Fig. 1: Feynman diagrams for e� �� � through an intermediate boson. I labels the intermediate boson field.One-half year later, Oneda and Pati 3) noted that the problem pointed out by Feinberg, that is the

non-observed radiative muon decay, could be overcome by making the neutrino associated with muon decay a different particle than that associated with beta decay, � �� �� .

This was followed by the article of Bruno Pontecorvo 4) in which he proposed specific experiments to check if the two neutrinos are the same or if they are different, in particular to make beams of neutrinos produced in the decay of muons, and to look if these neutrinos associated with muons will produce electrons and positrons in interacting with nucleons,

n e p� �� � � � and p e n�� �� � � � ,in the same manner as beta-decay neutrinos would be expected to do, in the frame of the Fermi theory. This is the experiment which we did in 1962 5).

J. Steinberger, v2006, Santa Fe, June 2006

The electron and the muon neutrinos are different particles; the story of the second neutrino

This story begins with a paper of G. Feinberg 1). At the time, the neutrino, the Fermi theory of �-decay, as well as the universal Fermi interaction, which linked the muon-neutrino current to the electron-neutrino and the nucleon-neutrino currents, and so created the Puppi triangle of three weak interactions, were well established and confirmed. But it had also already been noticed that at highenergies the Fermi interaction violates unitarity, and the idea of an intermediate meson had been put forward. The paper of Feinberg pointed out that this theory would predict also the radiative decay of the muon and, on the basis of the Feynman diagrams of Fig. 1, predicted the branching ratio �/24� =10–4. However, Lokanathan and I 2) had already searched for this decay and observed an upper limit of 2 × 10–5, that is five times smaller.

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 221 (2011) 273–280

0920-5632/$ – see front matter © 2011 Published by Elsevier B.V.

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doi:10.1016/j.nuclphysbps.2011.09.016

Fig. 2: Bruno Pontecorvo (Courtesy JINR, Dubna)

In some sense, our experiment began on one of the customary Friday afternoon Columbia University physics department coffees, this one early in 1960. Unfortunately for me, I was not present. The work of Pontecorvo had not yet travelled to the US. T.D. Lee asked the question: All we know of the weak interaction is based on the study of particle decays. How can we learn about the weak interaction at higher energies than are involved in the decays of pions, kaons and hyperons? Stimulated by this question, Mel Schwartz reflected and came to the idea of doing experiments using beams of neutrinos of higher energy 6). This short note by Mel in Physical Review Letters was followed by a considerably longer letter by Lee and Yang 7), in which they detail nine specific questions on the weak interaction which might be answered by neutrino beam experiments:

1. The identity of the neutrinos.

2. Conservation of leptons.

3. Possible existence of a neutral lepton current.

4. Point structure of the lepton current.

5. Universality of weak interactions involving electrons and muons.

6. S-symmetry.

7. Conserved vector current and proportionality with electromagnetic current.

8. Possible existence of weakly coupled Boson W+/-.

9. Interactions with extremely large momentum transfers.

J. Steinberger / Nuclear Physics B (Proc. Suppl.) 221 (2011) 273–280274

Certainly neutrino experiments contributed to the resolution of several of these questions. But it is also true that not even Lee and Yang could anticipate all that could be learned with the help of neutrino beams, for instance the nucleon structure functions (before it was known that the nucleons were not elementary particles), or the first validation of the theory of the strong interaction, QCD, in the neutrino deep-inelastic scaling violations.

It should also be remembered that here, as is often the case in the history of advances in physics, the moment was ripe. On the basis of the invention of the alternating gradient synchrotron by Courant, Livingston and Snyder 8) in 1952, two accelerators, the PS at CERN and the AGS at Brookhaven Natlional Laboratory, were nearing completion, and these, for the first time, offered sufficiently high proton beam energies and intensities to make neutrino beam experiments a practical possibility.

