THE unknown pervades the universe. That which people can see, with the aid of various sorts of telescope, accounts for just 4% of the total mass. The rest, however, must exist. Without it, galaxies would not survive and the universe would not be gently expanding, as witnessed by astronomers. What exactly constitutes this dark matter and dark energy remains mysterious, but physicists have recently uncovered some more clues, about the former, at least.
One possible explanation for dark matter is a group of subatomic particles called neutrinos. These objects are so difficult to catch that a screen made of lead a light-year thick would stop only half the neutrinos beamed at it from getting through. Yet neutrinos are thought to be the most abundant particles in the universe. Some ten thousand trillion trillion—most of them produced by nuclear reactions in the sun—reach Earth every second. All but a handful pass straight through the planet as if it wasn't there.
According to the Standard Model, the most successful description of particle physics to date, neutrinos come in three varieties, called “flavours”. These are known as electron neutrinos, tau neutrinos and muon neutrinos. Again, according to the Standard Model, they are point-like, electrically neutral and massless. But in recent years, this view has been challenged, as physicists realised that neutrinos might have mass.
The first strong evidence came in 1998, when researchers at an experiment called SuperKamiokande, based at Kamioka, in Japan, showed that muon neutrinos produced by cosmic rays hitting the upper atmosphere had gone missing by the time they should have reached an underground detector. SuperKamiokande's operators suspect that the missing muon neutrinos had changed flavour, becoming electron neutrinos or—more likely—tau neutrinos. Theory suggests that this process, called oscillation, can happen only if neutrinos have mass.
Since then, there have been other reports of oscillation. Results from the Sudbury Neutrino Observatory in Canada suggest that electron neutrinos produced by nuclear reactions in the sun change into either muon or tau neutrinos on their journey to Earth. Two other Japanese experiments, one conducted at Kamioka and one involving the KEK particle-accelerator laboratory in Tsukuba, near Tokyo, also hint at oscillation.
Last week, researchers working on the MINOS experiment at Fermilab, near Chicago, confirmed these results. Over the coming months and years, they hope to produce the most accurate measurements yet. The researchers created a beam of muon neutrinos by firing an intense stream of protons into a block of carbon. On the other side of the target sat a particle detector that monitored the number of muon neutrinos leaving the Fermilab site. The neutrinos then travelled 750km (450 miles) through the Earth to a detector in a former iron mine in Soudan, Minnesota.
By comparing how many muon neutrinos arrived there with the number generated, Fermilab's researchers were able to confirm that a significant number of muon neutrinos had disappeared—that is, they had changed flavour. Thus the neutrino does, indeed, have mass and a more accurate number can be put on it.
That number is tiny—0.00001% of the mass of an electron. But it is significant because neutrinos are so plentiful. While their mass is so small that neutrinos cannot be the sole constituent of dark matter, they have an advantage in that they are at least known to exist.
The same cannot be said for sure of another possible form of dark matter being studied by a group of physicists in Italy. In a recent issue of Physical Review Letters, Emilio Zavattini and his colleagues at the National Institute of Nuclear Physics in Legnaro report an unusual signal in an experiment that goes by the unwieldy name of PVLAS. Like the good, sceptical scientists they are, the team has spent the past two years trying to explain the signal away—for example, as an artefact produced by the instruments. So far, however, they have failed. If the result continues to withstand scrutiny, it would appear to be evidence for an exotic new sort of fundamental particle, known as an axion, that could also be a type of dark matter.
The experiment itself is simple. The team sends a laser beam through a vacuum that sits in the centre of a powerful magnet. The laser light is polarised, meaning that it vibrates more in one direction than the other (for instance, from side-to-side rather than up-and-down). When the light emerges from the other side of the magnet, the team measures its polarisation to see what has happened.
According to the Standard Model, the answer should be very little, for the light has simply passed through empty space. Instead, Dr Zavattini and his colleagues found that the direction in which the emerging light vibrates is rotated ever so slightly from its original alignment. The effect is so small—and the measurement so precise—that a similar rotation in the minute hand of a clock would represent a billionth of a second.
Small though it is, this signal may be evidence for a brand new type of particle. Light itself is made up of particles called photons. The magnetic field in the experiment is composed of photons too, though unlike those of light, the photons of a magnetic field are continually flickering into and out of existence. If the signal Dr Zavattini has found is not an artefact, then its most likely explanation is that photons from the laser are interacting with the photons of the magnetic field in a way that produces axions.
Life is never quite so simple, though. A particle with the properties that the PVLAS experiment may have observed contradicts several astrophysical experiments. Since axions can be produced from light, the sun and other stars should generate them copiously. Unfortunately, nobody has seen such particles directly. And several other experiments looking for dark-matter axions have failed to observe them.
To help resolve the issue, Raul Rabadan of the Institute for Advanced Study in Princeton and his colleagues propose, in the same issue of Physical Review Letters, an independent test that could begin as soon as the end of this year. They suggest shining a bright beam of X-rays through a magnet, into a thick wall of material that is opaque to X-rays, and then through another magnet. Since the wall is opaque, it should block the x-rays completely.
However, x-rays are high-energy photons, so if PVLAS is producing axions, some of these photons should also turn into axions along the way. Unlike photons, axions would pass directly through the wall to the other side. Once there, they could change back into detectable x-ray photons by the reverse of the process that generated them in the first place (hence the need for the second magnet). And, although such an experiment would not be cheap, it would not require a machine of Fermilab proportions to carry it out.
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