[Rivet] Madgraph (and alpgen) in Rivet

[Andy Buckley]

yodamerge already knows the pre-normalization numbers in each run's
histos before merging, via the ScaledBy annotation. I think I got the
scaling logic correct so that if you pass the scalefactors on the
yodamerge command line, they will be applied after unrolling the
normalization and before the merging and re-normalization.

But I might have missed something. Will read more closely later but have
to run now...

Andy


On 23/12/14 17:07, Gavin Hesketh wrote:
> Hi Andy,
> don't think yodamerge does what is needed. If we had the xsec and sum of
> weights stored in the yoda file, and the area of each plot before
> unit-normalising, it would be possible. The cross section for each run
> alone does not help.
> 
> Ok, simplest case scenario: we have two jet slices we need to combine
> (eg W+0jet, W+1jet), with cross sections x1 and x2. Each sample is a
> different size, and has sum of weights s1 and s2. To combine them, we
> need to scale each sample to the same lumi (1 pb-1, say). The generated
> lumi for each sample is L1=s1/x1 and L2=s2/x2. So we need to divide each
> plot by L1 or L2.
> 
> Now say we have a plot which has two bins. Due to the cuts, the first
> bin is only filled by sample 1 (with N1 events), and the second bin is
> only filled by sample 2 (with N2 events). N1!=s1 and N2!=s2, due to the
> cuts.
> 
> After scaling and merging the two samples, the first bin will have
> N1*x1/L1 events, and the second will have N2*x2/L2.
> 
> Then after unit-normalising the merged plot, the bins will contain:
> 1) N1*x1/s1 / (N1*x1/s1 + N2*x2/s2)
> 2) N2*x2/s2 / (N1*x1/s1 + N2*x2/s2)
> 
> 
> With the current code, the plots are unit-normalised at the end of each
> run (ie for each sample). After merging the samples, the plot contents
> are (1,1). Redoing the unit-normalisation, they will be (0.5, 0.5). ie
> there is no way to get the right answer...
> 
> In order to get the right answer using yodamerge, we need N for each
> plot, x and s for each run. Storing just x and s is not enough.
> 
> Alternatively, as with the hack I've implemented, we normalise each plot
> to N.x/s, and can then do anything needed with yodamerge.
> 
> Hope that's clearer :)
> 
> Gavin
> 
> 
> 
> On 23/12/14 16:29, Andy Buckley wrote:
>> 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
>>> _______________________________________________
>>> Rivet mailing list
>>> Rivet at projects.hepforge.org
>>> https://www.hepforge.org/lists/listinfo/rivet
>>
>>


-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN