[Rivet] rivet: normalizing histograms to variable bin width

[Andy Buckley]

Hi Oldrich (and CC'ing the Rivet list -- please send to that rather than
me directly!),

In the API of YODA the bin width is taken into account when calling the
HistoBin::height() method:

https://yoda.hepforge.org/trac/browser/include/YODA/HistoBin1D.h#L97

(and a corresponding divide by x-y bin area in HistoBin2D)

By "normalize to bin width", I'm guessing that you mean "divide by
integral = sum of bin & overflows weight sums". For bins, the sum of
weights is the area rather than the height. Is that what you had in
mind? If so (and I'm sure this is the physically correct approach) then
this is already what YODA does:

Histo1D::normalize:
  https://yoda.hepforge.org/trac/browser/include/YODA/Histo1D.h#L146
calls Histo1D::integral:
  https://yoda.hepforge.org/trac/browser/include/YODA/Histo1D.h#L298
calls Histo1D::sumW:
  https://yoda.hepforge.org/trac/browser/src/Histo1D.cc#L43

Does that answer your question? In the mkScatter function that converts
Histo1D etc. to TGraphAsymmErrs-like Scatter2Ds, we use the
Bin::height() function so that the plotting will look correct, but
internally the area() == sumW() are much more important.

Andy


On 17/03/14 12:38, Oldrich Kepka wrote:
> Hi Andy,
> 
> sorry for this basic question. Is there already an implementation of a
> method normalizing histograms to produce differential cross section,
> i.e. normalizing to bin width.
> 
> Presumably, there must be, because there are several differential
> cross sections implemented. I am probably missing something, but I
> cannot find the place where the bin widtf is taken into account
> (looking through Histo1D, Bin1D, Dpn1D ... ). The usual Analysis
> scale/normalize methods boil down to scaleW of Dbn1D; scaling
> according to global desired factor.
> 
> Otherwise, I would go for something like this.
> 
>      _h_ZZ_ZpT.scale( norm);
>       for(size_t i = 0; i< _h_ZZ_ZpT-> numBins(); i++) {
>                 _h_ZZ_ZpT->bin(i). scaleW( 1/_h_ZZ_ZpT->bin(i). width() );
>          }
>        normalize(_h_ZZ_ZpT    , 1);
> 
> http://hepdata.cedar.ac.uk/view/ins1203852
> 
> 
> Thanks,
> Oldrich
> 


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