Life of µ: The Observation of the Spontaneous decay of Mesotrons and its Consequences, 1938–1947

Summary

The mesotrons, or mesons, were the first elementary particles observed to be inherently unstable. This essay offers a reconstruction of the stream of researches related to mesotron decay, and examines how these researches shaped some of the basic concepts and practices of the emerging field of particle physics. Mass measurements could not settle the question of whether the mesons were a homogeneous kind of particles or an assortment of particles with different masses. The assumption of a single mass prevailed not on experimental grounds but because the mesons were identified tentatively with the carriers of the nuclear force according to a theory formulated by Hideki Yukawa. The identification gained currency because it entailed the prediction of meson decay, and thereby upheld the promise of a unified explanation of nuclear and cosmic-ray phenomena. In turn, the observation of decay and the measurement of the mean lifetime created the conditions for investigating the nuclear interactions of mesons at rest. Interest in these interactions was heightened, immediately after WWII, by the prospect of building and using accelerators to acquire knowledge about fundamental nuclear processes. Using decay to study nuclear capture, however, led to the realization that there exist not only different kinds of mesons but also two nuclear forces.

It is a pleasure for me to thank Craig Fraser, Ian Hacking, Sungook Hong, and Trevor Levere. They were invaluable sources of insightful criticism, assistance, and encouragement. I wish also to express my appreciation to Giovanni Battimelli and Michelangelo De Maria, the curators of the Marcello Conversi and Edoardo Amaldi Archives at the Istituto di Fisica of the University ‘La Sapienza’ of Rome, for their work and hospitality. I owe special thanks to Bruno Ferretti, who shared with me his recollections and offered useful references. Thank you to an anonymous referee for helpful criticism. This work was made possible by the financial support of the Social Sciences and Humanities Research Council of Canada.

Notes

¹S. S. Schweber, QED and the Men Who Made It: Dyson, Feynman, Schwinger, and Tomonaga (Princeton, NJ, 1994).

²M. De Maria and A. Russo, ‘The Discovery of the Positron’, Rivista di Storia della Scienza, 2 (1985), 237–86; Xavier Roqué, ‘The Manufacture of the Positron’, Studies in History and Philosophy of Modern Physics, 28 (1997), 73–129, and references therein. See also Schweber, QED and the Men Who Made It, xxii and 5 (note 1). For the conceptual lineage of particle-antiparticle annihilation, see Joan Bromberg, ‘The Concept of Particle Creation before and after Quantum Mechanics’, Historical Studies in the Physical Sciences, 7 (1976), 161–91.

³Roger H. Stuewer, ‘Mass-Energy and the Neutron in the Early Thirties’, Science in Context, 6 (1993), 223–27.

⁴The history of mesotron decay has been examined, as a part of the history of meson theory, in Laurie M. Brown and Helmut Rechenberg, The Origin of the Concept of Nuclear Forces (Bristol, 1996), 177–97. Brown and Rechenberg characterize it as a ‘typical theory–experiment confrontation’ (p. 178). My reconstruction focuses on how observations of mesotron decay were the core of a theory–experiment alliance that created new directions of scientific enquiry.

⁵Experiments on mesotrons and their relation to theory are the subject of many participant recollections in L. M. Brown and L. Hoddeson, eds, The Birth of Particle Physics; Based on a Fermilab Symposium (Cambridge, 1983); David Cline and Gail Riedasch, eds, 50 Years of Weak Interactions; Wingspread Conference (1984) (Madison, WI, 1984); B. Foster and P. H. Fowler, eds, 40 Years of Particle Physics; Proceedings of the International Conference to Celebrate the 40th Anniversary of the Discovery of the p- and V-particles, held at University of Bristol, 22–24 July 1987 (Bristol, UK, 1988). An account that is partly a recollection and partly an historical analysis is in Abraham Pais, Inward Bound: of Matter and Forces in the Physical World (Oxford and New York, 1986), 452–56.

⁶J. R. Oppenheimer, ‘Thirty Years of Mesons’, Physics Today, 19 (1966), 58.

⁷An overview of the early history of particle physics—characterized as the ‘turbulent confluence’ of nuclear physics, cosmic-ray studies, and quantum field theory (p. 4)—is Laurie M. Brown and Lillian Hoddeson, ‘The birth of elementary particle physics: 1930–1950’ in The Birth of Particle Physics (note 5), 3–36. Laurie M. Brown, Max Dresden, and Lillian Hoddeson, ‘Pions to quarks: particle physics in the 1950s’, in Pions to Quarks; Particle Physics in the 1950s; Based on a Fermilab Symposium, edited by L. Brown, M. Dresden, and L. Hoddeson (Cambridge, 1989), 3–39 is an introduction to the following period of particle physics history. Early meson physics, especially from the point of view of theory, is discussed in Brown and Rechenberg, The Origin of the Concept of Nuclear Forces (note 4). The mesotron–muon episode is touched upon, with different emphases and for different purposes, in several discussions concerning aspects of the early history of particle physics, for example, in Peter L. Galison, How Experiments End (Chicago, 1987), 124–6; Peter L. Galison, Image and Logic: a Material Culture of Microphysics (Chicago, 1997), 202–10; Allan Franklin, Are There Really Neutrinos? An Evidential History (Cambridge, MA, 2001), 100–4. A fairly detailed summary in a larger historical perspective is provided i n Helge Kragh, Quantum Generations; A History of Physics in the Twentieth Century (Princeton, NJ, 1999), 194–205.

