Double-winding Wilson loops in SU(N) Yang-Mills theory
We examine how the vacuum expectation value (average) of double-winding Wilson loops depends on the number of color $N$ in the $SU(N)$ Yang-Mills theory. In the case where the two loops $C_1$ and $C_2$ are identical, we derive the exact operator relation which relates the double-winding Wilson loop operator in the fundamental representation to that in the higher dimensional representations depending on $N$. By taking the average of the relation, we find that the difference-of-areas law for the area law falloff recently claimed for $N=2$ is excluded for $N \ge 3$, provided that the string tension obeys the Casimir scaling for the higher representations. In the case where the two loops are distinct, we argue that the area law follows a novel law $(N-3)A_1/(N-1) + A_2$ with $A_1$ and $A_2$ ($A_1 < A_2$) being the minimal areas spanned respectively by the loops $C_1$ and $C_2$, which is neither sum-of-areas ($A_1+A_2$) nor difference-of-areas ($A_2-A_1$) law when $N\ge 3$. Indeed, this behavior can be confirmed in the two-dimensional $SU(N)$ Yang-Mills theory exactly.
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Vacuum Structure and Confinement