WW scattering in the lep-lep final state at 14 TeV including pile-up

Transcription

1 WW scattering in the lep-lep final state at 4 TeV including pile-up Philipp Anger, Jan Schumacher, Michael Kobel, Anja Vest philipp.anger@physik.tu-dresden.de Institut für Kern- und Teilchenphysik der Technischen Universität Dresden ATLAS-D Meeting Mainz 3/9/ Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

2 Overview Naive Standard Model without Higgs: Scattering of longitudinal W bosons rises infinitely: sww σ (W L W L W L W L ) Unitarity violation above s WW. TeV Vector Boson Scattering at LHC reaches this limit in parts of the phase space Higgs observed: Unitarity conserved No Higgs observed: Unitarity conservation requires physics beyond the Standard Model Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

3 EWChL and Unitarization Effective Electroweak Chiral Lagrangian (EWChL): Leff = L LO + α 4 (Tr[V µ V ν ]) + α 5 (Tr[V µ V µ ]) +... Approximates the rising edge of a resonance beyond the accessible mass range (anomalous couplings) No longer valid at LHC energies Resonances and Unitarity in Weak Boson Scattering A. Alboteanu, W. Kilian and J. Reuter (arxiv:86.445v) Need resonance(s) with masses M and couplings g to weak bosons Spin J Weak Isospin I I = I = I = J = σ ϕ, ϕ ±, ϕ ±± J = ρ, ρ ± J = f a, a ±, a ±± K-Matrix formalism guarantees unitarization Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 3 / 9

4 Event Generator WHIZARD WHIZARD: W. Kilian, T. Ohl, J. Reuter. FR-THEP-7-, SI-HEP-7-7, Aug 7. Only generator that implements K-Matrix unitarization with resonances No Effective W Approximation Quark splitting W/Z pdf Full matrix element for the six fermion final state Angular correlations preserved Irreducible backgrounds included Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 4 / 9

5 Signal and Irreducible Backgrounds with qqlνlν final state Signal: Resonance Background: EW Also all SM triple and quartic boson vertices (except Higgs) included Background: QCD All generated with WHIZARD for 4 TeV Signal entangled with irreducible background WHIZARD qqlνlν samples available: EW... Resonances and QCD switched off Signal+EW... QCD switched off QCD+EW... Resonances switched off Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 5 / 9

6 Disentangling Signal and Irreducible Backgrounds Samples reweighted from high to low coupling values S(g) =n Signal+EW (g) n Signal+EW (g = ) Reasonable couplings for strong EWSB: g < π.5 Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 6 / 9

7 Reducible Backgrounds Top pairs tt Two-lepton filter ATLFAST-II Single top () AcerMC Two-lepton filter ATLFAST-II no pile-up available W / Alpgen Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 7 / 9

8 Analysis Assumed integrated luminosity: fb (not an early study) All samples with 4 TeV ATHENA release 4..5 Pile-up In-time-pile-up: More than one proton-proton interaction per bunch crossing First studies with available samples to see general influence of pile-up Poisson distributed mean number of pile-up collisions per filled bunch crossing: 6.9 Luminosity: 33 cm s (low luminosity pile-up) Goal: High luminosity pile-up Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 8 / 9

9 Experimental Signature 4 ζ η 3 4 φ 3 Tagjets (large p T, large distance in η) Few jets between tagjets Final state lνlν: Missing ET Decay products between tagjets Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 9 / 9

10 Event Selection - Boosted Decision Tree TMVA (Toolkit for Multivariate Analysis), Release Input variables Distributions... b-tag p T of leptons invariant tagjet mass η between tagjets transverse mass p T of tagjets missing E T lepton centrality ζ p T balance minijet veto Training: Pile-up samples trained with pile-up samples and vice versa Signal (Test Sample) Signal (Training Sample) Background (Test Sample) Background (Training Sample) BDT Discriminator Response Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

11 BDT Cutflow WHIZARD irreducible background most important background Reducible backgrounds disappear for cuts >.6 W+jets already well suppressed MC statistical uncertainty to be taken into account Cut factorization problematic for MVA Detailed statistical analysis on signal, irreducible background, top pair and Expected Number of Events [/fb] σ, m=85gev/c, g = Cut on BDT output r cut (r >= r cut ) Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

12 φ between Leptons Signal shows clear lepton angular separation Preserved by WHIZARD Motivation for lνlν final state Lepton angular separation Φ ll no input variable of BDT No BDT cut Possible control region at low Φ ll After BDT cut Separation power of Φ ll lost Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

13 Statistical Analysis - Discovery Significance Example: Discovery significance for scalar isoscalar σ with m = 85 GeV Profile Likelihod method BDT cut >.7 r cut Assumed experimental luminosity: fb Amount of Monte Carlo scaled to fb Optimize BDT cut for best 5σ discovery significance coupling g BDT cut Z5 Sensitivity Z Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 3 / 9

14 Discovery Significance Results for 85 GeV Discoverable minimal couplings g per resonance: Type 3σ 5σ σ ϕ.96.4 ρ.6.78 f.4.77 t.7. 5σ discovery sensitivity for: All resonances with g = π.5 apart from ρ SM Higgs (σ, g = ) Expected Significance Z 4 σ φ ρ f t Coupling g Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 4 / 9

15 Discovery Significance Results for 5 GeV Discoverable minimal couplings g per resonance with m = 5 GeV: Type 3σ 5σ σ.6.64 ϕ.85. ρ f.44. t Expected Significance Z 4 σ 8 φ ρ f t Coupling g Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 5 / 9