⁸Bruno B. Rossi, ‘The decay of ‘mesotrons’ (1939–1943): experimental particle physics in the age of innocence’, in The Birth of Particle Physics (note 5), 183–205 (p. 185).

¹¹Neddermeyer and Anderson, ‘Note on the Nature of Cosmic Ray Particles’, 886 (note 9).

¹³Seth H. Neddermeyer, ‘The Penetrating Cosmic-Ray Particles’, Physical Review, 53 (1938), 102.

⁹Seth H. Neddermeyer and Carl D. Anderson, ‘Note on the Nature of Cosmic Ray Particles’, Physical Review, 51 (1937), 884–6; J. C. Street and E. C. Stevenson, ‘New Evidence for the Existence of a Particle of Mass Intermediate Between the Proton and the Electron’, Physical Review, 52 (1937), 1003–4.

¹⁰The history of the intermediate particles discovery is analysed in Peter Galison, ‘The Discovery of the Muon and the Failed Revolution against Quantum Electrodynamics’, Centaurus, 26 (1983), 262–316. Also, in Galison, How Experiments End, 75–133 (note 7).

¹²Y. Nishina, M. Takeuchi, and T. Ichimiya, ‘On the Nature of Cosmic Ray Particles’, Physical Review, 52 (1937), 1198–9.

¹⁴Neddermeyer and Anderson, ‘Note on the Nature of Cosmic Ray Particles’, 886 (note 9).

¹⁵H. J. Bhabha, ‘On the Penetrating Component of Cosmic Radiation’, Proc. Roy. Soc., A164 (1938), 290. Bhabha was concerned, in particular, with the energy losses of cosmic ray particles in lead and aluminium measured by P. M. S. Blackett and J. G. Wilson at Birkbeck's College in London. These measurements were in disagreement with the QED formulae for electrons above a critical energy, which seemed to decrease with increasing atomic number of the medium (over 10¹⁰ MeV in air, around 2×10³ MeV in aluminium, and between 2 and 4×10² MeV in lead). At this stage, Bhabha's analysis reflected Blackett's re-interpretation of his own data in the light of the heavy electrons hypothesis. Blackett had moved from regarding the measurements as a demonstration that QED broke down at high energies to interpreting them as evidence that heavy electrons, if they existed, had to turn into ordinary electrons when losing energy. On this point, see Martha Cecilia Bustamante, ‘Blackett's Experimental Researches on the Energy of Cosmic Rays’, Archives Internationales d'Histoire des Sciences, 47 (1997), 132–7.

¹⁶Enrico Fermi, ‘Tentativo di una teoria dei raggi beta’, Nuovo Cimento, 2 (1934), 1–19. Reprinted in Enrico Fermi: Collected Papers (Note e Memorie), edited by E. Amaldi et al., 2 vols (Chicago and Rome, 1962), I, 559–4.

¹⁷J. R. Oppenheimer and R. Serber, ‘Note on the Nature of Cosmic Ray Particles’, Physical Review, 51 (1937), 1113.

¹⁸For historical analyses of Heisenberg's, Fermi's, and Yukawa's theories, see Joan Bromberg, ‘The Impact of the Neutron: Bohr and Heisenberg’, Historical Studies in the Physical Sciences, 3 (1971), 307–41; Olivier Darrigol, ‘The quantum electrodynamical analogy in early nuclear theory or the roots of Yukawa's theory’, Revue d'Histoire des Sciences, 41 (1988), 225–97; Helmut Rechenberg and Laurie M. Brown, ‘Yukawa's Heavy Quantum and the Mesotron (1935–1937)’, Centaurus, 33 (1990), 214–52; Cathryn Carson, ‘The Peculiar Notion of Exchange Forces—II: From Nuclear Forces to QED, 1929–1950’, Studies in History and Philosophy of Modern Physics, 27 (1996), 99–131; Brown and Rechenberg, The Origin of the Concept of Nuclear Forces (note 4).

¹⁹For a detailed examination of how forces came to be thought of as mediated by the ‘exchange’ of field quanta, and the roles of Heisenberg's, Fermi's, and Yukawa's theories in the evolution of this concept, see Carson, ‘Exchange Forces, 1929–1950’, 99–131 (note 18).

²⁰In a field theory, the ‘coupling constant’ is a parameter that quantifies the strength of the interaction because it determines the probabilities for all the processes allowed by the interaction term. In Fermi's formulation, g had the dimensions of volume×energy, and Fermi calculated that for β-decay the coupling constant had to assume typical values of the order of g=4×10⁻⁵⁰ cm³ erg. (Fermi, ‘Tentativo’, 18 (note 16).)

