[Rivet] DressedLeptons and photons from tau decays

[Markus Seidel]

Hi Andy, hi Frank,

I was not so much concerned about the photons directly from the tau but 
about those from the tau daughter leptons.

If we look at these lines:
>     return _hasRelativeWith(HepMC::ancestors, [](const Particle& p){
>         return p.genParticle()->status() == 2 && isTau(p);
>       });

It checks if there was a tau in the photon ancestors.
So if we have a chain tau->electron->photon, the photon is not used for 
the dressed electron because there is a tau upwards?

Is my interpretation correct, or do I just misunderstand the meaning of 
HepMC::ancestors?

Cheers,
Markus

Am 07.02.2017 um 21:49 schrieb Andy Buckley:
> On 06/02/17 15:07, Markus Seidel wrote:
>> Dear all,
>>
>> what is the reason for excluding photons from tau decays from the
>> DressedLeptons?
>>
>> Doesn't this prevent to cluster photons from electrons from taus?
>
> Hi Markus,
>
> I think the projection was probably written considering (as is usually
> the case, in my experience) leptonic taus to only be observed via their
> muon or electron child -- and it is the child rather than the parent
> whose momentum vector is used as the centre of the dressing cone.
>
> DressedLeptons isn't designed for hadronic taus, and I suspect this rule
> is in there to avoid "contamination" from hadronic tau decay photons
> into prompt electron or muon dressing.
>
> You're probably right that there's an issue in how it's implemented,
> though: it does seem reasonable, or at least would be a reasonable
> option, to include photons directly from a prompt leptonic tau when
> dressing that tau's electron/muon child. The neutrinos are still
> "missing", so it may not help much, but is physically well-defined. I
> *could* add an option flag of that sort, to restrict the veto to
> *hadronic* taus only: is that what you had in mind?
>
> Frank Siegert may know more: he originally implemented DressedLeptons
> (then called ClusteredLeptons), and I can'd remember if he or I added
> the tau photon veto! Frank??
>
> Andy
>