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Takaaki Kajita and Arthur McDonald Share Nobel in Physics for Work on Neutrinos Takaaki Kajita and Arthur McDonald Share Nobel in Physics for Work on Neutrinos
(about 11 hours later)
Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University in Canada were awarded the Nobel Prize in Physics on Tuesday for discovering that the ubiquitous but elusive subatomic particles known as neutrinos have mass. Takaaki Kajita of the University of Tokyo and Arthur B. McDonald of Queen’s University in Ontario were awarded the Nobel Prize in Physics on Tuesday for discovering that the enigmatic subatomic particles known as neutrinos have mass.
Neutrinos are the second most abundant subatomic particles in the universe, after photons, which carry light. Their existence was predicted in 1930, but for decades they remained some of the most enigmatic elements of astrophysics. Their experiments rewrote the balance sheet of the universe. Neutrinos were once thought to be massless, but decades of study have led astronomers to conclude that their collective weight in the cosmos is about equal to the collective weight of stars.
The ghostly neutrino — it means “small neutral one” in Italian — carries no electric charge and is so light that it had been assumed for many years to have no mass at all. The scientists showed that neutrinos, which are found in three “flavors,” could oscillate from one flavor to another — changing identities like a spy on the run, as they traveled through the atmosphere or through space from the sun — demonstrating that they have mass. After the light-carrying particles known as photons, neutrinos are the most abundant particles in the universe. Seas of them are left over from the Big Bang, and many more are produced in stars and in nuclear reactors. They drift through the earth and our own bodies like wind through a screen door.
Dr. Kajita was part of a team of researchers who in 1998 discovered that neutrinos from the atmosphere switched between two identities on their way to the Super-Kamiokande detector, nearly two-thirds of a mile below the earth’s surface. They also come in three different identities, or “flavors,” in the jargon — the key to their eventual unmasking.
In 1999, Dr. McDonald announced that neutrinos from the sun were not disappearing, but merely changing disguises, on their way to the earth. He and his colleagues had captured the neutrinos using a uniquely sensitive new detector 6,800 feet below ground, at the Sudbury Neutrino Observatory, which is part of Queen’s University in Kingston, Ontario. Experiments by Dr. Kajita’s team at the Super-Kamiokande Collaboration in Japan and Dr. McDonald’s team at the Sudbury Neutrino Observatory in Ontario showed that neutrinos could oscillate, changing identities like a spy on the run, as they traveled through the atmosphere or through space.
“The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe,” the Royal Swedish Academy of Sciences, which awards the prize, said in a statement. That they can change identities means they have mass, according to the rules of quantum physics, and knowing that has helped cosmologists understand how the universe evolved, how the sun works and how supernova explosions occur.
The universe is swamped in neutrinos that are left over from the Big Bang, and many more are created in nuclear reactions on earth and in the thermonuclear reactions that power the sun. “For particle physics, this was a historic discovery,” the Royal Swedish Academy of Sciences, which awards the prize, said in a statement. “Its Standard Model of the innermost workings of matter had been incredibly successful, having resisted all experimental challenges for more than 20 years. However, as it requires neutrinos to be massless, the new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.”
Once thought to travel at the speed of light, they drift through the earth and our own bodies like moonlight through a window. Knowing that they can change identities — and that they have mass — has helped cosmologists understand how the universe has evolved and how the sun works, and it will perhaps help them improve their attempts to create fusion reactors on earth. In an interview posted by the Nobel organization, Dr. McDonald said, “I hugged my wife,” after the call from the academy had awakened her, too.
“For particle physics, this was a historic discovery,” the Royal Swedish Academy said in its statement. “Its Standard Model of the innermost workings of matter had been incredibly successful, having resisted all experimental challenges for more than 20 years. However, as it requires neutrinos to be massless, the new observations had clearly showed that the Standard Model cannot be the complete theory of the fundamental constituents of the universe.” In a news conference at the University of Tokyo, Dr. Kajita said: “I want to thank the neutrinos, of course. And since neutrinos are created by cosmic rays, I want to thank them, too.”
Michael S. Turner, a theoretical cosmologist at the University of Chicago, agreed that the Standard Model, a suite of equations that has dominated physics for the last half-century, was not complete. He suggested that neutrinos left over from the Big Bang might make up part of the dark matter that dominates the universe. “Neutrinos contribute about as much to the mass budget of the universe as do stars!” he wrote in an email. “Hooray for neutrinos,” he added. Dr. Kajita and Dr. McDonald will share the prize of about 8 million Swedish kronor, or about $960,000.