Following the suggestions by Pontecorvo 4) and Schwartz 6), neutrino beam experiments were initiated both at the Columbia University Nevis Laboratory, for the Brookhaven AGS, and at CERN, for the PS. The Columbia team 5) consisted of Mel Schwartz, two faculty colleagues, Leon Lederman and myself, one post-doc from France, Jean-Marc Gaillard, two doctoral students, Dinos Goulianos and Nariman Mistry, and one Brookhaven colleague, Gordon Danby.

In order to fit the experiment into the available space at the AGS, the proton beam energy had to be reduced to 15 GeV, one-half of its maximum. The layout, see Fig. 3, (Fig. 1 of the publication 5)).

Fig. 3: Plan view of AGS neutrino beam experiment

The proton beam strikes an internal beryllium target, since the technique of beam extraction was still waiting to be invented by Piccioni a year or two later. The 31 m decay path for the pions and kaons, whose spectrum is shown in Fig. 4, Fig. 2 of the paper, is followed by 13.5 m of iron shielding, the scrap of battleships, kindly supplied by the US Navy, which at the time funded the NevisLaboratory. One of the problems in doing the experiment was that the initial shielding was found to be inadequate; the shielding was improved during the experiment.

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E BEV�

Fig. 5: Spark chamber and counter arrangement. A are the triggering slabs, B, C, and D are anticoincidence slabs. This is the front view, seen by the four-camera stereo system.

Fig. 4: Energy distribution of the neutrinos expected, with 15 GeV protons on a Be targert

Given the successful Fermi theory of weak interactions, the neutrino interaction cross-sections were known, and given their small size, the target mass had to be maximized and the target incorporated into the detector. For the detector we consequently chose the newly invented spark chamber, with the metal foils replaced by 2.5 cm thick aluminium plates. The detector consisted of ten chambers, five sitting on the other five. Each was made of nine square plates, each side 112 cm long,for a total (target) mass of 10 tons. Between the spark chambers, and following them, scintillator planes for triggering were inserted, as well as anticoincidence counters in front, on top, and behind a shield, as can be seen in Fig. 5, Fig. 3 of the paper.

J. Steinberger / Nuclear Physics B (Proc. Suppl.) 221 (2011) 273–280276

The experiment ran for several months, early in 1962. With a total of 3.5 × 1017 protons on target, 113 events were obtained, which originated within a fiducial volume 10 cm from front and back walls and 5 cm from top and bottom. If an event consisted of a single track, this was required to be at an angle of less than 60º with respect to the neutrino beam. Ignoring single track events with lengths corresponding to less than 300 MeV/c momentum, there remained:

a) 34 single track events,b) 22 ‘vertex’ events, characterized by more than one track, and c) 8 ‘showers’, that is events, in general single tracks, but too irregular in structure to be

normal mu mesons, and which perhaps could be electron or photon showers. Of these, sixwere so located that, for comparison with events of type a) and b), their potential ranges within the chambers corresponded to muons of more than 300 MeV/c.

An event of type b) is shown in Fig. 6.

Fig. 6: Type b) event, consisting of a penetrating track as well as an additional shorter track

Events of type c) constituted the candidates for events in which the neutrinos produced electrons or positrons. To analyse these, an auxiliary exposure of the detector in a 400 MeV electron beam was performed. Figure 7, Fig. 9 of the paper, compares what would have been expected for the case of equal production rates for electrons and muons, with the actual observations, that is, with the sixevents of type c). Comparing these two distributions, it could be concluded that the neutrinos accompanying muons in pion and kaon decay are different from those accompanying electrons in beta decay.

Following this experimental result, it was clear that there are two lepton families, each composed of a charged and neutral lepton. Following the SLAC discovery in 1969 that hadrons are complex particles, made of ‘partons’, a few years later understood to be third integral electric charge quarks, it was clear that each ‘family’ consists of four types of particles: a lepton doublet plus a quark doublet. With the discovery of the tau lepton, in 1973, and later the bottom and top quarks, it became clear that there are at least three families. One of the most important results achieved at the LEP collider, in 1989 and the years following, based on the measurement of the decay width of the Z0, is that the contribution to this width by Z0 decay to neutrinos corresponds exactly to three neutrino families, so that the total number of families is three.

