Half-Life: The Divided Life of Bruno Pontecorvo, Physicist or Spy

by Close, Frank

  Bruno’s role in invigorating the experimental physics program at Dubna culminated in 1958, with his election to the Soviet Academy of Sciences. Members of the Academy and their families enjoyed privileges that were utterly exceptional. In a country where quality goods were in short supply, an Academician had access to special stores that sold Western furniture, perfumes, and wine. Vacations in exclusive hotels also became possible, along with access to first-rate medical care and other benefits shared by those in the highest echelons of the party.

  In terms of privilege and status, this lifestyle far exceeded that of most Western scientists. However, in one area Bruno still lost out: Western scientists were free to take international vacations, and to perform experiments anywhere they liked. In the USSR, foreign travel was all but forbidden, even to celebrated Academicians. Thus the occasion of a major international conference on high-energy particle physics, to be held in Kiev in 1959, gave Bruno a rare opportunity to meet colleagues from around the world.

  BY THIS POINT, THE EXISTENCE OF THE NEUTRINO HAD BEEN ESTABLISHED. However, Bruno still had questions about the mysterious particle. For example, under what circumstances do neutrinos maintain an identity, and what determines it?

  At Chalk River, Bruno had pondered the relationship between the electron and the muon. By 1959 he was beginning to extend this line of thinking to neutrinos. He asked the following question: Are neutrinos that are produced along with electrons or positrons in beta decay the same as those produced along with a muon in the pion decay? In other words: Are “electron-neutrinos” the same as “muon-neutrinos”? At the conference, Bruno proposed that there might indeed be more than one variety of neutrino, and suggested ways of testing this idea in experiments, which will be described in the next chapter.

  Bruno’s presentation at the conference was a triumph; his interactions with his Western colleagues, however, were mixed. Nino Zichichi, then a young Italian researcher, met Bruno at the conference for the first time. He told me of his excitement at meeting Fermi’s former student, who had done such important work in his youth and then disappeared.18 Those who had known Bruno in his former life were less welcoming.

  Edoardo Amaldi, who had developed the slow-neutron method with Bruno in 1934, acknowledged his former colleague only with a nod, according to Bruno’s great friend from his Via Panisperna days, Gian Carlo Wick.19 Wick himself, according to Bruno, was “very cool” toward him. Luis Alvarez, a leading physicist from Berkeley, who had worked with Fermi and was a colleague of Segrè, had “on many occasions expressed his suspicions” about Bruno, following his defection. When Wick acknowledged Bruno’s presence, Alvarez gave him an “evil look” for doing so—or at least this is how Wick remembered the occasion later. Emilio Segrè—in the opinion of Bruno and others who were present—was especially harsh, snubbing his old friend entirely. Bruno’s impression, as he told Miriam Mafai years later, was that people he’d regarded as real friends ten years before, were now “very icy” and had “never forgiven him” for his move to the Soviet Union.20

  IMAGE 16.2. Bruno Pontecorvo, c. 1980, with signature in Cyrillic script. (COURTESY GIL PONTECORVO; PONTECORVO FAMILY ARCHIVES.)

  SEVENTEEN

  MR. NEUTRINO

  WHEN THE MYSTERIES OF NEUTRINOS WERE FINALLY SOLVED, IT enabled other puzzles in particle physics and cosmology to be solved as well. Bruno Pontecorvo made several seminal contributions in this field. During his time in the West he identified the weak force, and during the second half of his life he realized that neutrinos were the key to learning more about this fundamental force, which is the key to the production of elements in stars. Thanks to neutrinos emitted from the heart of the sun, and to techniques inspired by Bruno Pontecorvo, we have established how the sun creates those elements. Neutrinos emitted by a supernova have even revealed what happens when a star collapses.

  Bruno Pontecorvo not only inspired the use of neutrinos as a tool in cosmology; he also thought deeply about the nature of the neutrino itself. He was fascinated by the mystery of how a particle that is as close to nothing as anything we know—with no electric charge, a mass so small that it has yet to be determined, and an extraordinary aversion to being detected—could nonetheless come in two distinct varieties: those made of matter and those made of antimatter. How does a neutrino know its identity?

