Documentation

OpenLoops is an automated program for the numerical evaluation of tree and one-loop scattering amplitudes within the Standard Model. It is based on the Open Loops algorithm, which employs a hybrid tree-loop recursion to construct cut-open loops as tree structures supplemented with full loop-momentum information.

This approach can be combined either with tensor-integral or OPP reduction algorithms (see here) and permits to achieve, in both cases, very high CPU performance. CutTools (based on OPP reduction) is currently the default reduction library. It is shipped with OpenLoops, together with OneLOop for the evaluation of scalar integrals.

OpenLoops offers an automated and flexible stability system for the evaluation of numerical stability and potential treatment of unstable phase-space points.

The kernel of the OpenLoops recursion is universal and depends only on the Lagrangian of the model at hand. The OpenLoops algorithm is thus applicable to any process within any renormalisable theory. The present implementation supports a wide range of Standard Model processes at NLO QCD. It will soon be extended to NLO electroweak corrections.

OpenLoops can either be used as stand-alone program or as one-loop provider (OLP) in the framework of Monte Carlo tools. For the latter option OpenLoops offers various interfaces:

1. The native interface is designed in a simple and transparent way. At the same time it offers a maximum amount of flexibility and convenience.
2. The standardized BLHA interface for the combination of OLPs with Monte Carlo programs is available.
3. OpenLoops can be used as a plugin for Sherpa thanks to a dedicated and fully automated interface, which is integrated in Sherpa.
4. Similarly, OpenLoops can be used as a plugin for Herwig++.

If you are interested in interfacing your Monte Carlo program with OpenLoops do not hesitate to contact us.

Generation of tree and one-loop amplitudes is fully automatized in the OpenLoops framework. Still, for convenience of the user, processes are provided as pre-generated libraries that can be accessed through a simple download mechanism. This approach guarantees the internal consistency of the distributed code, its thorough validation, as well as the reproducibility of the results. The set of available processes will be extended continuously.