[Rivet] Partonic top support?

[Andy Buckley]

I am similarly optimistic about our users, and I agree with Ben that 
technical barrier will just lead to people making fiddly & fragile 
workarounds which are then provided as "fixed Rivet" in some public area.

Top quarks are not a perfectly defined final state and we don't need to 
rehash the arguments why that is. But they are also not a perfectly 
useless final state, and for various reasons there are many useful 
historical analyses that have used them as their truth definition. I 
think it is perfectly possible for us to maximise usefulness by 
permitting that (rather than being the self-appointed custodians of what 
is right) while still highlighting its defects and strongly making the 
case for fiducial definitions.

Hannes, just to clarify: the ability to access HepMC objects directly 
will always be part of Rivet, but what I'm suggesting is to *minimise* 
the extent to which users do that. Direct access to HepMC makes the 
analyses more complex and fragile, and I would far rather deal with the 
maintenance issues of different generator record idiosyncracies in a few 
centralised projections than in 100 different slightly broken user 
implementations ;-)

Andy


On 21/06/16 10:50, Frank Siegert wrote:
> Hi all,
>
> I am with Andy and Hannes here, in the sense that we are not a moral
> instance but rather a tool. It is not about maximising the number of
> analyses, but about letting the community decide on whether they want
> to use partonic analyses in certain cases. I don't even see the need
> to introduce any technical (compile flag) barriers, but would be
> confident that the community of Rivet users will on its own decide to
> not use such analyses as they will never really be able to know what
> exactly has been "measured".
>
> Cheers,
> Frank
>
> On 20 June 2016 at 22:23, Andy Buckley <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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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow