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Why is antimatter absent? Hunt heats up

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6:01pm, April 7, 2003
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Scientists searching for the Achilles' heel that wiped out antimatter have taken a big step, but they're not yet certain where they're headed.

The researchers want to know why the universe today appears to contain almost no antimatter, although presumably, in the Big Bang, both matter and antimatter formed in equal quantities.

Studies of the decay of particles and antiparticles known as B mesons promise vital clues. So, scientists in Japan and California have built electronpositron colliders, called B factories, for making B mesons and exploring their breakdowns (SN: 5/29/99, p. 342).

Since the 1960s, physicists have known that physical laws occasionally operate slightly differently for matter than for antimatter, an asymmetry dubbed the charge-parity, or CP, violation. In the early universe, CP violation may have made some antiparticles disintegrate a little more readily than their matter counterparts, creating a tiny excess of matter. In this scenario, because matter and antimatter annihilate each other, nearly all antimatter would eventually vanish, leaving just enough matter to make up essentially everything in the universe today.

On Monday, at a meeting in Osaka, Japan, the B-factory teams unveiled their first scientific results. Their findings indicate that they're quickly closing in on a measurement of CP violation among B mesons. In the past year alone, each of the two machines has come up to speed and generated about 10 million B decays, the scientists reported. That's as many as the formerly most prolific B-making machine, an accelerator at Cornell University, produced in a decade.

"The performance has been extraordinary," says David G. Hitlin of the Stanford (Calif.) Linear Accelerator Center (SLAC), site of the B factory in California. The Japanese machine's team expects to have enough data "very soon" to better discern the effect of CP violation on B mesons, adds Hirotaka Sugawara, director general of the KEK High Energy Accelerator Research Organization in Tsukuba.

At the 30th International Conference on High Energy Physics, the two teams presented preliminary figures for a parameter known as sin2b or sin2j1, which can have values between -1 and +1. Those extremes represent maximum CP violation, whereas 0 implies none. Physicists don't expect a negative result, which would mean that antimatter should have won the cosmic demolition derby.

Pinning down this parameter "is a high-stakes game," Hitlin notes. After all, the result may challenge the prevailing standard model of particle physics, which calls for sin2b to be roughly 0.7.

In contrast, SLAC's new value of sin2b, based on 120 events, is 0.12. With 98 events, the KEK group found 0.45. Last year, a group at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., announced a value of 0.79, derived from 400 events (SN: 2/20/99, p. 118).

Given that all the sin2b figures so far are greater than 0, there's "a reasonably high chance that we've already observed CP violation in the B system," comments Fermilab team member Joel G. Heinrich of the University of Pennsylvania in Philadelphia.

He and other B-meson specialists caution, however, that large, overlapping uncertainties spread the three measurements over a wide range that includes 0. So, researchers can't yet determine how much, if any, CP violation occurs.

An ultimate answer of 0, which the new SLAC result falls near, would be "staggering," says SLAC director Jonathan M. Dorfan. It would have "major implications" for why the known universe is here, he adds. He predicts the SLAC team will nail down "a more precise number" by year-end.

Planned upgrades to both B factories may boost their collision rates more than fivefold. A retrofit of the Fermilab machine is expected to up its rate by a factor of 20, Heinrich says.

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