[Rivet] Fwd: e+ e- from photon decay

[Vladyslav Danilov]

Hello Andy,

Sorry for missing that file last time.

During event-by-event comparison i found a virtual photon, from hard muon bremsstrahlung, that produced e-e+ pair.

In attached file you can find a piece of event:
- from vertex 128 we can see that W+ decayed into muon-(bar code 177) and muon antineutrino(barcode 178).
- vertex 129 shows that muon scatters into muon and virtual gamma(barcode 180).
- from vertex 131 we see that v.gamma produces virtual e- and Final State e+
- in vertex 134 we have FS e-

According to Your previous(see below) answer, it only makes sense in resolution of shower photon virtuality > 2m_e, 
so is it a bug in a code or i did a wrong interpretation?

Please write me if You have any questions.

Best regards,
  Vlad




----- Вихідне повідомлення -----
Від: "andy buckley" <andy.buckley at cern.ch>
Кому: "Chris Pollard" <cpollard at cern.ch>, "Vladyslav Danilov" <vladyslav.danilov at desy.de>
Копія: "rivet" <rivet at projects.hepforge.org>
Надіслано: Середа, 3 Лютий 2016 р 16:02:24
Тема: Re: [Rivet] e+ e- from photon decay

I thought a bit more on what Chris has rightly said, and I think (but 
please prove me wrong) that there's a deeper issue in that isolated 
photon conversion is fundamentally Lorentz-symmetry-violating. So it 
only makes sense in the resolution of shower photon virtuality > 2m_e 
and even then is dodgy because that off-shellness is there more by 
accident than design.

The real physical source of conversions in experiments is on-shell 
photon interactions with detector material, which doesn't apply to 
Rivet. Or you may have been referring to matrix element amplitudes with 
a photon propagator and final-state electrons: that's not a well-defined 
event "history", so to be physical you need to make the "photon" 
identification based on some other properties of those final-state 
electrons.

Please let us know if we can help further...

Andy


On 03/02/16 14:37, Chris Pollard wrote:
> Hi Vlad,
>
> We don't have a method that does this, no, largely because this is not a
> generator-independent thing to ask. It's in principal possible for
> off-shell photons to be created in the event shower and to pair produce
> electrons, but I don't think there's any guarantee that the photon is
> actually written to the event record.
>
> Does that make sense?
>
> Chris
>
> On Wed, Feb 3, 2016 at 12:12 PM, Vladyslav Danilov
> <vladyslav.danilov at desy.de <mailto:vladyslav.danilov at desy.de>> wrote:
>
>     Hello!
>
>     My name is Vlad, i work in ATLAS group at DESY.
>     Could You please help me with one question - i wonder to know
>     whether Rivet has some class or method that determines e+ e- to be
>     from photon decay?
>     I spent some time for search it among Rivet classes, but i didn't find.
>
>     Sorry for disturbing.
>
>     Best regards,
>        Vlad
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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow
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