Box ladders in non-integer dimension

TL;DR

This paper generalizes the Belokurov-Usyukina loop reduction technique to non-integer dimensions, enabling explicit calculation of triangle and box ladder diagrams in momentum space using hypergeometric functions.

Contribution

It introduces a novel extension of the loop reduction method to non-integer dimensions, linking position and momentum space diagrams through conformal transformations.

Findings

Derived recursive relations for L-loop triangle ladder diagrams.

Expressed box ladder diagrams explicitly in terms of Appell's hypergeometric function F_4.

Enabled calculations without expanding in powers of epsilon.

Abstract

We construct a family of triangle-ladder diagrams which may be calculated by making use of Belokurov-Usyukina loop reduction technique in d = 4 -2e dimensions. The main idea of the approach proposed in the present paper consists in generalization of this loop reduction technique existing in d = 4 dimensions. The recursive formula relating the result for L-loop triangle ladder diagram of this family and the result for (L-1)-loop triangle ladder diagram of the same family is derived. Since the method proposed in the present paper combines analytic and dimensional regularizations, at the end of the calculation we have to remove the analytic regularization by taking the double uniform limit in which the parameters of the analytic regularization are vanishing. In this limit on the left hand side of the recursive relations we obtain in the position space the diagram in which the indices of the rungs are 1 and all the other indices are 1-e. Fourier transformation of the diagrams of this type gives the momentum space diagrams which have indices of the rungs equal to 1-e and all the other indices 1. Via conformal transformation of the dual space image of this momentum space representation we relate such a family of the triangle ladder momentum diagrams to a family of the box ladder momentum diagrams in which the indices of the rungs are equal to 1-e and all the other indices are 1. Since any diagram from this family can be reduced to one-loop diagram, the proposed generalization of the Belokurov-Usyukina loop reduction technique to non-integer number of dimensions allows us to calculate this family of box-ladder diagrams in the momentum space explicitly in terms of Appell's hypergeometric function F_4 without expanding in powers of parameter e in an arbitrary kinematic region in the momentum space.