Associated SM Higgs search in H bb,WW (*) final states @ ATLAS

1. Associated SM Higgs searchin Hbb,WW(*) final states @ ATLAS Huaqiao Zhang IHEP/CPPM On Behalf of the ATLAS experiment H. ZHANG(CPPM/IHEP) SUSY08

2. Outline • Introduction • ttH,Hbb analysis • Signature and preselection • Event reconstruction and methods comparison • Systematics and results • ttH,HWW(*) analysis • Signature and event selection • Systematics and results • WH,HWW(*) analysis • Signature and event selection • Systematics and results • Summary H. ZHANG(CPPM/IHEP) SUSY08

3. SM Higgs searching in experiments http://lepewwg.web.cern.ch/LEPEWWG/ • Higgs particles is the only particle that predicted in SM but not found yet experimentally • LEP direct search exclude light Higgs below 114 GeV • With recent Tevatron results, SM electroweak fit prefers Higgs less than 190 GeV (including LEP) and 160 GeV without. • Higgs Coupling to top quarks and W boson are important properties of Higgs H. ZHANG(CPPM/IHEP) SUSY08

4. The SM Higgs at the LHC Difficult for this mass region: we need to associate more than one channel • Associate production: • Possible discovery channel: ttH,Hbb • Coupling measurement for Higgs mass [120, 200] GeV => SM Higgs or not • Decay modes: • For mH > 135 GeV/c2 H → WW(*) dominates (BR : 0.91 @ 160 GeV/c2 ) • H→bb is the main channel for mH < 135GeV/c2(BR : 0.68 @ 120 GeV/c2 ) H. ZHANG(CPPM/IHEP) SUSY08

5. The LHC and ATLAS • LHC: • Proton-Proton collisions @ 14 TeV • First run @ 10 TeV expected at the end of this summer • Luminosity: • Low luminosity regime ~1033cm-2s-1 • ~ 30 fb-1 between 2008 and 2011 • High Luminosity regime ~1034cm-2s-1 • ~ 300 fb-1 by 2014/2015 ATLAS detector • ATLAS: • General purpose experiments • Classic detectors composed mainly by 3 sub-systems • Inner tracker • Calorimeter system • Muon spectrometer • Very good Trigger/DAQ System • Good e/g/m/tau/missEt/b-jets identification H. ZHANG(CPPM/IHEP) SUSY08

6. ttH,Hbb Signature and preselection LO cross section (only tt+jets NLO+NLL) • ttH,Hbb: potential discovery channel for light Higgs boson, top quark Yukawa Coupling measurement • Hbb, t  bjj, t  bℓv t  bℓv, t  bℓv t  bjj, t  bjj • Main backgrounds • tt+jets : reducible background using b-tagging • ttbb (EW/QCD) : irreducible background • Strategy: Reconstruct the tt system to look at the rest 2 b-jets at the final state • b-tagging is one of the most important keys for this channel • Jet pairing and JES are also very important • Efforts mainly for lepton + jets channel • Isolated electron or muon trigger • One isolated lepton is required, Tau is not considered • Electron, muon: Acceptance, identification and isolation cuts • At least 6 jets, of which 4 tagged as b-jets: • cone 0.4 jet algorithm • Done for 30 fb-1 H. ZHANG(CPPM/IHEP) SUSY08

7. Cut-Based Reconstruction Invariant mass of reconstructed hadronic W boson (red for good combination) ATLAS preliminary • Hadronic W reconstruction • After requesting 4 b-tag jets, remaining jets are considered as light jets • W boson candidates are formed of all combinations of light jet pairs Invariant mass of reconstructed top quarks (red for good combination) • top quarks candidates are formed using one W boson candidate and one b quark • Combination with |m(jj) – m(W)| > 25 GeV/c2 or |m(treco) – m(ttrue)| > 25 GeV/c2 are removed • Combination minimizing a c2, based on the top quark masses, is chosen • The two remaining b jets used to reconstruct the Higgs candidates ATLAS preliminary ATLAS preliminary • Leptonic W reconstruction • Force ln mass to the W mass • Solve 2nd degree equation to get neutrino pz • 28% no solution → neglect imaginary part H. ZHANG(CPPM/IHEP) SUSY08

