A nonlocal charge for cylindrical gravitational waves

The classical scattering of cylindrical gravitational waves is exactly solvable. The motivation for this paper is to understand if the quantum scattering problem is also exactly solvable. The classical dynamics is governed by a two dimensional sigma model. We study this sigma model's $S$-matrix. We construct a conserved nonlocal charge and derive the associated tree-level $S$-matrix conservation law. We check our conservation law directly using Feynman diagrams. The existence of this symmetry is a hint that cylindrical gravitational waves might have an exactly solvable $S$-matrix.