Figure 1. The Sudbury Neutrino Observatory in Canada, where physicists first found evidence of the missing solar neutrinos. Source: Sudbury Neutrino Observatory.

Jetting through your eyeballs at this instant are 60 million particles traveling near the speed of light. Most are beamed directly out of the center of the sun, but some are debris from black holes, pieces of long-gone supernovae, or ghosts of the Big Bang. They pass through you, your computer, the earth, and just about everything else as if none of it existed. They are neutrinos — elusive apparitions of the particle world — and, until recently, they cast enormous doubt on how well we understand our own Sun.

The neutrino is an energetic little particle with no charge and almost no mass, which allows it to travel at the very edge of the speed of light. Tentatively proposed in 1930 by Wolfgang Pauli, the idea of the neutrino did not gain acceptance until 1934, when Enrico Fermi illustrated its role in beta decay. According to Fermi's theory, when a neutron decays, it produces a proton, an electron, and a neutrino. This process happens hundreds of billions of times every second in the core of the sun as hydrogen is converted into heavier elements. Neutrinos from these reactions tear out of the sun by the billions, as side effects of fusion.

Detecting the Neutrino

Most neutrinos that reach Earth are produced in the deepest parts of the sun, so they provide solar physicists with information about how the sun’s core operates. This makes solar neutrinos extremely valuable to scientists and provides plenty of incentive for studying the elusive particles.

Unfortunately, neutrinos are notoriously difficult to observe. A neutrino has no electric charge and is so small and fast that it could travel through hundreds of thousands of light years of solid lead without touching a single atom. Nevertheless, with enough material and enough time, physicists are able to detect them. Today, about a half a dozen countries host neutrino observatories, which consist of enormous tanks of liquid, usually buried deep within the Earth. If enough neutrinos flow through a large enough fluid-filled tank, an occasional one will strike an atom in that fluid, prompting a decay process that can be detected by physicists.

Homestake, the first of the neutrino observatories, was constructed in 1967, and consisted of a 100,000-gallon tank of chlorine-37 under South Dakota. Homestake was built under the assumption that a neutrino would collide with one of the chlorine atoms in the underground tank about once a day, causing the atom to decay into the isotope argon-37, which a detector would report. This assumption was based on sound theory: In the early 1960s, the physicist John Bahcall had calculated that, based on our understanding of solar physics, 30 million neutrinos should pass through every cubic inch of earth every second. If solar physicists had the correct models for the interior of the sun, and if Homestake worked as it was designed to, then one neutrino encounter should be recorded every day in the chlorine tank beneath South Dakota.

To the shock of theorists and experimentalists alike, Homestake detected one neutrino encounter every three days. Somehow, two-thirds of the solar neutrinos were missing.

Figure 2. A view of the giant heavy water tank at the Sudbury Neutrino Observatory in Canada. Source: Sudbury Neutrino Observatory.

Searching for the Neutrino

A 33-year battle ensued over the missing neutrinos. Stellar physicists insisted that their models of the sun’s interior were accurate and that the experimentalists had constructed a poor observatory. Experimentalists insisted that the reason they were not finding the “missing neutrinos” was because there were no missing neutrinos — the theorists had simply miscalculated. As data from
helioseismology (a technique for studying the interior of the sun) came in and our understanding of the inner workings of the sun continued to improve, it became increasingly clear that the problem of the missing neutrinos did not lie in the theory. The neutrinos should be there.

Larger neutrino observatories were constructed, using different methods to detect the elusive particles. Over the course of three decades, governments funded and physicists built five new observatories, including the Super Kamiokande (Super-K) in Japan and the Sudbury Neutrino Observatory (SNO) in Ontario. The results were always disappointing — still two-thirds short.

