optimization of vector-boson-fusion higgs candidate
DESCRIPTION
Optimization of vector-boson-fusion Higgs candidate. Search for the Higgs particle. Master in Particle and astroparticles. Bárbara Millán Mejías. Amsterdam Master of Physics Symposium 2008. Supervisor: Max Baak. Outline. The Higgs Boson The Atlas Experiment - PowerPoint PPT PresentationTRANSCRIPT
Optimization of vector-boson-fusion Higgs candidate
Bárbara Millán Mejías
Master in Particle and astroparticles
Supervisor: Max Baak
Amsterdam Master of Physics Symposium 2008
Search for the Higgs particle
UvA Master Symposium, 25/05/08
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Outline
• The Higgs Boson• The Atlas Experiment• Vector Boson Fusion as Higgs candidate• TMVA• Signal and Background Characteristics• Results• Conclusion
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Standard Model
τ
ντμ
νμ
bt
sc
eνe
Lept
ons
WZgγ
Bos
onsQ
uark
s
du
H
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The Higgs boson - Holy grail! Plays the key role
explaining the origin of the mass of all known particles
Only particle not observed yet from Standard Model.
This particle is often called the Holy grail of particle physics
Higgs at the Atlas experiment
• 114 < Hmass <182 GeV/c2• W mass = 80GeV
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Big ring = 27 km circumference•LHC is part of CERN …•CERN is dedicated to the study of elementary particle and their interactions•Start end of summer 2008 …
The experiment will have at the center of mass 7 times more energy than the biggest experiment now (Tevatron)
Because this energy is so high it will certainly help to find or exclude the Higgs Boson.
100m
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Atlas weight = Eiffel-tower weight
Atlas size = Dam palace size
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Atlas Detector•40 million proton-proton collisions per second
2000 Thousand people
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What a Higgs boson might look like in the Atlas detector
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VBF H WW (ll)Vector Boson Fusion
e, muon
e, muon
Jet: Spray of particle originating from a quark or gluon
quark
quark
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Characteristic Signal
• 2 isolated leptons (electron or muon) pointing in same direction• 2 Tagging Jets with high momentum.• Missing transverse energy from the neutrinos• Clean events (few jets per event)
qqH qqW+W- qql+ l - pT
Tagging JetTagging Jet
Leptons
Neutrinos -> Missing Et
Proton
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Expected backgrounds processes
Anything that produces 2 jets, 2 leptons, and missing transverse energy … (and can be confused as a Higgs boson)
• top – anti-top quark pair (top->W+b)• Z+ 2 jets• WW + 2 jets• W + 3 jets
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Needle in a haystack
Simulated Samples
Expected cross section (fb)
SignalVBF
Hm=170GeV 150
top-antitop 449820
Background WW+2j 1273
W + 3j 593343
Z + 2p 28762
Signal events= 150 Background events= 1073198
My research: optimize the filtering of Higgs events based on Higgs decay characteristics.
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TMVA
“The Toolkit for Multivariate Analysis (TMVA): a machine learning environment for the processing and parallel evaluation of sophisticated multivariate classification techniques. ”
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A linear boundary? A nonlinear one?
Event Classification
We have found discriminating input variables x1, x2, …
What decision boundary should we use to select events of type H1 ?
Rectangular cuts?
H1
H0
x1
x2 H1
H0
x1
x2 H1
H0
x1
x2
How can we decide this in an optimal way ? Let the machine learn it !
Suppose data sample with two types of events: H0, H1
http://tmva.sourceforge.net/talks/
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Nonlinear Analysis: Artificial Neural Networks
Works like interacting neurons in a brain …
The neural network optimize the weights between the neurons
1
i
. . .N
1 input layer k hidden layers 1 ouput layer
1
j
M1
. . .
. . . 1
. . .Mk
2 output classes (signal and background)
Nvar discriminating input variables
11w
ijw
1 jw. . .. . .
( 1)1,2kx
Feed
-forw
ard
Mul
tilay
er P
erce
ptro
n
http://tmva.sourceforge.net/talks/
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Nonlinear Analysis: Boosted Decision Trees (BDT)
http://tmva.sourceforge.net/talks/
The Data comes into nodes, a condition is evaluated. On the result and exam is done and it decides to continue on one of two branches.
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TMVA methods I appliedMethod pros cons
Cuts Rectangular cuts Old fashion
Most simple Most simple,I do not have results yet
Fisher Linear discriminants Simple, robust against overtraining
Cannot be tuned
MLP New advanceComplex (non-linear)
Good results overtraining
BDT Newer more advance, complex (non-linear)
Superior results Need more statistics / overtraining
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Simulated Samples used in Higgs signal optimization
• Tagjets:• 2 jets with highest pT
• Higgs:• 2 leptons between tagjets, with opposite charge
• Event selection• 2 leptons, 2 or 3 jets
• b tagging jet • Identified the best b jet • (2 b-jets from top-antitop quark pair decays)
Preselection throws away any other background, except for the ones already mentioned …
We use simulated proton-proton collisions …
•Top-anti top
•W +Jets
•WW +
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Observables used in the signal optimization
• TagJets ||• TagJets invariance mass• Significance of the best b jet• Missing transverse energy from neutrinos
(We are using small number observables to prevent overtraining)
=0
=infinity
=1.5
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Higgs signal event
Background Background
Higgs signal event
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Signal
Background
Signal
Signal
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Preliminary results
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Each method gives 1 output observable:
0 means background,
1 means signal
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Fisher significance
Fisher:
Accepted number of signal events 7.1
Accepted number of background events 20.7
Significance of result: S/sqrt(B) 1.47
S= number of signal events
B=number of background events
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MLP significance Genetic Algorithm
S= number of signal events
B=number of background events
MLP:
Accepted number of signal events: 5.9
Accepted number of background events: 11.1
S/sqrt(B) : 1.62
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BDT significance
s/sqrt(b) > 5
S= number of signal events
B=number of background events
BDT:
Accepted number of signal events: 5.0
Accepted number of background events: 2.3
S/sqrt(5/2+B) : 2.30
Higgs can be claim
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Conclusion and to do
• Optimization results not yet finished.• Yet: the current results are that BDT method
works best at finding the Higgs boson using vector-boson-fusion.
• To do: Still need to fine-tune optimization methods and get overtraining under control.
• For 100 days of continuous LHC running (1/fb data), expect a signal significance of 2.3 sigma (for a Higgs with a mass of 170 GeV/c2).
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Thank you!
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Pick a number, the same number and shows the efficiency, for the training and signal sample
Overtraining example
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Mass boundaries
• The highest possible Hmass is at the scale of TeV/c2
• 114 < Hmass <182 GeV/c2
• If Hmass> 130Gev/c2 rule out most of the super symmetry models
• No-lose theorem
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Output (preliminary)
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Higgs W–W+
l+ l–