[Rivet] MissingMomentum and WFinder problem & plans for 2.4.0

[Andy Buckley]

Done, and pushed. I added an optional "mass" arg to the 
MissingMomentum::visibleMom and ::missingMom methods, and fixed up 
WFinder to use the zero-mass target rather than subtracting the visible 
momentum from (sqrtS,0,0,0) -- cunning, but I think it meant that the 
vector still had the mass of what went down the beampipe, i.e. high!

This definitely needs a validation run. Maybe we could even try running 
a few WFinder analyses in the system ~now, Chris, to make sure I didn't 
completely balls it up?

Andy


On 15/09/15 15:17, David Grellscheid wrote:
> Hi all,
>
> I agree with zero mass as the default. Maybe we can introduce a target
> mass in the MissingMomentum projection, to give theory input in the
> analyses that may need it. But in the absence of any other information
> the m=0 assumption is certainly a good one.
>
>    David
>
> On 15/09/2015 10:09, Chris Pollard wrote:
>> I'm in favor of only exposing the MissingMomentum 3-momentum.
>>
>> I also agree that setting the neutrino mass to zero when solving for
>> the W
>> mass is the right way to go, although David G should comment on whether
>> this would break BSM reinterpretations.
>>
>> Chris
>>
>> On Tue, Sep 15, 2015 at 1:50 AM, Andy Buckley <andy.buckley at cern.ch>
>> wrote:
>>
>>> Hi all,
>>>
>>> A summer student working with Chris and I spotted last week that the W
>>> returned from WFinder is still nonsensical.
>>>
>>> The cuts are ok, and the lepton is also fine, but the use of the full 4D
>>> missing ET vector to make the "pseudo-neutrino" messes up because that
>>> vector has an energy component equal to the energy sum of all the
>>> in-acceptance particles used to calculated the *visible* momentum
>>> against
>>> which the missing one is balanced.
>>>
>>> One option would be to reduce the MissingMomentum to the transverse
>>> vector
>>> only, but that seems too far because it should also be usable in e+e-
>>> colliders. It also seems wrong to give the missing momentum vector a
>>> null
>>> mass, because while that's fine for neutrinos it's wrong for BSM
>>> invisibles.
>>>
>>> I suggest that we change the MissingMomentum interface (again... I
>>> already
>>> made some enhancements for 2.4.0) to only expose a 3-momentum. The E
>>> component is really a misleading accident. What do you think? Any
>>> objections or alternative suggestions? (A user-specified missing
>>> momentum
>>> mass target, maybe?)
>>>
>>> Similarly, we can't just restrict the WFinder to use transverse mass due
>>> to e+e- compatibility. But there we do have a definite hypothesis
>>> that the
>>> missing momentum vector should be a neutrino, so we could set the
>>> pseudo-nu
>>> mass to zero, and feed that into the pseudo-W mass in the case that full
>>> rather than transverse mass is used. Again, thoughts?
>>>
>>> Andy
>>>
>>> PS. Still waiting for analysis integration volunteers. I'll start
>>> forcibly
>>> assigning them if I don't hear soon (this is the procedure we agreed
>>> on!),
>>> so this is your chance to take a quick look at the tarballs and pick a
>>> couple of easy ones ;-)
>>>
>>> --
>>> Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
>>> Particle Physics Expt Group, University of Glasgow
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>>
>>
>>
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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow