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.
A previous model-independent variational approach considers a tetraquark with two heavy antiquarks and two light quarks as a heavy antidiquark with the color field of a quark bound to the two light quarks with a wave function like that of a heavy baryon.
Results indicate that a charmed-strange tetraquark $\bar c \bar s u d$ or a bottom-strange tetraquark $\bar b \bar s u d$ with this "diquark-heavy-baryion" wave function is not bound, in contrast to "molecular-type" $D-K$ and $B-K$ wave functions.
In this work we study tetraquark bound states in the framework of the constituent quark model of Ref. [2], which has been used for the description of non-strange two- and three-baryon systems and later on applied to the hadron spectra.
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