  Jack Steinberger, who won the Nobel Prize in 1988 for his own work with neutrinos, summarized Pontecorvo’s contributions as follows: “There are few of us who can boast of a single original and important idea. Bruno’s wealth of seminal suggestions establish him as a truly unique contributor to the remarkable advances of high energy physics in the latter half of the twentieth century.”1

  Of all Bruno’s ideas, perhaps the most famous is his insight that neutrinos exist in more than one variety. This great contribution to physics is forever recorded on his memorial at the Campo Cestio in Rome, with an equation that declares the separate identities of the electron-neutrino and the muon-neutrino.2

  Such work is why Bruno Pontecorvo has been given the sobriquet “Mr. Neutrino.” The reasons he never won a Nobel Prize for any of these contributions are secreted in the closed archives of the Nobel Foundation in Stockholm. However, there is a consensus that this may be the price he paid for his flight to the USSR. Once in the Soviet Union, he was forced to publish in Russian journals, which meant that his work only appeared in English after a gap of about two years—a disastrous delay in a competitive and fast-moving field. Also, because Bruno was not free to travel outside the USSR, he was unable to perform various crucial experiments. These restrictions limited his ability to test his ideas about the enigmatic neutrinos, and other scientists ended up gaining the spoils.

  ANTIMATTER NEUTRINOS

  In 1956, American physicists Fred Reines and Clyde Cowan confirmed the existence of the neutrino. The discovery owed nothing to Pontecorvo, but it would stimulate him to come up with a series of ideas.

  Pontecorvo’s 1946 paper advocated the use of chlorine as a target, and predicted that the impact of a neutrino would convert a neutron into a proton, thus changing the chlorine into a radioactive form of argon, which lies immediately next to chlorine in the periodic table of the elements. The neutrino, meanwhile, would turn into a negatively charged electron to conserve the total amount of electric charge throughout the process. This was not the approach that Cowan and Reines used. And, in fact, the neutrinos produced by a reactor do not generate this sequence of events. Instead, they convert a proton into a neutron, which would change chlorine into sulfur. And instead of turning into an electron, the neutrino becomes a positron—the positively charged antiparticle of an electron.3 The conventional way to differentiate between these two alternatives is to say that, in the former case (where an electron emerges), a neutrino has struck, whereas in the latter case (where a positron emerges), it was an antineutrino that made the impact. This makes sense if the terms matter and antimatter have some intrinsic meaning: a neutrino (matter) turns into an electron; an antineutrino (antimatter) turns into an antielectron, or positron. However, this raises the question of what it is about the antineutrino, as it flies through space, that identifies it as such, and differentiates it from a neutrino.

  Pontecorvo began to ponder this question, and in 1957 gave a talk at the Dubna laboratory in which he suggested that a neutrino might transform into an antineutrino, or vice versa, in midflight. His idea stemmed from two circumstances. The first was a rumor, which turned out to be false, about Ray Davis’s quest for neutrinos; the second was a discovery relating to a specific strange particle: the neutral kaon, or K-zero.

  By this time it was clear that there were two electrically neutral strange particles, one with positive strangeness and the other with negative, known as the K-zero and the anti-K-zero, respectively. As their names suggest, one is the antiparticle of the other. Experiments had shown that when either of them decays, the debris can be a pair of pions. Quantum theory implied that a pair of pions from a K-zero deca
y could then come back together—this time forming an anti-K-zero. Through this two-step process, a piece of electrically neutral matter could turn into antimatter. This possibility fascinated high-energy physicists. In 1957, Bruno wondered if this idea could also apply to neutrinos and antineutrinos.4

  The motivation for his conjecture seems to have been a game of telephone. In 1957, a rumor arrived in Dubna to the effect that the American experimentalist Ray Davis, who had been using Bruno’s chlorine method, had detected neutrinos from a nuclear reactor. Bruno deduced that if the report was correct, the antineutrino that left the reactor must have have changed into a neutrino capable of triggering Davis’s chlorine detector. Unaware that the rumor was false, he proposed that neutrinos and antineutrinos could “oscillate” back and forth, shifting their identities from one to the other.

