The Second Lepton Family Klaus Winter, CERN The Nobel Prize for Physics for 1988 was awarded to L. Lederman, M. Schwartz and J. Steinberger for work on neutrinos in the early 1960s. In a letter [1] addressed to the "dear radioactive ladies and gentlemen", writ­ ten in December 1930, Wolfgang Pauli proposed, as a "desperate remedy" to save the principle of conservation of energy in beta-decay, the idea of the neutrino, a neutral particle of spin 1/2 and with a mass not larger than 0.01 proton mass. "The continuous beta-spectrum [2] would then become understandable by the assumption that in beta-decay a neutrino is emitted together with the electron, in such a way that the sum of the energies of the neutrino and electron is constant." Pauli did not specify at that time Fig. 1 — A recent photograph taken at CERN of Leon Lederman (left), whether the neutrino was to be ejected Jack Steinberger (centre) and Melvin Schwartz. or created. In his famous paper "An attempt of a theory of beta-decay" [3] the muon not decay into e + at the rate ween 1 and 2 GeV should be achievable. E. Fermi used the neutrino concept of predicted if such a non-locality exis­ Would these synchrotrons though, deli­ Pauli together with the concept of the ted ? ". On this view the muon would vir­ ver enough neutrinos? According to nucleon of Heisenberg. He assumed tually dissociate into W + v, the charged their specifications they should accele­ that in beta-decay a pair comprising an W would radiate a and W + v would rate 1011 protons per second, an unpre­ electron and a neutrino is created, analo­ recombine to an electron. What was cedented number, yet it would still re­ gous to photons in transitions between wrong with this picture? The non­ quire a detector of 10 tons weight to nuclear states. This theory accounted locality looked like a must to avoid the detect one neutrino reaction per day. successfully for the observed conti­ unitarity catastrophe. Why not postulate The first question was whether the nuous electron spectrum in beta-decay. two neutrinos, one associated with the neutrinos from pion decay would be Detection of the recoils of nuclei pro­ electron family and the other with the identical with those originating in 3- duced in beta-decay in coincidence with muon family. This would explain the ap­ decay and produce equal numbers of the electron had already given reality to parent absence of the decay µ → e + . electrons and muons in their capture the neutrino. Cowan and Reines [4] first These were exciting and most funda­ reactions, or would they be different and detected antineutrinos from fission frag­ mental questions. They were investiga­ produce only muons. The detector ments by observing the inverse beta- ted, independently, by Bruno Pontecorvo would have to be shielded against the decay of protons. [7] and by Melvin Schwartz [8]. Mel enormous flux of pions and muons The universal V-A theory of Feynman Schwartz has related his own discovery which would otherwise mask the rare and Gell-Mann [5] described successful­ story [9], He had the idea during the neutrino reactions. Both at CERN and ly all weak decays by a local current- night following a coffee break discus­ at Brookhaven, beams were designed current interaction. According to this sion at the Pupin Laboratory of Columbia using internal targets in the vacuum theory, the cross-sections of neutrino University in New York in November chambers of the synchrotron. A big race reactions would increase with energy 1959 of using high energy neutrinos to between the newly founded European and lead to a unitarity catastrophe at the probe the weak interaction at high ener­ Laboratory (CERN) in Geneva, and so-called Fermi scale, GF1/2 = 300 GeV. gy, T.D. Lee had asked the question. This Brookhaven started. Who would have An experimental investigation of the was the answer: use high energy neu­ the best beam and the best detector? behaviour of the weak interaction at trino beams. Mel Schwartz also gave the The best detector would be a heavy li­ high energy therefore became a central method of making such a beam. Pro­ quid bubble chamber which could show question. duce high energy pions in proton colli­ all details of the reaction, but it was to be Following Feynman and Gell-Mann sions with a target and let them decay. another 10 years before Gargamelle, a the divergency inherent in a point-like Depending on the decay angle, the neu­ 10 ton bubble chamber could be built coupling of the weak current could be trinos would have energies up to 43% of for CERN. Meanwhile, a 1 ton bubble avoided by postulating that the current- the pion energy. At the new strong fo­ chamber was set up at CERN by Bernar- current coupling is transmitted by the cussing proton synchrotrons at CERN dini and Steinberger and a liquid scin­ exchange of massive charged bosons (26 GeV) and at Brookhaven National tillator detector combined with a ma­ W±. However, there was already a pro­ Laboratory (30 GeV), then under con­ gnetic cloud chamber by a group headed blem. Feinberg [6] asked: "Why does struction, muon-neutrino energies bet­ by Helmuth Faissner. In Brookhaven, 10 Gaillard, Lederman and Schwartz made a lucky choice. They adopted the spark chamber which had just been success­ fully used in an experiment at Berkeley by J. Cronin. The plates could be made massive and it had the required space resolution for discriminating between muons and electron cascade showers. In the Spring of 1961, preparations for the CERN experiment had to be inter­ rupted. Because of the shorter straight sections between the magnets of the synchrotron the neutrino flux was just too low; it had been overestimated. Jack Steinberger returned to Colum­ bia University and gave up his bubble Fig. 2 — Schwartz in 1963 with his spark chamber (photo Life Magazine). chamber plans, recognizing the decisive nos as probes produced a rich harvest. first high energy neutrino experiment advantage of the longer straight sec­ The prediction based on the theory of and the discovery of a second neutrino tions of the Brookhaven AGS. A steel weak interactions that the cross-section family will probably not be the last shielding wall of 13 m thickness was for neutrino interactions would increase chapter in neutrino physics. We are lack­ built, but tests showed excessive by five orders of magnitude from the ing direct detection of the third neutrino leakage and it turned out to be neces­ reactor neutrino energies used by (vτ), we are ignorant about the ques­ sary to reduce the synchrotron energy to Cowan and Reines to 1 GeV was drama­ tions of neutrino mass and the level at 15 GeV. A team of seven physicists, con­ tically confirmed. In inelastic reactions which lepton number conservation will sidered to be a large group at that time, above 1 GeV the cross-section was fail, and whether neutrinos are Dirac or performed the experiment in 1962 [10], found to increase linearly with energy Majorana particles. (G. Danby, J.M. Gaillard, K. Goulianos, [12]. This unexpected behaviour was L.M. Lederman, N. Mistry, M. Schwartz later on attributed to a new elementary and J. Steinberger). In 800 hours of process, the deep inelastic scattering of REFERENCES beam time, unprecedented by any other neutrinos on quarks. A jet of hadrons [11 Wolfgang Pauli, Collected papers, Vol. II high energy experiment, it was demons­ results from the fragmentation of the (Interscience, N.Y.) p. 1313. trated that neutrinos from pion decay [2] Chadwick J., Z.f. Physik 31 (1914) 210; quarks because of their confinement. Ellis C.D., Wooster W.A., Proc. Roy. Soc. produce only muons. Today we know of Comparing results of deep inelastic three lepton families, (e, ve), (µ, vµ ) and (London) MM (1927) 109; Meitner L„ Orth- muon and neutrino scattering, the mann W„ Z.f. Physik 60 (1930) 143. (τ , v τ ). The notion of fermion families CDHS Collaboration [13] found a ratio of was introduced when the existence of [3] Fermi E„ Z.f. Physik 88 (1934) 161. 5/18 for the structure functions, evi­ [4] Cowan C.L. jr. and Reines F., Phys. Rev. the muon-neutrino was uncovered. dence for the alleged electric charge of 107 (1957) 528. At CERN the sudden need to interrupt quarks of -1/3 and 2/3. [5] Feynman R. and Gell-Mann M. the first neutrino experiment led to an In December 1972 another dramatic [6] Feinberg A., Phys. Rev. 110 (1958) 1482. advance in technique. The invention of discovery was made using the intense [7] Portecorvo B., Proc. IXth Int. Conf. High the magnetic horn by Simon van der CERN neutrino beam and the gigantic EnergyPhysics\Jo\. 1, p. 233, Kiev 1959 and Meer [11] together with the ejection of Soviet Physics JETP 37 (1959) 1236. heavy liquid chamber Gargamelle con­ [8] Schwartz M., Phys. Rev. Lett. 4 (1960) the proton beam from the synchrotron ceived and built by André Lagarrigue. An onto an external target gave an increase 306. event in which an electron was recoiling [9] Schwartz M., Adventures in Experimen­ in the neutrino flux of a factor of 10. An from the interaction with an antimuon- tal Physics, Ed. B. Maglich, 1972, p. 82. enlarged heavy liquid bubble chamber neutrino was discovered by the group of [10] Dany G., Gaillard J.M., Goulianos K., and a spark chamber set-up recorded a Helmuth Faissner in Aachen [14], This is Lederman L.M., Mistry N., Schwartz M.