Image description: Quarks and gluons inside protons are connected by color magnetic flux tubes. If a tube breaks, new tubes form between the remaining quarks. Currently, we only know two types of hadrons: mesons, where a quark is paired with an antiquark of opposite color, and baryons, where three quarks of red, green, and blue colors combine to form a white particle (this property is the origin of the term "color" for the charge of strong interactions, as it resembles the additive synthesis of "true colors").
Quarks, leptons, and bosons are the elementary particles that make up all the matter we know.
These quantum constituents are described by the Standard Model of elementary particles, validated by the experimental confirmation of quarks (1995), neutrinos (2000), and the Higgs boson (2012).
The constituents of protons and neutrons are composite, non-elementary particles, part of a strange assembly of quarks and gluons. The binding that holds quarks together is the strong nuclear interaction, sometimes called the color force.
However, protons and neutrons are not the only particles made 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 lifetime between 10^-8 and 10^-23 seconds. But the only truly stable particle among this variety is the proton, which has a lifetime of about 10^29 years. Although 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, explains the composition of nuclei, and allows for the calculation of quark and gluon masses is called Quantum Chromodynamics (QCD). QCD was proposed in 1973 by H. David Politzer (1949-), Frank Wilczek (1951-), and David Gross (1941-).
When measuring the mass of a proton (2 up quarks + 1 down quark) and the mass of a neutron (1 up quark + 2 down quarks), we do not find the mass of their constituents.
Mass of the proton = 1.673 yg (1.673x10^-24 g).
Mass of the neutron = 1.675 yg.
Mass of an up quark = 0.004 yg.
Mass of a down quark = 0.009 yg.
The mass of the quarks represents only about 0.02% of the mass of nucleons!!
So where has the missing mass gone?
The missing mass is the kinetic energy and strong interaction energy (E=mc^2) that stirs and holds the quarks together.
Due to a property called confinement, quarks cannot be isolated. They are strongly bound by the exchange of electrically neutral particles carrying a color charge, called gluons.
A single quark cannot be conceived because the more we try to separate quarks, the more the strong nuclear interaction retains control, and the coupling between quarks increases with distance. In other words, the closer they are to each other, the less they interact. Conversely, the further apart the quarks are, the stronger the nuclear force behaves elastically, forcing them to stay together.
This phenomenon, called "quark confinement," is related to the property of asymptotic freedom of strong interactions acting on particles with color charge. For couplings due to other fundamental interactions (electromagnetic, weak, and gravitational), the reverse is true; they decrease with distance.
The theory tells us that even when quarks dissociate, the strong interaction forces them to re-associate to form hadrons. That is, mesons formed from a quark and an antiquark or baryons formed from three quarks like protons and neutrons.
Inside hadrons, the phenomenon of particle appearance and disappearance occurs at a frantic pace. It resembles a sea of quarks and gluons in varying numbers, constantly deforming the nucleus in a coexistence of shapes. New quark-antiquark pairs materialize at any moment. Quarks and antiquarks constantly appear in the nucleon in a frenzied dance, never able to leave the stage.
If, following an energetic collision, a quark exits the nucleon, it immediately creates a new assembly of quarks and gluons (according to the relation E=mc^2), 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.) produced by the collision did not break the nucleon; the quarks remained confined in the nucleus as before the collision.
The force of the strong nuclear interaction increases with the separation of quarks and decreases when they are brought very close together, hence the representation of springs.
If you pull very hard on the spring, the gluon disintegrates, and the energy it contains transforms into a quark-antiquark pair. Conversely, a quark-antiquark pair can merge and disappear, giving energy back to the gluon. The correct image of the internal structure of a proton or neutron would not be that of three distinct quarks connected by gluons, but rather a diffuse sea of quarks, antiquarks, and gluons appearing and disappearing, binding and unbinding constantly. But in the end, there are always three more quarks than antiquarks, 2 up + 1 down for a 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 assemble.
N.B.: Color confinement is a property of elementary particles with a color charge: these particles cannot be isolated and are observed only with other particles such that the combination formed is white, meaning its total color charge is zero. This property is the origin of the existence of hadrons. The phenomenon is described within the framework of Quantum Chromodynamics.
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