IR finite S-matrix by gauge invariant dressed states

Abstract: Dressed states were proposed to define the infrared (IR) finite $S$-matrix in QED or gravity. We show that the original Kulish-Faddeev dressed states are not enough to cure the IR divergences. To illustrate this problem, we consider QED with background currents (Wilson lines). This theory is exactly solvable but shares the same IR problems as the full QED. We show that naive asymptotic states lead to IR divergences in the $S$-matrix and are also inconsistent with the asymptotic symmetry, even if we add the original Kulish-Faddeev dressing operators. We then propose new dressed states which are consistent with the asymptotic symmetry. We show that the $S$-matrix for the dressed states is IR finite. We finally conclude that appropriate dressed asymptotic states define the IR finite $S$-matrix in the full QED.