Pythia and HERWIG require a mass for all particles, while in MadGraph several are set to zero. How do I fix it?

The !ME2Pythia interface included in the Pythia/PGS package automatically adds masses to particles which are massless in the LHE file, but have mass in Pythia. This implementation is crude, and simply modifies the z-momentum component to account for the added mass. For normal collider processes this should be good enough, since the masses in question here typically are very small compared to the momenta.

There is also a more refined implementation by Steve Mrenna, in Template/bin/addmasses.py, which boosts each particle pair to the rest frame of their mother, modifies the kinematics, and boosts them back. At the moment this only works when the mother is explicitly given in the event file. This produces a new event file, which can be used with any shower Monte Carlo.

-- Main.JohanAlwall - 02 Mar 2009