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Vector Boson Scattering At High Mass
Hanzhe Liu (USTC) Advisor:Junjie Zhu (UM)
REU final presentation 2012.8.10
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Introduction
• What is VBS (vector boson scattering)
• Why VBS is important
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Overview• Monte Carlo Generation of signal: Pythia• Event kinematics studies• Using Boosted Decision Tree (BDT) to separate
signals from the Standard Model backgrounds
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High Mass Resonance Models
• exchange a SM Higgs boson• 1 TeV scalar ( used for BDT studies )• 1.4TeV vector • 1.9TeV vector • a 800 GeV scalar and a 1.4 TeV vector
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MWW distribution with a SM Higgs boson
• Exchange a SM Higgs boson• minimum WW invariant mass -> 200GeV
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Inclusive cross sections
Cross sections (fb):SM:2.116 (black)
1TeV Scalar:2.65 (red)
1.4TeV vector:1.528 (green)
1.9TeV vector:1.746 (blue)
800GeV scalar &1400TeV vector:5.164 (pink)
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Event Pre-selection
• 2 isolated leptons with pT>25 GeV, |η|<2.5• 2 forward jets with pT>30 GeV, |η|<4.5
• Backgrounds: WZ, ZZ, Wγ, W+jets, Z+jets, WW(SM), ttbar, singletop
b-tagging
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Kinematic Variable
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Kinematic Variables 2
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Kinematic Variable 3
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Input correlation matrix
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Multivariate method• In fact, compared to cut based methods, the advantage of BDT is to analyze the
correlations of different variables • Input data MVA (multivariate)
A large variety of multivariate classification algorithmsExtract a maximum of the available information from data
Output: each events has a scoreOutput distribution looks like this:
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Boosted Decision Tree
• A decision tree• Define the purity in a node:
• Define:• When a node is split into two nodes, one minimizes:
• The resulting tree is a decision tree• One decision tree is not enough!
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Boosted Decision Tree• notations:
• adaptive boost (adaboost):• Define for the mth tree:
• Change the weight: renormalisation: training events……(takes a long time) Finally, the score for a given event is:
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BDT training
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Cut efficiency
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BDT application
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Reference
[1] von Jan W. Schumacher PhD-CERN-THESIS-2010-140_VBS[2] Hai-Jun Yang , Bryon P.Roe , Ji Zhu Studies of Boosted Decision Trees for MiniBooNE Particle Identification
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Thank you !