Image: Standard model of elementary particles.
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The quark, a strange particle!
Quarks, leptons, and bosons are the elementary particles that make up all matter that we know.
These quantum constituents are described by the standard model of elementary particles made credible by the experimental confirmation of quarks (1995), neutrinos (2000) and the Higgs boson (2012).
The constituents of the proton and the neutron are composite, non-elementary particles, and are part of a strange assemblage of quarks and gluons. The bond that binds quarks together is the strong nuclear interaction sometimes called the color force.
However, protons and neutrons are not the only particles made up of quarks. About a hundred other very ephemeral particles (mesons) are made of quarks and gluons (pion, muon, kaon, eta, rho, phi, upsilon, lambda, etc.). All mesons are unstable and have a very short lifespan between 10-8 and 10-23 seconds. But the only truly stable particle of this diversity is the proton, which has a lifespan of around 1029 years. Although the neutrons bound in an atomic nucleus are relatively stable, when free they decay after 880.3 seconds (≈ 15 minutes).
The physical theory that describes the strong interaction, which explains the composition of nuclei, and which enables the mass of quarks and gluons to be calculated, is called quantum chromodynamics (QCD). The QCD was proposed in 1973 by H. David Politzer (1949-), Frank Wilczek (1951-) and David Gross (1941-)
When we measure the mass of a proton (2 quarks u + 1 quark d) and the mass of a neutron (1 quark u + 2 quarks d) we do not find the mass of their constituents.
Proton mass = 1,673 yg (1,673x10-24 g).
Neutron mass = 1.675 yg.
Mass of a quark u = 0.004 yg.
Mass of a quark d = 0.009 yg.
The mass of quarks only represents about 0.02% of the mass of nucleons!!
So where has the missing mass gone?
The missing mass is the kinetic energy and the strong interaction energy (E=mc2) which stir and hold the quarks together.
Due to a property known as confinement, quarks cannot be isolated. They are strongly linked by an exchange of electrically neutral particles, carrying a color charge, called gluons.
Quarks cannot be separated
Image: The confinement of quarks.
Quarks and gluons inside protons are connected by tubes of colored magnetic flux. If the tube is broken then new tubes form between the quarks present.
At present, only two types of hadrons are known: mesons, where a quark is associated with an antiquark with its anticolor, and baryons, where three quarks with the colors red, green and blue combine to form a white particle (this property is at the origin of the term color for the charge of strong interactions, since it recalls the additive synthesis of "true colors").
We cannot design a quark on its own because the more we try to separate the quarks and the more the strong nuclear interaction keeps control, the coupling between quarks increases with the distance. In other words, the closer they are to each other, the less they interact. Conversely, the more the quarks move away, the more the nuclear force acquires an elastic behavior forcing them to stay together.
This phenomenon called "quark confinement" is linked to the property of asymptotic freedom of strong interactions which act on particles with a color charge. For couplings due to other fundamental interactions (electromagnetic, weak and gravitational) it is the reverse, they decrease with distance.
The theory tells us that even when the quarks dissociate, the strong interaction forces the quarks to re-associate with each other to form hadrons. That is to say mesons formed by a quark and an antiquark or baryons formed by three quarks such as protons and neutrons.
Inside hadrons, the phenomenon of appearance and disappearance of particles occurs at a tremendous rate. It looks like a sea of varying numbers of quarks and gluons continually deforming the nucleus into a coexistence of shapes. New pairs of quark and antiquark materialize all the time. Endlessly quarks and antiquarks appear in the nucleon in a frantic dance without ever being able to leave the track.
If, following an energetic collision, a quark leaves the nucleon it immediately creates a new assembly of quarks and gluons (according to the relation E=mc2) which can give rise to a pion, a kaon, a rho... without ever leaving a quark alone.
The strangest thing is that the particle (pion, kaon, etc.) which was produced by the collision did not break the nucleon, the quarks remained confined in the nucleus as before the collision.
The strong nuclear interaction force increases with the separation of the quarks and decreases when they are strongly brought together, hence the representation of the springs.
If the spring is pulled very hard, the gluon disintegrates and the energy it contains is transformed into a pair of antiquark quark. Conversely, a pair of antiquark quark can merge and disappear by restoring energy to the gluon. The correct image of the internal structure of a proton or a neutron would not be the image of three quite distinct quarks linked by gluons but rather the image of a diffuse sea of quarks, antiquarks and gluons which appear and disappear, which bind and loosen unceasingly. But in the end there are always three more quarks than antiquarks, 2 up + 1 down for 1 proton and 2 down + 1 up for a neutron.
It is this mysterious structure inside nuclei that allows atoms to find the best way to come together.
NB: Color confinement is a property of elementary particles possessing a color charge: these particles cannot be isolated and are observed only with other particles such that the combination formed is white, that is to say that its total color charge is zero. This property is at the origin of the existence of hadrons. The phenomenon is described in the context of quantum chromodynamics.