In 2001, scientists at SNO decided that perhaps they were looking for the wrong type of neutrino. Since the mid-1970s, researchers have known that the neutrino comes in three “flavors,” depending on how it forms. Electron-neutrinos are produced during beta decay; the muon-neutrino is emitted when a pion decays into a muon and a neutrino (all three of which are subatomic particles); and a tau-neutrino is formed when a neutron decays into a tau and a neutrino. Muon- and tau-neutrinos usually require extremely high-energy events in order to form: matter falling into a black hole, for example, or a supernova explosion. The sun produces only electron-neutrinos, so the observatories built to solve the missing neutrino problem were designed to look for only electron-neutrinos. The SNO Collaboration, which consists of more than 150 scientists looking for neutrinos, decided to look for the other two types as well.

Since the SNO can only detect electron-neutrinos, SNO researchers combined their data with those from the Super-K in Japan. SNO’s tank is filled with heavy water, which is water that has had its hydrogen atoms replaced by deuterium, a hydrogen atom with an extra neutron. At the SNO, an electron-neutrino collides with a deuterium atom in the heavy water about 10 times a day, and the atom will be ripped into a proton and neutrons, which are then detected by the observatory. Muon- and tau-neutrinos are unable to break up a deuterium atom, so they pass right through. The Super-K observatory, however, uses regular water, and detects electron-neutrinos by their collisions with electrons. Occasionally, muon- and tau-neutrinos will also bounce off electrons, giving the observatory a false detection. If Super-K had been detecting muon-, tau-, and electron-neutrinos, SNO scientists reasoned, then the number of neutrinos detected by Super-K should be just slightly larger than the number detected by SNO.

They were right—Super-K was recording more neutrino encounters than SNO.

Old Problem Solved — Bring on the New Ones!

On June 18, 2001, the SNO Collaboration announced that it had found the “missing neutrinos” — physicists had just been looking for electron-neutrinos, and had overlooked the other two flavors. Vindicated theorists were pleased to see that the number of neutrinos now detected were 35 million per cubic inch per second — just as they had predicted. As a result of their leadership in the discovery, physicists Raymond Davis and Masatoshi Koshiba were awarded the Nobel Prize in Physics in 2002.

But the problem still remains: why the muon and tau flavors? The sun produces only electron-neutrinos; however, fully two-thirds of the neutrinos from the sun that encounter Earth are of the muon and tau varieties. Researchers concluded that something is turning electron-neutrinos into muon- and tau-neutrinos during their flights to Earth.

Fortunately for theorists, transforming an electron-neutrino into another flavor is possible. Two decades ago, researchers discovered that neutrinos can undergo oscillations, changing their flavors to and from electron, tau, or muon types.

However, in order to be able to dance among the different flavors, a neutrino must have some tiny amount of mass, and each flavor of neutrino must have a slightly different mass. Figuring out how an electron-neutrino at the center of the sun changes into a muon- or tau-neutrino in a tank under Canada is now the challenge facing solar and particle physicists.

Theories abound on how this transformation occurs. Some of the neutrinos produced in the sun’s core could possibly encounter material in the sun's outer layers that could convert them to muon- or tau-neutrinos; however, this mechanism would produce only a small percentage of the required muon- and tau-neutrinos. Others have suggested that perhaps a fourth neutrino is causing the confusion, or our understanding of the standard model in particle physics is flawed.

The problem is further complicated by news from Stanford University and NASA's Ames Research Center researchers, who, in 2002, found that the flux of solar neutrinos varies as the sun rotates. This has led some scientists to think that perhaps the rotation of the solar magnetic field is responsible for the neutrino oscillations. The team of NASA and Stanford scientists hypothesized that the neutrino has a tiny bit of magnetic moment, which, when encountering a monumentally large magnetic field, could change its flavor. Another theory suggests that the neutrino flux and oscillation problems might be related in some way to the sun's 11-year cycle, made famous by sunspots.

Theories attempting to solve the problem are now just as abundant as theories attempting to explain the “missing neutrino” problem that had troubled physicists for three decades. Seventy years from their discovery, questions on the nature of neutrinos continue to plague researchers. Once thought to be too elusive for accurate measurements, the neutrino has proven to be a more complex particle, switching personalities with ease. The invisible ghost particle, the most abundant elementary particle in the universe, still has mysteries to reveal.