New Scientist 29 Aprile 2009

L’antimateria: forse non esiste. Altra falsa scienza?

If you were to list the imperfections of the standard model - physicists' remarkably successful description of matter and its interactions - pretty high up would have to be its prediction that we don't exist.

According to the theory, matter and antimatter were created in equal amounts at the big bang. By rights, they should have annihilated each other totally in the first second or so of the universe's existence. The cosmos should be full of light and little else.

And yet here we are. So too are planets, stars and galaxies; all, as far as we can see, made exclusively out of matter. Reality 1, theory 0.

There are two plausible solutions to this existential mystery. First, there might be some subtle difference in the physics of matter and antimatter that left the early universe with a surplus of matter. While theory predicts that the antimatter world is a perfect reflection of our own, experiments have already found suspicious scratches in the mirror. In 1998, CERN experiments showed that one particular exotic particle, the kaon, turned into its antiparticle slightly more often than the reverse happened, creating a tiny imbalance between the two.

That lead was followed up by experiments at accelerators in California and Japan, which in 2001 uncovered a similar, more pronounced asymmetry among heavier cousins of the kaons known as B mesons. Once the LHC at CERN is back up and running later this year, its LHCb experiment will use a 4500-tonne detector to spy out billions of B mesons and pin down their secrets more exactly.

But LHCb won't necessarily provide the final word on where all that antimatter went. "The effects seem too small to explain the large-scale asymmetry," says Frank Close, a particle physicist at the University of Oxford.

The second plausible answer to the matter mystery is that annihilation was not total in those first few seconds: somehow, matter and antimatter managed to escape each other's fatal grasp. Somewhere out there, in some mirror region of the cosmos, antimatter is lurking and has coalesced into anti-stars, anti-galaxies and maybe even anti-life.

"It's not such a daft idea," says Close. When a hot magnet cools, he points out, individual atoms can force their neighbours to align with magnetic fields, creating domains of magnetism pointing in different directions. A similar thing could have happened as the universe cooled after the big bang. "You might initially have a little extra matter over here and a little extra antimatter somewhere else," he says. Those small differences could expand into large separate regions over time.

These antimatter domains, if they exist, are certainly not nearby. Annihilation at the borders between areas of stars and anti-stars would produce an unmistakable signature of high-energy gamma rays. If a whole anti-galaxy were to collide with a regular galaxy, the resulting annihilation would be of unimaginably colossal proportions. We haven't seen any such sign, but then again there's a lot of universe that we haven't looked at yet - and whole regions of it that are too far away ever to see.

Finding anti-helium or other anti-atoms heavier than hydrogen would be concrete evidence for an anti-cosmos. It would imply that anti-stars are cooking up anti-atoms through nuclear fusion, just as regular stars fuse normal atoms. The Alpha Magnetic Spectrometer is a $1.5 billion piece of kit built to scour cosmic rays for just such signs. It is grounded at the moment, waiting for a lift up to the International Space Station, but will hopefully hitch a ride on one of NASA's final space shuttle launches in 2010 or 2011.

Source >  New Scientist | Apr 27