  The idea was extremely audacious, and was regarded by many as the “fantasy of a prominent physicist.”5 Scientists were skeptical because, according to quantum theory, such a transmutation could occur only if the neutrino had mass, and the received wisdom at the time was that it had none. This point became moot when the rumor about Davis’s experiment was found to be false. In reality, Davis had not recorded any neutrinos at the reactor. This seemed to confirm the emerging conventional picture: a reactor produces antineutrinos, and there was no reason to suspect that an antineutrino can switch to become a neutrino.6

  Thus, in 1957, Pontecorvo’s idea about oscillation was forgotten, though not by him. A few years later, he would resurrect it in a new guise—one that we now know to be correct. For neutrinos are not massless, and although their mass is so trifling that it has not yet been measured, Pontecorvo’s theory of “neutrino oscillations” has today become the focus of a whole branch of science. I shall return to this point later.

  STELLAR NEUTRINOS

  During his time at Chalk River Bruno Pontecorvo had been intrigued by the muon. Indeed, it was he who had established that it was a relative of the electron—and also that there was some unique character, other than its mass, that made it distinct from an electron. This enigmatic quality troubled Bruno, as it did many others, and continues to be troubling even today. The image of the muon invaded his mind and would not go away. He did not determine what makes a muon different from an electron, other than its mass, but the kaleidoscope of confusion settled into an unexpected picture: Bruno realized that neutrinos might play an important role in astrophysics and cosmology.

  I can only guess how Bruno reached this conclusion, but we do know the basic pieces of his puzzle, and from these a possible path emerges. It is as follows:

  Muons are heavier than electrons. Due to Einstein’s relation between mass and energy, a muon therefore has more energy locked within its mass than an electron. A muon can convert this energy into an electron, a neutrino, and an antineutrino. This is the traditional way that a muon decays. Bruno now imagined what might happen if an electron had a lot of energy: Could it shed that energy by “radiating” a neutrino and an antineutrino, analogous to the decay of a muon? There was no reason why not. How would an energetic electron do this, while maintaining the sacrosanct laws of energy- and momentum-balance? Bruno knew the answer: when an electron passes near an atomic nucleus, it picks up energy from the atom’s electric field. It can then shed that energy by transmuting into a lower-energy electron, neutrino, and antineutrino.

  Bruno’s insight was truly novel, and momentous not just for particle physics but for cosmology as well.

  The link lies inside of stars. The production of neutrinos involves the weak force. As its name suggests, the effects of this force are feeble relative to those of the electromagnetic force. This is why the production of neutrinos is normally so rare compared to the radiation of light. However, at high energies the relative power of the weak force grows. Bruno realized that inside stars that are much hotter than the sun, the weak force becomes more powerful, potentially comparable to the electromagnetic force, in which case the production of neutrinos could, in theory, occur as easily as the radiation of photons.

  Neutrinos penetrate matter much more easily than photons do. This leads to a startling consequence. Whereas most photons are absorbed within the stellar mass, neutrinos produced in the heart of a star can escape. The implication, which Bruno pointed out in his paper of April 1958, is this: “At some stage in the evolution of a star, it may well be that the energies radiated into space in the form of neutrinos and photons become comparable.”7 Thirty years later this insight would be confirmed when a burst of neutrinos was detected coming from a distant supernova—SN1987A.

  THE FLAVORED NEUTRINO

  Bruno’s work on neutrinos became vital in saving a developing theory of the weak force.

  According to Fermi’s theory of beta decay, the interactions of neutrinos (and weak-force interactions in general) are not always feeble. Instead, the chance of a reaction depends sensitively on the energies of the particles involved. Double their energy and the chance increases fourfold; triple their energy and the chance increases by a factor of nine. In general, the growth is proportional to the square of the energy.

  However, this chance cannot grow indefinitely in reality, for if it did, at a certain point it would imply that some processes occur with greater than 100 percent probability, even infinite probability, which is nonsense. So Fermi’s theory can only be an approximation of some more complete explanation.

