[Rivet] Question on normalisation of histograms containing weighted events

[Andy Buckley]

A very quick reply...

This plot is marked as thrust, but the shape suggests to me that it's
actually 1-T, i.e. the perfect back-to-back dijet config is at 0 rather
than 1: is that correct?

If so, then it looks like your glued distribution is more spherical than
the true inclusive distribution, which would imply that too much weight
is going on high #MPI, or too little on #low MPI. So, some questions:

* have you included a #MPI = 0 "bin"?
* I think it's worth double checking that everything is consistent in
the normalisation, since Rivet now includes the overflow and underflow
bins in calls to normalize()
* double-check that there's no bin-width silliness going on!

Finally, just a comment that I don't think #MPI is not really a physical
variable, but more an artefact of the modelling with multiple
perturbative scatters. Is the analysis attempting to *measure* <#MPI>,
or is that just an intermediate thing that you're e.g. reweighting?

Andy


On 17/07/12 18:33, Holger Schulz wrote:
> Hi,
> 
> I am working on a template fit for some event shape
> distributions. In each event, I ask for the number of MPI scatters
> and thus get the distributions differential in #MPI.
> Those distributions are not normalised in finalize().
> 
> I also obtain a histogram with the occurences of
> the number of MPI-scatters, which I normalise to one.
> 
> Further I fill a histogram that contains the inclusive
> distribution, which I normalise to 1.
> 
> When I try to "glue" the differential distributions
> together, using the relative #MPI contributions
> and afterwards scale the glued distribution to 1,
> I do not get the same shape as is present in the
> inclusive histo (see attachment).
> 
> The event generator I use is Sherpa, generating
> weighted events and this is rivet still using the AIDA
> histogramming.
> 
> Does anyone have a feeling if the weighted events
> and my normalisations do not get along well?
> 
> Cheers,
> Holger
> 
> 
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-- 
Dr Andy Buckley, SUPA Advanced Research Fellow
Particle Physics Expt Group, University of Edinburgh



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