[Rivet] CMS dressed top question

[Lars Sonnenschein]

Hello Andy,
so if I understood you right we should only look at the
vector sum of all neutrinos in a generated event and then reconstruct
the tops up to an eight-fold ambiguity in the ttbar dilepton 
channel, assuming that we are not able to 
distinguish a b-jet from a bbar-jet?

 	Lars



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Dr. rer. nat. habil. Lars Sonnenschein
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On Wed, 12 Nov 2014, Andy Buckley wrote:

> On 12/11/14 09:45, Lars Sonnenschein wrote:
>> Hello Andy,
>
> Hi Lars,
>
> Please contact the Rivet list rather than me personally. I've CC'd it
> (again ;-) )
>
>> we are still struggling with the first top analysis getting into Rivet.
>> In the ttbar dilepton channel we get two oppositely charged leptons and
>> two neutrinos we are touching in Rivet to reconstruct the two W bosons.
>>
>> Are we allowed to make use of the information if it is a neutrino or an
>> anti-neutrino we are dealing with?
>
> It's up to you, but I'd advise against it unless CMS has some
> extraordinary subdetector that I'm not aware of which can do detailed
> neutrino particle ID ;-)  The experimental analysis needs to resolve the
> ambiguity, therefore it's best that the equivalent MC analysis does so, too.
>
> And of course it's actually a worse problem than you say -- you can only
> see one missing energy vector, not two separate (anti)neutrino
> components within it, so you can't treat the two tops independently.
>
>> This would remove one ambiguity.
>> The temporary version of the Rivet code is right now making a choice:
>>
>>   |(lepton1+neutrino1).M()-W.M()|
>>   + |(lepton2+neutrion2).M()-W.M()|
>>   < |(lepton1+neutrino2).M()-W.M()|
>>   + |(lepton2+neutrion1).M()-W.M()|
>>
>> for the pair which matches better the W boson mass without exploiting
>> neutrino/anti-neutrino information.
>
> Again, how can you experimentally determine the momentum vectors of
> multiple neutrinos in a single event? If you think you can do that
> safely, then being able to tell whether they are nu's or anti-nu's is
> not a big leap. The important thing to think about is whether you can
> safely use this sort of experimentally unavailable truth information at
> all -- naively the answer is no.
>
> The (only?) situation in which it *is* appropriate is where the data has
> been unfolded, using MC models and adding significant modelling
> systematics, to an MC analysis which used this full information. Then
> you would be writing a Rivet analysis using identical logic to the MC
> truth analysis that went into the data analysis. That is fine, but doing
> the data analysis that way (i.e. relying on unfolding to perfectly deal
> with an intrinsic experimental analysis ambiguity) is rather
> unsatisfactory and I suspect is not the case.
>
> Andy
>
> -- 
> Dr Andy Buckley, Royal Society University Research Fellow
> Particle Physics Expt Group, University of Glasgow / PH Dept, CERN
>