The concept of free quarks refers to quarks existing in isolation, outside the confines of composite particles such as hadrons (e.g., protons and neutrons). The reason free quarks cannot be theoretically observed is due to a phenomenon known as color confinement.
Quarks carry a property called color charge, which is analogous to electric charge in electromagnetism. However, unlike electric charge, which can exist as isolated particles (such as free electrons), color charge is confined within composite particles. The strong nuclear force, mediated by gluons, binds quarks together, and the force increases with distance, preventing the separation of individual quarks.
As quarks move apart, the energy stored in the gluon field between them increases. This energy is sufficient to create new quark-antiquark pairs, forming new hadrons. This process, known as hadronization or hadron formation, ensures that color charge is always confined within composite particles, resulting in the absence of free quarks.
Experimental evidence strongly supports the theory of color confinement. No experiment to date has been able to directly observe isolated quarks. Instead, quarks are always observed in bound states within hadrons, making the concept of free, isolated quarks a theoretical construct that does not exist in the observable universe.