8. Multivariate Based Reconstruction • Pairing likelihood: • Using tt system kinematic properties to build a pairing likelihood • 6 variables are used • b-jets and light jets are treated separately • Choose the combination that maximize the likelihood output ATLAS preliminary Invariant mass of reconstructed Higgs boson using likelihood for signal sample (red for good combination) • Constrained fit: • Fit jet pT and ETmiss to give the mass of the top quarks • Pairing likelihood is formed using the c2output of the constrained fit together with b-tagging and kinematic quantities • 14 variables are used • 3D likelihood is used to take into account the correlations • Final selection likelihood is used to separate signal and physics background f ijet: scale factor for jet momentum H. ZHANG(CPPM/IHEP) SUSY08

9. Comparison of Reconstruction Algorithms • S/sqrt(B) differences relatively small among the 3 methods • Multivariate techniques increase Higgs bb pair purity by ~5% • Analyses suffer from low purity mainly coming from b exchange between top and Higgs • Wide distribution for reconstructed Higgs mass • No clear signal peak on top of background distribution • Combinatorial background dilutes differences between ttbb and ttH sig ATLAS preliminary ATLAS preliminary Invariant mass of reconstructed Higgs boson using constrained fit for all samples, a cut at -4.2 in Ls/b for better stat. significance 90 < mH < 150 GeV/c2 H. ZHANG(CPPM/IHEP) SUSY08

10. Systematic uncertainties and results @ 30 fb-1 main systematic errors, background sample ATLAS preliminary S/sqrt(B+dB**2) • Systematic errors on background higher than signal • If nothing is done for the background extraction, the significance will decrease to: ~0.5 (mH=120GeV/c2) Main systematic errors come from the JES, jet resolution and b-tagging efficiency Theoretical errors on background cross sections, especially top anti-top production, are sizable H. ZHANG(CPPM/IHEP) SUSY08

11. ttH,HWW(*)Signatures • ttH2b4j2l2Vl • 6jets + 2samecharge lep. + MissEt • Possible BKGs: • tt(1l), tt(2l), ttZ, ttW, tttt, ttWW, ttbb... • ttH2b2j3l3Vl • 4jets + 3leptons + MissEt • Possible BKGs: • tt(2l),ttZ,ttW,tttt,ttWW,ttbb • tau not considered in signal • Complex final states=>Number counting experiment • At least 2 neutrinos • Multi jets + Multi leptons + MissEt • Background control important • Main BKG tt suppressed by lepton isolation=>Lepton isolation is crucial • ttZ suppressed by Z mass veto • QCD BKGs neglectable gt b P 2 leptons 4 light jets P b b P 3 leptons 2 light jets P b H. ZHANG(CPPM/IHEP) SUSY08

12. ttH,HWW(*) events selection • One high Pt Isolated lepton trigger • Eff: 82% (2L),91% (3L) • offline significance impact < 1% • Phys. Obj. kinematic region: • |η|<2.5; Pt>15.GeV • Electron identification: • Calorimeter seeded Alg. • Matching to ID track • Muon identification: • Seeded in Muon Spectrometer • Combined with ID track • Jet identification: cone based, size 0.4 • Lepton Isolation: • Electron: • Muon: • Number of leptons >= 2/3, Number of jets >= 6/4 • Final tighten cut: • Zveto, MuonPt>20GeV, Number of lepton==2/3, sameCharged/null Eff:70.5% Eff:92.7% Calorimeter Isolation + Tracker Isolation + Cone Isolation H. ZHANG(CPPM/IHEP) SUSY08

13. Systematic uncertainties and results @ 30 fb-1 ATLAS preliminary • Accuracy of σttH * BRHWW: Best at 160 GeV: 47.4%, if 10% uncertainties of BKG • Biggest uncertainties from JES and BKG (especially ttbar) normalization. • Without systematics, the statistical accuracy can achieve 28% • Systematics dominate the accuracy H. ZHANG(CPPM/IHEP) SUSY08

