Optimization of vector-boson-fusion Higgs candidate

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

B. Millán Mejías 2

Outline

• The Higgs Boson• The Atlas Experiment• Vector Boson Fusion as Higgs candidate• TMVA• Signal and Background Characteristics• Results• Conclusion



Standard Model

τ

ντμ

νμ

bt

sc

eνe

Lept

ons

WZgγ

Bos

onsQ

uark

s

du

H



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



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



Atlas weight = Eiffel-tower weight

Atlas size = Dam palace size



Atlas Detector•40 million proton-proton collisions per second

2000 Thousand people



What a Higgs boson might look like in the Atlas detector



VBF H WW (ll)Vector Boson Fusion

e, muon

e, muon

Jet: Spray of particle originating from a quark or gluon

quark

quark



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



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



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.



TMVA

“The Toolkit for Multivariate Analysis (TMVA): a machine learning environment for the processing and parallel evaluation of sophisticated multivariate classification techniques. ”



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/



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




Nonlinear Analysis: Boosted Decision Trees (BDT)


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.



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



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 +



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



Higgs signal event

Background Background

Higgs signal event



Signal

Background

Signal

Signal



Preliminary results



Each method gives 1 output observable:

0 means background,

1 means signal



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



MLP significance Genetic Algorithm



MLP:

Accepted number of signal events: 5.9

Accepted number of background events: 11.1

S/sqrt(B) : 1.62



BDT significance

s/sqrt(b) > 5



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



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).



Thank you!

[email protected]

mailto:[email protected]



Pick a number, the same number and shows the efficiency, for the training and signal sample

Overtraining example



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



Output (preliminary)





Higgs W–W+

l+ l–

Documents

Optimization of vector-boson-fusion Higgs candidate