The world of neutrino physics has come a long way in the last 30 years. Once a ghostly afterthought of particle physicists, introduced to explain something that was missing rather than something that was there, neutrinos have proved to be every bit as fascinating as quarks, gluons and all the other fundamental particles. Indeed, they might even be able to explain one of the biggest puzzles in physics: where did the matter in the universe come from?

According to the Big Bang model, the universe began 13.7 billion years ago as a tiny region of pure energy that expanded and cooled to create the cosmos we see today. Among the many successes of this model, however, there is at least one glaring problem: the universe is dominated by matter and contains very little antimatter. The laws of physics do allow energy to be converted into matter, but require that almost equal quantities of antimatter are produced in the process.

It is now becoming clear that the answer to this puzzle could come from a very unexpected quarter: the behaviour of neutrinos. How we have come to this startling conclusion is a fascinating tale and, as is so often the case in science, the story begins with a completely different problem.

The birth of the neutrino

The concept of neutrinos dates back to the 1930s, when researchers noted that energy seemed to disappear when one atomic nucleus decayed into another nucleus plus an electron. Wolfgang Pauli hit upon a "desperate remedy" to explain the situation, proposing that the missing energy was being carried away by a third particle emitted in the decay. To the modern reader this might not seem particularly revolutionary, but in Pauli's day there were only two known particles - the electron and the proton - so introducing a third particle was radical in the extreme.

Understandably Pauli was initially reluctant to publish the idea, and even apologized afterwards for predicting a particle that he thought would be impossible to detect (if only modern theorists were similarly concerned!). This was because his "neutrinos", as Enrico Fermi christened them, had no electric charge and would interact only weakly with other matter. Happily, Pauli was proved wrong and lived to witness the Nobel-prize-winning detection of neutrinos in 1953 by the late Fred Reines and Clyde Cowan.

But the emerging field of particle physics did not, of course, end there. We went on to discover the positron (anti-electron), the pion, the muon and many other novel particles. Along with the muon came another neutrino, now called the muon neutrino, νμ, which was found to be distinct from the electron neutrino proposed by Pauli (νe). Then, after the discovery of the tau lepton in 1975, it was apparent that there was a third "flavour" of neutrino: the tau neutrino, ντ, which was finally detected in 2000 by the DONUT experiment at Fermilab in the US.

Meanwhile, the plethora of strongly interacting particles such as protons and pions was brought into order by the quark model. Combined with a handful of other particles that could explain the forces between elementary particles, this left us with a rather simple but extremely powerful picture of particle physics called the Standard Model. In this model, neutrinos were initially considered to be strictly massless and to interact only via the weak interaction though the exchange of the "Z" and the "W" particles. Reality, however, has turned out to be somewhat more complicated than this.

A problem with the Sun

In the 1960s, while other particle physicists were investigating all these newly discovered particles, Ray Davis at the Brookhaven National Laboratory in the US was pursuing the idea of using neutrinos as a probe. For decades astronomers had thought that the most likely power source for the Sun and other stars was thermonuclear fusion, but no direct proof was available. Davis believed he could observe the fusion reactions directly by detecting the neutrinos they produced.

In the basic fusion reactions in the Sun, four protons are converted into a helium-4 nucleus, emitting two positrons and two electron neutrinos in the process. These neutrinos have a wide range of energies and vast numbers of them escape from the Sun without interacting with anything, hurtling towards the Earth at close to the speed of light. But it is precisely this extremely low probability of interacting with matter that makes neutrinos so hard to detect.

Davis tackled this problem using a technique from radiochemistry proposed by Bruno Pontecorvo while at the Chalk River Laboratory, Canada, in 1946: assemble a large mass of some target atom that will very occasionally undergo a nuclear reaction triggered by a solar neutrino. Davis chose an isotope of chlorine as the target, which he managed to obtain at an acceptable cost in the form of 600,000 litres of cleaning fluid. Neutrinos from the Sun would then react with the chlorine to produce radioactive argon atoms, which could be "gathered up" and individually counted. Knowing the probability that a neutrino would trigger the reaction in the first place, it would then be possible to infer the solar-neutrino flux.

The first results of this audacious experiment were announced in 1968, and surprised almost everyone. Davis' team only detected about 30% of the neutrinos predicted by the best solar models available, namely those developed by John Bahcall and colleagues. At first, researchers were sceptical as to whether a few atoms of argon could really be seen in such a huge volume of liquid, but after rigorous tests it seemed the experiment was not at fault. Nevertheless, the real proof that the "solar-neutrino problem" was here to stay did not come until twenty years later, when the Kamiokande experiment in Japan confirmed Davis' results. Davis and Kamiokande spokesperson Masatoshi Koshiba of the University of Tokyo shared the 2002 Nobel Prize for Physics for their pioneering work in neutrino astrophysics.

The Kamiokande experiment consisted of several thousand tonnes of pure water in a tank deep underground, and was originally built to search for proton decay. However, its designers realized that the experiment might also be able to detect highly energetic neutrinos from the Sun that interact with electrons via scattering reactions. These electrons can travel faster than the local speed of light in the water, causing them to emit the optical equivalent of a sonic boom - a glow of blue light called Cerenkov radiation that can be detected by ultra-sensitive photomultiplier tubes around the tank.

