A Finite S-Matrix

When massless particles are involved, the traditional scattering matrix (S-matrix) does not exist: it has no rigorous non-perturbative definition and has infrared divergences in its perturbative expansion. The problem can be traced to the impossibility of isolating single-particle states at asymptotic times. On the other hand, the troublesome non-separable interactions are often universal: in gauge theories they factorize so that the asymptotic evolution is independent of the hard scattering. Exploiting this factorization property, we show how a finite ''hard'' $S$ matrix, $S_H$, can be defined by replacing the free Hamiltonian with a soft-collinear asymptotic Hamiltonian. The elements of $S_H$ are gauge invariant and infrared finite, and exist even in conformal field theories. One can interpret elements of $S_H$ alternatively 1) as elements of the traditional $S$ matrix between dressed states, 2) as Wilson coefficients, or 3) as remainder functions. These multiple interpretations provide different insights into the rich structure of $S_H$. For example $S_H$ exhibits symmetries, such as dual conformal invariance, that are not symmetries of the traditional infrared-divergent S-matrix.