data based method for background subtraction: z · 2006-04-20 · jetset herwig pythia. halina...
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Data based method for background subtraction:
Z →μ+μ-
Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni Nicoletti
Laboratori Nazionali di Frascati, Università di Roma La Sapienza, INFN
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
OutlineMotivation.Example of application: bb Z → µ+ µ-
Method proposedSoftware & Generators• Simulation, Reconstruction, Analysis.
Sample Conclusion and future development
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
MonteCarloIsajet Ariadne Sherpa
Les Houches Guidebook hep-ph/040345Cojest
JetsetPythiaHerwig
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Shower MonteCarlo
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Motivation
It is then extremely important to relyon alternative method to MonteCarlo, as much we
canWe propose a method based on a different
data sample of experimental dataOnly hypothesis: lepton universality.Example of application: bb Z → µ+ µ-
(most copious background of bb h0/A → µ+ µ- studies (14/12/2005) S.G. presentation, ATLAS-PHYS-2003-015, June 2003.)
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Signal & background
h0 bb→ µ− µ+bb
σ~ 0.2 - 0.01 pb(even lower)
σ ~ 22.8 pb
Zbb → µ-µ+ bb
(Pythia 6.226)AcerMC (v.2.3) interfacedwith Pythia 6.2 (hep/ph0405247).
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Background Subtraction Method
Experimental methodproposed based onZ→ µ+ µ− and Z→ e+ e-
Relying on experimental data
Br( h0→ e+ e-) neglegible
Different Inner Bremsstrahlung
Precise Knowledge of background is crucial
2
⎟⎟⎠
⎞⎜⎜⎝
⎛∝
emm µ
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Z production
bbbhbgg −+→→ µµ
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Software Release & SampleGeneration Athena 9.0.4 - Pythia (v.6.226) - Acer (v. 2.3)Simulation, Digitalization, Reconstruction Athena 10.0.1Analysis based on CBNT (root –ntuples).Final study in scenario
~ 600000 eventsIn this presentation Lint ~ 20 fb-1
~ 400000 events
∫ −= 130 fbLdt
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Study performedDifferent Inner Bremsstrahlung
Zbb → µ-µ+ bb (γ) Nγ ~ 0.086 Nev=464522Zbb → e-e+ bb (γ) Nγ ~ 0.157 Nev=442828Different Detector ResponseThe most significative variable of
bb h0/A → µ+ µ- analysis has be chosen Minv as test .NB. The combination of a same flavor lepton originating from b and one of Z are included inevitably in the plots
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Bremsstrahlung Photons
NγMC ~ 0.157Nγ
MC ~ 0.086
Zbb → µ−µ+ bb Zbb → e−e+ bb
e
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Bremsstrahlung PhotonsZbb → e−e+ bbZbb → µ−µ+ bb
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level: Zbb → µ-µ+ bbMinv(µ+µ-) Minv(µ+µ-)γ
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level: Zbb → µ-µ+ bbMinv(µ+µ-) Minv(µ+µ-)γ
Only events with a γ of Bremsstrahlung
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level: Zbb → µ-µ+ bb
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level: Zbb → e-e+ bbMinv(e+e-) Minv(e+e-)γ
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level: Zbb → e-e+ bbMinv(e+e-) Minv(e+e-)γ
Only events with a γ of Bremsstrahlung
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level: Zbb → e-e+ bb
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level
Bremsstrahlung events
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generator level
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Leptonsmuons electrons
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Photons: Zbb → µ-µ+ bbBremsstrahlung photonsAll photons
PT vs. Distance between γ and µ watch the different scale!Subsample(~25000 ev)
22 φηR ∆+∆=∆
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Photons: Zbb → µ-µ+ bb
Distance between γ and µ watch the different scale!
