[Rivet] Histogram normalisation

[Frank Siegert]

Hi Jon,

Thanks for the comments.

Jonathan Butterworth, Tuesday 20 October 2009:
> - I'd be very wary of the the KFactor. I am particularly worried that
> people don't start applying multiple scale factors and losing track of
> what has been applied. I suggest that the default output is ALWAYS just
> use the "truth" (either xsec proportional or normalised by your Norm
> factot if applicable) and any other scaling is done later, transiently,
> with plotting tools. The KFactor could be stored and written out so it
> can be applied by the plotting tools if desired (?)

My proposal was meant to have truth output plus the KFactor=x.xx written 
out. But thinking about the complications and dangers of this approach, I 
agree: Let's drop the LO mode I proposed, and ...

> - We also discussing having a plotting tool which steps over various
> scale factors for a combined run and works out the optimal scale factor
> based on the Chi2 between data and MC. This could also (optionally)
> apply the KFactors. Is that still in the plan?

... replace it with this more general and easier to implement solution of 
automatic KFactor finding.
In any case we just have to make sure, that histograms which already have 
a Norm=x.xx or Scale=x.xx are ignored (or does anybody have an analysis 
use case where a histogram is scaled with anything else than 
crossSection()/sumOfWeights() and _still_ should have a kfactor?).
And for all others the automatically determined kfactor should somehow be 
plotted for each MC run in the histogram, maybe together with the legend, 
or above the top edge?

> - How do plotting tools know whether a histogram has a cross-section
> (i.e. semi-floating) normalisation or is fixed? Is Norm=0 or some other
> special value for the xsec type histograms? Or is Norm just not written
> out?

I would suggest Norm to not be written out in such a case.

Cheers,
Frank