### Can I just calculate the amplitudes for a (sub)process and check them point-by-point in phase space points using MadGraph ?

The easiest way to do this is to create a Standalone package:

generate p p > e+ e- output standalone PATH

You can then easily choose which diagrams to include in the matrix.f files of the SubProcesses/P_xx_xxxx directories. There is also an example program (check_sa.f) in the SubProcesses directory which shows how to calculate the amplitude at specific phase space points or by using PS configurations from RAMBO.

You can also returns a valid c++ standalon code by running

generate p p > e+ e- output standalone_cpp PATH

### Pythton

If you want to have acces to the matrix-element via python. The easiest is to have the fortran code wrapped into python (evaluating a matrix element in python is just too slow to be usable). Each of the SubProcesses/P_xx_xxxx of the standalone directory can be compiled to be python linkable (note this require f2py part of numpy and to have python-devel package installed):

make matrix2py.so

Then you can link this from your python code. Here is an example:

import matrix2py def invert_momenta(p): """ fortran/C-python do not order table in the same order""" new_p = [] for i in range(len(p[0])): new_p.append([0]*len(p)) for i, onep in enumerate(p): for j, x in enumerate(onep): new_p[j][i] = x return new_p matrix2py.initialisemodel('../../Cards/param_card.dat') p = [[ 0.5000000E+03, 0.0000000E+00, 0.0000000E+00, 0.5000000E+03], [ 0.5000000E+03, 0.0000000E+00, 0.0000000E+00, -0.5000000E+03], [ 0.5000000E+03, 0.1109243E+03, 0.4448308E+03, -0.1995529E+03], [ 0.5000000E+03, -0.1109243E+03, -0.4448308E+03, 0.1995529E+03]] P =invert_momenta(p) alphas = 0.13 nhel = -1 # means sum over all helicity me2 = matrix2py.get_value(P, alphas, nhel) print me2

### Full library of process in python

If you are interested to have a single python library for a set of matrix-element. This is also possible in fortran/python. This is possible both for loop output ([sqr=virt]) and for tree level.

generate p p > t t~ output standalone PATH --prefix=int

then you can compile the library by going to the SubProcesses directory and

make allmatrix2py.so

Here is one example in that case

import allmatrix2py allmatrix2py.initialise('../Cards/param_card.dat') def invert_momenta(p): """ fortran/C-python do not order table in the same order""" new_p = [] for i in range(len(p[0])): new_p.append([0]*len(p)) for i, onep in enumerate(p): for j, x in enumerate(onep): new_p[j][i] = x return new_p p =[[ 0.5000000E+03, 0.0000000E+00, 0.0000000E+00, 0.5000000E+03], [0.5000000E+03, 0.0000000E+00, 0.0000000E+00, -0.5000000E+03], [ 0.5000000E+03, 0.1040730E+03, 0.4173556E+03, -0.1872274E+03], [ 0.5000000E+03, -0.1040730E+03, -0.4173556E+03, 0.1872274E+03] ] p =invert_momenta(p) nhel = -1 # means sum over all helicity pdgs = [21,21,6,-6] #which pdg is requested scale2 = 0. #only used for loop matrix element. should be set to 0 for tree-level alphas=0.13 ans = allmatrix2py.smatrixhel(pdgs,p,alphas,scale2,nhel) print ans

### Python for version MG5_aMC version 2.5.x

For version of the code older than 2.6.0, the syntax was:

import matrix2py def invert_momenta(p): """ fortran/C-python do not order table in the same order""" new_p = [] for i in range(len(p[0])): new_p.append([0]*len(p)) for i, onep in enumerate(p): for j, x in enumerate(onep): new_p[j][i] = x return new_p matrix2py.initialise('../../Cards/param_card.dat') p = [[ 0.5000000E+03, 0.0000000E+00, 0.0000000E+00, 0.5000000E+03], [ 0.5000000E+03, 0.0000000E+00, 0.0000000E+00, -0.5000000E+03], [ 0.5000000E+03, 0.1109243E+03, 0.4448308E+03, -0.1995529E+03], [ 0.5000000E+03, -0.1109243E+03, -0.4448308E+03, 0.1995529E+03]] P =invert_momenta(p) alphas = 0.13 nhel = 0 # means sum over all helicity me2 = matrix2py.get_me(P, alphas, nhel) print me2