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Asymmetric Dancing Partnersby James Weber
Physicists have known for several decades that antimatter is not a perfect mirror-image of matter. They've been trying to discover how extensively the behaviour of antimatter varies from matter and what causes the deviations, known as asymmetries. The Standard Model, which describes the behavior of the atom and its sub-atomic particles, makes allowance for small asymmetries. Recently, Peter Shawhanrepresenting the KTEV projectpresented surprising dramatic experimental evidence of an asymmetry much greater than many physicists had predicted. The experimental data, derived from observing millions of k-meson particle decays at the Tevatron accelerator at the Fermi National Laboratory (Fermilab), raised questions about the completeness of the Standard Model as a detailed description of the smallest particles.
The energies, directions, and affinities of sub-atomic particles appear as mysterious and complex as any combination of moves and motives on a dance floor. Researchers have been investigating a simple form of sub-sub-atomic particles, called mesons, which consist of a quark and an anti-quark. The KTEV project team are particularly intrigued by the gyrations of the k-meson. Unlike other mesons, the neutrally charged k-meson (also called a kaon) does not behave symmetrically with respect to its cousin antiparticle, a subatomic particle, and its corresponding antimatter partner, the anti-k. These odd kissing cousins did not exhibit the upside-down and backwards "parity reversal" symmetry noticed in the relative mirror-images of other pairs of matter-antimatter particles. Because the kaons are very short-lived, physicists also speculated that their asymmetries may be associated with the decay of the particles into more stable states. The KTEV team was able to test the decay theory with the Fermilab's newly available, extremely sensitive instrumentation. They used the Tevatron accelerator, which has an enhanced capability of producing and collecting kaons, and sophisticated computer systems that permit a better "focus" in following decays "in flight." The Fermilab researchers found the decay ratios of kaons into more stable pi-mesons to be much higher than expected, giving a strong impetus to the decay theory as the source of the asymmetry. Val Fitch, who with collaborator James Cronin won the Nobel Prize for the 1964 discovery of kaon asymmetries, was excited: "It is a most astonishing result. It is quite unexpected and very, very interesting." An earlier experiment at the European Laboratory for Particle Physics (CERN) suggested an asymmetry in particle decay, but the Fermilab results were clearly more definitive. Because these results were obtained using only 20% of the data collected, future analyses may be interesting, as well. In addition, the KTEV experiment at Fermilab's Tevatron accelerator this year is expected to generate twice as much data as in 1998. And Fermilab is not alone in its quest. The CERN Laboratory's NA48 experiment is due to produce calculated results of similar work very soon, and staff conducting the K-Long Experiment (KLOE) at Frascati National Laboratory (Italy) are taking yet another approach to examining kaon asymmetry. For more information, see the ut KTEV website at
http://fnphyx-www.fnal.gov/experiments/ktev/ktev.html. |
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Chris Quigg of the Fermilab staff
recounted the moment of revelation: |
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www.pnl.gov/energyscience/ |