progress on tt spin correlations at cppm goal : update the tt spin correlation study in semileptonic...
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![Page 1: Progress on tt spin correlations at CPPM Goal : Update the tt spin correlation study in semileptonic channels using production and decay density matrices](https://reader035.vdocuments.net/reader035/viewer/2022062807/5697bfee1a28abf838cb9b86/html5/thumbnails/1.jpg)
Progress on tt spin correlations at CPPM
Goal :
Update the tt spin correlation study in semileptonic channels using production and decay density matrices
First :
Using the same approach, reproduce our Prague colleagues dileptonic results
Since last meeting :
• Independent tool developed
• Dileptonic channel analysis performed
F. Hubaut, E. Monnier, P. Pralavorio
CPPM10/2003 Top WG meeting
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• Standard PYTHIA does not include spin correlations
• Interfaced Toprex3.28-Pythia6.156 with Atlfast2.22 (thanks to K. Assamagan)
• Then moved to Toprex4.1-Pythia 6.214
Tools
• Use TOPREX* (developed by CMS) which includes full density matrices
• Pythia for hadronization, fragmentation and decays
• Tauola package for the τ decay
• Independent tool
• Generalist: can do single top, Higgs resonance, rare top decays (FCNC, t H+b), …
• Evolutive (future integration in Athena framework…)
URL = http://cmsdoc.cern.ch/~spitskyc/toprex/toprex.html and note hep-ph/0201292
*
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• 2 processes with different theoretical spin correlation implementations are well suited in Toprex :
• CTEQ5L structure function set
• ~20fb-1 of events (~1M events) in the dileptonic channel with process 20, with process 50, and with standard Pythia6.2
• Events reconstructed with Atlfast2.22
Event generation and reconstruction
• process 20 : Standard model tt production (Breit-Wigner approach with off-shell W bosons)
• process 50 : tt production in the on-shell approximation, with the possibility to switch on an intermediate Higgs resonance (switched off in the following)
can help to understand the theoretical systematics
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Theoretical framework and notations largely discussed by Prague colleagues (e.g. see K. Smolek talk given at Prague Atlas week or Atlas note ATL-PHYS-2003-012) :
• Θ+ (Θ-) = angle between chosen spin axis and lepton direction in the t(t) rest frame. Spin axis is t(t) direction in the parton c.m.s. (helicity basis)
• 9 <CosΘ+ · CosΘ-> = unbiased estimator for the asymmetry coefficient A, which measures the spin correlations
Asymmetry coefficient
9 * CosΘ+ · CosΘ-
En
t ri e
s Standard Pythia
Mean = -0.0010.003
= A
In the following, statistical errors < 0.01
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Standard Pythia vs Toprex
Toprex reproduces the LHC expected asymmetry
A=0.00
A=0.01 A=0.46
A=-0.45
A=0.00 A=-0.32
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Toprex 20 vs Toprex 50
Very good agreement between both processes
A=-0.45
A=0.46 A=0.46
A=-0.45
A=-0.32 A=-0.32
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• Signal : events with both W e or μ
• Background : events with at least one W τ e or μ
• Event topology reconstruction in the dileptonic channel is out of our scope (done by Prague colleagues)
Event analysis
Look at the angles between partons
• without any cut (S/B ~ 1/0.382)
• with standard cuts on partons • with standard cuts on reconstructed objects (S/B ~ 1/0.116) – only acceptance effects, no reconstruction efficiency (lepton id., B-tagging, …)
and compute the related asymmetry (for S+B, S, B)
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Standard Pythia vs Toprex
Necessary to study the systematics induced by the cuts
A=0.00
A=0.09 A=-0.14
A=0.04 A=-0.17
A=-0.32
Signal + background
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Toprex 20 vs Toprex 50
Very good agreement at all levels
A=-0.32
A=-0.14 A=-0.14
A=-0.32
A=-0.18 A=-0.18
Signal
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Idem with ISR/FSR
Changes are small with respect to previous plot
A=-0.32
A=-0.13 A=-0.16
A=-0.33
A=-0.18 A=-0.19
Signal
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• All these results (at the parton level) are in fair agreement with Prague results :
• Comparisons between Prague, Toprex20 and Toprex50 give a first guess on some systematics :
Systematics (1)
No cut
Prague Toprex20 Toprex50
S+B -0.332 0.002
-0.321 0.003
-0.331 0.003
Signal -0.341 0.002
-0.323 0.004
-0.330 0.004
No spin
-0.005 0.004
-0.001 0.003
Even before cuts, differences ~0.02
(statistical errors only)
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• Besides this systematics due to event generation, other systematics
– related to the cuts – related to the event topology reconstruction – …
still need to be studied (out of our scope for dileptonic channel)
• Systematics are much larger than statistical errors
Systematics (2)
Control of systematics is one of the key point of this kind of
study
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• A new tool (Toprex+Pythia6.2+Atlfast) is under control for tt spin correlation studies
• Results on dileptonic channel have been reproduced with 2 generation processes : good agreement with Prague, but necessity to study the systematics
Conclusions and outlook
Same study will be done on semileptonic channels including :
• event topology reconstruction
• study of systematics