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Heavy finding

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12:38pm, November 20, 2006
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The most massive subatomic cousins of protons and neutrons ever detected have made fleeting appearances in a U.S. particle accelerator.

The weightiest parts of the particles—known as sigma-b baryons—are called bottom quarks, one of the six types of quarks that are fundamental constituents of matter.

Physicists at Fermi National Accelerator Laboratory in Batavia, Ill., announced the exotic particles' discovery at the lab on Oct. 23. The scientists found telltale traces of the sigma-bs in the debris produced by more than 100 trillion collisions of protons and antiprotons that had occurred in the accelerator in the past 5 years.

"You think, 'What kinds of jewels can you build out of these quarks?'" says physicist Jacobo Konigsberg of the University of Florida in Gainesville, a spokesman for the discovery team. When the team examined their data, the researchers realized that the sought-after quark combinations had shown up.

Ordinary matter, such as protons and neutrons, contains only up quarks and down quarks, whereas the sigma-bs each contain a bottom quark and either a pair of up quarks or a pair of down quarks.

Once abundant in the Big Bang, sigma-bs—and the bottom quarks that make them special—show up today only in high-energy events such as particle collisions, notes Fermilab physicist Rob Roser, also a spokesman for the sigma-b discoverers.

Sigma-b particles weigh about six times as much as protons. The prevailing theory of particle physics accurately predicted the sigma-b masses. It calls for still heftier baryons that haven't yet been found.

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