Higgs discovery potential at the LHC: channels relevant for SM Higgs

1. Status of the LHC machine, CMS and ATLAS detectors • SM Higgs at the LHC: cross-sections and branchings • SM Higgs Searches in CMS and ATLAS using channels: • Hgg, HZZ*4l, VBF Htt, HWW (gg-fusion and VBF) • Statistical combination of ATLAS+CMS analyses • Summary and outlook Higgs discovery potential at the LHC: channels relevant for SM Higgs Ilya Tsukerman, ITEP, Moscow, Russia On behalf of the ATLAS and the CMS collaborations International Symposium on Multiparticle Dynamics 15-20 September 2008, DESY, Hamburg, Germany

2. The Large Hadron Collider The LHC will collide protons at √s = 14TeV It is starting operation at √s = 10 TeV Two general-purpose experiments: ATLAS, CMS Aim to study the origin of the electroweak symmetry breaking 2

3. LHC Status and timeline http://lhc.web.cern.ch/lhc/ • all sectors at 2°K from August • First collisions expected on October at 5 TeV • Commissioning run at √s=10 TeV until November (43-156 bunches) • Increase energy to 7 TeV for 2009 run (75 ns bunch crossing then 25 ns) LHC first beam: Full success on Sep.10: historic moment! 450GeV beam from SPS was circulated through all LHC sectors 3

4. Cross-sections and rates at the LHC Total inelastic cross-section  ~100mb bb-production cross-section  ~100mb Wln production cross-section  ~10nb Higgs (mH =150 GeV) cross-section  ~10pb Higgs production cross section is TEN orders of magnitude smaller than total inelastic cross-section

5. SM Higgs production cross section at LHC Disfavoured by EW precision fits Excluded by LEP

7. The ATLAS Experiment 45 m 24 m 7000 T Higgs results: Computing System Challenge notes, to be published soon

8. Higgs results: physics TDR, published in J.Phys.G 34 (2007) 995

11. HZZ*4l: selection cuts and mass reconstruction Preliminary • The “golden channel” : H->ZZ*->4e/4μ/2e2μ • Good for a wide mass range, except mH ≈ 2 mW • Backgrounds: • qqZZ* and ggZZ*dominant (irreducible) • Zbb, tt, ZW, Z + X (reducible) at low masses • Analyses: • Isolated muon and electron pairs with opposite charge • Reject Zbb, tt, etc using quality cuts: isolation, lepton track impact parameter, vertex constraints • At least one Z->ll on shell • Reconstruct 4-lepton invariant mass • Estimate background from sidebands • Estimated effects of overlapping events at L=1033 cm-2s-1: 3 – 11% selection efficiency loss (ATLAS) CMS Preliminary CMS Toy experiment superposed H4e reconstruction

12. HZZ*4l: invariant mass spectra for ATLAS 150GeV 130GeV 180GeV 300GeV 600GeV 400GeV

13. HZZ*4l: discovery potential CMS CMS • Four CMS analyses, one combined ATLAS analysis • Varied level of systematic uncertaintiestreatment • Data-driven methods to estimate backgrounds, efficiencies, resolutions etc • Comparable significance • Differences explained by different selection and detector resolution • Potential for 5σ discovery in much of the allowed mH range with less than 30fb-1 CMS

14. Hgg: phenomenology and experiment Experimental issues: Need good γ-jet separation (10-3÷10-4 to get σγj+σjj<< σγγ) Need good mass resolution (~1.5 GeV/c2) Vertex reconstruction Conversion analysis (very high dead material in inner detector) Phenomenological issues: • Small branching ratio (BR≈0.002 @mH=120-140GeV/c2) • But very clear signature “easy” to separate from background • Important for low-mass region (120-140 GeV/c2) • Backgrounds: • Irreducible: γγ, γγ+jets • Reducible: γ+jets, jets, Drell-Yan

16. Hgg: invariant mass spectra for ATLAS Inclusive Example: m(H)=120 GeV H+1jet ZHllgg WHlngg H+2jet

17. Hgg: discovery potential ATLAS: • Vertex determined from extrapolation using calorimeter samplings • Converted photons used to improve vertex determination: 57% Higgs events have ≥1 conversion • Signal divided into categories according to ηγ, #jets, #converted photons • Signal significance: 3.6 (2.8) for floating (fixed) mH and 10fb-1 CMS: • Signal categories according to ηγ and lateral shower shape variable • Very good EM energy resolution • 5σ discovery possible at mH=120GeV/c2 with 7-8fb-1 Both experiments use cut-based analyses and multivariate (NN, likelihood) methods Both experiments estimated effects of pileup and enhanced dead material CMS

