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1 Victor F. Weisskopf SEARCH FOR HEAVY LEPTONS FROM TIME-LIKE PHOTONS AT CERN From Lepton Physics at CERN and Frascati - Edited by Nicola Cabibbo World Scientific Series in 20th Century Physics - Vol. 8 - 1995 - pp. 45-47 WORLD SCIENTIFIC SINGAPORE - NEW JERSEY - LONDON - HONG KONG 1995 2 The "heartbeat of the proton" Victor F. WEISSKOPF Once Nino came to my office to tell me about his ideas of studying lepton pair production at PS. I was still not Director General, but Research Director at CERN. In addition to (ee) and () pairs, he wanted to search for (e) pairs as a signature of a new lepton carrying its own lepton number. He told me that if such a lepton existed with one GeV mass, it would have escaped detection in hadron accelerator experiments for two reasons: i) it would decay with a lifetime of order 10 11 sec and ii) because there is no mechanism for such a heavy new lepton: for its production a time-like photon would be needed. Time-like photons could be produced in hadronic interactions: for example in (p̅p) annihilation. This was before Lederman-Schwartz and Steinberger had discovered the two neutrinos. To think of a ''sequential'' Heavy Lepton and to work out the possible ways to get it in a hadron machine was for me extremely interesting. Nino had just finished his first high precision work on the muon (g2). It was some time after the Rochester Conference in 1960. I gave Nino the following suggestion: if you want to search for something so revolutionary as a Heavy Lepton carrying its own lepton number you should work out a proposal for a series of experiments where the study of lepton pairs (ee) and () could be justified in terms of physics accepted by the community. In addition a high intensity antiproton beam was needed. He came later to tell me that he had two very good friends, both excellent engineers: Mario Morpurgo and Guido Petrucci. A very high intensity antiproton beam could be built to study the electromagnetic form factor of the proton in the time-like region. If the proton was "point-like" in the time- 3 like region, the rate of time-like photons yielding (ee) and () pairs could be accessible to experimental observation, thus allowing to establish some limits on the new Heavy Lepton mass, or to see it, via the (e ) channel. The "official" theme was: to establish if the proton had a structure or not in the time-like region. Thus a powerful system able to detect (ee) and () pairs could be built. Nino established in 1963 the existence of a time-like structure of the proton studying the (ee) channel and in 1964 studying the () channel. The set up was able to do what he wanted: a simultaneous detection of electrons and pairs, therefore (e) as well. Unfortunately the proton was not a point-like particle in the time-like region and therefore the source of time-like photons originated in (p̅p) annihilation was very depressed. In fact, using the (ee) and the () channels, Nino established that at 6.8 (GeV/c)2 time-like four momentum transfer, the crosssection was 500 times below the expected point-like value. This result had attracted a lot of attention. Bogoliubov was very interested when in 1964 Nino went to Dubna to present the () results at the International Conference on "High Energy Physics". Yang had a model that predicted a point-like structure of the proton in the time-like region. I called this series of experiments as measuring the "heartbeat of the proton". Of course there were no (e) events, neither in the (p̅p) nor in the (p) channel. Nevertheless a series of experiments was performed on "standard" physics, such as the discovery of many rare decay modes of mesons and the measurement of the () mixing. All these experiments could be done because Nino had invented what is now known as the ''preshower'' method to reject with high efficiency pions in favor of "electrons". Once it was clear that in hadronic interactions there are very few time-like photons, he asked me if I would give the green light 4 in order to consider the use of the (e) technology in the newly being developed Frascati (ee) collider. There the "time-like" photons were very abundant and the (e) method would have been the best in order to see if a Heavy Lepton carrying its own lepton number existed. Of course he got the green light and in 1970 he got the first limit on the Heavy Lepton mass together with a series of high precision QED measurements. But one story I Will never forget in connection with the "heartbeat of the proton". After he succeeded with his friends Mario (Morpurgo) and Guido (Petrucci) to build the highest intensity antiproton beam at CERN, Nino came to my office and said more or less the following: "Viki, by changing the voltage of the electrostatic separator and a few other trivial details, in one night, I will be able to establish if the antideuteron exists with the correct expected deuteron mass". I told him that this was an experiment where he would get the Nobel prize if he found nothing. "But, there is a but", I added. "If you do not succeed in one night and if you destroy the beam, then I will not defend you. My green light is only valid if you really can check the existence of the antideuteron in a single night”. Next morning, when I arrived at CERN, Nino was there With his graph where the antideuteron signal was exactly where it was expected to be. I remember the year, 1965, not the day. It was (and is) the birthday of Peter Standley who was at that time the PS Division Leader. I called him in my office and the antideuteron discovery at CERN was my and Nino's gift to our mutual friend Peter. I decided not to have a press-release and Nino agreed. A few weeks later we read in the newspaper that the antideuteron had been discovered by Lederman and Ting in the United States. They had decided to have a press-release. Nevertheless Nino's paper in Nuovo Cimento preceeds Leon's and Sam's publication in Physical Review.