[Rivet] CMS dressed top question

[Andy Buckley]

You should do whatever is the appropriate MC analysis strategy for CMS
W/top/b-jet reco. That sounds like a reasonable approach to me, but of
course there are (imperfect) ways to distinguish single- and
double-tagged b-jets -- jet shapes, subjets, maybe pflow tricks -- and
it's presumably analysis-specific whether they are being used.

This is why it's so valuable to get the people who did the analysis
involved in implementing the Rivet analysis -- typically only they will
know exactly what is appropriate for their analysis. (Helped by people
who know the Rivet technicalities and who appreciate the sorts of issues
involved in making robust and model-independent MC analyses I mentioned
in my last email.)

Andy


On 12/11/14 12:37, Lars Sonnenschein wrote:
> 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
> 
> 
> 
> ______________________________________
> Dr. rer. nat. habil. Lars Sonnenschein
> ______________________________________
> Home Institution:
> RWTH Aachen
> III. Phys. Inst. A, 26A204
> Physikzentrum
> 52056 Aachen
> Germany
> --------------------------------------
> ______________________________________
> CERN:
> PH/UCM, 32/2C-07
> CH-1211 Geneve 23
> Switzerland
> Tel.:+41(22)767-9875
> --------------------------------------
> ______________________________________
> FNAL:
> D0, PK151
> Mailstop #352
> Fermilab, P.O.Box 500
> Batavia, IL 60510-500
> USA
> Tel.: +1(630)840-8740
> ______________________________________
> 
> 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
>>


-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN