What exactly are quarks?

Richard Muller:

Quarks are the main components of protons. We have concluded through experimental probing of the proton with high energy electrons that inside the proton there are three massive objects, and based on Murray Gell-Mann’s theory, we call these quarks.

(There are other things inside the nucleus, such as gluons, but they are lighter in weight. The mass is dominated by the quarks.)

The biggest surprise about quarks is that we cannot extract them. If we put enough energy to pull one out, that energy is transformed into the creation of additional quarks, including an antiquark, which binds with the one we extracted, to make (for example) a quark-antiquark pi meson. This feature is a consequence of the fact that the forces between quarks do not decrease with distance. Pi mesons consist of quark-anti-quark pairs.

This property is called confinement, and it means that in the macroscopic world, we will never see a free quark. Quarks can be semi-free in what we call a quark-gluon plasma, but that’s similar to saying that an electron is free when it conducts inside a metal. And unlike the electron, we can’t pull the quark off the surface of the plasma.