²¹Hideki Yukawa, ‘On the Interaction of Elementary Particles. I’, Proceedings of the Physico-Mathematical Society of Japan, 17 (1935), 48–57. Reprinted in Hideki Yukawa, Tabibito. (The Traveler), translated by L. Brown and R. Yoshida (Singapore, 1982), 209–18.

²²Yukawa's general aim is revealed not only by the paper's title but also by its introduction, which closes with the following remark: ‘Besides such an exchange force and the oridinary [sic] electric and magnetic forces there may be other forces between the elementary particles, but we disregard the latter for the moment’. (Yukawa, ‘On the Interaction of Elementary Particles. I’, 210 (note 21).)

²³According to the current Standard Model, nuclear binding is due to the strong nuclear force and β-decay to the weak nuclear force, the two forces being irreducible to one another in the domain of presently attainable energies. For this reason, Fermi's theory is sometimes characterized as the first theory of weak interactions, and Yukawa's theory as the first theory of strong interactions. (See, for example, Pais, Inward Bound, 580 (note 5).) Of course, neither Fermi nor Yukawa meant such a distinction.

²⁴H. J. Bhabha, ‘On the theory of heavy electrons and nuclear forces’, Proceedings of the Royal Society of London, A166 (1938), 501–27 (p. 503).

²⁵Gian Carlo Wick, ‘Range of Nuclear Forces in Yukawa's Theory’, Nature, 142 (1938), 993–4 (p. 993).

²⁶The significance of the discovery of heavy electrons for the acceptance of Yukawa's theory has been examined by Stephen G. Brush, as one of three case studies on the role of theoretical predictions in theory evaluation. Brush's analysis supports the philosophical claim that empirical confirmation of a prediction provides corroboration for the theory, not in the sense of increasing the probability that the theory is true, but in the sense of making it ‘more reasonable to pursue that hypothesis than one that has not been corroborated’. (Stephen G. Brush, ‘Prediction and Theory Evaluation: Subatomic Particles’, Rivista di Storia della Scienza, Serie II, 1 (1993), 47–152 (p. 116).)

²⁷C. D. Anderson and H. L. Anderson, ‘Unraveling the particle content of cosmic rays’, in The Birth of Particle Physics, edited by Brown and Hoddeson, 131–54 (p. 149) (note 5).

²⁸Anderson and Anderson, ‘Unraveling the particle content of cosmic rays’, 149 (note 27).

²⁹Neddermeyer and Anderson, ‘Note on the Nature of Cosmic Ray Particles’, 886 (note 9).

³⁰Yukawa's theory was affected by the same problem of divergent quantities as QED; the problem, however, was more serious because of the larger coupling between heavy particles and field.

³¹Wick, ‘Range of Nuclear Forces’, 993–4 (note 25). Wick reasoned that exchanges of field quanta cannot be ‘actual’ emission and absorption processes because they violate the conservation of energy with the creation of an amount of energy at least equal to the rest energy of the quanta. They are what physicists call ‘virtual’ transitions, that is, processes that ‘exist’ in formal representation but are unobservable as a matter of principle. An energy imbalance of the order of ΔE can ‘exist’ only as long as it is not observable. That is possible, by virtue of the uncertainty principle, if the emitted quantum of rest energy ΔE=mc ² lasts no longer than the time Δt∼h/ΔE. This means that a virtual quantum of mass m=ΔE/c ² can travel no farther than the distance λ=h/mc before being absorbed.

³²H. Euler and W. Heisenberg, ‘Theoretische Gesichtspunkte zur Deutung der kosmischen Strahlung’, Ergebnisse der exakten Naturwissenschaften, XVII (1938), 1–69 (p. 24). (‘Wenn man diese Annahme nicht macht, so gibt es einstweilen noch keine theoretischen Gesichtspunkte, die zu Aussagen über das Verhalten dieser Teilchen führen könnten. Wenn man jedoch die Existenz einer bestimmten Teilchensorte von einer Ruhmasse von etwa 160 Elektronenmassen annimmt, so liegt es nahe, diese Teilchen in Verbindung zu bringen mit einer Theorie der Kernkräfte, die im Jahre 1935 von Yukawa vorgeschlagen und von ihm und verschiedenen anderen Forschern ausgearbeitet worden ist’.)

³³Carl D. Anderson and Seth H. Neddermeyer, ‘Mesotron (Intermediate Particle) as a Name for the New Particles of Intermediate Mass’, Nature, 142 (1938), 878.

³⁵H. A. Bethe, ‘The Meson Theory of Nuclear Forces. I. General Theory’, Physical Review, 57 (1940), 260–72 (p. 262). The perfunctory ‘if’ in Bethe's pronouncement reads as a premonition because after 1947, mesotrons did return to be seen as heavy electrons, although by then the particle world was becoming more complicated. In present terms, the mesotrons were muons, and muons are ‘second generation leptons’. This means that they are almost like higher mass states of ordinary electrons. The presently accepted structure of the lepton ‘family’ recalls the old view of multi-mass electrons to a remarkable extent but not exactly. A muon does have the same spin as an electron; however, it does not simply decay into its lower mass state, the electron, shedding the excess energy in the form of a photon or a neutrino. It decays into an electron, a neutrino (muon neutrino), and an antineutrino (electron antineutrino). This three-body disintegration is believed to be necessary because muons (and muon neutrinos) carry—besides rest energy, electrical charge, and spin—a ‘lepton number’ different from the lepton number of electrons (and electron neutrinos), and in each elementary process the different lepton numbers must be separately conserved. (The electron antineutrino carries the lepton number of the electron but with an opposite sign.)

³⁴G. E. M. Jauncey, ‘Possible Origin of the X Particle’, Physical Review, 52 (1937), 1256. See also Paul Weisz, ‘Zenith Angle Distribution of the Hard Component of Cosmic Rays and the Mass of the Mesotron’, Physical Review, 55 (1939), 1266–7.

³⁶See, for example, Yukawa, quoted in Brown and Rechenberg, The Origin of the Concept of Nuclear Forces, 111 (note 4). See also Takehiko Takabayasi, ‘Some characteristic aspects of early elementary particle theory in Japan’, 294–303 (p. 295), and Takabayasi's remarks in ‘Second round-table discussion’, 278, in Brown and Hoddeson, eds., The Birth of Particle Physics, 278–92 (note 5).

³⁷E. C. G. Stückelberg, ‘On the Existence of Heavy Electrons’, Physical Review, 52 (1937), 41–2.

³⁸Bhabha, ‘Heavy electrons and nuclear forces’, 502 (note 24).

³⁹Bhabha, ‘Heavy electrons and nuclear forces’, 501–2 (note 24). See note 15 on the parallel between Bhabha's and Blackett's views.

⁴⁰H. J. Bhabha, ‘Nuclear Forces, Heavy Electrons and the b-decay’, Nature, 141 (1938), 118.

⁴⁴Euler and Heisenberg, ‘Theoretische Gesichtspunkte’, 42 (note 32). (‘Dieser Wert ist etwa 5mal so groß wie der aus der Yukawaschen Theorie berechnete. In Anbetracht der Unsicherheit mancher Einzelheiten in der Yukawaschen Theorie, insbesondere der Masse des schweren Elektrons, ist diese Übereinstimmung durchaus befriedigend’.)

⁴⁶Blackett to Heisenberg, 10 September 1938, quoted in Brown and Rechenberg, The Origin of the Concept of Nuclear Forces, 183 (note 4). Blackett's enthusiasm might have been compounded by the fact that Euler and Heisenberg were able to explain his observations on the different energy spectra of heavy electrons and ordinary electrons (see note 15). They clarified that the disappearance of heavy electrons at low energies was a combined effect of decays and of increased energy losses by ionization. (Euler and Heisenberg, ‘Theoretische Gesichtspunkte’, 40–1 (note 32).)

⁴⁷Blackett, ‘High Altitude Cosmic Radiation’, 693 (note 43).

⁴¹At least, Euler and Heisenberg, attributed this value to Yukawa and, in a letter to Heisenberg Yukawa, referred to it as ‘our calculation’. (Yukawa to Heisenberg, 15 July 1938, quoted in Brown and Rechenberg, The Origin of the Concept of Nuclear Forces, 182 (note 4).) The first published value of Yukawa and his collaborators was τ=1.3×10^–7 s. (Hideki Yukawa and Shoichi Sakata, ‘Mass and Mean Life-Time of the Meson’, Nature, 143 (1939), 761 and references therein.)

⁴²Euler and Heisenberg, ‘Theoretische Gesichtspunkte’, 26 (note 32).

⁴³The decay explanation for anomalous absorption measurements was first put forward, in a qualitative manner, by H. Kulenkampff. (H. Kulenkampff, ‘Bemerkungen über die durchdringende Komponente der Ultrastrahlung. (Zum Teil nach Messungen von H. Kappler und H. Martin.)’, Verhandlungen der Deutschen Physikalischen Gesellschaft, 2 (1938), 92. Reviews of existing absorption observations, together with their re-interpretation in terms of the decay hypothesis, are given in P. M. S. Blackett, ‘High Altitude Cosmic Radiation’, Nature, 142 (1938), 692–3; P. M. S. Blackett, ‘Further Evidence for the Radioactive Decay of the Mesotrons’, Nature, 142 (1938), 992; Bruno Rossi, ‘Further Evidence for the Radioactive Decay of Mesotrons’, Nature, 142 (1938), 993; Bruno Rossi, ‘The Disintegration of Mesotrons’, Reviews of Modern Physics, 11 (1939), 296–303.

⁴⁵Thomas S. Kuhn, ‘The Function of Measurement in Modern Physical Science’, in The Essential Tension (Chicago, 1977), 179–224 (p. 184).

