Comment 2 for bug 1089199

Hello Thibaud,

Many thanks for the report. This was due to a poorly formatted read-in in the file SubProcesses/P.../auto_dsig.f, which only affects decay processes (of the type A > B C ...) when some intermediate resonance is unable to go onshell due to the masses of initial and final state particles AND there are multiple identical particles in the final state, giving symmetric diagrams. So basically, only for exactly the process you are calculating. So indeed the factor 2 should be there, and the zero-width result is the correct one.

You can fix this once and for all in your MG5 directory by replacing the following lines in the file madgraph/iolibs/template_files/auto_dsig_v4.inc:

On line 41, insert the following line:
      DOUBLE PRECISION RCONF

On line 78, replace the line
          READ(LUN,*,ERR=10,END=10) ICONF, IFACT
with
          READ(LUN,*,ERR=10,END=10) RCONF, IFACT
          ICONF=INT(RCONF)

I have committed this fix to the branch 1.5.6. This version should get released soon.

Thanks again, all the best,
Johan