Matthew Schwartz | What is the S-matrix?

[ All Things EFT 53 | Feb 2, 2022 ] The S-matrix is the basic object of interest in quantum field theory. We use S-matrix elements for phenomenological applications as well as to learn about symmetries and for hints of deeper structure in quantum field theory. Unfortunately, the traditional definition of the S matrix is only valid for theories in which all particles are massive. With massless particles, and associated long-range interactions, the S matrix is singular and formally does not exist. Although we have work-arounds for phenomenology, it is hard to justify why such an ungainly singular object should manifest any useful formal properties. The problems of the S matrix are due with universal soft and collinear interactions in initial and final states. Thus it is possible to study an improved and finite S matrix by removing these interactions in a systematic way using ideas from Soft-Collinear Effective Theory. In this talk, Matthew Schwartz will discuss problems with the S matrix, proposed solutions,