J. Steinberger / Nuclear Physics B (Proc. Suppl.) 221 (2011) 273–280 277

Fig. 7: Spark distribution of 400 MeV/c electrons normalized to expected number of showers. Also shown are the ‘shower’ events.

Fig. 8: The second neutrino team in 1962, and 26 years later, in Stockholm. From left to right: myself, Goulianos, Gaillard, Mistry, Danby, W. Hayes (technician), Lederman, Schwartz.

The last of the particles constituting the three families, the tau neutrino, was observed in 2001 at Fermilab by the DONUT Collaboration9). In the experiment, as shown in Fig. 9, a tau-neutrino beam is produced by 800 GeV protons striking a 1 m long tungsten beam-dump target. The neutrinos are invited to interact in a hybrid target of emulsion modules interleaved with scintillating fibre planes. Up to four emulsion modules, each 7 cm thick and separated by 20 cm were used. The emulsion modules were of two types, either 1 mm stainless steel plates interleaved with 100 micron emulsions layers on each side of a 200 micron plastic sheet, or entirely of emulsion plates with 350 microns of emulsion on each side of a 100 micron plastic sheet. The emulsion neutrino target was followed by a magnetic spectrometer to track the decay products of the tau leptons produced in the interaction, and by a muon identifier.

J. Steinberger / Nuclear Physics B (Proc. Suppl.) 221 (2011) 273–280278

Fig. 9: (Fig 1. of Ref. [9].) Experimental beam and spectrometer. At the left, 800 GeV protons were incident on the beam dump, which was 36 m from the first emulsion target. Muon identification was done by range in the system at the right.

The observed muon tracks were followed back into the emulsion, in the search for possible tau decays. Four events of tau leptons produced in neutrino interactions were identified. These are shown in Fig. 10.

Fig. 10: (Fig 2 of Ref. [9]). The four tau-neutrino charged-current interaction events. The neutrinos are incident from the left. The scales are given by the perpendicular lines, with the vertical line representing 0.1mm and the horizontal line 1.0 mm. The target material is shown by the bar at the bottom of the figure, steel is shaded, emulsion is crosshatched, and plastic has no shading.

J. Steinberger / Nuclear Physics B (Proc. Suppl.) 221 (2011) 273–280 279

To conclude, the past century has witnessed a formidable progress in our understanding of the particles which everything is made of, and their interactions. Neutrino-beam experiments have played a major role in this advance. It is very sad that, for reasons of health, this historical review of the experiment which demonstrated the existence of a second neutrino, could not be given by Melvin Schwartz, who not only had the idea for this experiment, but also dominated it technically.

References:[1] G. Feinberg, Decays of the � meson in the intermediate-meson theory, Phys. Rev. 110, 1482

(1958). [2] S. Lokanathan and J. Steinberger, Phys. Rev. 98, 240(A) (1955). [3] S. Oneda and J.C. Pati, V-A four-fermion interaction and the intermediate charged vector

meson, Phys. Rev. Lett. 2, 125 (1959). [4] B. Pontecorvo, Electron and muon neutrinos, Zh. Eksp. Teor. Fiz. 37, 1751 (1959). [5] G. Danby, J-M. Gaillard, Zk. Goulianos, L.M. Lederman, N. Mistry, M. Schwartz, and J.

Steinberger, Observation of high-energy neutrino interactions and the existence of two kinds of neutrinos, Phys. Rev. Lett. 9, 36 (1962).

[6] M. Schwartz, Feasibility of using high-energy neutrinos to study the weak interaction, Phys. Rev. Lett. 4, 306 (1960).

[7] T.D. Lee and C.N. Yang, Theoretical discussion on possible high-energy neutrino experiments,Phys. Rev. Lett. 4, 307 (1960).

[8] Ernest D. Courant, M. Stanley Livingston and Hartland Snyder, The strong focusing synchrotron—a new high-energy accelerator, Phys. Rev. 88, 1190 (1952).

[9] K. Kodema et al., Observation of tau neutrino interactions, Phys. Lett. B 504, 218 (2001).

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