[Rivet] normalisation to cross-sections

[Hendrik Hoeth]

Thus spake Frank Siegert (frank.siegert at durham.ac.uk):

> So if you read this to refer only to histograms where the _data_ is
> proportional to the cross section, how would my proposal make any
> difference to what we have done so far?

It makes a difference to, well, all the distributions which are
proportional to the cross-section. Simplest example: Jet cross-sections.

> For them we already do finalize with crossSection()/sumOfWeights() and
> nothing will have to be done a posteriori to change that.

No, many of those are still normalised to the data, and I thought that
fixing this is what the discussion is all about.

> I understand that in the ideal world the histogram numbers coming out
> of Rivet would be directly the same as in the experiment. But please
> also consider that we have a desperate need to run higher statistics
> through Rivet by merging runs. That is mainly relevant for validation
> rather than tuning though.

Sure, but I can't see how for merging event shapes the proposed
normalisation would help you. You'd need something like the integrated
luminosity, not the cross-section.

> So I thought my suggestion was a fair (and temporary) compromise. Do
> you have any better suggestion?

At least for the distributions that don't depend on the cross-section
(event shapes) I would like to see Rivet not requiring a cross-section.
Avoiding an unneeded dependency. That's all. For all those distributions
like jet cross-sections, particle pT spectra, etc., that have a "pb" as
unit on the y-axis I agree those should be normalised to cross-section.

> > [... setNeedsCrossSection() ...]
> 
> Which is exactly what I did in my recent commit unless I have missed one, 
> in which case I would appreciate if you pointed it out.

D0_2004 doesn't have it -- this and the D0_1996_S3214044 analysis are
the only ones where I looked at the code after the commit.

     Hendrik

-- 
"You have to take the most direct road to go instead of your
 meeting, you have to, this one ended, leave at once the CERN
 domain."         (imprint on the CERN visitor ID cards)