How do I change the mass of a particle that is assumed massless in MadGraph/MadEvent ?

Several particles (light quarks, charm, he electron and the muon,..) are assumed to be massless in the SM implementation.

BEFORE CODE GENERATION

One can change this by dowloading the full package and defining a new model.

> cd Models
> cp sm sm_mc
> cd sm_mc

and then edit the ​particles.dat by changing

c       c~        F        S      ZERO  ZERO    T    c    4. 

into

c       c~        F        S      CMASS ZERO    T    c    4. 

You then need to make sure this parameter is read in the lh_readin routine in ​couplings.f. You have to add it also in the ​param_card.dat if it is not already there. Then you're ready to generate processes and events.

AFTER EVENT GENERATION

One can change this by acting directly on the event file with suitable script that change the mass and the four momenta of the interested particle in the final state by also reshuffling all other momenta to conserve overall momentum. Pythia will infact reject events where the energy momentum checkcount is not satisfied.

-- Main.FabioMaltoni - 02 Mar 2009