14. Multivariable methods for improvement • Likelihood method on isolation • Projective likelihood estimator • Only 3 isolation variables used ATLAS preliminary • Mass constrained fit in ttH2L analysis • A system with 2 neutrinos (one virtual W at Higgs mass <=160 GeV) • 11 free parameters • 8 enter chi^2,3 from neutrino,5 Windows to determine combinatory • 11 constraints • Only 4 equality, other 7 domains • Extra 8 implicitly : 0.5 < cali_E < 1. ATLAS preliminary ttH,HWW(*) 2L signal More work needed to improve H. ZHANG(CPPM/IHEP) SUSY08

15. WH,HWW(*) Signature and preselection Higgs physics Supplement to the Discovery Necessary confirmation for the property Couplings to gauge boson WH,HWW(*) physics analyses One High Pt isolated lepton trigger Two and three lepton final states (2L/3L) WHWWW(*)lv lv jj WHWWW(*)lv lv lv Full analysis over WH against ttbar, WZ, ttW, ZZ Number counting experiments • Number of jets and leptons • Same charge(2L), Zveto • Missing Transverse Energy(3L) • Lepton isolation • Calorimeter isolation • Tracker isolation • Cone isolation • Impact parameter • B-jet/jet veto • W mass range • Higgs-Spin cut H. ZHANG(CPPM/IHEP) SUSY08

16. Preliminary results @ 30 fb-1 * Suffer from the statistics of MonteCarlo samples , BKG Systematics dominated by JES: 19%(2L), 17%(3L) HWW(*) coupling measurement need more/hard works H. ZHANG(CPPM/IHEP) SUSY08

17. Summary • Studies on channels of ttH,Hbb, ttH,HWW (*) and WH,HWW (*) with ATLAS real detector geometry full simulation • Coupling measurement • ttH,Hbb (gt, Higgs~120GeV), potential discovery channel • ttH,HWW (*) (gt, Higgs ~160GeV) • WH,HWW (*)(gW, Higgs ~170GeV) • Number counting experiments => BKG control is crucial • Mass peak reconstruction: • possible in ttH,Hbb • Hopeless in ttH,HWW(*), not yet tried in WH,HWW(*) • Background uncertainties • JES(Btaging in ttH,Hbb) dominate the uncertainties of BKGs • ttbar or ttjj normalization important • Data driven method needed • More advanced methods during 30fb-1 data gathering • Statistical part of systematics reduced with 30fb-1 data • Still rooms for improvement of these analysis • Good feasibilities, more efforts needed H. ZHANG(CPPM/IHEP) SUSY08

18. Bak up: Main Issues for ttH,Hbb Channel There is still room for improvement Trigger Dedicated or combined trigger (e.g. btagging, jets + soft leptons) Combinatorial background More powerful jet pairing techniques (e.g. BDT) JES, Jet resolution Energy flow algorithms B-tagging/Light jet rejection More advanced taggers that should be well understood after 3 years of data taking Cross section uncertainties Background extraction from data • After analysis Higgs purity is ~30% • large tails and width in the bb invariant mass • No visible peak on top of physical background, side band extraction very difficult • The main problem is the b-jets exchange between top quarks and the Higgs • b-jet fromHiggsused for thehadronic top : ~36% • b-jet from Higgs used for the leptonic top :~30% • Higgs boson reconstructed with only one correct b-jet: ~55% (The other wrong jet is mainly coming from the top quarks) • Big uncertainty for the tt+jets cross section • Extracting the background shape and normalization in data is crucial for this channel • Using loose and tight b-tagging cuts looks promising • Background shape independent from b-tagging cuts H. ZHANG(CPPM/IHEP) SUSY08

19. Bak up: Main Issues for ttH/WH,HWW(*) Channel • Higgs reconstruction difficult • At least two neutrinos, one from Higgs W decay, one from top W decay • With a virtual W, which could decay leptonic or hadronic • Large combinatory in complex final states • Lepton isolation is crucial in background suppression • Several isolation methods are tried • Could improve by Multivariable methods, with better understanding of real data • Big uncertainty for the backgrounds cross section normalization • Theoretical uncertainties could improve with understanding of real data • Statistical part of systematics could be reduced with full 30fb-1 real data H. ZHANG(CPPM/IHEP) SUSY08