⁴⁸The terminology of ‘indirect’ and ‘direct’ observations was widely used. See, for example, Rossi, ‘The Disintegration of Mesotrons’, 296 (note 43). For an historical analysis of experiments on the decay of mesotrons and their degrees of directness, see D. Monaldi, forthcoming.

⁴⁹Yukawa to Heisenberg, 15 July 1938, quoted in Brown and Rechenberg, The Origin of the Concept of Nuclear Forces, 182 (note 4).

⁵⁰L. W. Nordheim, ‘Lifetime of the Yukawa Particle’, Physical Review, 55 (1939), 506.

⁵¹C. Møller, L. Rosenfeld, and S. Rozental, ‘Connexion between the Life-time of the Meson and the Beta-Decay of Light Elements’, Nature, 144 (1939), 629.

⁵²C. Møller and L. Rosenfeld, ‘The Electric Quadrupole Moment of the Deuteron and the Field Theory of Nuclear Forces’, Nature, 144 (1939), 476. One of the computational troubles of meson theory was that the nuclear potential was divergent at small distances. Møller and Rosenfeld's field mixture achieved the cancellation of this infinite term, but only at the lowest order of the perturbation expansion.

⁵³H. A. Bethe and L. W. Nordheim, ‘On the Theory of Meson Decay’, Physical Review, 57 (1940), 998–1006 (p. 1004).

⁵⁴Julian Schwinger, ‘On a Field Theory of Nuclear Forces’, Physical Review, 61 (1942), 387.

⁵⁵The perturbation method presupposed that the interaction probability was dominated by the most elementary interaction mechanisms (the lowest-order terms of the perturbation expansion series). This was evidently inappropriate in the case of strong coupling, in which more complex processes (higher-order terms) became increasingly significant. For an extended and detailed examination of meson theory difficulties and the rich variety of approaches to solve them, including strong and intermediate coupling theories, and meson pair theories, see Brown and Rechenberg, The Origin of the Concept of Nuclear Forces, 253–91 (note 4).

⁵⁶Brown and Rechenberg, The Origin of the Concept of Nuclear Forces, 277–80 (note 4); R. E. Marshak, ‘Particle physics in rapid transition’, in The Birth of Particle Physics, 376–401 (p. 378) (note 5).

⁵⁷Seth H. Neddermeyer and Carl D. Anderson, ‘Nature of Cosmic-Ray Particles’, Reviews of Modern Physics, 11 (1939), 191–207 (p. 207).

⁵⁸John A. Wheeler and Rudolf Ladenburg, ‘Mass of the Meson by the Method of Momentum Loss’, Physical Review, 60 (1941), 754–61 (p. 760).

⁵⁹M. A. Pomerantz, ‘The Instability of the Meson’, Physical Review, 57 (1940), 3–12 (p. 11).

⁶⁰Paul Weisz, ‘The Rest Mass of the Mesotron’, Physical Review, 59 (1941), 845–9 (p. 849).

⁶¹C. G. Montgomery et al., ‘Slow Mesons in Cosmic Radiation’, Physical Review, 56 (1939), 635–9 (p. 638).

⁶²Montgomery et al., ‘Slow Mesons in Cosmic Radiation’, 639 (note 61).

⁶⁵Williams and Roberts, ‘Evidence’, 102 (note 63). Cloud-chamber images of stopping mesotrons were of little use for measuring the lifetime. Williams and Roberts estimated a mesotron mass of µ=(250±70)m _e. For the lifetime, they could only provide a very loose upper limit of approximately 2×10⁻⁴ s.

⁶³E. J. Williams and G. E. Roberts, ‘Evidence for Transformation of Mesotrons into Electrons’, Nature, 145 (1940), 102–3.

⁶⁴E. J. Williams and G. R. Evans, ‘Transformation of Mesotrons into Electrons’, Nature, 145 (1940), 818–9.

⁶⁶S. Tomonaga and G. Araki, ‘Effect of the Nuclear Coulomb Field on the Capture of Slow Mesons’, Physical Review, 58 (1940), 90–91.

⁶⁷Franco Rasetti, ‘Disintegration of Slow Mesotrons’, Physical Review, 60 (1941), 198–204. It was known that mesotrons were positively and negatively charged, with a 20% excess of positives.

⁶⁸Rossi's own recollections are in Bruno B. Rossi, ‘The decay of “mesotrons” (1939–1943): experimental particle physics in the age of innocence’, in The Birth of Particle Physics, 183–205 (note 5); Bruno Benedetto Rossi, Moments in the Life of a Scientist (Cambridge, 1990).

⁶⁹Bruno Rossi and Norris Nereson, ‘Experimental Determination of the Disintegration Curve of Mesotrons’, Physical Review, 62 (1942), 417–22.

