Recola FAQ

This is a list of Frequently Asked Questions about Recola

Why …

… do I get different numbers with other tools?

There are many reasons for finding disagreement with other tools. The most common mistake is a different set of input parameters. With Recola2 you can print all internal and derived parameters by setting

from pyrecola import *

which allows for a direct comparison of all used parameters.

Recola has a different convention for finite parts of loop-amplitudes. Comparing to OpenLoops or Madgraph requires to set

from pyrecola import *
from math import pi

Furthermore, for a technical comparison we recommend switching off the Complex-Mass scheme (CMS) and setting all widths to zero.

How do I…

… compute loop-squared amplitudes?

By default, Recola computes tree-loop interferences when defining and computing processes with the ‘NLO’ flag. If for some reason the tree-amplitude is non-existent, the loop-squared is computed and returned instead of the tree-loop interference. In practice, one can therefore define a process and unselect all tree orders via unselect_all_powers_BornAmpl_rcl().

… visualize contributions to processes?

Recola can draw branches with set_draw_level_branches_rcl(), which, after invoking generate_processes_rcl(), will generate TeX files in the current working directory. The TeX files can be compiled and transformed to postscript with the programs latex and dvips and require the Axodraw package.

… check the consistency of a process?

Recola supports various ways to check, in particular, virtual amplitudes. The first thing that should be done is to compare Recola1 against Recola2. They slightly differ in the construction of the amplitudes and this partly validates renormalization and rational terms, as they were derived independently. Another check is to compare the virtual amplitudes against the Background-Field method (see Recola2 model files). In this method the construction of the amplitude, the bare-loop, counterterm and rational amplitude significantly differ to the conventional method, but the sum has to agree. One may see small differences when using the complex-mass scheme which have to disappear with the widths set to zero. Finally, we support the QED Ward-identity check for the tree and separately only the loop-part of squared amplitudes via set_longitudinal_polarization_rcl().