[Rivet] Weights in Rivet

[Andy Buckley]

On 20/10/14 12:10, Rikkert Frederix wrote:
> Hi Andy,
> 
> We are trying to figure out how to use the weights from scale and PDF variation, i.e. those that are written in the LHE event file, in an (existing) rivet analysis. What are the possibilities? Any suggestions are more than welcome.

Hi Rikkert,

So far there hasn't been any support for these in HepMC-writing
generators, so we haven't made a public release of Rivet that uses them.
I understand that the latest Pythia8 release does copy LHE weight
vectors, though, and hopefully others will follow soon, so we should get
back to working on Rivet support for it.

Some work in this direction has taken place on a development branch, and
we have a design for how to handle weights (and counter-event groups) in
a coherent way. But it will take a bit of development work. I've copied
this to the Rivet developer mailing list so those who expressed interest
in working on it are in the loop.

It would be great if you could let us know how you would like them to be
used by default: our plan at the moment is that we write out N copies of
every histogram as a weight-variation, and build some logic into our
default plotting tools to turn these groups of variations into displayed
error bands. This requires that we can clearly identify the groups and
tell how to combine them... which is specified at the LHE level, but not
the HepMC one -- right? So the first version might be a bit approximate,
e.g. always a min-max envelope rather than sometimes a 68% interval,
sometimes a quadrature sum, etc.

Andy

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