Physicists from the experience of Belle KEK laboratory in Japan have discovered a curious meson called Z. According to some, this would be a particle consisting of four quarks. Such a thing seems almost impossible under the theory of quantum chromodynamics.
Since their introduction in the world of particle physics in the early 1960's by Gell-Mann, Ne'eman and Zweig, the quarks physicists have consistently intrigued by their abnormal behavior in relation to that of other elementary particles. Yet, the theory of strong interactions dominating the world of hadrons built with them has been particularly successful in describing experiences accelerators.
However, the equations of QCD (quantum chromodynamics) describing the exchange of gluons between quarks, and responsible for the composite structure of
protons and
neutrons, are notoriously difficult to resolve because of their nonlinear structure. The fact that there was still not quite sure why quarks are confined within hadrons, although there has been much progress since the late 1960's, and it is still almost impossible to predict the mass of protons and neutrons without using computers.
Nevertheless, the theory implies quite strong that quarks can gather that paired
particle- antiparticle to form mesons, and three to train baryons.
It is therefore with some surprise that the experimenters employed to analyze the reaction products of collisions between electrons and positrons, with the experiences BaBar at Stanford Linear Accelerator Center and Belle at KEK laboratory, have found strong indications of the presence meson consisting of four quarks.
An excited state of charmonium?
At first glance, it did not seem to be the most plausible explanation. Indeed, mesons, as baryons, as composites like atoms, they have energy levels and may find themselves in an excited state. The first assumption was made that it was precisely in the presence of this phenomenon with a charmed meson said, still called charmonium because consisting of a charmed quark and an antiquark charmed (charm denoting a quantum state similar to the spin this type of quark). We found just as a state of disintegration of an unstable meson can be interpreted as consisting of 4 quarks.
However, the meson Z (4430) today is discovered while the charmonium is neutral! It seems difficult to believe that this is indeed an excited state. In addition, the Z (4430) is désintégre in charmonium and a pi meson charge. It is therefore in the presence of a candidate meson four quarks, which seems a discernible excited state of charmonium unlike another meson: X (3872).
All physicians are not yet convinced, and some believe that further experiments are still necessary. Indeed, if there is a four - quark meson were to be confirmed, we should reconsider the equations of QCD, if this is the theory of strong interactions nuclear itself.

Note for Quantum chromodynamics

Quantum chromodynamics (abbreviated as QCD) is the theory of the strong interaction (color force), a fundamental force describing the interactions of the quarks and gluons found in hadrons (such as the proton, neutron or pion). QCD is a quantum field theory of a special kind called a non-abelian gauge theory. It is an important part of the Standard Model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years.
QCD enjoys two peculiar properties:

Asymptotic freedom, which means that in very high-energy reactions, quarks and gluons interact very weakly. That QCD predicts this behavior was first discovered in the early 1970s by David Politzer and by Frank Wilczek and David Gross. For this work they were awarded the 2004 Nobel Prize in Physics.

Confinement, which means that the force between quarks does not diminish as they are separated. Because of this, it would take an infinite amount of energy to separate two quarks; they are forever bound into hadrons such as the proton and the neutron. Although analytically unproven, confinement is widely believed to be true because it explains the consistent failure of free quark searches, and it is easy to demonstrate in lattice QCD.

Note for Strong interaction

The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD). The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. Of the four fundamental forces, the strong interaction is the most powerful.
The strong force only acts directly upon elementary particles. However, a residual of the force is observed between hadrons (the best known example being the force that acts between nucleons in atomic nuclei) as the nuclear force. Here the strong force acts indirectly, transmitted as gluons which form part of the virtual pi and rho mesons which classically transmit the nuclear force (see this topic for more). As has been shown by many failed free quark searches, the elementary particles affected are unobservable directly. This phenomenon is called confinement, a theory which allows only hadrons to be seen.

Note for Quark

A quark is a generic type of physical particle that forms one of the two basic constituents of matter, the other being the lepton. Various species of quarks combine in specific ways to form protons and neutrons, in each case taking exactly three quarks to make the composite particle in question.

There are six different types of quark, usually known as flavors: up, down, charm, strange, top, and bottom. (Their names were chosen arbitrarily based on the need to name them something that could be easily remembered and used.) The strange, charm, bottom and top varieties are highly unstable and died out within a fraction of a second after the Big Bang; they can be recreated and studied by particle physicists. The up and down varieties survive in profusion, and are distinguished by (among other things) their electric charge. It is this which makes the difference when quarks clump together to form protons or neutrons: a proton is made up of two up quarks and one down quark, yielding a net charge of +1; while a neutron contains one up quark and two down quarks, yielding a net charge of 0.
Quarks are the only fundamental particles that interact through all four of the fundamental forces.

Pictures overview

In figure 1, Three examples of mesons formed a quark and an antiquark designated by a bar on top.

In figure 2, Collisions électron-positrons produce many different types of particles that désintégrent in different modes chain.
Here a beautiful meson (B) désintégre Z (4430) which itself provides a charmonium also called meson J / psi.

In figure 4, The proof of the existence of the Z (4430) with a resonance in the rate of production at 4430 MeV.