# Soft factorisation Soft factorisation in QFT is the statement that the S-matrix factorises into a hard part and a soft part. The soft part contains information about [[0295 Infrared divergences in scattering amplitude|infrared divergences]], and the hard part contains physical quantities that can be measured in experiments. The fact that they factorise has far-reaching consequences. In particular, since there are infinitely many low-energy particles (soft particles) that escape detection in real experiments, it would be impossible to compare experimental data with theoretical calculations if the scattering amplitudes of the hard particles depends on the arrangements of the soft ones. Soft factorisation is also intimately related to dressing. It is possible to remove the infrared divergences in a theory if the states are appropriated addressed with Goldstone bosons of spontaneously broken [[0060 Asymptotic symmetry|asymptotic symmetries]]. This has been worked out in QED and gravity at least for massless particles. ## Refs - original - [[1983#Bassetto, Ciafaloni, Marchesini]] - important - [[2014#Feige, Schwartz]]: all orders; provides intuition - celestial soft factorisation - QED: [[2017#Nande, Pate, Strominger]] - gravity: [[2020#Himwich, Narayanan, Pate, Paul, Strominger]] - celestial IR-finite amplitudes via dressing - QED: [[2017#Kapec, Perry, Raclariu, Strominger]] - gravity: [[2017#Choi, Akhoury]] ## Related - [[0272 Faddeev-Kulish]]