16 Upper Limit Results 85 GeV 5 GeV Expected Significance Z σ φ ρ f t Expected Significance Z σ φ ρ f t Coupling g Coupling g Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 6 / 9

17 Discovery Significance Results with Pile-up Modified BDT options: Including negativ weights Less input variables Modified input sample definition Pile-up trained with pile-up Yet no systematic uncertainties Resonances at m = 85 GeV BDT training with pile-up nearly compensate loss in discrimination power of input variables Significance Z σ 8 ϕ ρ 6 f 4 t pile-up coupling g Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 7 / 9

18 Summary These Analyses: ATLAS has discovery and limit setting potential in the Vector Boson Scattering two lepton channel at fb at 85 GeV and 5 GeV in relevant coupling range g <.5 Small contributions of low luminosity pile-up 5σ discovery already for fb (ϕ resonance, m = 85 GeV, g >.) Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 8 / 9

19 Follow Up 4 TeV Analysis: Systematic uncertainties for the pile-up case [almost done] Jet-Energy scale as main contribution Modified Profile Likelihood method Jet-vertex association Look at high luminosity pile-up Plans for my PhD thesis: Switch to 7 TeV and current ATHENA release 6 Look at data Update analyses with other final state to take advantage of WHIZARD and angular correlations Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 9 / 9

20 BACKUP Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

21 Training options Number of trees: Boosting type: Gradient Shrinkage:.3 Separation type: Gini index Pruning method: Cost Complexity Pruning strength: 5 Maximum number of nodes: 5 Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

22 Event Selection - Fiducial Precuts η between tagjets > 3. p T of tagjets > GeV Generator level p T of st and nd lepton > 3 GeV Generator level Trigger plateau m leplep > 5 GeV Removing Z+jets background Caveat: Sample has a cut m leplep < GeV Triggers: Electron trigger: 5 GeV Muon trigger: GeV Fraction of events passing triggers p of lower p lepton [GeV] T T Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ / 9

23 Input Variables Distributions Back to Boosted Decision Tree σ, m=85gev/c, g=. - - σ, m=85gev/c, g=. - - σ, m=85gev/c, g= Flavour Tag Weight b Flavour Tag Weight b Lepton p T [GeV/c] - σ, m=85gev/c, g=. - σ, m=85gev/c, g=. - σ, m=85gev/c, g= Lepton p T [GeV/c] tagjet Tagging Jets Invariant Mass m [GeV/c ] tagjet Tagging Jets η Separation η Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 3 / 9

24 Input Variables Distributions Back to Boosted Decision Tree σ, m=85gev/c, g=. - - σ, m=85gev/c, g=. - - σ, m=85gev/c, g= VBS Transverse Mass m T [GeV/c ] Missing Transverse Momentum p miss [GeV/c] T Flavour Tag Weight b - - σ, m=85gev/c, g=. - - σ, m=85gev/c, g=. - - σ, m=85gev/c, g= Flavour Tag Weight b Lepton p T [GeV/c] Lepton p T [GeV/c] Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 3 / 9

25 Input Variables Distributions Back to Boosted Decision Tree... - σ, m=85gev/c, g=. - σ, m=85gev/c, g= minijet Minijet Transverse Momentum p [GeV/c] T minijet Minijet Transverse Momentum p [GeV/c] T σ, m=85gev/c, g=. σ, m=85gev/c, g= Lepton Momentum Fraction x Lepton Momentum Fraction x Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 3 / 9

26 Boosted Decision Tree Results evts/fb - ϕ(g=.) - ϕ(g=.) ϕ(g=.) - ϕ(g=.) pile-up W+jets evts/fb - ϕ(g=.) - ϕ(g=.) ϕ(g=.) - ϕ(g=.) pile-up Z+jets evts/fb - ϕ(g=.) - ϕ(g=.) ϕ(g=.) - ϕ(g=.) pile-up QCD:= [QCD+EW] - [EW] W+jets pile-up Z+jets pile-up QCD:= [QCD+EW] - [EW] pile-up pu - nopu pu + nopu.5 pu - nopu pu + nopu.5 pu - nopu pu + nopu BDT-cut BDT-cut BDT-cut W+jets background Z+jets background WHIZARD QCD evts/fb - ϕ(g=.) - ϕ(g=.) ϕ(g=.) - ϕ(g=.) pile-up EW evts/fb - ϕ(g=.) - ϕ(g=.) ϕ(g=.) - ϕ(g=.) pile-up tt evts/fb - ϕ(g=.) - ϕ(g=.) ϕ(g=.) - ϕ(g=.) pile-up EW pile-up tt pile-up pu - nopu pu + nopu pu - nopu pu + nopu pu - nopu pu + nopu BDT-cut BDT-cut BDT-cut WHIZARD EW t t background background Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 4 / 9

27 TMVA Training Tests Trust a multivariate method? Significance Z 6 original subsample subsample 4 subsample subsample 3 subsample 4 Significance Z 6 original sample random retraining 4 random retraining random retraining 3 random retraining 4 Significance Z 6 original subsample with 6 events 4 subsample with 4 events subsample with 3 events coupling g coupling g coupling g Retraining with equivalent subsamples of the same size Retraining with random picking of training events inside TMVA ϕ resonance with m = 85 GeV and pile-up Retraining with samples with different number of events Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 5 / 9

28 Luminosity studies Significance Z - Ldt = fb Ldt = fb - Ldt = fb - Ldt = fb pile-up.5.5 coupling g Philipp Anger (IKTP TU Dresden) WW scattering including pile-up ATLAS-D 3/9/ 6 / 9