by Close, Frank
Bruno evidently suspected that the two neutrinos were indeed different, because in his paper he introduced the nomenclature and notation that is universally used today. The particles became known as “muon-neutrinos” and “electron-neutrinos” forever onward. In the shorthand notation of particle physicists they are written νμ and νe, respectively.
IT IS NOT UNUSUAL IN SCIENCE FOR A GREAT IDEA TO OCCUR TO MORE than one person, independently. The fact that one person is remembered and another forgotten can be due to many factors: chance, opportunity, or the confidence to follow through on what others might regard as crazy. And when that “crazy” idea turns out to be sensible after all, and winner and loser have talked to each other along the way, versions of history can diverge, as memories differ of who did what.
The saga of how the idea of two neutrinos matured into established lore is a paradigm of such divergence. This tale begins in Moscow, in 1957, with a colleague of Bruno’s named Moisy Markov.
Markov was interested in the neutrinos that are produced when cosmic rays hit the upper atmosphere. The collisions produce pions and muons, which in turn shed neutrinos. These can have considerable energy, far more than the neutrinos from nuclear reactors, the main source in the 1950s. Markov wondered how scientists could detect these “atmospheric neutrinos.” He decided this might be a good project for his student Igor Zheleznykh to investigate.
Zheleznykh designed a detector. It contained one cubic meter of lead, and was placed deep underground, where it would be shielded from other cosmic ray particles. By 1958, Zheleznykh had shown that the chance of neutrinos interacting with the target grows considerably with their energy. He also remarked, “Different numbers of electrons and muons induced by neutrinos in a detector could give evidence of the existence of two types of neutrino.” Like Feinberg, he did not pursue this any further.
During his research, Zheleznykh had stumbled onto a second profound question: Why make calculations only for high-energy atmospheric neutrinos? Why not consider performing neutrino experiments at high-energy accelerators? One evening, late in 1957, Zheleznykh visited Markov at home and asked him.
It turns out that Markov had already asked himself this very question, in connection with the Synchrophasotron, the new higher-energy accelerator planned for Dubna, but had dismissed the idea as impractical. Impractical at Dubna, maybe, but it was an interesting challenge in principle: the experiment might be feasible somewhere, someday. Zheleznykh’s question led Markov to reconsider the problem. He went to discuss the idea with Bruno Pontecorvo: “I told [Pontecorvo] that I would like to suggest neutrino experiments at accelerators. [He] liked such an idea very much.”
Inspired by Pontecorvo’s enthusiasm, Markov gave the problem to another student, Docho Fakirov, who included it in his thesis at Moscow State University in 1958. Markov decided to write a report, titled “On High Energy Neutrino Physics,” which he planned to present in 1959 at the conference on high-energy physics in Kiev. However, several colleagues were skeptical, with one influential physicist asking him, “Are you serious?” in a manner that clearly suggested the answer to be no.14 Markov withdrew the paper. It was a big mistake: Bruno Pontecorvo would be braver, and as a result it was he who was remembered for this advance.
FINDING THE NEUTRINO HAD BEEN HARD ENOUGH; PROVING experimentally that there was more than one variety of the phantom particle would present a new level of difficulty. Bruno’s idea was to replicate the Cowan-and-Reines discovery of the antineutrino, using a source of muon-antineutrinos instead of a nuclear reactor, which produces the electron variety.
In Cowan and Reines’s experiment, the collision of an antineutrino with a proton converted the proton into a neutron and a positron. This led to the discovery of the electron-antineutrino. Bruno wanted to initiate this same process using antineutrinos produced in association with a muon. If neutrinos have distinct flavors, as he suspected, the subsequent collision with a proton should release a positive muon, not a positron; the production of a positron, on the other hand, would demonstrate that the two neutrinos are identical.
