Evidence of first four-quark particles

Nature : In combinations of two ( mesons ) and three ( baryons ), quarks are the building blocks of many other particles, but there is no reason why four or more quarks can’t combine. Such tetraquarks have been hypothesized to have existed in the period shortly after the Big Bang. Now, two different particle accelerators have found evidence of four-quark particles. The Belle detector at the KEK research institute in Japan and the Beijing Spectrometer III (BESIII) at the Beijing Electron–Positron Collider in China, both of which collide electrons and positrons, have confirmed detection of a particle dubbed Z _c (3900). However, there is some debate whether the four-quark arrangement is actually a unique tetraquark particle, or a molecule-like arrangement of two mesons. Proponents of the tetraquark possibility argue that there is no evidence of the individual mesons that would result from the unstable molecule-like particle breaking apart. But the margin of error is still too large to completely rule out the molecule-like option. Although the researchers at both accelerators are still examining the data, confirmation of the arrangement of the quarks may not be possible until the upgraded Belle detector comes on line in 2015.