MadGraph Interface

A small library of matrix elements obtained with MADGRAPH version 4 (MG4) are included with the standard VINCIA release, along with the corresponding HELAS routines. Through its MG interface, VINCIA is able to match to these matrix elements during the shower evolution.

flag  Vincia:MGInterface   (default = on)
The MGInterface must be switched on for matching to work properly.

Note: see the section on Useful References for additional references to include when using the matrix elements provided by this interface.

Importing new matrix elements from MadGraph

The procedure for including a new matrix element is not intended for general users at this point, but is documented here for reference:

The MADGRAPH matrix elements included with VINCIA are located together with the actual interface code in VINCIA's MGInterface/ subdirectory. They are ordinary MADGRAPH matrix element routines, which have been renamed and modified slightly for use by VINCIA. Most of this conversion is done automatically, by a script. Implementing new matrix elements should therefore be fairly trivial, as long as the final state corresponds to something that VINCIA can actually generate. (E.g., implementing matrix elements for DIS or hadron-collider processes will not suddenly make VINCIA able to handle such cases, since the shower is not yet developed for that.)

Procedure:

Enumeration of Helicity Amplitudes in MadGraph

To extract individual helicity amplitudes (squared) from MG, it may be useful to know that the helicity information for an amplitude involving n partons (numbered sequentially from [1,...,n], with the two first ones normally being the incoming lines for a 2→n scattering process) is encoded in the single digit IHEL, which for the simple case of massless partons (all having helicities hi = +/- 1) simplifies to:

If scalars are involved, note that MG4 writes their "helicities" as -1. If massive vectors are involved, the 0 helicity mode also enters. In both cases, the formula above is invalid, but trivially generalises for summing over 1 or 3 helicity states respectively, instead of 2, for each particle.