Quarks cannot exist independently as free particles due to a phenomenon called confinement. The reason for this lies in the fundamental nature of the strong nuclear force, which is responsible for binding quarks together.
The strong nuclear force is mediated by particles called gluons. Unlike other fundamental forces, such as electromagnetism, the strong force does not weaken as particles move apart. Instead, it becomes stronger as quarks are pulled apart from each other. This behavior is known as asymptotic freedom, a property described by the theory of quantum chromodynamics (QCD).
As quarks are separated, the energy stored in the gluon field between them increases. The strong force acts like a rubber band, exerting a force that prevents the quarks from separating beyond a certain distance. The energy required to separate the quarks becomes so large that it is energetically unfavorable to create isolated quark pairs.
This phenomenon of confinement means that the strong force effectively "confines" quarks within composite particles called hadrons, which include protons, neutrons, and other particles. In a hadron, the quarks are bound together by the exchange of gluons, forming a color-neutral combination.
While confinement has been observed experimentally, our understanding of its precise mechanisms is complex and relies on the mathematical formalism of QCD. Quarks are only observed indirectly through the particles they compose, and their confinement within hadrons is a fundamental characteristic of the strong force and the way quarks interact.