Nuclear Physics B142 (1978) 29-52 © North-Holland Publishing Company ELECTRON PAIR PRODUCTION AT THE CERN ISR A.G. CLARK, P. DARRIULAT, K. EGGERT *, V. HUNGERBUHLER, H.R. RENSHALL, J. STRAUSS ** and A. ZALLO CERN, Geneva, Switzerland B. AUBERT +, M. BANNER, J.C. CHEZE, T. MODIS, P. PEREZ, G. SMADJA, J. TEIGER, C. TUR, J.P. VIALLE ¢, H. ZACCONE and A. ZYLBERSTEJN CEN-SA CLA Y, France P. JENNI, P. STROLIN and G.J. TARNOPOLSKY ETH t, Zarich, Switzerland Received 12 June 1978 The electron pair production cross section from pp collisions at c.m. energies x/s = 52 and 63 GeV has been measured for masses above 1.4 GeV[c 2. The J/TJ is dominant in this mass range and its production properties have been studied. An esti- mate of the qJ' production cross section is given. Limits for T ~ e+e- and × ~ J/C0' productions are presented. The behaviour of the continuum cross section as a function of mass is compared to current theoretical estimates. 1. Introduction The study of lepton pair production in hadron-hadron collisions has provided a powerful way to study vector meson production, and additionally has yielded im- portant information on the quark distribution inside hadrons. Results on the process pp -+ e+e - + anything, at the CERN ISR, are presented for c.m. energies x/s = 52.4 GeV and x/s = 62.7 GeV, for an integrated luminosity of 1.5 X 1037 cm -2. The electron pairs are detected at large angle with respect to the initial proton direction in the c.m. of the reaction. The experimental difficulty is to reject the very large background due to pion production, and to select electron * III. Physikalisches Institut der Technischen Hochschule, Aachen, Germany. ** Institut for Hochenergie Physik der Osterr. Akademie der Wissenschaften, Vienna, Austria. LAL Orsay, France. t Supported in part by the Swiss National Science Foundation.