[Rivet] Treament of Z/W Bosons in Rivet

[Christian Gutschow]

Hi all,

hang on a second here. While I’m all in favour of adding something like PROMPT vs ALL enums to the W/ZFinder, I would be a little bit cautious about just changing past routines.

There may be cases where adding the promptness requirement will have basically no effect and that’s fine, but there will be cases where it does have an effect and for those a change would invalidate the routine. Track-vertex association is something that’s usually done at the detector level, but that’s essentially irrelevant here. What matters is what the data have been unfolded to: If the particle-level definition includes non-prompt leptons and the data have been unfolded to that, then that’s what should go into the routine. Unfortunate, but still a valid particle-level definition.

Cheers,
Chris


On 14 Jul 2016, at 16:10, Andy Buckley <andy.buckley at cern.ch<mailto:andy.buckley at cern.ch>> wrote:

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
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 —

 Dr. Christian Gütschow

 University College London
 Department of Physics and Astronomy
 Gower Street
 London WC1E 6BT

 > D10 Physics Building
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