18. Vector Boson Fusion (VBF) Higgs production Established by Zeppenfeld et al. for low-mass region Earlier studies by Dokshitser, Khoze, Sjöstrand, Troyan, Kleiss, Stirling and others Important for low-mass region (to improve significance and measure parameters of H) Studied in Higgs decays to WW, ττ (andgg) |h|<3.2 Two high-pTtag jets with large rapidity gap in between No color flow between tag jets – central jet veto is rather effective to reduce BGR’s

19. VBF Htt: selection and mass reconstruction • Important channel for low Higgs mass • Dominant backgrounds are: Z/γ*ττ; also tt Estimated from data: selectZmm, decay m’s in Tauola and merge back into event • Three subchannels: lepton-lepton, lepton-hadron, hadron-hadron • Forward jet tagging and central jet veto used to reject QCD backgrounds • Mττ can be reconstructed (δm≈8-10 GeV) in the collinear approximation CMS, ttlh, 30 fb-1

20. VBF Htt: discovery potential CMS, lepton-hadron channel Combination of lepton-lepton and lepton-hadron should allow 5σ measurement with 30 fb-1 in the range 115<mH<125 GeV/c2 20

21. HWW*lnln (+lnqq) Signal region BGR region S BGR CMS, HWWlnqq+ 2 jets, 60 fb-1 CMS, HWWll+ 0 jets, 165GeV, 10 fb-1 • Main search channel for mass in range: 2mW < mH < 2mZ (also good at lower masses) due to large branching ratio • Analyses: • H + 0 jets  lνlν (dominated by gluon fusion) • H + 2 jets  lνlν; H + 2 jets  lνqq (dominated by VBF) • Main backgrounds: WW, Wt, tt with mentioned final states 21

22. HWW*: discovery potential Full extra jet veto Loose extra jet veto CMS, 30fb, HWWlnqq + 2 jets CMS

23. Summary of SM Higgs boson discovery in CMS.

24. http://council-strategygroup.web.cern.ch/council-strategygroup/BB2/contributions/Blaising2.pdfhttp://council-strategygroup.web.cern.ch/council-strategygroup/BB2/contributions/Blaising2.pdf

25. Conclusions and Outlook After 20 years of planning and construction, the LHC and it’s experiments have become a reality The Higgs searches in ATLAS and CMS are ready for the data CMS and ATLAS SM Higgs discovery potentials are comparable 5s discovery is possible already in 2009 25

26. ATLAS Inner Detector Tracking ||<2.5 B=2T TRT SCT Silicon pixels (Pixel): 0.8 108 channels Silicon strips (SCT) : 6 106 channels Transition Radiation Tracker (TRT) : straw tubes (Xe), 4 105 channels e/ separation /pT ~ 5x10-4 pT  0.01 Pixel

27. ATLAS Calorimetry Calorimetry ||<5 Barrel Endcap Electromagnetic Calorimeter barrel,endcap: Pb-LAr ~10%/√E energy resolution e/γ 180000 channels: longitudinal segmentation Hadron Calorimeter barrel Iron-Tile EC/FwdCu/W-LAr (~20000 channels) /E ~ 50%/E  0.03 pion (10 ) Trigger for e/γ , jets, Missing ET Tile

28. ATLAS Muon System Stand-alone momentum resolution Δpt/pt < 10% up to 1 TeV 2-6 Tm ||<1.3 4-8 Tm 1.6<||<2.7 ~1200 MDT precision chambers for track reconstruction (+ CSC) ~600 RPC and ~3600 TGC trigger chambers

31. SM Higgs: current limits on its mass • Electroweak symmetry breaking needed to explain e.g. masses of fundamental fermions and W/Z bosons • Simplest model of EW symmetry breaking predicts the existence of a Higgs scalar – Higgs boson mass is only free parameter in theory • Masses of Higgs, top and W connected through loop diagrams  precision electroweak fits sensitive to Higgs mass mH = 87 +34 -26 GeV/c2; including LEP: mH < 185 GeV/c2 • From LEP: mH>114.4 GeV/c2 @ 95% CL • Tevatron experiments expected to be able to exclude SM Higgs @ 95%CL up to mH≈200 GeV/c2 or make 3σ observation ICHEP08, Philadelphia 31 http://tevnphwg.fnal.gov/results/SM_Higgs_Winter_08hepex/ http://lepewwg.web.cern.ch/LEPEWWG/