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

[Andy Buckley]

Hi again Vlad (and hi again to the Rivet list -- please keep it in CC),

You forgot to attach the file! But anyway...

What is your distinction between "shower photons" and "bremstrahlung 
photons"? Strictly bremstrahlung is actually a material interaction 
effect, on a much longer timescale than what is modelled in the 
generator. These are QED shower splittings which dress the hard process 
without any implication of time-ordering: they are as prompt as the W decay.

So I think there can't be any a priori reason to say that these 
electrons are special and should not be labelled as prompt by Rivet -- 
they are produced on a pre-hadronisation timescale and directly 
connected to the electroweak objects in the hard process. It is 
dangerous to interpret the event record as strictly saying that these 
particles are produced "after" the muon because that is just the way 
that the shower is approximating a higher-order matrix element.

Probably these electrons *are* relatively soft compared to the muon, 
though... or are they not? And also probably fairly close to collinear? 
The more physical thing to do is to use final state kinematics to reduce 
the impact of such configurations, if that's what you want. For example, 
require the leptons to be above an energy or pT threshold, and perhaps 
also insist that they are isolated from their anti-particle. (In really 
near-collinear configurations, the pair interacts with the net change of 
zero, cf. the Chudakov effect -- and so would probably be experimentally 
detected as photons, if sufficiently energetic.) So the event only needs 
to be classified as di-lepton (or 3-lepton!) if you find more than one 
*hard & isolated* charged lepton... which is of course the same issue as 
the experiment has to work with, based on the same information. I think 
it comes down to having a robust fiducial acceptance definition that 
also works for MC events without needing special interpretation of the 
recorded shower structure.

Andy


On 16/02/16 17:27, Vladyslav Danilov wrote:
> Hello Andy,
>
> I found a virtual photon, from hard muon bremsstrahlung, that produced e-e+ pair.
>
> In attached file you can find a piece of event generated with PowHeg+Pythia8:
> - 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,
> but here it happens with Bremsstrahlungs photons. It seems that these electrons would be counted as prompt electrons in rivet, causing a single-lepton event to be classified as di-lepton.
>
> 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
>


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