Color-charged particles cannot be found individually. For this reason, the
color-charged quarks are confined in groups hadrons with other quarks. These
composites are color neutral. Strong interactions reflected that quarks combine only
into baryons three quark objects, and mesons quark-antiquark objects, but not,
for example, four-quarkobjects. Only baryons three different colors
and mesons color and anticolor are color-neutral.

Color-Force Field

The quarks in a given hadron madly exchange gluons. For this reason, physicists
talk about the color-force field which consists of the gluons holding quarks together.

If one of the quarks in a given hadron is pulled away from its neighbors, the color-force field "stretches" between that quark and its neighbors. In so doing, more and more energy is added to the color-force field as the quarks are pulled apart. At some point, it is energetically cheaper for the color-force field to "snap" into a new quark-antiquark pair. In so doing, energy is conserved because the energy of the color-force field is converted into the mass of the new quarks, and the color-force field can "relax" back to an unstretched state.

When a quark emits or absorbs a gluon, that quark's color must change in order to conserve color charge. For example, suppose a red quark changes into a blue quark and emits a red/antiblue gluon the antiblue manifests as yellow. The net color is still red. This is because - after the emission of the gluon the blue color of the quark cancels with the antiblue color of the gluon. The remaining color then is the red color of the gluon.

Quarks emit and absorb gluons very frequently within a hadron, so there is no way to observe the color of an individual quark. Within a hadron, though, the color of the two quarks exchanging a gluon will change in a way that keeps the bound system in a color-neutral state.

So what holds the nucleus together?
They don't call it the strong force for nothing. The strong force between the quarks in one proton and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force. This is called the residual strong interaction,
and it is what "glues" the nucleus together.

Weak

There are six kinds of quarks and six kinds of leptons. But all the stable matter of the universe appears to be made of just the two least-massive quarks (up quark and down quark), the least-massive charged lepton (the electron), and the neutrinos.

Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. When fundamental particles decay, it is very strange: we observe the particle vanishing and being replaced by two or more different particles. Although the total of mass and energy is conserved, some of the original particle's mass is converted into kinetic energy, and the resulting particles always have less mass than the original particle that decayed.

The only matter around us that is stable is made up of the smallest quarks and leptons, which cannot decay any further. The Standard Model has united electromagnetic interactions and weak interactions into one unified interaction called electroweak.

Electroweak
Weak and the electromagnetic interactions
have been combined into a unified electroweak theory.

Physicists had long believed that weak forces were closely related to electromagnetic forces. Eventually they discovered that at very short distances (about 10^-18 meters) the strength of the weak interaction is comparable to that of the electromagnetic. On the other hand, at thirty times that distance (3x10^-17 m) the strength of the weak interaction is 1/10,000^th thanthat of the electromagnetic interaction. At distances typical for quarks in a proton or neutron(10^-15 m) the force is even tinier.

In fact, the weak and electromagnetic forces have essentially equal strengths. This is because the strength of the interaction depends strongly on both the mass of the force carrier and the distance of
the interaction. The difference between their observed strengths is due to the huge difference
in mass between the W and Z particles, which are very massive, and the photon,
which has no mass as far as we know.

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