The next question was how best to conduct such an experiment. He noted that using antineutrinos of high energy would be advantageous, as the chance of interaction increases with energy. Thus a high-energy accelerator was needed. The basic idea was to smash a beam of high-energy protons into a target, which would liberate large numbers of positively charged pions. These decay into muons and antineutrinos. A steel shield would absorb the muons, but would be almost transparent to the antineutrinos. Several meters away, another large target would serve as an antineutrino detector. The antineutrinos would have high energy, and hence there would be a reasonable chance that occasionally one would hit an atom in the detector, pick up electric charge, and reveal itself.
He calculated that, with a detector similar to the one used by Cowan and Reines, one collision an hour might be detected “at new accelerators now being discussed in which the intensity of the protons may be [a thousand times larger than at previous accelerators].” He insisted that experiments to test the identity of muon- and electron-neutrinos “must be seriously thought over” when the new accelerators—at CERN in Europe, and Brookhaven in the US—became available.15
Bruno mentioned some of these ideas at the end of July 1959, during the International Conference on High Energy Physics, in Kiev. He wrote a report under the auspices of the Joint Institute for Nuclear Research (JINR), which was formally published in the Soviet Journal of Experimental and Theoretical Physics later that year. This article was written in Russian, of course, which limited its international reach. Bruno’s remarks at the conference would have been translated, but there is no record that they had any memorable effect.16
The following year Bruno wrote a second paper, in which he developed his earlier ideas. The first paper had dealt with antineutrinos; the 1960 paper considered how to test specifically for neutrinos of different flavors. He advocated using a high-energy accelerator to produce a beam of neutrinos, pointed at a lump of carbon. Any collisions would convert an atom of carbon into nitrogen.17
The beams in an accelerator come in pulses, so the neutrinos they produce also arrive in distinct bursts, separated by a fraction of a second. Bruno realized that with modern electronics it would be possible to record the instant when a burst of neutrinos arrived at the carbon target and see if this coincided with the appearance of an electron. In this way, one could determine if the electron was a genuine signal produced by the collision, or if instead it had strayed out of an atom in the detector itself, and as such was merely background. The nitrogen produced in such a collision would be radioactive and would decay by emitting a positron. This would occur marginally later than the actual collision, and the time delay between the electron and the positron could be used as another check. Bruno had done everything possible to design a realistic experiment; now all that was required was an opportunity to perform it.
Unfortunately, it would be impossible to do the experiment at Dubna. In November 1959, CERN’s powerful Proton Synchrotron began operation, and could have served Bruno’s purposes, but the Soviet authorities refused to allow him to leave the USSR. Two years would pass before Bruno’s ideas were made available in English. By this time, it was too late: he had been scooped.
IN THOSE DAYS, SOVIET IDEAS WERE LARGELY UNKNOWN IN THE US and Europe until they appeared in translation. This meant that ideas often developed independently in the two hemispheres. In New York, during November 1959, Chinese-American theorist T. D. Lee was pondering Fermi’s theory on the behavior of weak interactions. Lee was unaware of Bruno Pontecorvo’s ideas, let alone Markov’s.
As we have seen, Fermi’s theory was only an approximation of some complete explanation, since it gave impossible outcomes for the behavior of neutrinos at very high energy levels. Schwinger’s hypothesis of the W boson solved that problem but ran into its own difficulties regarding the decays of muons. Lee therefore wanted to reveal the solutions to these problems exper
imentally. The challenge was to find a way to probe the weak interaction in high-energy experiments.
While leading a discussion on this subject, Lee realized that such an experiment would be hard to perform because when particles collide at high energies, the effects of the electromagnetic and strong forces tend to obscure those of the weak force. Melvin Schwartz, a squat experimentalist with a bubbly personality, was one of those present. The lunchtime discussion must have entered his subconscious mind, as in bed that night Schwartz suddenly had the answer: “It was incredibly simple. All one had to do was to use neutrinos.”18 Neutrinos aren’t affected by the strong force, and, being electrically neutral, they aren’t affected by electromagnetic forces either. As such, Schwartz realized, they are ideal for probing the weak force. His idea was that the production of pions, and their subsequent decays, might produce neutrinos in sufficient numbers that they could be used in experiments.
