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

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 CM energy 14 TeV (design) Luminosity (cm-2s-1) Low: 2x1033 High: 1034 Bunch crossing 24.95 ns Particles/bunch 1.15x1011 Beam radius 16.7 μm Overlaid events 23 @ 1034cm-2s-1 Dipole field 8.33 Tesla Stored energy 360 MJ/beam 2

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) Year Energy bunches Lumi (cm-2s-1) Integrated lumi/year 2008 5 TeV 43 1030 10’s pb-1 156 1031 2009 7 TeV 936 1032 6 fb-1 2808 2010 12 fb-1 LHC first beam: Full success on Sep.10: historic moment! 450GeV beam from SPS was circulated through all LHC sectors 3

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

Disfavoured by EW precision fits SM Higgs production cross section at LHC Disfavoured by EW precision fits Excluded by LEP Main production channels: gluon fusion, Vector Boson Fusion, associated production with vector bosons, associated production with ttbar pair Animation

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

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

HZZ*4l: selection cuts and mass reconstruction 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) Preliminary CMS Preliminary CMS Toy experiment superposed H4e reconstruction

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

HZZ*4l: discovery potential CMS Four CMS analyses, one combined ATLAS analysis Varied level of systematic uncertainties treatment 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 CMS

Hgg: phenomenology and experiment 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 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)

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

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 * 94% trigger eff wrt offline cuts

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 (and gg) Two high-pT tag jets with large rapidity gap in between No color flow between tag jets – central jet veto is rather effective to reduce BGR’s |h|<3.2

VBF Htt: selection and mass reconstruction Important channel for low Higgs mass Dominant backgrounds are: Z/γ*ττ; also tt Estimated from data: select Z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

CMS, lepton-hadron channel 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

HWW*lnln (+lnqq) 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 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 21

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

Summary of SM Higgs boson discovery in CMS.

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

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

ATLAS Inner Detector /pT ~ 5x10-4 pT  0.01 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

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/Fwd Cu/W-LAr (~20000 channels) /E ~ 50%/E  0.03 pion (10 ) Trigger for e/γ , jets, Missing ET Tile

~600 RPC and ~3600 TGC trigger chambers 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

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://lepewwg.web.cern.ch/LEPEWWG/ http://tevnphwg.fnal.gov/results/SM_Higgs_Winter_08hepex/