1. 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 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
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
1034cm-2s-1
Dipole field
8.33 Tesla
Stored energy
360 MJ/beam 2

3. LHC Status and timeline
all sectors at 2°K from August
First collisions expected on October at 5 TeV
Commissioning run at √s=10 TeV until November ( 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

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. 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

7. 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

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

11. 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

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

13. 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

14. Hgg: phenomenology and experiment Phenomenological issues:
Small branching ratio
But very clear signature “easy” to separate from background
Important for low-mass region ( 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)

16. Hgg: invariant mass spectra for ATLAS Example: m(H)=120 GeV Inclusive
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
* 94% trigger eff wrt offline cuts

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 (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

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: 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

20. 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

21. 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

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

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 /pT ~ 5x10-4 pT  0.01 Tracking ||<2.5 B=2T
TRT
SCT
Silicon pixels (Pixel): channels
Silicon strips (SCT) : channels
Transition Radiation Tracker (TRT) :
straw tubes (Xe), 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
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

28. ~600 RPC and ~3600 TGC trigger chambers
ATLAS Muon System
Stand-alone momentum resolution Δpt/pt < 10% up to 1 TeV
2-6 Tm ||< 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 = GeV/c2; including LEP: mH < 185 GeV/c2
From LEP: mH> % CL
Tevatron experiments expected to be able to exclude SM 95%CL up to mH≈200 GeV/c2 or make 3σ observation
ICHEP08, Philadelphia 31