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Giovanni Cantatore is feeling rather troubled. On the face of it, he shouldn鈥檛 be: his experimental results suggest that he and his colleagues have succeeded in creating dark matter, and, although this is the stuff that is thought to make up about 85% of all the matter in the universe, no one has ever managed to see so much as a particle of it before. Detecting it would be a major breakthrough; working out how to make it in the laboratory should put him and his colleagues in the running for a Nobel prize.

And yet Cantatore, who works at the Italian institute for nuclear physics in Trieste, is troubled. Why? Because there鈥檚 something about his team鈥檚 results that makes no sense.

Their dark matter particles 鈥� called axions 鈥� aren鈥檛 behaving as they should. They seem to be endowed with a property that means they should have sucked the life out of the sun billions of years ago. Plainly this has not happened, so what is going on? 鈥淚t is very disturbing,鈥� says Cantatore.

鈥淭he experiment could be the world鈥檚 first dark matter factory鈥�

Axions might have been created when photons collided in the early moments after the big bang. Their existence was first invoked by theorists needing a particle to patch up a flaw in the theory of the strong force that binds atomic nuclei together. But it soon turned out that axions could fill another hole: they are among the lightest particles around and yet there could be so many of them that they outweigh all the visible matter in the universe. That makes them a candidate for explaining the anomaly of the universe鈥檚 鈥渕issing mass鈥� (see 鈥淒ark horses鈥�). Cantatore鈥檚 laboratory, which is in Legnaro, about 150 kilometres from Trieste, could be the world鈥檚 first dark matter factory.

If, that is, the Italians can get to the bottom of their axions鈥� behaviour. It was Cantatore鈥檚 Trieste colleague Emilio Zavattini who came up with the idea behind the Legnaro experiment. Decades ago he suggested that axions could be created in the lab by firing a laser through a magnetic field. If photons really do interact to produce axions, laser photons that happen to be polarised parallel to the field have a small chance of being converted into axions. The axions will escape, but the loss of those converted photons twists the overall polarisation of the laser beam.

Cantatore鈥檚 team has been looking for this twist using a magnet that produces a field of 5 tesla, 100,000 times stronger than the Earth鈥檚 magnetic field. Last year they reported success. When a laser beam was bounced back and forth through the field 44,000 times, its polarisation had rotated by a few parts in a hundred million.

Instead of filling the researchers with joy, however, the result so unnerved them that they spent the next three years checking it before reporting it. That rotation, slight though it sounds, is altogether too much for comfort. It means that far more axions were being created than the theory predicts. The Italian group seems to have discovered a mutant axion. And it has some rather undesirable properties.

For a start, the characteristics of this new particle mean that it cannot cure the problems with the strong force. The axion was originally thought up to fix a problem with quantum chromodynamics (QCD), the theory that describes the behaviour of the quarks inside protons and neutrons, and of the gluons that stick them together. Without axions, certain reactions involving gluons would look different depending on whether time runs forwards or backwards, and that has never been observed in experiments. Yet the mutant axion has the wrong properties to fix this.

Worse, its effects should have shown up in other experiments. All sorts of observations 鈥� including our own existence 鈥� seem to show that this particle doesn鈥檛 exist. An axion that is formed so easily in the lab would not only have been formed soon after the big bang, but should also be produced in huge quantities as photons collide in the sun鈥檚 core. The axions would fly straight out of the sun and into deep space, which would have drained all the sun鈥檚 fusion energy after only a few thousand years. The same is true of every star: Cantatore鈥檚 mutant axion would make the universe a pretty dark and lifeless place.

Invisible light

So is Cantatore sure he has not simply made some experimental error? Not at all. 鈥淎t the moment we are only reporting an anomalous observation,鈥� he says. 鈥淵ou should keep in mind the possibility that it is an instrumental effect.鈥� Indeed, he says, there is a puzzling amount of variation in the observed rotation that the team cannot explain.

But the fact remains that the effect just won鈥檛 go away, whatever the researchers do. Recently, they tried swapping the old infrared laser beam for a green laser in case the wavelength of the light had something to do with it, but they are still seeing the twist in the laser beam鈥檚 polarisation. Though Cantatore shies away from claiming to have proved the existence of the particle, the effect certainly seems real enough.