Dr. Kajita and Dr. McDonald will share the 8 million Swedish kronor, or about $960,000, prize. They joined 199 laureates, including Albert Einstein, Niels Bohr and Marie Curie, who have been honored with the prize since 1901. Neutrinos have led physicists and astronomers on a merry chase ever since 1930, when Wolfgang Pauli suggested their existence to explain a loss of energy when neutrons decayed.
Goran K. Hansson, permanent secretary of the Royal Swedish Academy, which appoints the prize committee, announced the prize in Stockholm. He later joked that he had done a terrible thing: “I have postulated a particle that cannot be detected.” A neutrino so rarely collides with ordinary matter that it can pass through a light-year of lead with no effect, or so the theory went.
Dr. McDonald, a Canadian citizen born in 1943, was awakened by the Nobel committee and asked to call in to the prize announcement in Stockholm. “It’s a very daunting experience, needless to say,” he told reporters by telephone. “Fortunately, I have many colleagues as well who share this prize with me.” But neutrinos do hit atoms once in a great while, producing a shower of debris that can be detected. In 1956 Frederick Reines and Clyde Cowan detected neutrinos streaming from a nuclear reactor. Experimenters later found that there were actually three kinds of neutrinos: electron, muon and tau neutrinos.
Informed by phone of the prize, Dr. Kajita, who was born in 1959 in Japan, and directs the Institute for Cosmic Ray Research at the University of Tokyo, said simply, “Kind of unbelievable!” Years later Raymond Davis, a researcher at Brookhaven National Laboratory, set out to capture neutrinos from the sun in a tank of cleaning fluid set up in the Homestake Gold Mine in Lead, S.D. He came up short of the predicted number, leading to speculation that something was wrong with our theories of physics or of the sun, or both.
Later, in a news conference at the University of Tokyo, Dr. Kajita said: “I want to thank the neutrinos, of course. And since neutrinos are created by cosmic rays, I want to thank them, too.” But an answer to the neutrino problem already existed. The Italian theorist Bruno Pontecorvo, inspired by the behavior of another subatomic particle, the kaon, suggested in 1957 that neutrinos could oscillate between different personalities.
In this scheme of things, one form of neutrinos generated in the sun might be morphing into another, undetectable form before they got to Homestake.
The quest to understand these particles drove physicists deep underground, where their experiments would be shielded from cosmic rays.
Dr. Kajita, who was born in Higashimatsuyama, Japan, in 1959, and got his Ph.D. from the University of Tokyo, had cut his teeth on an experiment known as Kamiokande to see if protons, the building blocks of ordinary matter, would decay (they have not, so far).
He got interested in neutrinos because they were a source of noise in his experiment. The Super-Kamiokande, a tank of super-pure water 120 feet deep, started up in 1996 in an old zinc mine, designed to identify muon neutrinos produced by cosmic rays. Its detectors recorded flashes of light caused by debris speeding away from a neutrino hit.
In principle, since the Earth is transparent to neutrinos, equal numbers should have been coming from overhead and from below. But in fact more particles were coming from above, suggesting that neutrinos passing through the earth were oscillating into a form unreadable by the detector.
This oscillation meant that the different flavors had different masses. “This is clearly the physics that is beyond the Standard Model of particle physics,” Dr. Kajita said.
The next bit of cosmic bookkeeping was performed by Dr. McDonald and his team at the Sudbury Neutrino Observatory. Dr. McDonald was born in Sydney, Nova Scotia, in 1943, and earned a Ph.D. at the California Institute of Technology.
His detector, also deep underground, could distinguish electron neutrinos from the other types, either measuring the electron neutrinos or recording the overall total.
Dr. McDonald’s team recorded only a third of the number of electron neutrinos that theory predicted. But when the physicists measured the total number, the deficit had been made up. Two-thirds of the electron neutrinos had transformed into muon and tau neutrinos on the eight-minute journey from the sun, the collaboration reported in 2002.
“We had an extremely clear result that showed neutrinos do change from one type to another,” Dr. McDonald said in an interview with the Nobel organization.
Tuesday’s prize announcement is hardly the end of the story. The Fermi National Accelerator Laboratory in Batavia, Ill., has staked much of its future on neutrino studies — in particular the Deep Underground Neutrino Experiment, or DUNE, which will beam neutrinos from Fermilab to a detector in South Dakota to study their oscillations.
Which is fitting, Joe Lykken, deputy director of Fermilab, said in an email. “DUNE,” he said, “returns us to the Homestake mine in South Dakota, where Ray Davis collected the first data on solar neutrinos back in the 1960s, inspiring the experiments being honored now with the Nobel Prize.”