Confinement
Quarks are never observed in isolation; they are always bound within composite particles due to the strong force's self-interaction.
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The statement of the theorem
Consider the expectation value of the Wilson loop \langle W(C) \rangle = \langle \text{Tr} \exp \left( i g \oint_C A_{\mu} d^\xi^\mu \right) \rangle for a large closed contour . Confinement dictates that for a large area enclosed by , the expectation value decays according to the area law: , where is the string tension, implying a linear potential at large distances.