Hello,
I'm a new beginner of genie neutrino generator, and have a question about how to weight genie events.
The "weight" I mean is NOT same thing as the field I can get from EventRecord with following function
evt->Weight();
which for now I set always 1.
I generated 2000 genie event with gevgen command, and converted result .root file to gst format root file.
When I draw 1D histogram of El parameter (energy of final-state lepton), the vertical axis will be number of generated events per energy bin. This is just unweighted MC histogram and can't be used for physics analysis.
Now I want to convert this number to physics plot, number of interactions actually happened per 2000 primary neutrinos. Due to the small cross section, the vertical axis should become very small (~1e-36).
According to Genie user manual, genie is using internal global probability scale which is chosen from highest input energy and longest path length. At first, I thought this number will be the constant event weight to get actual number of events :
event_weight = gmcj_driver->GlobProbScale()
...
gst->Draw("El", event_weight);
but after I read GMCJDriver carefully I noticed it won't work, because the event loop inside GMCJDriver actually loops much more than 2000. There is no way to know how many neutrinos are actually injected, thus no way to get normalization factor.
It is always simple to calculate event weight of a specific event : it's just the interaction probability (and easy to obtain from EventRecord), but weighting entire MC sets strongly depends on how these MC sets are generated...
and I couldn't find any document about weighting procedures, instead, I found re-weighting staff (which currently I don't need.)
Is there any document or example code?
thank you,
kotoyo
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