  In 1956, Julian Schwinger, the American theorist who shared a Nobel Prize for his work on the electromagnetic force, made a crucial proposal about the weak force, which went beyond Fermi’s theory.8 In a nutshell, Schwinger suggested that the weak force and the electromagnetic force have something in common.

  Electromagnetic radiation comes in particle bundles—photons. In quantum theory, the electromagnetic force between two particles arises when they exchange photons. Schwinger suggested that a similar process occurs for the weak force. He predicted the existence of the W (“weak”) boson, analogous to the photon. In Schwinger’s hypothesis, particles experience the weak force when they exchange W bosons.

  This was confirmed in 1983 with the discovery of the W boson, but in 1956 it was just a hypothesis. Nonetheless, the idea was compelling. In addition to its seductive implication that two fundamental forces (electromagnetic and weak) were analogous and could perhaps be united theoretically, it also avoided the nonsensical probabilities implicit in Fermi’s original model. When particles interact at high energies, W bosons can be produced. When this new process is included in the quantum accounts, the troubles with Fermi’s model disappear.9

  Almost immediately there was a problem. Soon after Schwinger had made his suggestion, another American theorist, Gerald Feinberg, noticed that if the weak force is indeed due to the action of W bosons, there is an unwanted implication for the decay of the muon. By this stage it had been established that a muon decays into an electron and two neutrinos. If this occurs through the intermediate action of a W boson, as Schwinger proposed, the laws of quantum mechanics imply that you can do away with the neutrinos and have the muon decay into an electron and a photon. However, no example of such behavior had ever been seen. Feinberg calculated that one in every ten thousand decays should result in an electron and photon. This is a small percentage, but the experimental data already showed that if there were any such decays at all, they at most accounted for one in a hundred million!10 Schwinger’s theory looked to be in trouble.

  Feinberg did point out, however, that there was a loophole in his argument: he had assumed that the neutrino associated with the muon and the neutrino paired with the electron were the same. If a “muon-neutrino” differs from an “electron-neutrino,” there is no problem. Having made this observation, Feinberg took it no further.

  BRUNO NOW MADE HIS FIRST INTERVENTION. HIS PAPER ON “ELECTRON and Muon Neutrinos” was written on June 29, 1959.11 He was not actually the first to have pursued the implications of Feinberg’s observation—Jogesh Pati and Sadao Oneda in the US had wri
tten a paper earlier that year, in which they “deliberately” denoted a distinct “neutral counterpart of the muon” (the muon-neutrino) as well as a neutral counterpart of the electron (the electron-neutrino).12 They pointed out that if the two were actually identical, then a muon could decay into an electron and a photon. If they were not identical, then this could not occur. Bruno also understood this. However, his efforts in Dubna would go further toward finding a definitive answer to the problem.

  Initially, the primary goal of the scientists at Dubna was to investigate how the strong forces of the atomic nucleus produce pions, which are the material embodiment of the energy latent in the nuclear field. A pion is not stable. It self-destructs, leaving either a muon or an electron, accompanied by a neutrino.13 The traditional decays of nuclear particles produce electrons (or positrons) and neutrinos. Once in every ten thousand decays, the pion does also, but most of the time it decays into a muon and a neutrino. In 1959 Bruno Pontecorvo started wondering: Are the neutrinos produced when a pion decays into a muon the same as those emitted in conventional beta decays?

  He began by systematically listing all processes in which neutrinos occur, then moved the subject forward by identifying practical experimental ways of identifying a neutrino’s character. He showed that although neutrinos and antineutrinos could be identified as distinct, the question remained open empirically as to whether muon- and electron-neutrinos are different or identical.

  In Bruno’s paper, one section was titled “Are muon-neutrino and electron-neutrino identical particles?” He acknowledged that the possibility that they are distinct would be “attractive from the point of view of symmetry and the classification of particles.” His point is that the muon and electron have distinct “flavors,” so if their neutral counterparts occur in two flavors also, there would be symmetry among these particles. Today this is a basic plank of the Standard Model, which classifies the fundamental forces and the interactions among them.