⁷⁰E. Amaldi, ‘Gli anni della Ricostruzione. Parte I’, Scientia, 114 (1979), 29–50; Edoardo Amaldi, Giovanni Battimelli, and Michelangelo De Maria, Da via Panisperna all'America: I fisici italiani e la seconda guerra mondiale (Rome, 1997); M. Conversi, ‘The period that led to the 1946 discovery of the leptonic nature of the “mesotron”’, in The Birth of Particle Physics, 242–50 (note 5); M. Conversi, ‘Early Study of Muons and Muon Decay’, in 50 Years of Weak Interactions, 154–67 (note 5); Marcello Conversi, ‘From the discovery of the mesotron to that of its leptonic nature’, in 40 Years of Particle Physics, 349–68 (note 5); Oreste Piccioni, ‘The observation of the leptonic nature of the ‘mesotron’ by Conversi, Pancini, and Piccioni’, in The Birth of Particle Physics, 222–41 (note 5); Oreste Piccioni, ‘The history of the discovery of the extended leptonic nature and a comment on an article in Scientia’, in 50 Years of Weak Interactions, 486–508 (note 5); Oreste Piccioni, ‘The Discovery of the Muon’, in History of Original Ideas and Basic Discoveries in Particle Physics, 143–62.

⁷¹M. Conversi and O. Piccioni, ‘Misura diretta della vita media dei mesoni frenati’, Il Nuovo Cimento, 2 (1944), 40–70; M. Conversi and O. Piccioni, ‘On the Mean Life of Slow Mesons’, Physical Review, 70 (1946), 859–73.

⁷²M. Conversi and O. Piccioni, ‘Sulla disintegrazione dei mesoni lenti’, Il Nuovo Cimento, 2 (1944), 71–87; M. Conversi and O. Piccioni, ‘On the Disintegration of Slow Mesons’, Physical Review, 70 (1946), 874–81.

⁷³Conversi and Piccioni, ‘Sulla disintegrazione dei mesoni lenti’, 87 (note 72).

⁷⁴M. Conversi, E. Pancini, and O. Piccioni, ‘On the Decay Process of Positive and Negative Mesons’, Physical Review, 68 (1945), 232.

⁷⁵Edoardo Amaldi, ‘Sulle ricerche di fisica nucleare eseguite a Roma nel quadriennio di guerra. Relazione presentata al Convegno dei fisici ed elettrotecnici, Como, novembre 1945’, La Ricerca Scientifica, 16 (1946), 61–5; Gilberto Bernardini, in Report of an International Conference on Fundamental Particles and Low Temperatures, held at the Cavendish Laboratory, Cambridge, on 22–27 July 1946, vol. I (London, 1947).

⁷⁶ Science, the endless frontier: a report to the President by Vannevar Bush, director of the Office of Scientific Research and Development, July 1945 (Washington, DC, 1945). Whether or not atomic weaponry fitted the definition of common good was, of course, another important issue in physicists’ reflections. Separating pure science from its applications was also a way to deal with this problem. The themes that I outlined here are also considered, in a larger historical perspective, in S. S. Schweber, ‘Some Reflections on Big Science and High Energy Physics in the United States’, Rivista di Storia della Scienza, 2 (1994), 127–89.

⁷⁸John A. Wheeler, ‘Problems and Prospects in Elementary Particle Research’, Proceedings of the American Philosophical Society, 90 (1945), 36–47 (pp. 36–7).

⁷⁷‘The Italian Navigator has reached the New World’ was the message with which A. H. Compton (head of the Metallurgical Laboratory in Chicago) telephonically informed J. B. Conant (chairman of the National Defense Research Committee) that Fermi had successfully activated the first atomic pile on 2 December 1942. The anecdote is reported, with small variations, by many sources, for example, Laura Fermi, Atoms in the Family; My Life With Enrico Fermi Architect of the Atomic Age (Chicago, 1954), 198; E. Segrè, Enrico Fermi Physicist, paperback ed. (Chicago, 1972), 129; Daniel J. Kevles, The Physicists. The History of a Scientific Community in Modern America (New York, 1979), 326 and references therein.

⁷⁹Wheeler, ‘Problems and Prospects’, 39 (note 78).

⁸¹Wheeler, ‘Problems and Prospects’, 44 (note 78).

⁸⁰The magnetic moment of proton and neutron was another phenomenon of which meson theory failed to give a quantitative account. Wheeler, ‘Problems and Prospects’, 45 (note 78).

⁸²Karl K. Darrow, ‘1945 Annual Meeting at New York, January 24–26, 1946’. Proceedings of the American Physical Society, in Physical Review, 69 (1946), 246.

⁸⁴‘A Roma, con grande difficoltà abbiamo ripreso a lavorare. Anzi in un primo tempo, quando arrivarono le Phys. Rev. fino a tutto il 1944, avemmo l'illusione di aver mantenuto il passo. Purtroppo era solo un'illusione e ora, con le pile al Plutonio, i betatroni da 100 MeV (mi ha scritto Bruno Pontecorvo che hanno fatto i mesoni in casa e che sembra che vi siano tante masse mesoniche, da 20 masse elettroniche in su; se è vero, è cosa di eccezionale importanza per tutta la fisica delle particelle elementari), perdiamo terreno a chilometri al secondo e forse senza speranza di recupero’. Bernardini to Persico, 4 February 1946, in Amaldi, Battimelli, and De Maria, Da via Panisperna all'America, 168–9 (note 70).

