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Takaaki Kajita and Arthur McDonald share 2015 Physics Nobel Prize

7 October 2015
The two experimenters are honored for their respective teams' discovery that neutrinos oscillate in flavor.

Takaaki Kajita and Arthur McDonald

Takaaki Kajita and Arthur McDonald

The Royal Swedish Academy of Sciences announced on Tuesday that the 2015 Nobel Prize in Physics is to be awarded to Takaaki Kajita of Tokyo University and Arthur McDonald of Queen’s University in Kingston, Ontario.

After photons, neutrinos are the most abundant particle in the universe. Their ubiquity, however, comes with an infinitesimal propensity to react with matter. Every second, trillions of neutrinos—some created in the Big Bang, some spewing from distant supernovas, some emanating from inside the Sun—stream through our bodies without our noticing.

Detecting neutrinos is hard. Measuring their mass and some other properties is harder still. Kajita's and McDonald's teams designed, built, and ran huge experiments that sought to determine one of the most important neutrino properties of all: whether the particle oscillates among its three flavors—electron, muon, and tau.

Kajita was team leader at the Super-Kamiokande neutrino observatory, which is located near the city of Hida, 250 kilometers northwest of Tokyo. McDonald directed the Sudbury Neutrino Observatory, which is 400 km north of Toronto.

The affirmative and conclusive confirmations from the two facilities, obtained almost two decades ago, constituted proof that at least one neutrino eigentstate is endowed with mass. That finding has profound implications for physics. It requires that the standard model of particle physics, which had originally presumed neutrinos to be massless, be extended.

The finding also suggests that the solution to one of the biggest outstanding problems in physics—the preponderance of matter over antimatter—might be found in the nature and behavior of neutrinos.

A growing realization

The story of neutrino oscillations involves not just particle physics, but also nuclear physics and astrophysics. Wolfgang Pauli proposed the existence of a new neutral particle in 1930 to preserve the conservation of energy, momentum, and spin in beta decay. Whatever it was, the hypotheical particle—dubbed the neutrino by Enrico Fermi—was evidently light, fast, and feebly reactive.

The realization that neutrinos are produced in the nuclear reactions that power the Sun prompted Bruno Pontecorvo to propose in 1946 that the particle's feeble interactions could be an advantage: Neutrinos that reached Earth could carry with them information about the Sun's interior—provided they were detectable.

Neutrinos were detected 10 years later by Clyde Cowan and Frederick Reines. In a method that foreshadowed the ones that Kajita and McDonald used four decades later, Cowan and Reines arranged 110 photomuliplier tubes around a tank containing 200 liters of water. Neutrinos streaming into the tank from a nearby nuclear reactor reacted only rarely with protons, but often enough that the resulting positrons annihilated with ambient electrons to produce telltale flashes of gamma radiation.

The notion that neutral kaons oscillated between two distinct states gave Pontecorvo the idea in 1957 that neutrinos might also oscillate. When the muon neutrino was discovered in 1962, Ziro Maki, Masami Nakagawa, and Shoichi Sakata embodied Pontecorvo's suggestion in a theory encompassing the notion that the different neutrino flavors could be superpositions of different mass eigenstates. Oscillation between flavors is the result of the different mass eigenstates evolving at different rates.

Meanwhile, astrophysicists were incorporating nuclear reactions into realistic models of the Sun. Among the reactions included in the models was the beta decay of boron-8, which yields neutrinos of energies up to 15 MeV.

To detect those neutrinos, Raymond Davis made use of an abandoned South Dakota gold mine, where he built a tank that contained 500 tons of tetrachloroethylene. Neutrinos triggered the inverse beta decay of chlorine-37 to yield radioactive argon-37. By 1968 Davis had gathered enough 37Ar to publish. His conclusion: The Sun was emitting fewer electron neutrinos than the solar models predicted. The deficit became known as the solar neutrino problem.

