[Rivet] PartonicTops with em shower

[Andy Buckley]

Hi again Markus,

I finally got time to think about this, and I think the 
!p.hasAncestor(PHOTON) route is preferable to modulo-2 arithmetic. 
Perhaps/probably in time we will come across another generator which 
breaks this heuristic, but it seems pretty reasonable to me.

I think with this, there is no need for the prompt_e && !prompt_mu type 
of logic, but what do you mean by "for the hadronic part there needs to 
be an `xor`"? I know what an xor is, of course, but am not sure which 
line you're referring to, or whether it still applies in the 
p.hasAncestor() approach. I think everything should be ok (fingers 
crossed!) with just the hasAncestor(PHOTON) check, but please let me know.

Having just done this and a bunch of other fixes, I'm hoping that we can 
get the backlog of analyses integrated and go for a Rivet 2.5.4 release 
in the next week or so.

Cheers,
Andy


On 20/04/17 10:04, Markus Seidel wrote:
> Dear Rivet authors,
>
> as you know PartonicTops projection is important for many CMS top
> analyses at 8 TeV :s
> People stumbled upon the following problem: If there is a photon->l+l-
> within the shower, the top mother is flagged as leptonic.
>
> Maybe we can replace
>> const bool prompt_e/mu = any(...);
> by
>> const bool prompt_e/mu = (count(...) % 2);
> and
>> if (prompt_e/mu && (...)) return true;
> by
>> if (prompt_e/mu && !prompt_mu/e && (...)) return true;
>
> For the hadronic part there needs to be an `xor` between `prompt_e` and
> `prompt_mu` at least...
>
> If you prefer to not mess around with the logic, we may just require
> that the lepton does not have any photon mother...
>> ... && !p.hasAncestor(PID::PHOTON)
>
> It seems to be hopeless to account for all possibilities of a full weak
> shower but at least the case of em shower is very common (at least in
> Pythia 8?).
>
> Cheers,
> Markus
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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow