[Rivet] Treament of Z/W Bosons in Rivet

[Andy Buckley]

Hi David,

Yes, that seems very sensible to me. And it needs to be an *internal* 
application of the PromptFinalState because we have to respect the flag 
for including non-prompt photons in the lepton dressing.

Making it disableable is important, I think, but I'd be tempted to 
change the default behaviour to only use prompt charged leptons. Rather 
than use bools I think the interfaces should be extended to use new 
W/ZFinder::ChargedLeptons { PROMPT, ALL } enums in the constructors. To 
avoid clashes, maybe we should start using the more powerful C++11 "enum 
class" syntax.

Since this is an API & physics change, we probably need to target it for 
Rivet 2.6.0. I would hence like to tie it in with the proposed API 
changes to allow uniform access to composite particles, for the new 
smearing system to use.

Andy


On 14/07/16 15:55, Yallup, David wrote:
> Hi all,
>
>
> I've been looking at using the current library of Rivet analyses for BSM
> limit setting, which is going to be an exercise involving some 'safety
> proofing' of the analyses! One of the issues we've run into is spurious
> signals arising from fake Z/Ws that shouldn't be faking.
>
>
> For example a model adding additional heavy flavour pair production, the
> lepton pairs from heavy flavour hadron decays have the possibility to
> fake Z/W signals (has a noticeable statistical effect in cases). The way
> around is then to build the Z from promptfs projection, which has the
> right effect from our PoV (I have some modified versions of Z/W analyses
> of interest I've been running like this, written by David Grellscheid)
> and would be happy to propagate a change like this through through other
> analyses needed and hand them over for testing.
>
>
> I think this is a sensible safety feature to add, and is already
> something experiments have attempted to cater for in the definition of
> the fiducial cross section, via track-primary vertex association or
> something along those lines, so should be mirrored in the rivet routine.
> The question then is should there be a switch in the convenience
> Z/WFinders checking for isPrompt on the leptons by default? I think this
> would then be sensible for people going forward, as it would be sort of
> in the spirit of the Finders, doing the hard work for you and all that,
> and making the sensible choices needed by default. Obviously in quite a
> few cases the Finders aren’t even used, so sorting the projections out
> would still be done by hand.
>
>
> Anyway wanted to see if there are any opinions on this before I start
> churning through old analyses or anything like that.
>
>
> Cheers,
>
> David
>
>
>
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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow