[Rivet] Rivet: Accessing intermediate particles

[David Hall]

Hi Andy,

Its possible that they are not the incoming protons. I just checked the 
PDG code for the parents and it returned 2212. I can do this additional 
check though to see if they are incoming protons.

I am finding the electrons in the following way (sorry for the code, but 
I thought it was the simplest way to explain):

IdentifiedFinalState elecs(-100.0, 100.0, 0.0*GeV);
elecs.acceptIdPair(ELECTRON);
addProjection(elecs, "elecs");

...

ParticleVector elecs;
foreach(const Particle& elec, applyProjection<FinalState>(event, 
"elecs").particlesByPt()){
     GenVertex* prodVtx = elec.genParticle().production_vertex();
     if (prodVtx == 0) continue;
     bool isFromDecay = false;
     foreach (const GenParticle* parent, particles(prodVtx, 
HepMC::parents)){
           if (parent->status() == 2)
                 isFromDecay = true;
     }
     if (!isFromDecay)
           elecs.push_back(elec);
}

I'd still be interested to hear how to do this, but I believe it is too 
late for my deadline.

Thanks,
David


On 30/06/12 20:07, Andy Buckley wrote:
> 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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>