He wrote a short paper outlining his ideas, which was published in 1960. Pontecorvo’s paper had just appeared in English translation, and Schwartz included a comment at the end of his paper noting the “related paper which has just appeared” by Pontecorvo. He also thanked Lee, and Lee’s collaborator C. N. Yang, for emphasizing the importance of high-energy neutrino interactions.19
Pontecorvo’s ideas about the two distinct flavors of neutrinos were not included in Schwartz’s paper. However, in the meantime Lee and Yang had been thinking about what might be learned from these experiments. By the summer of 1960 they had reached the same conclusion as Pontecorvo: the absence of muon decay into electron and photon could be the smoking gun proving that the muon-neutrino and electron-neutrino differed. This became the quarry to chase.
In July 1960 the Alternating Gradient Synchrotron (AGS) began operations at Brookhaven. Slightly more powerful than CERN’s Proton Synchrotron, the AGS was for the next eight years the world’s highest-energy machine. One of the first experiments at the new accelerator led to Nobel Prizes for the three leaders—Melvin Schwartz, Jack Steinberger, and Leon Lederman. In essence they performed the experiment that, unknown to them, Bruno had proposed, and found the result that he was hoping for: muon-neutrinos and electron-neutrinos are distinct. In addition they demonstrated that, at the new high-energy accelerators, the neutrinos are less shy, as Zheleznykh had deduced in 1958, and thereby become useful tools for science. During the next four decades, beams of high-energy neutrinos at accelerators throughout the world opened up new vistas in our understanding of the structure of matter, and the profound patterns at work in the fundamental laws of nature.
The idea that neutrinos have distinct flavors, which parallel those of their electrically charged siblings, electrons and muons, is one of the basic ingredients in the modern theory of fundamental particles, and today Bruno Pontecorvo is recognized as its parent. The American team had the grace to mention his independent insight when they accepted the Nobel Prize in 1988, with Schwartz remarking that Pontecorvo’s “overall contribution to the field of neutrino physics was certainly major.”20
THE TEAM’S NOBEL PRIZE WAS AWARDED FOR THE EXPERIMENTAL “demonstration” of distinct types of neutrino and for the “neutrino beam method.” Although Bruno’s insights were central to the former, his claims on high-energy neutrino beams are peripheral. As we have seen, others in Moscow, whose work he must have been aware of, had taken the idea somewhat further, but had lacked confidence in their conclusions. Furthermore, as we shall now see, Bruno did not really advocate high-energy beams either.
Although the experiment he proposed could have been done at CERN as well as at the AGS in Brookhaven, it is not clear whether Bruno would have chosen to perform it himself, even if he had been allowed to leave the USSR. His 1960 paper suggests that he was rather pessimistic about the practicality of the enterprise. He noted that the chance of a neutrino interacting grows with its energy, but he worried that, at very high energies, the quantity of neutrinos produced would be smaller. This is due to the time-dilation effect of relativity, which causes fast-moving particles to experience time slowed down, which in turn causes them to live longer. Thus high-energy pions, which are faster, live longer than lower-energy ones. It is the decays of pions that give rise to the beams of neutrinos, so if fewer pions decay, the number of neutrinos falls as well. As a result, Pontecorvo focused on neutrinos with moderate energies, rather than on the very high energies available at CERN.
In fact, he was overly pessimistic. At these high energy levels, the increased chance of interaction more than compensates for the reduction in the number of neutrinos. Also, the effects of collisions at high energy are easier to diagnose. Overall, the rule is: the higher the energy, the better.21
BRUNO CONCEIVES THE STANDARD MODEL
Even after the American trio confirmed Bruno’s theory that there is more than one variety of neutrino, there remained a further question: Do the two varieties respond to the weak force in precisely the same way? This would be the case if the weak interaction were truly a fundamental force of nature. In 1962 Bruno came up with a way to test the question.
Bruno’s idea grew out of work that he and Frédéric Joliot-Curie had done twenty-five years before.
