Optimization of vector-boson-fusion Higgs candidate

1. 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

2. Outline • The Higgs Boson • The Atlas Experiment • Vector Boson Fusion as Higgs candidate • TMVA • Signal and Background Characteristics • Results • Conclusion B. Millán Mejías

3. u c t γ Bosons d s g Z νμ ντ W μ τ Standard Model Quarks b νe Leptons e H B. Millán Mejías

4. 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 B. Millán Mejías

5. 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 … 100m 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. B. Millán Mejías

6. Atlas weight = Eiffel-tower weight Atlas size = Dam palace size B. Millán Mejías

7. Atlas Detector • 40 million proton-proton collisions per second 2000 Thousand people B. Millán Mejías

8. What a Higgs boson might look like in the Atlas detector B. Millán Mejías

9. e, muon e, muon Vector Boson Fusion VBF H  WW (ll) quark quark Jet: Spray of particle originating from a quark or gluon B. Millán Mejías

10. qqH qqW+W- qql+ l - pT Characteristic Signal Leptons Tagging Jet Tagging Jet Proton Neutrinos -> Missing Et • 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) B. Millán Mejías

11. 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 B. Millán Mejías

12. Needle in a haystack Signal events= 150Background events= 1073198 My research: optimize the filtering of Higgs events based on Higgs decay characteristics. B. Millán Mejías

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

14. x2 x2 x2 H1 H1 H1 H0 H0 H0 x1 x1 x1 Event Classification • Suppose data sample with two types of events: H0, H1 • We have found discriminating input variables x1, x2, … • What decision boundary should we use to select events of type H1? Rectangular cuts? A linear boundary? A nonlinear one? • How can we decide this in an optimal way ?  Let the machine learn it ! B. Millán Mejías http://tmva.sourceforge.net/talks/

15. Nonlinear Analysis: Artificial Neural Networks • Works like interacting neurons in a brain … • The neural network optimize the weights between the neurons 1 input layer k hidden layers 1 ouput layer ... 1 1 1 2 output classes (signal and background) . . . . . . . . . Nvar discriminating input variables Feed-forward Multilayer Perceptron i j Mk . . . . . . N M1 B. Millán Mejías http://tmva.sourceforge.net/talks/

16. 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. B. Millán Mejías http://tmva.sourceforge.net/talks/

17. TMVA methods I applied B. Millán Mejías

18. 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) • Top-anti top • W +Jets • WW + Preselection throws away any other background, except for the ones already mentioned … We use simulated proton-proton collisions … B. Millán Mejías

19. Observables used in the signal optimization • TagJets |Dh| • TagJets invariance mass • Significance of the best b jet • Missing transverse energy from neutrinos • (We are using small number observables to prevent overtraining) h=0 h=1.5 h=infinity B. Millán Mejías

20. Higgs signal event Higgs signal event Background Background B. Millán Mejías

21. Signal Signal Signal Background B. Millán Mejías

22. Preliminary results B. Millán Mejías

23. Each method gives 1 output observable: 0 means background, 1 means signal B. Millán Mejías

24. Fisher significance S= number of signal events B=number of background events B. Millán Mejías

25. MLP significance Genetic Algorithm S= number of signal events B=number of background events B. Millán Mejías

26. BDT significance s/sqrt(b) > 5 Higgs can be claim S= number of signal events B=number of background events B. Millán Mejías

27. 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). B. Millán Mejías

28. Thank you! bmillan@nikhef.nl B. Millán Mejías

29. Overtraining example Pick a number, the same number and shows the efficiency, for the training and signal sample B. Millán Mejías

30. 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 B. Millán Mejías

31. Output (preliminary) B. Millán Mejías

32. B. Millán Mejías

33. Higgs W+ W– l+  l–  B. Millán Mejías