Interacting open Wilson lines from noncommutative field theories

In noncommutative field theories, it is known that the one-loop effective action describes the propagation of noninteracting open Wilson lines, obeying the flying dipole's relation. We show that the two-loop effective action describes the cubic interaction among ``closed string'' states created by open Wilson line operators. Taking d-dimensional $\ensuremath{\lambda}[{\ensuremath{\Phi}}^{3}{]}_{\ensuremath{\star}}$ theory as the simplest setup, we compute the nonplanar contribution at a low-energy and large noncommutativity limit. We find that the contribution is expressible in a remarkably simple cubic interaction involving scalar open Wilson lines only and nothing else. We show that the interaction is purely geometrical and noncommutative in nature, depending only on the size of each open Wilson line.