In 1937, Bruno and Joliot-Curie had tried to prove that the beta particles that emerge during radioactive decay are indeed electrons, and not just similar to electrons. They did this by firing electrons at atoms in the hope of inducing the transmutation in reverse—what has become known as the inverse beta process. Their results were inconclusive because there are so many electrons in matter that it was hard to distinguish between signals and background. However, twenty-five years later Bruno remembered their attempt, and now realized that it should be possible to perform the experiment with muons, rather than electrons, and initiate the inverse beta process that way. When it hit the target, the muon would turn into a muon-neutrino, and the experiment could show whether the probability of a muon-induced reaction was the same as that of an electron-induced reaction. Because the neutrino is electrically neutral and would leave no trace as it escaped, it would be important to keep track of the corresponding change in the target to make sure that the reaction had occurred. To do so, he proposed using nuclei of helium-3, in which two protons are bound to a single neutron. If the inverse beta reaction occurred, the helium would convert to tritium—made of one proton and two neutrons.
If a cloud chamber was filled with the helium-3 gas, any tritium formed by the reaction would recoil and leave a wispy trail. So the chamber would act as both a target and a track detector. The problem was that helium-3 was not easy to come by, especially in the Soviet Union. Fortunately, Igor Kurchatov was interested in Pontecorvo’s ideas and provided him with sufficient helium-3 (a product of Kurchatov’s nuclear weapon program) for the experiment to succeed.
The result was everything that Bruno had hoped for. He found that the weak interaction acts on muons and muon-neutrinos with the same strength as it does on electrons and electron-neutrinos. Pontecorvo had established the universal nature of the weak force: it acts on muon and electron flavors impartially.
This breakthrough, along with Bruno’s earlier idea of distinct neutrino flavors, forms the core of today’s Standard Model of particles and forces.22
SOLAR NEUTRINOS
Bruno Pontecorvo’s 1946 paper, which had inspired Ray Davis’s unsuccessful attempt to catch neutrinos coming from a reactor, only mentioned the sun in two sentences. However, the importance of the underlying concept is immense. Ten million solar neutrinos pass through your eyeballs every second, unseen. This is indeed a lot, but still not enough to be easily detected. Furthermore, the chlorine detector is blind to almost all of them.
To be seen, a neutrino first has to hit the nucleus of a chlorine atom. That is a rare enough occurrence, but it’s still not sufficient to guarantee that the chlorine will convert into argon. The reason is that the argon nucleus has more mass than chlorine, and this extra mass has to be created from the energy of the incident neutrino. The cat
ch is that the neutrinos produced by the main part of the solar fusion engine (the conversion of hydrogen into helium) have too little energy to do the task.
However, our sun makes more than just helium; other light elements are created in its nuclear furnace. After hydrogen has been converted into helium, the helium nuclei can fuse and, though a series of processes, produce elements such as boron and beryllium. This also liberates neutrinos, some of which have enough energy to activate a chlorine detector. Although these neutrinos are far outnumbered by those created in the hydrogen-to-helium process, the good news is that the chance of a neutrino being captured grows with energy, as we have seen, and so these higher-energy neutrinos are easier to capture.
As the 1950s came to a close, Ray Davis began his quest to capture neutrinos from the sun. This story would continue for forty years, with many twists and turns along the way, before its final chapter was written.23
In his first attempt in 1959, Davis used 4,000 liters of cleaning fluid, but he failed to detect anything. He realized that to have any chance he would need to be even more ambitious, so he set out to build a target containing 400,000 liters of the liquid, enough to fill a swimming pool. In 1964 Bruno Pontecorvo held a special seminar in Leningrad to report on Davis’s quest. There was a lot of interest in the seminar, but Pontecorvo later said that he was the only person present who believed that the experiment would be successful. Whether this belief was based on insight or simple optimism is hard to say.
By the end of summer 1966—twenty years after Pontecorvo first advertised chlorine as a way to detect neutrinos—Davis’s enlarged experiment was ready to begin.