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ATLAS Discovery Potential of the Standard Model Higgs Boson
Christian Weiser University of Freiburg
(on behalf of the ATLAS Collaboration) 14th Lomonosov Conference on Elementary Particle Physics Moscow, August 2009
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Content


Introduction Higgs search channels in ATLAS H VBF H H H H bb ZZ WW





Combined sensitivity Summary
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What do we already know?


Direct searches at LEP MH > 114.4 GeV/c2 (95% CL)



Direct searches at the TEVATRON (9 5% C L )

Exclude 160 GeV/c2 < MH < 170 GeV/c2



electroweak precision measurements 163 GeV/c
2

MH <

(95% CL) incl. LEP Limit)

(191 GeV/c2

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Higgs Boson Production
(i) Gluon-Fusion

(ii) Vector Boson Fusion (VBF)

(iii) Associated Production (W/Z, tt)

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[pb]
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Higgs Boson Decays

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Search Channels
X BR [fb] Main channels studied: Lo w mass: H qqH, H ttH, H bb W/Z H, H bb NEW!

Also medium (& high) mass: - H WW(*) - H ZZ(*)

For low mass: Combination of large production cross section (gg-fusion) and large BR (H->bb) does not work because of QCD background

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Higgs Studies in ATLAS
Studies presented here: - Realistic (at least we believe ...) GEANT4 based detector description simulation - Real reconstruction algorithms - Trigger simulation - Effects of additional minimum bias events (Pile-Up), misalignment etc. studied - Improved generators (NLO etc.) - Background estimation from data in many cases For all details see CERN-OPEN-2008-020 Focus in this talk: Discovery potential in the low and medium mass range (mH 200 GeV) @ s = 14 TeV To be (re)studied: Parameter determination (Mass, Spin, CP, Couplings, ....) Heavy Higgs bosons
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H
No direct coupling tiny BR () x BR < 100 fb Backgrounds:


Irreducible (+X)
( 30 pb for pT,>25 GeV, ||<2.5 80 GeV < m < 150 GeV)



Reducible +jet, jet+jet

(j : 180 nb for pT,>25 GeV jj : 480 b for pT>25 GeV)

Need excellent Photon identification Photon-jet separation Mass resolution

50% of events: at least 1 photon conversion reconstruct from e+, e- tracks in Inner Detector

mH 1.2 % mH

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H
Inclusive (ask for photons only) analysis: + 1,2 jet analyses (enhance VBF): 10 fb-1 + 1 Jet S/B 8% 1.8

S/B 3% 2.6

10 fb

-1

Discovery Potential (counting) 10 fb
-1

Pile-Up (1033 cm-2s-1): drop 10%

S/B 50% 1.9

+ 2 Jets

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Vector Boson Fusion qqH, H
Signature of Vector Boson Fusion (VBF) Production process: Two hard forward/backward "Tagging Jets" with large rapidity gap and high invariant mass Jet activity suppressed in central part of the detector Central Jet Veto Large discrepancies between generators

Jet

Rapidity Gap

Central Jet Veto
s ig nal Z

Jet
Main Backgrounds: Z ( ) + jets W ( ) + jets tt
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Vector Boson Fusion qqH, H
decay channels: Lepton-lepton (ll) Lepton-hadron (lh) [Hadron-hadron (hh)]

p p
2

vis 2

Invariant mass computation: (> 1 neutrino in final state) "Collinear Approximation" (Crucial: MET resolution)

p

1

p

vis 1

Dominating Background Z : Estimate on data from Z events with transformation
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(No Pile-Up)

hadron-hadron final state: No reliable estimate of QCD background without data No expected significance quoted
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H bb
tt H
a promising search channel some years ago:

Access to top-Higgs Yukawa coupling!
Complex final state:

Now:

MH = 120 GeV L = 30 fb
-1


Main Backgrounds: ttbb, ttjj
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Need precise background normalization! Has to come from data! Pile-Up: impact on selection efficiency and mass resolution
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ttH has disappeared from latest sensitivity plots!
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W/Z H
The most important channels at the TEVATRON at low mass! But: Signal to background ratio less favourable at the LHC History: Follow idea of J.Butterworth et al. [PRL 100:242001,2008]:

! EW N

Select events (5% of cross section), in which H und W bosons have large transverse momenta: pT > 200 GeV m b
H

"mono"-jet
H W l

b

-> b-Quarks in one "fat" Jet

+ Acceptance (more central in detector) + Lepton-Identification, B-Tagging
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W/Z H
Analyse jet structure:

ATL-PHYS-PUB-2009-088

l

ll



S L = 3 0 fb : = 3. 0 B
in t . -1

MH = 120 GeV


S = 1. 5 B

S = 1. 6 B

Combined:

S = 3 .7 B



(Pile-Up not yet included)
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S/B much Different Still good (e.g. S/B

better than for ttH backgrounds for different channels sensitivity including systematics = 3.0 for 15% uncertainty on all backgrounds)
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H ZZ
"Golden Channel": Clear, narrow mass peak on top of background Covers wide mass range Backgrounds: Irreducible: ZZ Reducible: Zbb, tt Lepton isolation and impact parameters

Effect of Pile-Up (1033 cm-2s-1) and Cavern Background: Loss in signal efficiency of 10% (studied for mH = 130 GeV)

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H ZZ
MH = 130 GeV
MH=180 GeV

MH=600 GeV

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H WW


Accessible for dominant production process (gg fusion) AND decay mode (mH > 135 GeV) Final state considered: H WW e Two analyses: + 0 jet (gg fusion), +2 jets (vector boson fusion) 2 neutrinos no mass peak reconstructable, only transverse mass Use angular correlation between leptons:

Fit transverse mass and transverse momentum of The WW system in signal and control regions

(ll)

(No Pile-Up)

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Combined Discovery Potential

H ZZ* HWW* 5 qqH (WW*) qq H ( H



5 discovery seems possible with 10 fb

-1

for m

H

> 127 GeV

Adding the W/Z H, H bb channel will improve situation for low masses
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Exclusion

2 fb
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-1

to exclude mH > 115 GeV @ 95% CL
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ATLAS SM Higgs


Conclusions


Much more realistic detector simulations, Monte Carlo generators, reconstruction tools etc. have improved the quality of the predictions for ATLAS Higgs boson searches significantly Strong effort has been put in studying data driven methods to become less dependent on Monte Carlo generators A discovery (or exclusion) should be possible over the full mass range from 115 GeV ­ 1 TeV However, the low mass region (just above the LEP limit) is very challenging and several channels may contribute







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