What is the smallest subatomic particle?

University of Hawai'i at Manoa physicists recently reported the discovery of a new and unusual elementary particle. This particle, which they have dubbed the X(3872), weighs about the same as a single atom of helium and exists for only about one billionth of a trillionth of a second before it decays to other longer-lived more familiar particles. Although extremely short-lived by human standards, this is nearly an eternity for an elementary particle of this kind.

The new particle was discovered by UH Manoa Professor of Physics and Astronomy Stephen Olsen and Gyeongsang University (Korea) Professor Sookyung Choi among the decay products of the so-called beauty meson that is produced in large numbers at KEKB, a huge “atom smasher” at the High Energy Accelerator Research Laboratory in Tsukuba Science City, Japan.

“The discovery is very exciting because there are some indications that the X(3872) may be the first example of a new type of sub-atomic particle, one where two more ordinary particles attach to each other similar to the way atoms stick together to form molecules,” said Olsen. “If so, this is the first glimpse of a whole new realm of sub-atomic physics, with many new particles to discover and understand.”

Particles produced at KEKB are studied at the Belle Detector, a complex assortment of highly sensitive radiation detectors located inside of a very large super-conducting electromagnet. Faculty and students from UH Manoa regularly participate in an international consortium of researchers from 11 different countries that collaborated on the construction and operation of the Belle Detector. They also take turns helping to operate the equipment which runs continuously with only a short summer break for improvements. The device took nearly ten years to design and build and has been operating since 1999. Olsen and UH Manoa Professor of Physics and Astronomy Thomas Browder are leaders of the Belle team.

“Like a giant telescope with unprecedented light gathering power that allows astronomers to peer further and further into the cosmos, the Belle Detector and the KEKB accelerator have enabled us to penetrate previously hidden aspects of nature at the smallest sub-atomic scales,” said Browder. “As is often the case when the full power of a new experimental instrument is harnessed, big surprises are found. The discovery of the X(3872) particle is a good example of this.”

There are hundreds of elementary particles and the discovery of a new one is not unusual. However, the X(3872) particle is peculiar in that it does not easily fit into any known particle scheme. Olsen and Browder initially thought the new particle was a member of the charmonium family of particles, which are comprised of a charmed-quark and an anticharmed-quark held together by the "color" force, the most powerful force in nature. Many different charmonium particles have been found and their properties reflect the many different ways that these charmed-quark anticharmed-quark combinations can be accomplished. However, theoretical expectations for all possible charmonium particles are very well formulated and the mass and other features of the X(3872) do not match well to any of them.

http://www.hawaii.edu/cgi-bin/uhnews?20031202101040
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