[Rivet] Madgraph (and alpgen) in Rivet

[Gavin Hesketh]

Hi,
We are validating madgraph for ATLAS, and are having to generate samples 
of jet multiplicities. This causes problems for some analyses where 
histograms are unit-normalised or ratios are taken at the end of a run. 
I've raised this problem in the past for alpgen, and unfortunately seems 
like it's not going to go away any time soon...

So I wondered if there are good ideas for how to handle it, or if yoda 
has some features I'm not aware of that we can use for work-arounds?


An example: I'm looking at ATLAS_2013_I1217867 (kt splitting scales in W 
events). At the end of the run, each plot is unit-normalised:
	normalize(_h_dI[flav][i], 1.0, false);
So, when later adding up 0-jet, 1-jet, 2-jet, 3-jet samples, we end up 
with a histogram with area 4, and completely the wrong shape.

To solve this, I've hacked the routine to do a more standard 
cross-section normalisation:
       double normfac = crossSection()/sumOfWeights();
       scale(_h_dI[flav][i], normfac);
then will yodamerge the jet samples and write a little yoda script to 
unit-normalise the plot at the end.

While I think it would be a nightmare to try to generalise and solve for 
all possible situations, it would be very useful to have a rivet 
command-line flag which turns off unit-normalisation / ratios, and just 
normalises all plots to cross sections. Still not a small amount of work 
to modify all existing analyses of course... Could then provide an 
example script to merge and unit-normalise plots in yodamerge.

But, like I said, I'm hoping someone has a better idea...

Merry Christmas :)
Gavin