Resolving the solar problem

The nuclear reactions in the Sun produce neutrinos of one flavor, electron. So long as experiments, like Davis's, were sensitive to just one flavor, two explanations remained in play to account for the deficit: Either the solar models were wrong, or the models were right and electron neutrinos oscillated into other, undetectable flavors.

Established in 1984 in an abandoned nickel mine, the Sudbury Neutrino Observatory was conceived to resolve the solar neutrino problem. Like Davis's experiment, SNO sought to detect neutrinos produced from the decay of 8B. Unlike Davis's experiment, its active ingredient, deuterated water (D2O), is sensitive to different neutrino flavors.

The flavor sensitivity arises because when a neutrino of any flavor reacts with a deuteron, it can produce a proton and a neutron. But only an electron neutrino reacts with a deuteron to produce two protons and an electron. The neutrons are detected when their capture yields a gamma ray. The electrons are detected through the Cherenkov radiation that they emit.

At SNO a 12-meter-diameter acrylic sphere containing 1000 tons of D2O was monitored by 9500 photomultiplier tubes. The sphere and its detectors are housed 2 km below Earth's surface.

Under McDonald's direction, SNO became operational in 1999. By 2001, it had collected enough data to prove that the deficit of electron neutrinos from the Sun is caused by neutrino oscillations.

Resolving the atmospheric problem

SNO was designed expressly to resolve the solar neutrino problem. Super-Kamiokande was designed, at least originally, to investigate proton decay. The evidence for neutrino oscillations came from the experiment's ability to detect energetic neutrinos produced in Earth's atmosphere.

Located 1 km below Earth's surface in an abandoned zinc mine, Super-Kamiokande consists of a cylindrical stainless steel tank containing 50 000 tons of ultra-pure water.

As in the case of SNO, the detector medium is monitored by thousands of photomultiplier tubes, which discriminate between electron and muon neutrinos based on the shape of the flash of Cherenkov radiation engendered by the particles: neat and circular for muon neutrinos; blobby for electron neutrinos.

When energetic cosmic rays slam into atomic nuclei in Earth's atmosphere, a cascade of nuclear reactions takes place that yields neutrinos in a ratio of approximately two muon neutrinos for each electron neutrino. Whether those neutrinos have the chance to change flavor before they reach a detector below depends on their energy: The higher the energy, the farther a neutrino must travel before it completes one flavor oscillation.

It turns out that for neutrinos of 1 GeV and lower, Earth's atmosphere is just high enough to allow neutrinos to oscillate. In the early 1990s Kamiokande—Super-Kamiokande's smaller and less sensitive predecessor—and other experiments had measured a ratio of muon neutrinos to electron neutrinos that was closer to 1:1 than to 2:1. The likeliest explanation: Muon neutrinos transform into another flavor on their way down.

As was the case with the solar neutrino problem, measuring a deficit in the expected number of neutrinos does not by itself constitute proof that neutrinos oscillate. By 1998, Super-Kamiokande had clinched the case.

In principle, atmospheric neutrinos of 5 GeV and higher that arrive at a detector directly beneath them preserve their original muon-to-electron ratio of 2:1. But that's not the case for energetic neutrinos produced in the atmosphere above a detector's antipodal point. By the time those neutrinos have passed right through Earth to reach the detector, they could have oscillated several times.

Super-Kamiokande determined that at high energies, the muon-to-electron ratio did indeed depend on whether the neutrinos came from above or from below (and through Earth). What's more, the experiment also determined that the muon neutrinos transformed into a flavor other the electron.

Neutrino oscillation experiments are sensitive to mass differences, not to absolute masses. Still, from SNO, Super-Kamiokande, and other experiments, it is possible to assert that there exists at least one neutrino mass eigenstate whose mass is at least 0.04 eV. From observations of the cosmic microwave background, the mass is at most 0.3 eV.

That value—at most of order a millionth of the mass of the electron—is tiny. Yet because of neutrinos' abundance, the particles have exerted, and continue to exert, an influence on the cosmos. Whether they are also the key to why the cosmos contains more matter than antimatter remains to be seen.

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