[Rivet] Madgraph (and alpgen) in Rivet

[Andy Buckley]

Hi Gavin,

Have you tried yodamerge? It allows you to give multiplicative
coefficients for the merging (even of unit-normalised histograms and
profiles) of YODA files from different np-X runs.

You would still need to know the xsecs for each run, but that is not
unusual! In future we'll make this a bit more automatic by writing the
xsec into each .yoda file as one of several "hidden" data structures,
but that's not in place yet -- you know the usual manpower story. We'd
be *very* happy to have some effort donated if you need this to work,
though!

Let me know if yodamerge works for you, or if not, what I've
misunderstood this time ;-)

Merry Christmas!
Andy


On 23/12/14 16:06, Gavin Hesketh wrote:
> 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
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-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN