[Rivet] Partonic top support?

[Hannes Jung]

Hi Andy et al

thanks very much for this initiative, I appreciate this very much.

I think it would be very good to have the option of using HepMC in Rivet,
for top analyses but also for just generator studies.

I fully agree with you, that an experimental analysis should be based on
what can be really measured, but if experiments decide to correct to
parton level, one should still have the possibility to use those results
also in Rivet, as they are also in HepData.
Rivet should not be the instance which decides what is right or wrong,
but should be a tool, which includes as many analyses and results as
possible.

So, I fully support your initiative and compromise :)

Best
Hannes


On 20.06.2016, at 22:23, Andy Buckley <andy.buckley at cern.ch<mailto:andy.buckley at cern.ch>> wrote:

Hi all,

As we all know, we *massively* favour writing Rivet analyses based on post-hadronisation particles. And that approach has had increasing purchase in the experiments, with the likes of fiducial "pseudo-top" measurements increasing.

But for top analyses in particular, there are many useful analyses that rely on parton-level tops. For example, we were sent a CMS analysis a few months ago which included a parton-top finder digging around in HepMC... and I've not included it in the official analysis collection because it doesn't fit with our philosophy. I don't need to repeat the many reasons that this approach is suboptimal, but the measurements will continue to be made, there is still useful physics in them, and it seems unfortunate for Rivet to not be able to include them.

I wonder if this situation is sufficiently nuanced that we should swallow our distaste and provide an official "DodgyPartonFinder" to avoid repetition of that fragile code? I'd want to make it print out some warning messages to flag up the dangerous unportability, and clearly mark as dangerous in the .info file of any analysis that uses it... but it's still better than needing to maintain n *different* implementations of dirty HepMC-walking parton finder algorithms.

I'm convinceable either way, but (as having initiated this thread suggests) I'm leaning toward thinking that analysis coverage and pragmatism are sufficiently valuable to allow a compromise... in the case of top physics.

Thoughts & feelings? I expect controversy -- please deliver ;-)

Andy

PS. As Rivet v3 approaches we also need to develop a plan for how future analysis distribution, separated from the core library, can work without destroying the quality control that we've made a key feature. Maybe we'll grasp that nettle in person in September, but I just note here that we could have several "grades" of approval, and hence put partonic top top analyses in a "use with caution" category.

--
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow
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Hannes Jung
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