Diagrammar of physical and fake particles and spectral optical theorem

TL;DR

This paper establishes spectral optical identities in quantum field theories involving physical and fake particles, enhancing understanding of unitarity through algebraic derivations applicable to complex loop diagrams.

Contribution

It introduces spectral optical identities for physical and fake particles, providing a new algebraic approach to analyze unitarity in quantum field theory diagrams.

Findings

Spectral optical identities hold for all thresholds and frequencies.

Skeleton diagrams obey a spectral optical theorem.

Formulas for loop integrals with fakeons are derived and related to physical particle integrals.

Abstract

We prove spectral optical identities in quantum field theories of physical particles (defined by the Feynman $i\epsilon $ prescription) and purely virtual particles (defined by the fakeon prescription). The identities are derived by means of purely algebraic operations and hold for every (multi)threshold separately and for arbitrary frequencies. Their major significance is that they offer a deeper understanding on the problem of unitarity in quantum field theory. In particular, they apply to "skeleton" diagrams, before integrating on the space components of the loop momenta and the phase spaces. In turn, the skeleton diagrams obey a spectral optical theorem, which gives the usual optical theorem for amplitudes, once the integrals on the space components of the loop momenta and the phase spaces are restored. The fakeon prescription/projection is implemented by dropping the thresholds that involve fakeon frequencies. We give examples at one loop (bubble, triangle, box, pentagon and hexagon), two loops (triangle with "diagonal", box with diagonal) and arbitrarily many loops. We also derive formulas for the loop integrals with fakeons and relate them to the known formulas for the loop integrals with physical particles.