A tetraquark is a subatomic particle composed of four quarks.

One of the most bizarre properties of quarks is that they can never exist individually. They always manifest themselves in groups -- such as groups of two (mesons) or groups of three (protons, neutrons and other baryons) -- and if you try to smash these groups apart, new groups are formed. An intriguing aspect of the Standard Model is that it does allow for larger formations of quarks. It allows for groups of 4 quarks, 5 quarks and beyond, but despite years of searching, these bigger quark clumps remained elusive until 2003 with the discovery of the pentaquark by the SPring-8 experiment in Japan and the Jefferson Lab in Virginia, and the discovery of the 4-quark state, X(3872), by the Belle experiment in Japan. The evidence is strong that the X(3872) is the first discovery of a 4-quark formation.

The name X is a temporary name, indicating that there are still some questions about its properties to be tested. The 3872 is the mass of the particle measured mass units of "MeV" (as is usual in particle physics). For perspective, this mass is about four times the mass of a proton.

See also

• List of particles

External links

• The Belle experiment
  • Belle press release
• SPring-8 project website (English)
• Thomas Jefferson National Accelerator Facility (Jefferson Lab)