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

[Andy Buckley]

Hi Vlad,

Your interpretation is fine... although do be aware that the kinematics 
of intermediate particles with non-standard status codes says as much 
about the generator code implementation as about any robust definition 
of virtuality.

Anyway, in this case you end up with a "visible" final state from the W 
of one muon, one photon (the very, very soft one emitted at V133), and 
the e+e- pair. If you take the virtuality of photon 180 seriously, then 
it is ~60 GeV off-shell, so well above the 2*electron threshold. By my 
calculation the angle between e+e- pair 183,187 is 0.02 degrees: 
definitely unresolvable! So there is a final-state kinematic criterion 
for these things. The pair has about 45 GeV, so would probably be 
resolved as something like a hard photon.

I don't think this represents a bug in any code. Parton and QED showers 
with 1->2 splittings have to violate Lorentz symmetry, particularly 
obviously in gluon- or photon-splitting to quarks or leptons, when 
masses are included. Those accidental off-shellnesses have to be got rid 
of somehow, and it looks like Pythia will use lepton pair creation as 
part of that procedure.

Andy


On 16/02/16 21:29, Vladyslav Danilov wrote:
> 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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>>
>>
>>
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>


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