[Rivet] Shape comparisons

[Andy Buckley]

Hi Jim,

Rivet's been able to do that for some time, but it requires the version
of HepMC being used to support cross-section passing.

So are you saying that passing an argument of -x 1.0 to the rivet
command line tool does not override the value that comes from the HepMC
input? I'd personally consider that to be a bug.

However, modifying the analysis to hard-code the cross-section may not
be necessary. Is this a personal analysis or a built-in one? If a
personal one, and you always want to just do shape comparisons, then the
obvious thing to do is to put some normalize(...) calls in the
finalize() method. If it's an "official" analysis, or you don't *always*
want your plots to be normalised, then the rivet-rescale script allows
you to do post hoc normalisation: it's a bit hacky, since making a
complete programmatic interface via the command line is ~impossible, but
should be enough for simple cases like this. People on this mailing list
should be able to help with the latter if you have trouble.

Finally, just a note that if you're only interested in shapes, then
there's no need for your analysis to make use of cross-section
information: just don't make any calls to crossSection(), and remove any
use of "NeedsCrossSection" from your .info file. But it's always a good
idea to at least normalise histograms to a fixed area rather than the
number of events used -- I see the latter all the time, but it just
makes it harder / more error prone to compare to other samples which may
be a different size -- perhaps to a subtle extent which doesn't
immediately set off warning bells!

Cheers,
Andy


On 13/07/12 13:29, James Henderson wrote:
> Dear Rivet Experts,
> 
> I have recently updated my Rivet version to 1.8.0 and it seems that now
> Rivet is able to automatically read in the cross-section of a sample
> straight from the HepMC file. I was trying to normalise two sample with
> different actual cross-sections to 1pb in order to do a shape comparison
> but I can't seem to overwrite the automatic cross-section finder.
> 
> Is there a way to do such a shape comparison without explicitly altering
> the .cc file in order to not normalise to the cross-section?
> 
> Thanks,
> Jim
> 
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-- 
Dr Andy Buckley, SUPA Advanced Research Fellow
Particle Physics Expt Group, University of Edinburgh



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