Leslie Rosenberg of the University of Washington in Seattle, another physicist trying to detect dark matter axions, also thinks there could well be something in it. 鈥淎lthough it would be premature to throw your hands in the air and cry hallelujah, it would be foolish to ignore this result,鈥� he says. 鈥淚t really is a head-scratcher.鈥�

If the axion is real, then it must be a truly bizarre object, far beyond the bounds of the standard model of particle physics. 鈥淚t would be remarkable and a major discovery,鈥� says Massachusetts Institute of Technology physicist Frank Wilczek, who co-wrote the paper that first predicted the axion, naming the particle after a brand of detergent because it cleaned up the problem with QCD.

A few people have already tried to reconcile the Italian measurement with other evidence. One idea comes from Eduard Mass贸 and Javier Redondo of the University of Barcelona in Spain. They suggest that the particle detected at Legnaro could be made of two as yet unknown quark-like particles that are loosely bound together. This fragile composite could form easily in the cool environment of Cantatore鈥檚 lab, but not in the turbulent furnace of a stellar core or a supernova, so it would not suck out all their energy. 鈥淵ou need something very exotic, either our idea or something else,鈥� Mass贸 says. 鈥淚f everything is confirmed, this will be a little revolution.鈥� Mass贸鈥檚 composite axion is probably the nearest anyone has got to a solution so far, but it is rather messy. It needs not only the new constituent particles, but also a new fundamental force to bind them together, and all the properties of this composite creation have to be carefully chosen.

Even then the hypothesis may not work, according to Matthew Kleban of the Institute for Advanced Study in Princeton. 鈥淚nside the sun there would be a hot soup of all the new particles that compose the axion,鈥� he says. 鈥淎s you move out from the centre and the temperature falls, these particles will combine and form axions.鈥� He has calculated that more energy would be stolen by axions than comes out in photons, so the sun would still be snuffed out. Kleban has also tried devising theoretical models to fix the problem by making a readily forming axion that is star-safe, but all of his attempts have failed. 鈥淚 think it鈥檚 pretty difficult to come up with one that works,鈥� he says.

鈥淎 consistent theory would not be hard but it would be ugly鈥�

Others are slightly more hopeful. Particle physicist Ann Nelson of the University of Washington in Seattle is one 鈥� but she has a caveat. 鈥淚 don鈥檛 think it would be hard to come up with a consistent theory, but it would be ugly,鈥� she says.

Maybe physicists should accept that nature is ugly. But maybe not just yet 鈥� it is still entirely possible that Cantatore鈥檚 troubling measurement is a mistake. Adrian Melissinos of the University of Rochester in New York thinks it is far more likely that the Legnaro result is just a mirage, some flaw in the experimental set-up. 鈥淢y opinion is that it is absolutely wrong,鈥� he says.

Melissinos鈥檚 confidence comes from the fact that he worked with Cantatore on a similar experiment at Brookhaven National Laboratory in Upton, New York, in the 1980s. That experiment saw no sign of axions, despite being more sensitive than the set-up at Legnaro. However, it had one small disadvantage. The Brookhaven experiment was only suited to seeing axions with a mass of less than 0.8 millielectronvolts. Cantatore鈥檚 experiment is sensitive to axions a shade heavier, at 1 millielectronvolt. And the mass of the detected axions appears to be right on that upper threshold, out of reach of the Brookhaven team.

The best way to resolve the controversy and to discover whether mutant axions really are flying about the universe is an experiment called photon regeneration, also known by the rather Zen-like term of 鈥渋nvisible light shining through walls鈥�. The idea is to aim a laser like the one in Legnaro through a magnet at a solid wall and to put another magnet on the other side. The wall will stop the laser beam from passing through, unless some of its light is turned into axions, which should fly straight through the wall.

鈥淚f the Legnaro result is correct, there will be hundreds of millions of axion-like particles crossing the wall every second,鈥� says Paul Rabadan of the Institute for Advanced Study in Princeton. When they then pass through the magnetic field on the other side of the wall, a few of them should turn back into photons with exactly the same frequency as the original laser beam. 鈥淚t would be an irrefutable signal of the existence of this particle,鈥� says Rabadan.

Cantatore鈥檚 team is planning a photon regeneration experiment this year. An independent test is to be carried out at the DESY laboratory in Hamburg, Germany, by early 2007, and others are planned by CERN and by a collaboration between the universities of Toulouse and Lyon in France. If any of these labs find that invisible light does shine through walls, it will be a bombshell; physicists will be faced with trying to explain how these impossible particles can exist. They might find that the mutant axion leads to a fundamental change in our picture of reality, perhaps an unwelcome, ugly one. Still, at least the sun will carry on shining.