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
Python
If you want to have access 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 (from version 2.8.0)
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) proc_id = -1 # if you use the syntax "@X" (with X>0), you can set proc_id to that value (this allows to distinguish process with identical initial/final state.) 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,proc_id, p,alphas,scale2,nhel) print ans