⁸³Fermi to Amaldi and Wick, 24 January 1946, in Amaldi, Battimelli, and De Maria, Da via Panisperna all'America, 166 (note 70). In 1947, the project was changed into a 450 MeV synchrocyclotron. (Segrè, Fermi, 173 (note 77).)

⁸⁵See, for example, Robert N. Cahn and Gerson Goldhaber, The Experimental Foundations of Particle Physics (Cambridge, 1989), 20; Pais, Inward Bound, 479 (note 5).

⁸⁶H. A. Bethe, ‘Abstract H5. Influence of Multiple Scattering on Curvature Measurements’, Proceedings of the American Physical Society, in Physical Review, 69 (1946), 689.

⁸⁷Marcel Schein, A. J. Hartzler, and G. S. Klaiber, ‘Production of Heavily Ionizing Particles by X-Rays Generated by a 100-Mev Betatron’, Physical Review, 70 (1946), 436.

⁸⁸G. S. Klaiber, E. A. Luebke, and G. C. Baldwin, ‘Abstract C11. Range–Momentum Measurements of Particles Emitted in Nuclear Disintegrations Induced by 100-MeV X-Rays’, Proceedings of the American Physical Society, in Physical Review, 70 (1946), 789–90.

⁹¹Bethe, ‘Multiple Scattering and the Mass of the Meson’, 829 (note 90).

⁸⁹Bruno Rossi, ‘Abstract C7. Some Problems in the Study of Cosmic-Ray Mesons’, Proceedings of the American Physical Society, in Physical Review, 70 (1946), 788.

⁹⁰Emphasis in the original. H. A. Bethe, ‘Multiple Scattering and the Mass of the Meson’, Physical Review, 70 (1946), 821–9 (821).

⁹²L. Leprince-Ringuet and M. L'héritier, ‘Existence probable d'une particule de masse 990 m _e dans le rayonnement cosmique’, Comptes Rendus de l'Academie des Sciences de Paris, 219 (1944), reprinted in Cahn and Goldhaber, The Experimental Foundations of Particle Physics, 66–8 (note 85).

⁹⁴Philip Morrison, ‘Physics in 1946’, Journal of Applied Physics, 18 (1947), 133–52 (p. 136).

⁹⁵Morrison, ‘Physics in 1946’, 136 (note 94).

⁹³Brown, Dresden, and Hoddeson, ‘Pions to quarks’, 3 (note 7).

⁹⁶In 1945–46, the Rome physicists were planning the construction of a betatron. One of them, Bernardo Nestore Cacciapuoti, visited several laboratories in the USA from November 1945 to February 1946, where he collected information on the new accelerators. (Cacciapuoti to Amaldi e Bernardini, 8 December 1945, and Cacciapuoti to Amaldi, 1 January 1946, Archivio Amaldi, Box 136, file 3, Sezione Archivi del Dipartimento di Fisica dell'Università di Roma 1, ‘La Sapienza’. See also Amaldi, Battimelli, and De Maria, Da via Panisperna all'America, 108 (note 70).)

⁹⁷B. Ferretti, ‘The Absorption of Slow Mesons by an Atomic Nucleus’, in Cambridge Conference Report, 75–7 (note 75).

¹⁰⁶Fermi, Teller, and Weisskopf, ‘Decay of Negative Mesotrons’, 315 (note 105).

⁹⁸Different points of view on whether they intended to investigate the atomic number dependence or not are offered by Piccioni and Conversi in their recollections (note 70). According to all accounts, the result that they obtained was completely unexpected.

⁹⁹Piccioni, ‘Discovery of the extended leptonic nature’, 498 (note 70).

¹⁰⁰M. Conversi, E. Pancini, and O. Piccioni, ‘On the Disintegration of Negative Mesons’, Physical Review, 71 (1947), 209–10; M. Conversi, E. Pancini, and O. Piccioni, ‘Sull'assorbimento e sulla disintegrazione dei mesoni alla fine del loro percorso’, Il Nuovo Cimento, Ser. 9, 3 (1946), 372–90; M. Conversi, E. Pancini, and O. Piccioni, ‘Sul comportamento dei mesoni positivi e negativi alla fine del loro percorso’, Rendiconti dell'Accademia Nazionale dei Lincei, Ser. 8, Vol. 2 (1947), 54–7.

¹⁰¹Amaldi to Fermi, 28 November 1946, in Enrico Fermi Collection, Box 9, Series II (Special Collection Research Center, The University of Chicago Library).

¹⁰²T. Sigurgeirsson and A. Yamakawa, ‘Decay of Mesons Stopped in Light Materials’, Physical Review, 71 (1947), 319–20. See also Thorbjorn Sigurgeirsson and K. Alan Yamakawa, ‘Electron Emitting Power of Stopped Mesons’, Reviews of Modern Physics, 21 (1949), 124–32.

