SOMETIMES the smallest of signs heralds the greatest of changes. Last June, the Japanese-American SuperKamio-kande team announced that at least one of the three ghostly particles known as neutrinos must have some mass-perhaps less than a millionth that of an electron. Yet this tiny quantity has profound consequences.
“It’s a fantastically important result, the biggest breakthrough for particle physics in over a decade,” says Joel Primack, a physicist from the University of California, Santa Cruz. In the Standard Model of particle physics, none of the three varieties-the electron neutrino, the muon neutrino, and the tau neutrino-has any mass at all.
But if the Standard Model is wrong, what will replace it? The answer depends on exactly how hefty each kind of neutrino is. Their masses will tell us much about the next generation of fundamental theories. And they are also important for cosmology, as different neutrino masses would have had different effects on the evolution of the Universe. Neutrinos could be an important part of the invisible “dark matter” that astronomers believe is lurking everywhere. There could even be a shadow universe out there containing particles called sterile neutrinos that scarcely interact with normal matter.
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Fortunately, physicists have a good way to get a grip on neutrino mass. According to the laws of quantum mechanics, weighty neutrinos should “oscillate” from one kind to another and back again, like a quick-change artist in a Vaudeville comedy act. This happens if physical neutrinos are a mixture of two or more underlying quantum states that travel at different speeds because of their different masses. As the quantum waves of these underlying states have different frequencies, the different combinations fade back and forth into one another (“Cosmic changelings”, 91av, 16 March 1996, p 28). Without mass, however, neutrinos cannot dance this quantum jig.
To look for this process, physicists use large underground tanks of liquid, surrounded by sensitive light detectors. Neutrinos interact with matter via the weak force, a force so incredibly feeble that nearly all the neutrinos hitting Earth zoom right through the planet without leaving a trace, but a few do interact with atoms of ordinary matter, spawning charged particles that generate flashes of light.
For years there have been hints of oscillations in neutrino experiments. For example, electron neutrinos produced by nuclear fusion within the Sun’s core have been detected for decades. But there are fewer than predicted by theories of how the Sun burns (“High noon for solar neutrinos”, 91av, 15 August 1992, p 28). Perhaps the neutrinos are turning into another type before reaching the detectors.
Meanwhile, a group at Los Alamos National Laboratory in New Mexico has been creating neutrinos using a particle accelerator and looking for them in a nearby detector. In 1996, they claimed that they had observed muon neutrinos changing into electron neutrinos. But a similar experiment by a British-German team at the Rutherford Appleton Laboratory near Oxford has not revealed any such effect leading to fierce controversy over the Los Alamos claims.
So it is the compelling new results from Japan that have finally forced physicists to recognise that neutrinos are ponderable matter. The Super-Kamiokande experiment is built around a cathedral-sized tank containing 50 000 tonnes of ultrapure water, buried deep beneath the Japanese Alps. It searches for electron and muon neutrinos generated by cosmic rays striking Earth’s atmosphere.
Those muon neutrinos that arrive from directly overhead reach the tank in roughly the expected numbers. But only about half the anticipated number reach the detector from the opposite side of the world. As they travel thousands of kilometres through the Earth, these muon neutrinos have plenty of time to change identity. As no excess electron neutrinos appear in the detector, the muon neutrinos must be converting into either tau neutrinos-which the detector cannot observe-or into a possible fourth variety. The conclusion that had eluded physicists for years is now inescapable: at least one type of neutrino has mass.
This is the first unmistakable crack in the grand edifice of the Standard Model, which has dominated physics for 20 years. But it is not a sign that the model must be torn down-on the contrary, it can be extended.
Physicists have long been speculating about what theory might lie beyond, developing grand unified theories, which put electromagnetism and the strong and weak nuclear forces on an equal footing. Many of these theories imply massive neutrinos.
In some of these theories, there is a new particle called the N. Each of the three neutrinos spends an imperceptibly small fraction of its time as this supermassive object, trillions of times heavier than any atom (if one of these guys smacked you in the face, you’d feel it). This double life gives each neutrino some mass-but only a tiny amount, because quantum fluctuations into such a heavy state are desperately rare. The lightness of the three familiar neutrinos reflects the heaviness of their corpulent cousin, who acts like a fat person on the opposite end of a see-saw, levering them high into the air. Physicists call this the see-saw mechanism.
Such ponderous particles often appear in grand unified theories that have a special feature called supersymmetry. These theories require the existence of a forest of supersymmetric particles, partners of the known elementary particles. Some of these, now being sought by physicists using particle accelerators, could make up much of the dark matter in the Universe-especially the part that seems to congregate around galaxies.
In other theories, there are things called sterile neutrinos, extremely light versions of the N that, by a quirk of physics, have managed to shed nearly all their flab. Sterile neutrinos would not feel even the weak force, so they would interact with ordinary matter only via gravity.
Mirror universe
They may even inhabit a universe almost totally divorced from ours, suggests theorist Rabindra Mohapatra of the University of Maryland, College Park. Certain versions of superstring theory, which aims to unify all the forces of nature including gravity, permit the existence of a kind of “mirror universe” that parted company with our own familiar cosmos a split second after the big bang, thereafter interacting with us only by gravity.
In 1995, Mohapatra and Zurab Berezhiani of the University of L’Aquila in Italy pointed out that such a mirror universe could have three sterile neutrinos- mirror images of the familiar trio. Clouds of sterile neutrinos could be hovering around visible galaxies, or forming separate congregations with their own kind. But because they respond only to gravity, detecting them directly may be impossible.
