[Rivet] Advice for unusual truth analysis

[Daniel Weyh]

Hi Robert,

I'd guess it will not be possible "on the fly".
Rivet takes a single HepMC-Event, analyzes it and fills histograms with the result.
For what you want to do, you need to load two events from two different sources (I guess Sherpa cannot serve you alternatingly with MPI on/off and the same random seed for two consecutive events)...

The solution I have in mind - if you really need event-to-event comparison - is a bit dirty (I think) but should work:
1. In your Rivet analysis create a file-stream as member of your analysis (during init). (Remember to close in finalize() ;-) )
2. Do your analysis. During analyze append the number in question to your file (filestream << num_of_jets << endl;)
3. Run Sherpa another time with same random seed (I guess you know how to or check the Sherpa man), output as in 2. but in another file.
4. Analyze the two text files line by line externally. (<-- question to the others: are there python classes in Rivet which make creating an .aida by hand painless - to use Rivets plot facilities later on?)
Or: Change 3 to read from the file created in 2 and store the differences in a histogram.
As always depending on your needs. 
(How often you need it, automated or manually, how many observables (if not only #jets)...)

The files shouldn't be to big. I guess order(10^6) runs = 10^6 lines * (1byte (newline)+10byte (precision fp)) ~ order(10 MB).

This procedure requires you to write an analysis on your own (or at least to change the one you would have used for single-run).
I hope this is not a problem for you - otherwise don't hesitate to ask.


Good luck,
Daniel

 
Am 30.05.2011 um 17:54 schrieb Robert King:

> Dear Rivet Experts,
> 
> I have a slightly unusual truth level analysis and I hope you can advise me as to whether it is possible in Rivet. I have a limited experience with Rivet and it's not clear to me how one would/could do this.
> 
> I would like to generate identical Sherpa samples (i.e. use the same random number seeds) one with MPI on and another with it off. I would then like to compare event by event the number of jets each event.
> Is it possible to do this event by event comparison in Rivet?
> 
> Many thanks for you advice
> 
> Robert
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