In 1989 the Kamiokande team confirmed that the flux of neutrinos from the Sun was indeed much lower than expected. But experimental particle physicists take a lot of convincing, and there was still the possibility that the solar-neutrino problem arose not from the neutrinos but from the solar models themselves. This is because the neutrino flux measured by the Davis and the Kamiokande experiments was dominated by high-energy neutrinos from a small side reaction involving the decay of boron-8. The rate of this reaction depends critically on the core temperature of the Sun, so a small error in this temperature could explain the low neutrino fluxes seen in both experiments. We therefore had to confirm that all solar neutrinos were suppressed, not just those at high energies.

This required two new experiments called SAGE and GALLEX, which followed the basic idea of Davis' experiment except that they used gallium instead of chlorine as the target atom. Due to the more complex chemistry involved, these experiments were more difficult to perform, but in the early 1990s we eventually got the answer: the low-energy neutrinos were missing too. The problem did not lie with the solar models - it was something else.

Neutrino oscillations

If neither the solar models nor the experiments were at fault, then what was the source of the solar-neutrino problem? One solution, which was actually proposed by Pontecorvo the year before Davis had obtained his first results, was that neutrinos may change from one flavour to another on their journey from the Sun to the Earth (see box 1 below). Since the existing experiments were predominantly sensitive to electron neutrinos, rather than muon and tau neutrinos, this could explain why we only detected about a third of the solar neutrinos.

But there was one big problem with this neutrino-oscillation idea: it requires that neutrinos have mass, which they do not in the Standard Model. At the time this was an exciting prospect because it meant that neutrinos might explain the "dark matter" that is thought to dominate the universe. We now know that neutrino masses are too small to account for most of this strange, non-luminous substance (even so, there is roughly the same amount of mass in neutrinos as there is in all the visible matter in the universe). But these tiny neutrino masses are still of great interest because they might arise from some fundamentally different mechanism to the way the masses of other particles are generated - i.e. the Higgs mechanism.

The theory of neutrino oscillation contains a few underlying parameters: the masses of the three neutrino states ν1, ν2, and ν3 (see box 1 below) or rather the two independent differences between them, Δm122 and Δm232; three "mixing angles", θ12, θ23 and θ13; and a critical parameter called the phase, δ. Measuring this phase could be one of the keys to answering the riddle of why the universe contains more matter than antimatter. But before physicists could explore this possibility, we still had to determine whether neutrino oscillations were simply nice mathematics or real physics. In particular, we needed to measure the mass differences and mixing angles.

While proof of neutrino oscillations was being sought, a separate problem began to unfold in experiments searching for proton decay. Proton decay may or may not take place, but it is certainly very rare (the lifetime of the proton is predicted to be at least 1032 years). Experimenters have therefore had to worry about other processes that might hide or even mimic the decay of a proton in their detectors.

The largest source of such background events are neutrinos from cosmic rays, the high-energy particles that constantly bombard the Earth's atmosphere from sources in our galaxy and beyond. The debris of these collisions is dominated by pions, which decay into muons plus muon neutrinos in reactions such as π- → μ- + νbarμ, where the horizontal bar depicts a antineutrino. The muons themselves then decay into electrons and more neutrinos via the reaction μ- → e- + νbare + νμ.

This process should therefore produce two "atmospheric" muon-neutrino events for every electron-neutrino event. However, to the surprise of researchers working on a Kamiokande-like detector called the Irvine Michigan Brookhaven (IMB) experiment, and of the Kamiokande team itself, this ratio was not seen. Instead, the two experiments saw roughly the same number of both types of neutrino. As with the solar-neutrino problem, many physicists initially thought that this "atmospheric-neutrino anomaly" was simply due to a problem with the experiments, or possibly the models of atmospheric-neutrino generation. However, in 1998 a vastly larger version of Kamiokande called SuperKamiokande convinced almost everyone that the atmospheric anomaly must lie with the neutrinos themselves.

The breakthrough came because SuperK was able to compare the events that came "down" from the atmosphere with events that came "up" from below, and hence arose from interactions in the atmosphere on the other side of the planet (figure 1). The only significant difference between these two classes of neutrino is the distance they have travelled, but if neutrinos are massless this should make no difference.

However, for events coming from above, SuperK saw roughly the expected 2:1 ratio of muon to electron neutrinos, while for events coming from below it saw many fewer muon neutrinos. This was subsequently confirmed by the Soudan II and MACRO experiments, and demonstrated that nature really does satisfy the first condition for neutrino oscillations: that neutrinos have mass. But what about the second condition, that neutrinos change flavour?

Solving the solar-neutrino problem: SNO and KamLAND

Demonstrating that neutrinos can change flavour was the main purpose of the Sudbury Neutrino Observatory (SNO) in Canada, which was built by a large collaboration of Canadian, US and UK physicists (and which I have been a part of since 1988). SNO is a water Cerenkov detector like Kamiokande, but instead of using normal water it uses heavy water, D2O. The deuterons, D, in the heavy water are the most weakly bound of all nuclei, which gives SNO the chance to observe three different reactions induced by solar neutrinos.

The first of these processes is the charged-current reaction νe + D →  p + p + e-, which is detected by observing Cerenkov photons from the energetic recoil electron, e-. This reaction is only sensitive to electron neutrinos, which is good because these are the only type produced by nuclear reactions in the Sun's core. But what happens if these neutrinos oscillate on their way from the Sun to the Earth?