¹⁰³G. E. Valley, ‘The Radioactive Decay of Slow Positive a Negative Mesons’, Physical Review, 72 (1947), 772–83.

¹⁰⁴John A. Wheeler, ‘Abstract A4. Mechanism of Absorption of Negative Mesons’, Proceedings of the American Physical Society, in Physical Review, 71 (1947), 71; John A Wheeler, ‘Mechanism of Capture of Slow Mesons’, Physical Review, 71 (1947), 320–1.

¹⁰⁵E. Fermi, E. Teller, and V. Weisskopf, ‘The Decay of Negative Mesotrons in Matter’, Physical Review, 71 (1947), 314–5.

¹⁰⁷Ferretti, personal communication. See also B. Ferretti, ‘Sulla cattura atomica dei mesoni lenti’, Il Nuovo Cimento, 5 (1948), 325–66 (p. 326, footnote 5). For Fröhlich's argument, see H. Fröhlich, ‘Decay of Negative Mesons in Matter’, Nature, 160 (1947), 255.

¹⁰⁸E. Fermi and E. Teller, ‘The Capture of Negative Mesotrons in Matter’, Physical Review, 72 (1947), 399–408.

¹¹²R. E. Marshak, ‘Particle physics in rapid transition’, 381 (note 56).

¹⁰⁹S. S. Schweber, ‘A Short History of Shelter Island I’, in Shelter Island II. Proceedings of the 1983 Shelter Island Conference on Quantum Field Theory and the Fundamental Problems of Physics, edited by R. Jackiw et al. (Cambridge, MA, 1985), 302–43.

¹¹⁰J. R. Oppenheimer, ‘The Foundations of Quantum Mechanics. Outline of Topics for Discussion’, in Schweber, ‘Shelter Island’, Appendix, p. 339 (note 109).

¹¹¹V. F. Weisskopf, ‘Foundations of Quantum Mechanics. Outline of Topics for Discussion’, in Schweber, ‘Shelter Island’, Appendix, p. 338 (note 109).

¹¹³Møller, in Cambridge Conference on Fundamental Particles, 184 (note 75). See also Pais, Inward Bound, 449–50 (note 5). According to Pais, Møller also invented, in 1941, the term ‘nucleons’ for the heavy particles.

¹¹⁴B. Ferretti, ‘Sull'ipotetico mesone di vita media molto breve’, Il Nuovo Cimento, Ser. 9, 3 (1946), 307–19.

¹¹⁵The absence of strong interactions, even in experiments prior to the Rome experiment, was emphasized by Weisskopf. See Victor F. Weisskopf, ‘On the Production Process of Mesons’, Physical Review, 72 (1947), 510, and reference therein.

¹¹⁶This was the hypothesis discussed at Shelter Island and then developed by Marshak in collaboration with Bethe, as reported in R. E. Marshak and H. A. Bethe, ‘On the Two-Meson Hypothesis’, Physical Review, 72 (1947), 509 and footnote 18. For an example of how the β-decay of the heavy meson was left out of later accounts, see Robert E. Marshak, ‘The Multiplicity of Particles’, Scientific American, 186 (1953), 22–7 (p. 25).

¹¹⁷C. M. G. Lattes et al., ‘Processes Involving Charged Mesons’, Nature, 159 (1947), 694–7.

¹¹⁸The interpretive practices of the nuclear emulsion specialists are discussed in Galison, Image and Logic, 196–210. Galison concentrates on the creation of a ‘visual language’ through interaction between the instrumental images and the observers’ ‘tutored eye’. Here, I only wish to emphasize the dialogical relation between these experiments and other experiments and theoretical analyses.

¹¹⁹G. P. S. Occhialini and C. F. Powell, ‘Nuclear Disintegrations Produced by Slow Charged Particles of Small Mass’, Nature, 159 (1947), 186–90. The first picture of a meson-induced star had been published shortly before by Donald H. Perkins, of Imperial College, London, who had had his nuclear emulsion plates flown at 9000 m of altitude by an RAF airplane. (D. H. Perkins, ‘Nuclear Disintegration by Meson Capture’, Nature, 159 (1947), 126–7.)

¹²⁰Lattes et al., ‘Processes Involving Charged Mesons’, 696 (note 117).

¹²¹Marshak, ‘Particle physics in rapid transition’, 382 (note 56).

¹²²Marshak and Bethe, ‘On the Two-Meson Hypothesis’, 507 (note 116).

¹²³C. M. G. Lattes, G. P. S. Occhialini, and C. F. Powell, ‘Observation of the Tracks of Slow Mesons in Photographic Emulsions’, Nature, 160 (1947), 453–6.

¹²⁴Murray Gell-Mann and E. P. Rosenbaum, ‘Elementary Particles’, Scientific American, July (1957), 72–88 (p. 78).

¹²⁵Luis W. Alvarez, ‘Recent Developments in Particle Physics. Nobel Lecture, December 11, 1968’, in Evolution of Particle Physics, edited by M. Conversi (New York, 1970), 1–49 (p. 2).