti, ~) and simultaneously measure their energy or angle dependence at various PETRA energies. These measurements enable us to compare the data with predictions of quan- tum electrodynamics, to test the universality of these leptons at very small distances, and to set a limit on the charge radius of these particles. The detector we used, known as MARK-J, ' dis- tinguishes and measures the energies and the di- rections of the charged and neutral hadrons, elec- trons, photons, and muons. It covers a solid an- gle of p = 2& and 8 = 9' to 1'l1' (8 is the polar and y is the azimuthal angle). Lucite Cherenkov counters surround the intersection region, fol- lowed by two layers g and B) of three radiation lengths each of lead-scintillator shower counters, with one fast photomultiplier tube at each end. The counters A and B enable us to locate shower maxima in various 8 and p directions. They are followed by the sixteen C counters, consisting of twelve layers (twelve radiation lengths) of lead- scintillator sanchvich also with one phototube at each end. Surrounding the electromagnetic show- er counters are drift chambers which measure tracks from hadron showers and measure inci- dent muon angle. The next layers are hadron calorimeters consisting of magnetized-iron-scin- tillator sandwiches. The last layer of calorime- ter, composed of thea counters, is used for trig- gering on muons and for rejecting cosmic rays. The magnetic field in the iron is toroidal and its value is 17 kG. Finally, in the outermost layer there are drift chambers which are used to meas- ure single- and double-muon exit angles and thus mome nta. . In the small-angle region there are four layers of scintillation counters sandwiched between 10- cm-thick iron plates to measure shower energy at angles (12'&8 &30'). The total energy of each interaction and the di- rection of a particle or group of particles is com- puted from the time and pulse-height information of the shower counters and calorimeter counters. The azimuthal position is determined by the finely segmented shower counters. This method enables us to determine the 8 and p angles to an accuracy of & 5' for e or y and & 15 for hadrons. 1979 The American Physical Society 1915