Question: Processes with e+,e- as initial state have xec=0

[DvDongen]

When I set my incoming state to e+,e- (or some other incoming states, like 'gamma,gamma' or 'W+,W-') my pre-initialised estimated xsec is 0. It does work when i set my initial state to f.i. 'u,ubar'.
I'm working with the SM model with all masses turned on.
I've tried:
-Several different outgoing states (f.i. 'u,u,ubar,ubar', 't,tbar' and '4xgamma')
-With and without electron mass
-The initial_state set to partonic and eplus_eminus
-With and without minimal invariant masses
-With and without ps-cuts
-Different colour/helicity/phase_space-generators

[goord]

I believe you can check the status of the generator by calling print_status. Maybe pre-initialization is skipped because you have only a single subprocess?

[DvDongen]

Pre-init is not skipped. And print_status definitly helps:
The process doesn't generate any events with (positive/non-zero) weight...

[goord]

If you you the partonic initial state and uniform final state all weights are zero? If so, it is best to use gdb and inspect the pre-initialization phase event-by-event.

[DvDongen]

The issue turned out to be that the colour generator wasnt set to uniform. Only the uniform generator seems to work: both the 'summation' and 'flow_sampling' options give errors

[goord]

Then this issue qualifies as a bug: if no colored particles participate in the process, summation and flow sampling should just give the 'uncolored' configuration with weight 1.

[goord]

I cannot reproduce the reported behavior: e+e- -> ttbar at 600 GeV with RAMBO, uniform helicities and color flow sampling or summation yields a nonzero cross section after pre-initialization on my system. Please post your code (or send it to me) so I can figure out what is going on.