∆R ~ 0.15
All photons Bremsstrahlung photons
22 φηR ∆+∆=∆
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Photons: Zbb → e-e+ bb22 φηR ∆+∆=∆
PT vs. Distance between γ and e watch the different scale!Subsample(~25000 ev)
All photons Bremsstrahlung photons
5x5 cluster in ATLAS
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Photons: Zbb → e-e+ bbAll photons Bremsstrahlung photons
∆R ~ 0.25
22 φηR ∆+∆=∆
Distance between γ and e watch the different scale!
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
PhotonsIf the photon is emitted close to electron,
only one cluster is reconstructed and the combined4-momentum is measured. For the γ is effectively absorbed in the same cluster of the electron.
corresponds to a 5x5 cluster in ATLAS. The energy of electron is measured in e.m. calorimeter, θ and φ are measured in tracker.Photons, inside cone ∆R, are taken in account summing their 4-momentum to the 4-momentum of the nearest lepton. Photons reconstructed are e.m. clusters without track associated (∆η= 0.025,∆φ =0.05 ) and PT >5GeV, |η| >2.5.
Rφη ∆<∆+∆ 22
125.022 =∆+∆=∆ φηR
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
All Photons: Zbb → µ-µ+ bbZbb → µ−µ+ bb Zbb → e-e+ bb
PT γ> 5 GeV |η| >2.7 recontructed γ
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
ReconstructionMuons: combined reconstruction (inner detector + spectrometer), PT >10 GeV, |η| >2.5.Electrons: energy measured in calorimeter,(θ,φ) measured from tracker, PT >10 GeV , |η| >2.5.Photons: e.m. cluster without track associated :
∆η= 0.025, ∆φ =0.05, PT > 5 GeV, |η| >2.5.Brem photons: >2 leptons reconstructed +
γ reconstructed in a cone around lepton direction < ∆R. Electrons sample only one cluster is reconstructed. Muon sample: µ track near photon cluster, misidentification.
The number events reconstructed with γ brem identified is atpercent level.
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Reconstruction: Zbb → µ-µ+ bb
Minv(µ+µ-)γMinv(µ+µ-)
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Reconstruction: Zbb → µ-µ+ bb
Minv(µ+µ-)γMinv(µ+µ-)
Only events with a γ of Bremsstrahlung
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Reconstruction: Zbb → e-e+ bb
Minv(e+e-)γMinv(e+e-)
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Reconstruction: Zbb → e-e+ bbMinv(e+e-) Minv(e+e-)γ
Only events with a γ of Bremsstrahlung
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Reconstruction
Only events with a γ of Bremsstrahlung
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Reconstruction
The number γ bremsstrahlung reconstructed separatlyin muons events is higher than electron events, where are mostly absorbedin the electron cluster
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generation-reconstruction
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Generation-reconstruction
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
ConclusionsA study for an experimental method of background subtraction is developed based on fully simulated samples, Lint ~ 20 fb-1 of:
bb Z → µ+ µ- and bb Z → e+ e-
comparing:the Bremsstrahlung of the two samples.the ATLAS detector response.
The study is preliminary and needs careful tuning on data collected.∆R is an extremely important parameter, that should be carefully tuned.
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
ConclusionsIn the region Minv 98 -135 GeV the ratio is reasonablystable and the effect of Bremsstrahlung is negligible with respect to detector responseThe same method can be applied to H→ ττ and in general to all decays where the electron decay is negligible in the signal.The method relies on lepton universality and needs a good knowledge of detector response for different leptonsThe method doesn’t need an implementation on MC of higher order corrections
Acknowledgements: Borut Kersevan, Dirk Zerwas,Donatella Cavalli
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Riserve
Riserve
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
Matching track narrow∆η= 0.025 ∆φ =0.05
Isolation broad∆ η= 0.05 ∆φ =0.10
∆R =0.07 size of cluster 5x 5 ∆η= 0.025 ∆φ =0.05
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.
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Halina Bilokon, Vitaliano Chiarella, Simonetta Gentile, Giovanni NicolettiHiggs Meeting 18 April 2006,CERN.