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LHC Prospects on Standard Model Higgs

LHC Prospects on Standard Model Higgs. Riccardo Ranieri INFN and Università degli Studi di Firenze on behalf of ATLAS and CMS Collaborations ICHEP’04 32 nd International Conference on High Energy Physics Beijing, China 16-22 August 2004. ATLAS & CMS at LHC.

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LHC Prospects on Standard Model Higgs

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  1. LHC Prospects on Standard Model Higgs Riccardo Ranieri INFN and Università degli Studi di Firenze on behalf of ATLAS and CMS Collaborations ICHEP’04 32nd International Conference on High Energy Physics Beijing, China 16-22 August 2004

  2. ATLAS & CMS at LHC • Detectors optimised for Higgs boson and SUSY searches • very high energy: LHCpp √s=14 TeV • inelastic cross section: σpp=55 mb • interaction rate: 40 MHz • high luminosity: (2x)1033 cm-2s-11034 cm-2s-1 • per year: 20 fb-1100 fb-1 • BIG detectors • CMS: 15 m x 21.5 m • ATLAS: 25 m x 46 m CMS = Compact Muon Solenoid LHC = Large Hadron Collider first collisions in Summer 2007 ATLAS = A Toroidal LHC ApparatuS LHC Prospects on Standard Model Higgs

  3. Where will we be at LHC startup? Searches at TeVatron from now to 2007… • Design Projection: 4.4 fb-1 • mH<130 GeV/c2 covered to exclusion • 3σ evidence up to mH=125 GeV/c2 • Base Projection: 2.2 fb-1 • mH<125 GeV/c2 covered to exclusion ¿ Higgs boson signal mH=115 GeV/c2 from LEP2 data ? FERMILAB-PUB-03/320-E CERN-EP/2003-011 LHWG Note/2002-01 mH>114.4 GeV/c2 @ 95% CL LHC Prospects on Standard Model Higgs

  4. SM Higgs Production at LHC • Associated productionttHand bbH • high-pT lepton, top reconstruction,b-tag - - • Gluon Fusion • the highest cross section • Vector Boson Fusion • two high-pT forward jets • Associated ProductionWHand ZH • oneortwohigh-pT leptons useful for the trigger LHC Prospects on Standard Model Higgs

  5. SM Higgs Decays 2mZ LEP excluded “light” Higgs • hadronic and decays are favourite • …but difficult to select • decays into Vector bosons W and Z • “golden” channels • two-photon decays • extremely “clean” but rare and difficult to detect LHC Prospects on Standard Model Higgs

  6. Low Mass Higgs: H→ • This decay is very rare (Br≈10-3) • σ(pp→H115)xBr(H→)=76 fb (NLO) • S/B≈1/20 • good resolution mass peak • Electromagnetic Calorimetres crucial for H→: σ(m)/m≈1% needed • motivation for LAr (ATLAS) and PbWO4 (CMS) calorimetres • high granularity • response uniformity • 3 main background processes: • irreducibile: gg/qq→ 81 pb • +jet (with “real” or “fake” second photon) 9x104 pb • hadronic QCD jets (π0 decays) 108pb • ATLAS reach with 10 fb-1 and mH=115 GeV/c2: • Signal Significance: S/√B=2.0 (K-factors not included) CERN/LHCC 96-40 ATLAS TDR 1 CERN/LHCC 96-41 ATLAS TDR 2 CERN/LHCC 97-33 CMS TDR 4 LHC Prospects on Standard Model Higgs

  7. - Low Mass Higgs: ttH(→bb) - - • This is the favourite decay • σ(pp→H115)xBr(H→bb)=28 pb • S/B<10-7 • tagging the top quarks helps a lot • t→bW(→μν) • t→bW(→jj) • “crowded” final state • 6 jets (4 of them are b-jets) + additional ISR/FSR jets • 4 b-tagged jets needed to reduce combinatorics • 1 isolated lepton • it’s the key for trigger • optimised analysis • pz from W-mass constraints • likelihood pairing of jets • Final result for likelihood analysis (mH=115 GeV/c2): • 30 fb-1: S/√B=3.4 • 10 fb-1: S/√B=2.0 ν ATL-PHYS-2003-024 LHC Prospects on Standard Model Higgs

  8. Vector Boson Fusion: qqH(→) Higgs Decay products Forward tagging jets f h [VBF] Jet • hadronic jets in forward-backward regions • the forward jet tagging is a powerful background rejection tool • hadronic activity suppressed in low η region • emitted vector bosons are colour-singlets • Search for →ℓνℓ’ν’, ℓν+jet final states • S/√B≥5 in mH=120÷140 GeV/c2 range with 40 fb-1 • S/√B≈2.5 in one LHC year • this process offers the possibility for a direct measurement of Yukawa coupling H Jet Phys. Rev. D59(1999) 014037 ATL-PHYS-2003-004 CMS NOTE 2003/033 LHC Prospects on Standard Model Higgs

  9. Light Higgs Summary Three complementary channels Detector/performance requirements: • H→response uniformity of electromagnetic calorimeter • H→bbb-tagging capability of tracking system • qqH(→) efficient jet reconstruction in 3<|η|<5 • all good understanding (<10%) of the background Each channel contributes with ~2σ to the total significance Observation of all channels is very important to extract a convincing signal in the first year(s) - LHC Prospects on Standard Model Higgs

  10. VBF qqH→qqWW Di-lepton final state WW→ℓνℓν(ℓ=e,μ) b-jet and veto in the central region to reject tt background Drell-Yan background ee, μμ discarded with lepton cuts: Mℓℓ<75 GeV/c2; pTmiss>30 GeV/c Main decay channel around mH=170 GeV/c2 Isolated leptons WW(*)→ℓνℓν(ℓ=e,μ) Missing transverse energy ETmiss Dangerous background from top decays central jet veto WW spin correlations for the signal small ℓ+ℓ- opening angles High Mass Higgs: H→WW(*) 10 fb-1 are more than enough if mH is around 170 GeV/c2 ATL-PHYS-2003-005 - CMS NOTE 2003/033 LHC Prospects on Standard Model Higgs

  11. High mass Higgs: H→ZZ(*)→4μ • The “golden” channel • well defined peaks Z→μ+μ- • mH>2mZ: real Z’s • main backgrounds • reducible: tt, Zbb • μ isolation • Z reconstruction (mZ) • irreducible: ZZ • qq production mechanism dominates  softer muons - - - • Luminosity required for a 5σ discovery: • 10-30 fb-1 if mH>2mZ • 2-3 LHC years • up to 100 fb-1 if mH<2mZ • only one reconstructed Z • high luminosity runs CMS AN 2003-005 CMS AN 2003-007 LHC Prospects on Standard Model Higgs

  12. ATLAS & CMS Discovery Potential CERN/LHCC 99-15 ATLAS TDR 15 CMS NOTE 2003/033 • After detector calibration and LHC pilot run… • …almost all the “allowed” mass range can be explored during the first year (10 fb-1) • ...after 2 years (≈30 fb-1) 7σ significance over the whole mass spectrum, covered by more than one channel • LEP excess is near… LHC Prospects on Standard Model Higgs

  13. Conclusion • LHC has potential for Standard Model Higgs boson discovery already in the first year (months?) of operation • 1 LHC day at 1033 cm-2s-1≡ 10 years at previous machines • The ATLAS and CMS detectors are designed for Higgs boson search • no surprise that they can cover the full spectrum of Standard Model Higgs masses within 1 year of start of physics collisions • (however at the beginning a lot of time will be needed to reach the desired performance, optimise physics selection and measure backgrounds) …first collisions are near… LHC Prospects on Standard Model Higgs

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