Quarks are always found in bound states and are not observed as free particles in isolation. The phenomenon of confinement, governed by the strong nuclear force, prevents quarks from existing in isolation.
In the framework of quantum chromodynamics (QCD), the theory that describes the strong force and the interactions of quarks and gluons, quarks are always confined within composite particles called hadrons. The most familiar examples of hadrons are protons and neutrons, which are composed of quarks.
The strong force is unique in that it becomes stronger as quarks are pulled apart. As the distance between two quarks increases, the energy stored in the force field between them also increases. Consequently, the energy required to separate quarks becomes large enough that it exceeds the energy available in the system, preventing the creation of free quarks.
This phenomenon of confinement is supported by experimental evidence and is one of the fundamental aspects of QCD. Whenever quarks are produced in high-energy particle collisions, they immediately form bound states with other quarks to create color-neutral hadrons, such as mesons or baryons. The quarks are always confined within these composite particles.
While free quarks have not been observed in isolation, the effects of quark dynamics can be indirectly studied through the processes and properties of hadrons. Experimental observations and theoretical calculations of hadron properties provide insights into the behavior of quarks within bound systems.