My Top 10 Science Stories of 2006 – #7: Evidence for the Axion



Dark Matter near Trieste: Too Much of a Good Thing?

La propretĂ© integrale. By the way, shouldn’t this be “Axion Totale”?

By Davide Castelvecchi

At a lab near Trieste — a town also known for the best espresso in the world — physicists may have discovered the axion, a new particle that could help solve the mystery of the universe’s missing mass. The physicists themselves are still cautious about their findings; meanwhile, they are working on a follow-up experiment that will attempt to use axions to shine light through walls.

The particle was first hypothesized in the late seventies by Helen Quinn and Roberto Peccei. Frank Wilczek, the physics Nobel prize winner, later named it the axion in reference to a now-defunct brand of laundry detergent. “I saw it in American stores in the early seventies, or even earlier. I even have a framed box, courtesy of my wife,” Wilczek told me in an email. (The brand seems to have lived on for a while longer in France; in that case, the stress was presumably on the “o”, as an alternative spelling of the French word “action.”)

Wilczek’s idea was that the axion would wash out all the stains in the so-called standard model of particle physics — all the small inconsistencies in what has otherwise become the exemplar for what a successful scientific theory should be. Axions are now considered as a potential component of the dark matter, the stuff that’s much more abundant than ordinary matter but eludes detection as it flies around our galaxy and other galaxies as well.

Giovanni Cantatore and Guido Zavattini, of the Italian National Institute of Nuclear Physics, calculated that if axions exist, they should be easy to make by beaming a laser through an intense magnetic field. If the laser is polarized, the emission of axions — which otherwise zip through ordinary matter and disappear, undetected — would then leave its mark by slightly changing the polarization.

Over several years, the Italians’ team has indeed observed a tiny shift in their laser’s polarization. (In their results, they may have been helped by an unusual talisman. “We kept all along a colorful box of ‘Axion’ sitting on top of the magnets,” Cantatore says, referring to their lab machinery.)

Trouble is, the shift in the polarization was not tiny enough. It was too large to be consistent with other experiments and also with astrophysical data. If confirmed, the data they finally made public this year could upset physics as we know it and create more questions than it has answered.

The best way to resolve the issue, scientists have said, would be an experiment called “photon regeneration,” also known as shining light through a wall. The idea is that axions can go through ordinary matter, even when light can’t. So if some of the photons turn into axions, a small beam of these will carry on going through a wall even if the laser beam is stopped. But another magnetic field placed on the other side could perform the opposite trick and turn some of the axions back into photons.

The end result would be that — in the presence of strong magnetic fields — one could point a laser at a wall and essentially see it go through, though weakened. The Italians are now working hard on this new kind of experiment, Cantatore says, and plan to start collecting data in early 2007. “Stay tuned,” he says.

For now, the nature of the dark matter remains a mystery, though this year astronomers have found the most convincing proof yet of its existence — read on to the rest of this Top 10 if you want to know more. As for Axion, the brand of laundry detergent, “Maybe they’ll revive it once the particle is discovered,” Wilczek says. “I can see the ad campaign: ‘Makes your clothes as clean as dark matter.’ On second thought, maybe that needs a little work.”

Update February 12, 2007: Some of the discrepancies with older experiments may have been resolved. See article on PhysicsWeb

Update October 20, 2007: No light shining through a wall

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