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The strong interaction

or

Super-gluon glue

Strong interaction is responsible for the confinement of protons and neutrons inside atomic nuclei. Its intensity is considerable and it would dominate all of the other forces of nature if its range of action was not so miniscule (of the order of size of atomic nuclei, at 10^-15 metres). This explains why in the nucleus, two protons, of the same positive charge Q, are not violently separated by the electromagnetic force: the strong interaction is much more powerfull.

In fact, the strong interaction exerts itself more fundamentally between the constituants of the nucleons: the quarks (and the antiquarks). In the same manner that the electromagnetic interaction only acts on particles possessing a non zero charge Q, the strong interaction acts only on particles bearing a characteristic named "colour". The theory explaining this mechanism is called quantum chromodynamics (because chromo = colour) or QCD.
 

Quantum chromodynamics
 

Quarks and antiquarks are thus characterised by a colour:

• red, green or blue (the 3 primary colours) for quarks

• cyan, magenta or yellow (the 3 corresponding complementary colours or anticolours) for antiquarks

Don't imagine that these particles are coloured like billiard balls! this poetic term has been imagined to name a new charge which doesn't concern anything but the strong interaction.
With the quarks of a nucleon, everything goes in threes: The Q charges are fractional thirds and importantly nucleons don't carry a particular colour: they're white. This white cannot be obtained unless the three quarks in a nucleon are all of a different colour. As in optics:
 

Notice that the family of leptons (electron, muon, tau and their neutrino) is not sensitive to the strong interaction and therefore these particles are not coloured.
As for quarks, the composite particles that they form by bunching together are called hadrons. All of these hadrons are threefore sensitive to the strong interaction and they all have a colour charge of zero: they are therefore white.

There can be two different types:

• Baryons which contain 3 quarks of 3 different colours obey the following rule (nucleons are particular baryons containing quarks u and d) :

Antibaryons contain 3 antiquarks of 3 anticolours and obey the rule :
 

• Mesons (unstable and short lived particles) contain a quark of a primary colour and an antiquark of the corresponding complementary colour (the anticolour):

Gluons
 

The strong interaction exherts itself via the intermediary of a boson or mediator particle. This boson has been named a gluon and there exist eight different ones; each carrier of a colour and an anticolour.

Like photons, there have zero mass and like the graviton, they still haven't ever yet been detected.

A gluon is thus always associated with a colour and with an anticolour; whenever it is exchanged between two quarks, it will change their colours. In contrast to the mediator photon which does not change the Q charges of the interacting particles, gluons, change the colour of the quarks.

It is therefore necessary to represent a nucleon as an incessant transformation of 3 quarks permanently exchanging gluons. At any given moment, the 3 quarks have 3 different colours, but these colours will constantly and alternately switch properties.

More surprising still, seen from very close, the quarks of a nucleon seem free. On the other hand, if they move away from each other, the force which ties them augments and they cannot get out of the nucleon. Quarks and gluons are confined to the interior of  baryons and mesons. Thus two quarks of different colours are all the more attracted one to another when they are distanced one from the other! It's as if they were linked by elastic. This is the confinement of quarks.
We can never therefore observe a quark in isolation.
 

The inter-nucleon liaison
 

The fundamental strong interaction exercises itself between the quarks and the gluons of a nucleon.
The things which bind together the nucleons inside the atomic nucleus, are the "flights" of gluons, quarks and antiquarks.
In 1937, the japanese physicist Hideki Yukawa proposed that the liaisons between protons or neutrons are made by the intermediary of particles called mesons p (Pi). These mesons Pi would effectively be detected ten years later. In fact, we now know that mesons are a necessary unstable association of quarks, antiquarks and gluons: These meons are in some ways flights escaping from the nucleon and interacting with the neighbouring nucleon: this is the residual strong interaction acting secondarily between nucleons by virtue of the intermediary of mesons.