Foundation and generalization of the expansion by regions

TL;DR

This paper rigorously proves the correctness of the expansion by regions method for asymptotic expansions, clarifies the role of overlap contributions, and provides pedagogical examples to illustrate its application.

Contribution

It offers a general proof of the expansion by regions method, introduces a comprehensive formula including overlaps, and clarifies conditions for its correct application.

Findings

The expansion by regions reproduces exact results in asymptotic expansions.

Overlap contributions are essential in certain regularization schemes.

Pedagogical examples demonstrate the method's application and peculiarities.

Abstract

The "expansion by regions" is a method of asymptotic expansion developed by Beneke and Smirnov in 1997. It expands the integrand according to the scaling prescriptions of a set of regions and integrates all expanded terms over the whole integration domain. This method has been applied successfully to many complicated loop integrals, but a general proof for its correctness has still been missing. This paper shows how the expansion by regions manages to reproduce the exact result correctly in an expanded form and clarifies the conditions on the choice and completeness of the considered regions. A generalized expression for the full result is presented that involves additional overlap contributions. These extra pieces normally yield scaleless integrals which are consistently set to zero, but they may be needed depending on the choice of the regularization scheme. While the main proofs and formulae are presented in a general and concise form, a large portion of the paper is filled with simple, pedagogical one-loop examples which illustrate the peculiarities of the expansion by regions, explain its application and show how to evaluate contributions within this method.