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 32 nd International Conference on High Energy Physics Beijing, China 16-22 August 2004

2. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 2 ATLAS & CMS at LHC –Detectors optimised for Higgs boson and SUSY searches LHCpp √s=14 TeV  very high energy: LHC pp √s=14 TeV σ pp =55 mb –inelastic cross section: σ pp =55 mb 40 MHz –interaction rate: 40 MHz (2x)10 33 cm -2 s -1  10 34 cm -2 s -1  high luminosity: (2x)10 33 cm -2 s -1  10 34 cm -2 s -1 20 fb -1  100 fb -1 –per year: 20 fb -1  100 fb -1  BIG detectors –CMS: 15 m x 21.5 m –ATLAS: 25 mx 46 m CMS = Compact Muon Solenoid LHC = Large Hadron Collider ATLAS = A Toroidal LHC ApparatuS first collisions in Summer 2007

3. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 3 Where will we be at LHC startup? m H >114.4 GeV/c 2 @ 95% CL m H =115 GeV/c 2 ¿ Higgs boson signal m H =115 GeV/c 2 from LEP2 data ? CERN-EP/2003-011 LHWG Note/2002-01 Searches at TeVatron from now to 2007… Design Projection: 4.4 fb -1 Design Projection: 4.4 fb -1  m H <130 GeV/c 2 covered to exclusion  3σ evidence up to m H =125 GeV/c 2  Base Projection:2.2 fb -1  m H <125 GeV/c 2 covered to exclusion FERMILAB-PUB-03/320-E

4. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 4  Associated production ttH and bbH –high-p T lepton, top reconstruction,b-tag-- SM Higgs Production at LHC  Gluon Fusion –the highest cross section  Vector Boson Fusion –two high-p T forward jets  Associated Production WH and ZH –one or two high-p T leptons useful for the trigger

5. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 5 SM Higgs Decays “light” Higgs 2m Z –decays into Vector bosons W and Z »“golden” channels –two-photon decays »extremely “clean” but rare and difficult to detect LEP excluded –hadronic and  decays are favourite »…but difficult to select

6. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 6 Low Mass Higgs: H →  This decay is very rare (Br≈10 -3 )  σ(pp → H 115 )xBr(H →  )=76 fb (NLO) S/B≈1/20 mass peak –good  resolution  mass peak crucial σ(m  )/m  ≈1% –Electromagnetic Calorimetres crucial for H →  : σ(m  )/m  ≈1% needed  motivation for LAr (ATLAS) and PbWO 4 (CMS) calorimetres –high granularity –response uniformity CERN/LHCC 96-40 ATLAS TDR 1 CERN/LHCC 96-41 ATLAS TDR 2 CERN/LHCC 97-33 CMS TDR 4 –3 main background processes: 81 pb »irreducibile: gg/qq →  81 pb 9x10 4 pb »  +jet (with “real” or “fake” second photon)9x10 4 pb 10 8 pb »hadronic QCD jets (π 0 decays) 10 8 pb 10 fb -1 m H =115 GeV/c 2 »ATLAS reach with 10 fb -1 and m H =115 GeV/c 2 : S/√B=2.0 »Signal Significance: S/√B=2.0 (K-factors not included)

7. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 7 Low Mass Higgs: ttH( → bb) This is the favourite decay  σ(pp → H 115 )xBr(H → bb)=28 pb S/B<10 -7  tagging the top quarks helps a lot 1.t → bW( → μν ) 2.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 –p z from W-mass constraints –likelihood pairing of jets -- - ATL-PHYS-2003-024 ν »Final result for likelihood analysis (m H =115 GeV/c 2 ): »30 fb -1 : S/√B=3.4 »10 fb -1 : S/√B=2.0

8. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 8 Jet Vector Boson Fusion: qqH( →  )  hadronic jets in forward-backward regions –the forward jet tagging is a powerful background rejection tool low η  hadronic activity suppressed in low η region –emitted vector bosons are colour-singlets  → ℓ ν ℓ’ ν ’, ℓ ν +jet  Search for  → ℓ ν ℓ’ ν ’, ℓ ν +jet final states –S/√B≥5m H =120÷140 GeV/c 2 40 fb -1 –S/√B≥5 in m H =120÷140 GeV/c 2 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  Phys. Rev. D59 (1999) 014037 ATL-PHYS-2003-004 CMS NOTE 2003/033 Jet   Higgs Decay products Forward tagging jets [VBF]

9. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 9 Light Higgs Summary Detector/performance requirements: HHHH→response uniformity of electromagnetic calorimeter HHHH→bbb-tagging capability of tracking system qqqqqH(→)efficient j jj jet reconstruction in 3<|η|<5 aaaallgood u uu understanding (<10%) of the b bb background Three complementary channels 2σ 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) -

10. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 10 High Mass Higgs: H → WW (*) VBF qqH → qqWW –Di-lepton final state  WW → ℓ ν ℓ ν ( ℓ =e,μ)  b-jet and  veto in the central region to reject tt background  Drell-Yan background ee, μμ discarded with lepton cuts: –M ℓℓ 30 GeV/c –Main decay channel around m H =170 GeV/c 2 Isolated leptons WW (*) → ℓ ν ℓ ν ( ℓ =e,μ) Missing transverse energy E T miss Dangerous background from top decays –central jet veto WW spin correlations for the signal –small ℓ + ℓ - opening angles CMS NOTE 2003/033 ATL-PHYS-2003-005 10 fb -1 are more than enough if m H is around 170 GeV/c 2 -

11. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 11 golden –The “golden” channel Z → μ + μ -  well defined peaks Z → μ + μ - –m H >2m Z –m H >2m Z : real Z’s  main backgrounds –reducible: tt, Zbb »μ isolation »Z reconstruction (m Z ) –irreducible: ZZ »qq production mechanism dominates  softer muons 5σ  Luminosity required for a 5σ discovery: –10-30 fb -1 if m H >2m Z »2-3 LHC years –up to 100 fb -1 if m H <2m Z »only one reconstructed Z »high luminosity runs High mass Higgs: H → ZZ (*) → 4μ CMS AN 2003-005 CMS AN 2003-007 - - -

12. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 12 ATLAS & CMS Discovery Potential  After detector calibration and LHC pilot run… first year –…almost all the “allowed” mass range can be explored during the first year (10 fb -1 ) 2 years 7 σ more than one –...after 2 years (≈30 fb -1 ) 7 σ significance over the whole mass spectrum, covered by more than one channel »LEP excess is near… CMS NOTE 2003/033CERN/LHCC 99-15 ATLAS TDR 15

13. ICHEP’04 Beijing, 16-22 August 2004 LHC Prospects on Standard Model Higgs Riccardo Ranieri 13Conclusion already in the first year –LHC has potential for Standard Model Higgs boson discovery already in the first year (months?) of operation  1 LHC day at 10 33 cm -2 s -1 ≡ 10 years at previous machines ATLASCMSdesigned –The ATLAS and CMS detectors are designed for Higgs boson search  no surprise Higgs1 year  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…