Whether or not sterile neutrinos exist, however, the fact that neutrinos have mass means that they must have had some influence on the distribution of galaxies we see in the Universe today. Every thimbleful of space today holds hundreds of neutrinos, part of a vast cloud left over from the big bang. In the early Universe, fast-moving massive neutrinos would have smoothed away, discouraging galaxies from forming and gravitating into clusters.
The heavier the neutrinos, the smoother the Universe. There are already hints that a heavy neutrino might be present: simulations of the Universe’s structure by Primack and his colleagues suggest neutrino masses of a few electronvolts. Tighter constraints on the neutrino mass, or even an actual measurement, will be provided by the high-precision galaxy surveys already under way, say astrophysicists Wayne Hu, Daniel Eisenstein and Max Tegmark of Princeton University. For example, if one kind of neutrino is as heavy as half an electronvolt, or a millionth the mass of an electron, these surveys should see its effect on the distribution of galaxies.
The new result from Super-Kamiokande does not support or confound these possibilities, because neutrino oscillations provide information only about the difference in mass between two kinds-the greater the difference, the faster they change from one to the other. By looking through the Earth at several different angles, Super-Kamiokande shows that muon neutrinos from the atmosphere need a few hundred kilometres to oscillate, which implies that the mass difference between the muon neutrino and its dance partner (tau or sterile) is small-only about five hundredths of an electronvolt.
How do the earlier hints of oscillations fit in? The solar neutrino deficit can be explained by oscillations of electron neutrinos into another type that those detectors don’t see. But it implies a mass difference at least ten times smaller than that seen by Super-Kamiokande for atmospheric neutrinos. Either the oscillation length is about equal to the Earth-Sun distance (if much smaller, the deficit would vary seasonally as that distance changes), or another type of mechanism is responsible-matter-induced oscillations within the Sun itself. Both possibilities imply a very small mass difference.
In contrast, the embattled Los Alamos experiment implies that muon neutrinos are substantially heavier than electron neutrinos, perhaps by as much as a few electronvolts. Such a hefty neutrino is exactly what simulators need to smooth out the Universe.
If the Los Alamos results are true, says physicist David Caldwell of the University of California, Santa Barbara, then an overall picture emerges that accounts for all the neutrino-oscillation experiments and the cosmological observations, including the scarcity of galaxy clusters. In this model (see Diagram) there are four different neutrinos: the muon and tau neutrinos with nearly identical masses of 2 to 3 electronvolts, and the electron neutrino and a sterile neutrino with masses that are very close to zero. Oscillations between the heavier pair give the Super-Kamiokande results, whereas transformations between the lighter pair explain the observed deficits of solar neutrinos. And conversions of muon neutrinos into electron neutrinos yield the effects claimed at Los Alamos. If all three effects are caused by neutrino oscillations, sterile neutrinos are a necessity.
But everything in that tidy portrait hinges on the disputed Los Alamos results. If they do not hold up, then sterile neutrinos are a luxury that physicists can do without. The solar and atmospheric deficits could then arise from oscillations among the three known neutrinos. In this case they must all be very light neutrinos, with masses below 0.1 electronvolts-as specified in see-saw models, which require the electron neutrino to be much lighter than the muon, which in turn is much lighter than the tau. Gone would be the enticing possibility that massive neutrinos influence the structure of the cosmos.
How can we decide between these two pictures of the Universe? The experiment at the Rutherford Appleton Laboratory will continue to test the Los Alamos result for another year or so. And a definitive experiment is being prepared at the Fermi National Accelerator Laboratory near Chicago. In a few years, we should know whether sterile neutrinos exist.
Other groups are joining the effort to pin down the mass measurement. This month, physicists at the National Laboratory for High Energy Physics near Tokyo will begin firing a beam of muon neutrinos at Super-Kamiokande, 250 kilometres away. These artificially created particles have energies similar to the cosmic-ray neutrinos hitting the same detector, but in this case the neutrino source will be precisely known. On the other hand, 250 kilometres might not be far enough for the neutrinos to oscillate.
Another long-baseline neutrino experiment will aim an intense neutrino beam from Fermilab near Chicago at a detector in the Soudan iron mine in northern Minnesota. With 730 kilometres for the muon neutrinos to oscillate, this will be sensitive to smaller masses than the Japanese experiment, says project leader Stanley Wojcicki of Stanford University. And because they will use neutrinos that are ten times more energetic, this group may even be able to see tau neutrinos appearing in the detector-if these are, in fact, what the muon neutrinos missing from Super-Kamiokande have been turning into.
Things aren’t quiet on the solar neutrino front, either. Last year, the Sudbury Neutrino Observatory in Ontario, Canada, filled its detector with 1000 tonnes of heavy water, in which deuterium atoms replace hydrogen. Unlike other solar experiments, this will also be able to detect muon and tau neutrinos, which can shatter a deuterium nucleus into its component proton and neutron. If electron neutrinos created in the Sun oscillate into either of these two species en route, they will be detected. “That will be the smoking gun for solar neutrinos,” says astrophysicist John Bahcall of the Institute for Advanced Study at Princeton. But any sterile neutrinos produced will remain invisible.
Meanwhile, astronomers are searching the heavens for other clues to the mass of neutrinos. The Sloan Digital Sky Survey based in New Mexico and the Two Degree Field in Australia will spend several years mapping the positions of millions of galaxies, providing by far the most accurate and comprehensive atlas of the visible Universe. By analysing just how lumpy this looks on different scales, and combining with precise measurements of other cosmological parameters, astronomers may be able to calculate the mass of the heaviest neutrino.
Neutrinos have finally given us a glimpse of physics that lies beyond the Standard Model. In just a few years time, that glimpse should become a panorama, revealing vastly more about neutrinos and about the fundamental theory that can encompass their strange behaviour.