Summary

2. Summary The ATLAS detector at LHC The SM Higgs physics: - the experimental limits on the Higgs mass - the production and the decay at LHC Study of the main channels: - H gg - H tt (in VBF production) - H  ZZ  4l - H  WW Significance Conclusions

3. The ATLAS Detector at LHC The LHC will collide protons at √s = 14 TeV (at the start-up √s = 10 TeV) 24 m ATLAS is one of the two general-purpose LHC experiments mainly aimed to study the origin of the electroweak symmetry breaking 45 m • Solenoid in the inner cavity • Inner Detector • Si pixels + TRT for particle • identification • Solenoid B = 2T • R=12-18m, Z=60-580m • σ/pT~5x10-4pT⊕0.01 • Electromagnetic calorimeter • Pb-liquid Argon • σ/E~10%/√E⊕0.007 • Hadronic calorimeter • Fe-scint + Cu-liquid Argon(10λ) • σ/E~50%/√E⊕0.03 • 3 air-core toroids • Muon Spectrometer • Optimal performance • also standalone • σ/pT~2%@50GeV÷ • 10%@1TeV ( ID+MS)

4. Experimental limits to Higgs mass An indirect constraint is obtained performing a fit with the electroweak parameters measured with high precision at LEP, Tevatron and SLAC: MH<154 GeV/c2 (95% C.L.) From the direct research at LEP2 (result obtained by combining the data of the four LEP experiments): MH>114.4 GeV/c2 (95% C.L.) http://lepewwg.web.cern.ch/LEPEWWG/

5. The SM Higgs production at LHC A. Djouadi Phys.Rept.457:1-216 gluon-gluon Fusion Vector Boson Fusion (VBF) Higgs-strahlung gluon-gluon Fusion is the dominant process, Vector Boson Fusion (VBF) is ~10% of the total cross-section for MH<2MZ, but it has a clear signature in the detector due to the presence of two high pT jets in the forward region. Associated Production

6. The SM Higgs decay at LHC For MH<130 GeV, : H  gg (mainly in gg prod, studied also VBF and ass. prod. Golden channel in this region) H tt (in VBF prod) (dominant decay mode, large QCD background, studied in associated prod., complex final state) For MH>130 GeV: H WW(*) lnln (in gg or VBF prod) H WW(*) ln-had (in VBF prod) H  ZZ(*)  4 lep (in gg prod. Golden channel for MH>2MZ) A. Djouadi Phys.Rept.457:1-216 • What is the latest ATLAS framework to the Higgs studies? • • New Monte Carlo generators also for background (MC@NLO, ALPGEN, • HERWIG, PYTHIA, ...) • • Detailed GEANT4-based simulation of the ATLAS detector • Detailed event reconstruction (based also on test beam results).

7. H gg • Background : • – Irreducible: pp gg, pp gg+jets • – Reducible: pp g+jets, pp jets , Drell-Yan • Search for two isolated high pT photons • Experimental requirements: • – Excellent electromagnetic energy resolution; • – High photon efficiency and • strong p0/jet rejection (10-3÷10-4 to get σgj+σjj<< σgg) • – Good gg-vertex reconstruction • Good efficiency reconstruction of converted photons • (~57% of the events with at least one converted photon) It will be measured from data sidebands Preliminary H+ ETmiss+1lept Inclusive H+ 2j (VBF mainly) Preliminary Preliminary Preliminary

8. VBF H tt (1) The distinctive VBF signature: 1. Two forward “tag” jets (large h separation with high pT) with large Mjj, 2. No jet activity in the central region (due to no colour flow between tag jets) Typical selection for VBF channels: 2 high pT jets with large h separation, Mjj>0.5-1 TeV, Higgs decay products between tag jets in h central, jet veto in the central region. Experimental issues for VBF channels: good efficiency reconstruction for forward jet; robustness of the jet veto with respect to radiation in the underlying event and to pile-up. • distribution for the highest pT jet Preliminary Efficiency of central jet veto at different pileup level Preliminary

9. VBF H tt (2) The 3 topologies : qqH → qqtt , tt → lnn-lnn, lnn-had, had-had Dominant backgrounds : Specific issue of the tt channel: very good t-jet identification to have a low rate of fake t from jets. Good ETmiss measurement for mass resolution of tau-pairs Analysis: The H mass can be reconstructed using the collinear approximation: t mass neglected and assumed the n direction coinciding with the visible decay products of the t. Data-driven control samples are being explored for many backgrounds Preliminary Higgs Z bkg QCD bkg

10. H ZZ 4l • The “golden channel”: observation of a narrow peak on a continuous background. Backgrounds: Experimental issues: safe lepton identification and very good energy measurement Estimate background from sidebands Selection: Search for isolated muon and electron pairs with opposite charge, cuts on isolation variables, lepton track impact parameter (to reject reducible bkg), cuts on MZ,, reconstruction of M4l Track isolation Preliminary Preliminary Impact parameter Preliminary Preliminary

11. H  WW Studied topologies: H + 0 jets  lνlν (dominated by gluon fusion) H + 2 jets  lνlν or H + 2 jets  lνqq (dominated by VBF) Main backgrounds: For H  WW  lnln: - no mass peak, use transverse mass (MT) - select events with exactly 2 isolated leptons and ETmiss and jet veto - likelihood fit using MT, the transverse lepton opening angle Dfll and transverse momentum pTWW Need precise knowledge of background Develop data driven methods Results obtained with the ATLAS fast simulation code (2004) The new results obtained with the latest detailed simulation studies (including significance) will be available soon H + 2 jets  eνmν (VBF channel) (mH=120 GeV)

12. Higgs Significance H gg VBF H tt Combining various analysis strategies (inclusive,H+2j, etc)   Preliminary Preliminary H  ZZ  4l Combined significance results are in preparation (after finalizing WW analysis). They will be available before the end of the year Preliminary  Without systematic uncertainties

13. Conclusions • The Large Hadron Collider will start collisions in the 2009 Spring • ATLAS is well set up to explore the existence of a Standard Model Higgs boson • Many SM Higgs channels have been studied in detail with new MC generators, detailed apparatus simulation and reconstruction codes • The full Standard Model mass range can be covered and a good sensitivity could be achieved with few fb-1 (with a detector well calibrated and after background understanding)