[Rivet] how to handle multiple event weights with rivet

[Marco Zaro]

Hi Andy, all,
thanks for your answer. 
I played a bit with rivet (I have v2.5.4), and I found something which may be too simple to be true, although it looks like  it is working. I added a function to access the individual weights to the rivet code, in particular to the file src/Core/Event.cc (and of course its declaration to the header)
 

  double Event::weights(const int i=0) const {
    return (!_genevent.weights().empty()) ? _genevent.weights()[i] : 1.0;
  }

Via this function, the various weights in the .hepmc file can be accessed. In the rivet analysis, the number of histograms to be declared is N_observables * N_weights, and one can use a (global) Histo1DPtr array to easily loop over them all. The only thing I missed, and that I had to hardcode, is the tag for each weight (e.g. scale variation, PDF variation, …), but it should not be too much complicated to have it working in a general way (I am quite a newbie with c++).

In any case, if you think this can be a working solution, feel free to use it, and if you have comments or suggestions I would be very happy to hear.
Bye,

Marco

On 01 Feb 2018, at 22:04, Andy Buckley <andy.buckley at cern.ch> wrote:

> Hi Marco,
> 
> It's a painfully long-standing project! The Rivet 3.0.0alpha pre-release will do it, but is a year old now. David & Chris, any chance we can get a beta out soon, closer to sync with the current mainstream release?
> 
> Andy
> 
> Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
> Particle Physics Experiment Group, University of Glasgow
> On Jan 31 2018, at 9:24 am, Marco Zaro <m.zaro at nikhef.nl> wrote:
> 
> Dear Rivet authors,
> I have a question concerning the usage of rivet: is there any practical way to have the possibility to use the multiple event weights (e.g. for scale and PDF variations) within a rivet analysis? I currently have rivet 2.5.4 installed on my machine.
> Thanks in advance.
> 
> Best wishes,
> 
> Marco Zaro
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