On-the-fly reduction of open loops

TL;DR

This paper introduces an on-the-fly reduction method within the OpenLoops framework that enhances the efficiency and stability of multi-particle scattering amplitude calculations by integrating reduction steps into the recursive construction process.

Contribution

It presents a novel on-the-fly algorithm that unifies amplitude construction and reduction, improving computational speed and numerical stability for complex scattering processes.

Findings

Significant speed improvements demonstrated in multi-particle processes.

Enhanced numerical stability, eliminating Gram determinant instabilities.

Implementation of quadruple precision benchmarks and rescue system.

Abstract

We describe new developments in the OpenLoops framework based on the recently introduced on-the-fly method. The on-the-fly approach exploits the factorisation of one-loop diagrams into segments in order to perform various operations, such as helicity summation, diagram merging and the reduction of Feynman integrands in between the recursion steps for the amplitude construction. This method significantly reduces the complexity of scattering amplitude calculations for multi-particle processes, leading to a major increase in CPU efficiency and numerical stability. The unification of the reduction to scalar integrals with the amplitude construction in a single algorithm, allows to identify problematic kinematical configurations and cure numerical instabilities in single recursion steps. A simple permutation trick in combination with a one-parameter expansion for a single topology, which is now implemented to any order, eliminate rank-two Gram determinant instabilities altogether. Due to this any-order expansion, the numerical accuracy of the algorithm can be determined with a rescaling test. The on-the-fly algorithm is fully implemented for double and quadruple precision, which allows for true quadruple precision benchmarks with up to 32 correct digits as well as a powerful rescue system for unstable points. We present first speed and stability results for these new features. The on-the-fly algorithm is part of the forthcoming release of OpenLoops 2.