[Rivet] DressedLeptons and photons from tau decays

[Andy Buckley]

Hi Markus,

Sorry it's taken a while to get back to you on this -- I had a very busy 
period. Taking advantage of a train journey to reply!

It took a little digging to work out what code you were referring to: 
that's a couple of steps away, in Particle::fromTau, right? Which is 
called via Particle::fromDecay, if DressedLeptons is called with the 
fromDecay=true option. I think you're right about the unintended 
consequence, though.

I agree this stuff is a bit messy: when I wrote it, I didn't want to 
proliferate a multitude of options about vetoing photons 
directly/indirectly from hadrons and/or all/hadronic taus -- you get the 
idea from all the slashes, hopefully! But in retrospect I think I should 
have probably provided a isHadronicTau Particle classifier function*, 
and used the logic fromHadronicDecay = fromHadron || fromHadronicTau. 
"fromDecay" is too vague.

Would that meet your approval: if !fromDecay, don't use any photons for 
clustering which either have an unstable hadron or a prompt, 
hadronically decaying tau in their history?

Thanks,
Andy

* Or just a generic hasHadronic/LeptonicDecay? That's the problem with 
this stuff -- so many possible combinations, but which are the most 
natural/flexible/convenient?! I'm open to suggestions!



On 08/02/17 08:06, Markus Seidel wrote:
> 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
>>


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