Soft factorization in QED from 2D Kac-Moody symmetry

The soft factorization theorem for 4D abelian gauge theory states that the S-matrix factorizes into soft and hard parts, with the universal soft part containing all soft and collinear poles. Similarly, correlation functions on the sphere in a 2D CFT with a U(1) Kac-Moody current algebra factorize into current algebra and non-current algebra factors, with the current algebra factor fully determined by its pole structure. In this paper, we show that these 4D and 2D factorizations are mathematically the same phenomena. The soft 't Hooft-Wilson lines and soft photons are realized as a complexified 2D current algebra on the celestial sphere at null infinity. The current algebra level is determined by the cusp anomalous dimension. The associated complex U(1) boson lives on a torus whose modular parameter is tau = 2 pi i/e(2) + theta/2 pi. The correlators of this 2D current algebra fully reproduce the known soft part of the 4D S-matrix, as well as a conjectured generalization involving magnetic charges.