• Open Access

S-Matrix for massless particles

Holmfridur Hannesdottir and Matthew D. Schwartz
Phys. Rev. D 101, 105001 – Published 1 May 2020

Abstract

The traditional S-matrix does not exist for theories with massless particles, such as quantum electrodynamics. The difficulty in isolating asymptotic states manifests itself as infrared divergences at each order in perturbation theory. Building on insights from the literature on coherent states and factorization, we construct an S-matrix that is free of singularities order-by-order in perturbation theory. Factorization guarantees that the asymptotic evolution in gauge theories is universal, i.e., independent of the hard process. Although the hard S-matrix element is computed between well-defined few particle Fock states, dressed/coherent states can be seen to form as intermediate states in the calculation of hard S-matrix elements. We present a framework for the perturbative calculation of hard S-matrix elements combining Lorentz-covariant Feynman rules for the dressed-state scattering with time-ordered perturbation theory for the asymptotic evolution. With hard cutoffs on the asymptotic Hamiltonian, the cancellation of divergences can be seen explicitly. In dimensional regularization, where the hard cutoffs are replaced by a renormalization scale, the contribution from the asymptotic evolution produces scaleless integrals that vanish. A number of illustrative examples are given in QED, QCD, and N=4 super-Yang-Mills theory.

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  • Received 19 November 2019
  • Accepted 13 February 2020

DOI:https://doi.org/10.1103/PhysRevD.101.105001

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Particles & Fields

Authors & Affiliations

Holmfridur Hannesdottir* and Matthew D. Schwartz

  • Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

  • *holmfridur_hannesdottir@g.harvard.edu
  • schwartz@g.harvard.edu

Article Text

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Issue

Vol. 101, Iss. 10 — 15 May 2020

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