Frequently Asked Questions on MadGraph / MadEvent

How do I implement my own cuts in MadEvent ?

To do this you need to download the code (either after process generation or from our Download page) and run locally. If you just want to hard-code a cut, all you need to do is modify the file SubProcesses/cuts.f, where all external particle 4-momenta are given in the P(0:3,nexternal) array (energy in P(0,I) and 3-momenta in P(1:3,I)). The order of the external particles depends on the subprocess, and can be guessed from the name of the subprocess Pxxxxxxx directory or can be seen from the Pxxxxxxx/leshouche.inc file.<br/> If you want to be able to set the cut using the run_card.dat, you need to modify the files Source/setrun.f (which reads the run_card.dat), SubProcesses/setcuts.f and SubProcesses/cuts.f.

How do I add a mass for the charm quark in MadGraph/MadEvent ?

Since the charm quark is set to be massless in the standard model files for MadGraph, you need to download the developer package to do this. You then need to add a mass parameter (e.g. CMASS) in the file Models/sm/particles.dat and make sure this parameter is read in the lh_readin routine in couplings.f. Then you're ready to generate processes and events. (answered by Main.JohanAlwall)

Should I use different ids (@1,@2,...) in multiprocess proccard or could I use the same ?_

It is not necessary to make the process numbers different. The reason to do it is that the processes with different numbers are presented in different groups on the "results" web page, and in the event file (LHE file). So for processes that you prefer to have presented in one group, with the cross sections summed within that group, it is better to use the same number. The same applies for the event files - processes which you prefer to have grouped as one subprocess in the event file should have the same number, and vice versa. This feature can be particularly useful for matched processes, where it can be interesting to get the matching efficiency separately for the different multiplicities.

The only case when it is necessary to use different process numbers is if you have two different processes with identical final states (e.g. with different coupling orders) in the same proc_card. In this case, MadGraph will consider the processes as identical and only use the first process, unless they have different process numbers. (answered by Main.JohanAlwall)

How do I calculate the amplitudes of specific diagrams in specific phase space points using MadGraph ?

The easiest way to do this is to download the StandAlone MadGraph package and generate the process. 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 in the SubProcesses directory which shows how to calculate the amplitude at a specific phase space point (answered by Main.JohanAlwall)

Could you describe how to specify which diagrams are included? I'm not very familiar with the code, so I'm not sure where to start. Thanks -- Main.EmilyJohnson - 30 Apr 2008

How can I use MadGraph in the matrix element method? or How to add new function in the integrand?

MadGraph has a specific phase space generator for MatrixElement method: MadWeight. (answered by Main.OlivierMattelaer)

-- Main.JohanAlwall - 21 Feb 2008