Reluctant heroes
07 December 2002 by Frank Close
BY THE time you have read this sentence, over a trillion neutrinos will have passed through your body. These particles are one of the most pervasive forms of matter in the Universe: they are created in the Sun and in supernovas, by cosmic rays crashing into the upper atmosphere, and they are even made on Earth, streaming out from nuclear reactors and radioactive rocks.
Yet neutrinos are also the most elusive type of matter. Most of them pass straight through you without noticing a single atom of your body. Indeed they pass through the Earth and the stars virtually unhindered, taking with them all the information they carry.
And that information, it seems, is something that physicists would love to get their hands on. In the past decade, researchers have found growing evidence that neutrinos misbehave in ways that contradict our best theories of how the Universe works. These findings are forcing physicists to abandon their most cherished ideas about neutrinos: that they have no mass and can travel through space at the speed of light. And the alternatives are raising profound new questions about the nature of matter and even the structure and future of the entire Universe.
Serious doubts about the nature of neutrinos first emerged from studies of the Sun. Solar physicists have a deep understanding of our nearest star. They know that the fusion processes at its heart produce electron neutrinos - uncharged relatives of the electron, and one of the three known types of neutrino. The other two types are the muon neutrino and the tau neutrino, which are related to the muon and the tau particle respectively. From the Sun's temperature and other properties, they calculated the number of electron neutrinos it must emit and the number that should arrive on Earth. But experiments that had been recording electron neutrino levels for over 20 years only found between one-third and half as many as physicists expected. This enigma became known as the solar neutrino problem.
Inspired by these and other interesting findings, Masatoshi Koshiba at Tokyo University decided in the 1980s to design an experiment specifically to study neutrinos from the Sun. This work culminated in the mid-1990s with the start-up of the SuperKamiokande detector, located in a deep mine under the Japanese Alps. Neutrinos interact so rarely with matter that it takes something special to detect them, and SuperKamiokande is certainly remarkable. The heart of the detector is a huge tank filled with 50,000 tonnes of ultra-pure water. On the rare occasions when a neutrino hits a nucleus in the water, it converts to its charged particle partner - an electron if it's an electron neutrino, say - which goes on to produce a flash of light.
Watching for these tiny flashes are over 13,000 highly sensitive light detectors surrounding the tank of water. It would be great if every flash of light the detector picked up represented a neutrino, but in reality, most of the flashes are caused by particles emitted by radioactive rocks or by cosmic rays from outer space zipping through the water. And so a major part of the experiment involves sorting the needles from the haystack.
Before an accident last November that destroyed much of the detector and forced it to close temporarily, SuperKamiokande had picked up thousands of electron neutrinos from the Sun - more than any previous experiment. But still, these neutrinos amounted to less than half the number predicted by theorists. And the mystery deepened when particle physicists looked at neutrinos produced by cosmic rays.
High-energy cosmic rays hitting the atmosphere produce cascades of other particles. These eventually lead to a shower of neutrinos at ground level and even underground. Theory says there should be twice as many muon neutrinos as electron neutrinos in these showers. However, SuperKamiokande and other experiments found roughly equal numbers of electron and muon neutrinos.
So data from cosmic rays suggested that muon neutrinos were disappearing, and data from the Sun hinted that electron neutrinos were disappearing. Suspicion grew that something was happening to the neutrinos as they travelled. This was reinforced in 1999 when particle physicists at the KEK laboratory in Tsukuba used the lab's accelerator to create muon neutrinos and fire them towards the SuperKamiokande detector 250 kilometres away. The detector caught only a tiny fraction of the emitted neutrinos, but even so, the results showed a shortfall in the number of muon neutrinos detected and backed up the evidence from cosmic rays (New Scientist, 13 March 1999, p 32).
All these discoveries pointed towards the idea that neutrinos could change their identity as they travel through space. What starts out as a muon neutrino could somehow be transforming or "oscillating" into a tau or an electron neutrino over large distances. Such neutrino oscillations could also explain the shortfall of electron neutrinos from the Sun. If the three varieties of neutrino really do oscillate from one type to another during their flight, then it's possible that only one-third of them would be electron neutrinos by the time they reached Earth - as SuperKamiokande and other experiments had already found.
Although neutrino oscillations didn't fit well with some theorists' expectations, data from SuperKamiokande persuaded most sceptics that neutrino oscillations were real. Physicists hoped for detailed measurements of this process using the neutrinos produced at KEK. But the accident struck just as things were getting interesting, and it will be at least another year before the experiments restart.
There was a limitation to these experiments anyway: they were mainly sensitive to the electron neutrinos, so you wouldn't know for sure if the "missing" electron neutrinos had in fact simply changed into the muon or tau varieties. But earlier this year, all the uncertainty was washed away by dramatic results from a detector in Canada. Scientists at the Sudbury Neutrino Observatory (SNO) in Ontario are now able to unmask all the neutrinos that arrive from the Sun.
A pinch of salt
The SNO detector is similar to SuperKamiokande: it's a tank filled with 1000 tonnes of water and surrounded by nearly 10,000 light sensors. But there's a trick: SNO uses ultra-pure "heavy" water for part of the time; for the rest of the time it uses heavy water with a sprinkling of salt dissolved in it.
