Summary: This module of mine gives a fundamental idea of the theory of asymptotic freedom and the quark confinement hypothesis. It contains assimilated elements from various books and dictionaries.
The theory of asymptotic freedom states that the interaction between quarks reduces as the distance between them reduces (and hence energy increases), and tends to zero as the distance between them reduces to zero.Conversely, the interaction between them increases as they are separated by larger distances (and hence lower energies). If a theory requires the presence of Higgs Boson, asymptotic freedom is destroyed.Hence, the electroweak theory is not asymptotically free (or, the electromagnetic and weak forces are asymptotically not free).Also the strong nuclear force obeys the theory of asymptotic freedom.
Another result, originating from the theory of asymptotic freedom, is the quark confinement hypothesis.Recalling the implications of the asymptotic freedom, we ultimately see that the force between the two quarks (due to a special kind of charge called "colour charge"), increases at lower energies as they are separated more and more from one another.This leads to the idea that free quarks are never seen in isolation.Can you see why? As we try to increase the distance between, say a pair of quarks, they will have more affinity for each other and hence will be interacting even more strongly with each other.So, it is not possible to isolate a free quark. But it is possible that at very high temperatures, such as those of the early universe, quarks might have become free.The temperature at which quarks become free is known as "Deconfinement temperature".