Hints emerge of a four-quark particle

Physicists at accelerators in Japan and the United States have detected a subatomic particle that may be unlike any seen before.

Some evidence suggests that the particle contains two pairs of more fundamental particles–quarks and antiquarks–bound together. If that’s verified, then the new find would be the first four-quark particle known.

Until recently, quarks and antiquarks were observed only in groups of twos or threes. However, twice this year, researchers have reported possible five-quark particles (SN: 10/18/03, p. 245: New Quarktet: Subatomic oddity hints at pentaparticle family).

The discovery of yet other combinations of quarks and antiquarks could illuminate the force that binds quarks and antiquarks together. Physicists call this the strong force.

Experimenters at the High Energy Accelerator Research Organization (KEK) laboratory in Tsukuba, Japan, noted the first signs of the new particle while examining the aftermaths of hundreds of millions of electron-positron collisions. When the KEK investigators stumbled upon the intriguing new particle, they were seeking a previously undiscovered but mundane quark-antiquark duo–that remains unobserved.

A team at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., confirmed the KEK results, which are chronicled in an upcoming Physical Review Letters.

Some theorists suspect the new find may be deceptive. The calculations that guided the KEK team might have been inadequate for predicting the properties of the duo particle the team was seeking, proposes Fermilab theorist Estia J. Eichten. In that case, the newfound particle could actually be the duo, but with characteristics that only seem unexpected. On the other hand, Eichten notes, the large mass of the new particle closely matches that expected if two quark-antiquark D-mesons got hitched.


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