In heavy water, each hydrogen atom is replaced by the isotope deuterium, which has a proton and a neutron in its nucleus instead of just a proton. It's the proton and neutron acting in concert that helps to expose all three varieties of neutrino.
Electron neutrinos alone are revealed in the ultra-pure water when they turn into an electron, producing a flash of light (see Graphic). Meanwhile all three types of neutrino are revealed when heavy water with salt is used. They can all cause a deuterium nucleus to disintegrate when they strike it. The neutron liberated in this reaction can then be captured by a chlorine nucleus in the salt, giving off gamma rays in the process. It's these gamma rays that interact with electrons in the water to eventually produce telltale flashes of light. So although the detector can't tell the three types of neutrino apart, at least it can see them all. By subtracting the number of electron neutrinos from the total number found, researchers can work out the fraction of muon and tau neutrinos.
At a detection rate of one neutrino per hour, it took four years for SNO to get the first meaningful results. But now the collaboration has enough data to be sure: electron neutrinos only make up one-third of the total. This shows unambiguously that some of the electron neutrinos emitted by the Sun change into muon or tau neutrinos before they reach Earth. SNO researchers found that the total number of neutrinos, including muon and tau varieties, agrees with the predictions of the latest sophisticated models of the solar core. For the first time we have direct evidence that the Sun, and stars like it, are powered by thermonuclear fusion.
Solving the solar neutrino problem qualifies as one of the great moments in experimental science. And it clinched a share in this year's Nobel Prize for Physics for Ray Davis of the University of Pennsylvania and Masatoshi Koshiba of Tokyo University, who pioneered experiments to catch neutrinos. But with the breakthrough another enigma has emerged: neutrino oscillations can only happen if neutrinos - long thought to be massless particles - have different masses. This has major implications.
According to the standard model, which is our best description of all the matter and forces in the Universe, neutrinos are always "left-handed". Figuratively speaking, this means that if you think of a neutrino as a spinning particle, then it spins anticlockwise along its direction of motion, the opposite to the way a corkscrew moves into a cork. This has been backed up by detailed experiments: no one has ever detected a neutrino spinning clockwise along its direction of motion. Neutrinos and their antiparticle partners, antineutrinos, are unique in this respect - all the other matter particles come in both left and right-handed forms.
Theory says that a left-handed particle changes into a right-handed one when it interacts with the Higgs boson, the hypothetical particle thought to give other particles their masses. But right-handed neutrinos don't exist in the standard model, so this can't happen and neutrinos remain forever massless.
Now that neutrinos do appear to have mass, theorists believe they offer a first glimpse into a world beyond the standard model. Neutrinos may turn out to be just like other particles after all and have a right-handed partner that has so far escaped detection. This would imply that right-handed neutrinos interact only very rarely with matter, including the Higgs boson. Certainly they would interact even less than their left-handed relatives. This imbalance between left and right-handed particles is troubling. But there may be a way out.
Back in 1998, theorists suggested that right-handed neutrinos might move in dimensions other than the four dimensions of space-time that we live in. Extra dimensions arise naturally from string theory, the best candidate physicists have so far for a theory of everything. In these theories, the three space dimensions that we know are embedded in a 10 or 11-dimensional space-time. This raises the tantalising idea that we, and all the particles in the standard model, are trapped in three dimensions of a higher-dimensional world. In contrast, the right-handed neutrinos might be free to roam around the multidimensional Universe at will. This could explain why we have never seen right-handed neutrinos.
Quantum see-saw
Other theorists believe that the SNO and SuperKamiokande results point to the possibility that extremely heavy right-handed neutrinos exist without interacting with the Higgs particle. Quantum mechanics says that each of the three familiar neutrinos could spend a tiny yet significant fraction of their time in the supermassive right-handed state. The neutrino could teeter between an ultra-light left-handed form and a supermassive right-handed one. In the trade, this is known as the see-saw mechanism. The quantum fluctuations that tip the balance and take an ultra-light neutrino into a supermassive state are very rare and fleeting, and it's this that explains the neutrino's triflingly small mass.
The vast disparity in mass between the ultra-light and the supermassive neutrinos in this theory has intriguing implications. If these supermassive right-handed neutrinos really do exist, they could play a vital role in cosmology. With so many neutrinos present in the Universe, the mutual gravity of supermassive neutrinos could be large. So large, in fact, that they may have helped to seed the formation of galaxies. Massive neutrinos may even be part of the mysterious "dark matter" that seems to pervade our Universe.
The strange world of neutrinos appears to be our gateway to physics beyond the standard model. It is finally beginning to yield its secrets, and particle physicists are gearing up to uncover even more. Next year they will start firing muon neutrinos produced by an accelerator at Fermilab near Chicago towards a detector over 700 kilometres away in the Soudan mine in Minnesota. Meanwhile another group is planning to direct a neutrino beam a similar distance from the CERN nuclear research lab in Switzerland to the Gran Sasso Laboratory in Italy. And most daring of all are plans to build a neutrino generator that will churn out staggering numbers of these particles and send them to labs on other side of the Earth (see "Made on Earth"). Neutrino physics is about to enter a golden age.
Frank Close is based at Exeter College, University of Oxford, and is professor of astronomy at Gresham College in London. He is co-author of The Particle Odyssey, published by Oxford University Press
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