[Rivet] Rivet: Accessing intermediate particles

[Andy Buckley]

On 30/06/12 18:43, David Hall wrote:
> Hi Hendrik,
> 
> When I look at the parents of the electrons, it just gives me the
> incoming protons. It does this for >=2 electrons, so not just for
> electrons from the hard scatter. So it seems I can't differentiate
> between electrons this way.

Ok, so Sherpa says that the parents of *all* electrons are the protons,
and these have status 2? That doesn't sound right -- beam particles
should have status 4, and even in Sherpa there should be hadronic decay
chains in the event record: checking whether the parent satisfies the
PID::isHadron(pid) function would be another physically valid check that
you can't do in real experiments.

How are you accessing the lepton parents in HepMC? There are several
ways, one of which would really list *all* the upstream ancestor
particles in the event record, starting with the beams... but that
wouldn't be the right one to use.

> I agree that in an ideal world we can just treat everything exactly as
> we would in data. Unfortunately, due to time constraints and deadlines
> we often need a quick way to do things, so we need to be able to do
> truth-level studies like this relatively quickly.

That's not a good argument in general -- it doesn't take *that* long to
write a truth analysis (certainly not compared to doing the data
analysis part) and the "quick and obvious" way is often limited in
accuracy and physical meaning in ways that are not immediately
appreciated, especially if, e.g. your gamma* was really an admixture of
gamma* and Z0 and you wanted to distinguish those components. But
checking whether a particle was produced in a hadronic decay or a
pre-hadronisation part of the event should be possible and reliable, so
I hope this issue with Sherpa is just a technical one...

Cheers,
Andy


> On Sat 30 Jun 2012 19:25:52 CEST, Hendrik Hoeth wrote:
>> Hi David,
>>
>> Thus spake David Hall (David.Hall at physics.ox.ac.uk):
>>
>>> Unfortunately, when I check the status of each parent of the electron
>>> in an event, I find that they are _all_ status 2. This suggests that
>>> all the electrons are from decays, but I know this shouldn't be the
>>> case. Two should be from the hard scatter. Is it possible this is
>>> specific to Sherpa?
>>
>> Well, technically even in the hard scatter the electrons are decay
>> products of something. Though luck, eh? I mean ...
>>
>>>>> I understand the philosophy behind Rivet not providing access to the
>>>>> intermediate particles, but I believe that looking for the particles
>>>> >from the hard process in this way is not _so_ bad. :-)
>>
>> ... nature doesn't mark the particles from the hard process, either. So
>> the best thing is to resort to physically meaningful observables,
>> similar to what you do in the experiment.
>>
>> Cheers,
>>
>>      Hendrik
>>
>> PS: I assume you know you can walk through the whole HepMC tree in the
>> same way you asked for the electrons' parents. That's all the generator
>> supplies.
>>
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-- 
Dr Andy Buckley, SUPA Advanced Research Fellow
Particle Physics Expt Group, University of Edinburgh

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