Higgs Searches at the Tevatron
Kazuhiro Yamamoto (Osaka City Univ.)
On behalf of the CDF and DØ Collaborations

Strong Coupling Gauge Theories in LHC Era (SCGT09)
Nagoya, Japan
8-11 December, 2009

Outline
Tevatron and Collider Detectors
Standard Model Higgs Boson
Higgs Bosons Beyond the SM
Future Prospects
Conclusion

2The Tevatron Accelerator
Proton-antiproton collider at √ s = 1.96 TeV
Two major detectors at collision points : CDF and DØ
Tevatron and all upstream components are running very well.

CDFD∅Tevatron
Main Injector

Fermilab3

Tevatron Luminosity Progress
We are achieving typicalluminosity ofPeak : ~3 × 1032 cm-2s-1
Weekly integrated : 50~60 pb-1

Run II record luminosity
Peak : 3.7 × 1032 cm-2s-1
Weekly integrated : 74 pb-1

Integrated luminosity
Delivered : 7.4 fb-1
Acquired : 6.1 fb-1
Analyzed : 5.4 fb-1

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Collider Detectors

CDF and DØGeneral-purpose, cylindricallysymmetric detectors
Superconducting solenoid magnet
1.4T (CDF) , 1.9T (DØ)

Inner detectors for precision tracking
Calorimeters for energy measurement
Outer muon detectors
Well understood, stable operation overa long period of time
Accumulated ~6 fb-1 of good quality
data in both experiments.

5Standard Model Higgs Boson

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Status of SM Higgs
Indirect limit from global EW fit
mt = 173.1 ± 1.3 GeV/c2
mW = 80.399 ± 0.023 GeV/c2and precision EW measurements
at LEP and SLD

mH = 87+35−26 GeV/c2

Direct search at LEP
mH > 114.4 GeV/c2 (95% C.L.)
Combining with the limit from EW fit

mH < 186 GeV/c2 (95% C.L.)
CDF and DØ are probing Higgs in the mostprobable region : 100 < MH < 200 GeV/c2
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SM Higgs Production at Tevatron
Following 3 processes are dominant at the Tevatron energy.
Gluon fusionσ(gg → H) = 0.2 ~ 1 pb

Associated production with a vector boson
σ(qq → WH/ZH) = 0.01 ~ 0.3 pb

Vector boson fusion
σ(qq → qqH) = 0.02 ~ 0.1 pb

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Higgs Decays and Search Channels
Low mass Higgs ( < 140GeV/c2)
bb is dominant.
b-tagging is an effective way.

gg → H → bb swamps in QCD
multijet background.
Search in VH production
Need triggering with high-pT
lepton or missing ET

WH → ℓνbb
ZH → ℓ+ℓ−bb, ννbb

High mass Higgs ( > 140GeV/c2)
WW is dominant.
Use multi-lepton signature

Any single channel does not haveenough sensitivity for discovery.

gg → H → WW → ℓ+ℓ−ννCombine all available channels togain sensitivity.
9VH → E/ T bb
Signature contributed fromZH → ννbb
WH → ℓνbb, where ℓ is
missing from detector

Remove multijet background bymultivariate discriminantDØ : Boosted decision tree (BDT)
CDF : NN

Base selection
Lepton veto
Large missing ET + 2 or 3 jets
At least one b-tagged jetDØ : neural net (NN) tagger
CDF : secondary vertex
tagger (SECVTX) and jet
probability (JP) Background : W/Z+jets, tt,diboson, QCD multijets

10VH → E/ T bb (2)Second discriminant to separate
signal from other SM backgroundDØ : BDT, CDF : NN

For MH = 115 GeV/c2 w/ 5.2 fb-1
Expected limit : 4.6 × σSM
Observed limit : 3.7 × σSM

For MH = 115 GeV/c2 w/ 3.6 fb-1
Expected limit : 4.2 × σSM
Observed limit : 6.1 × σSM11

WH → ℓνbb
Most sensitive channel at lowmass

Dijet mass
(2 jet sample)

Base selection
Single isolated high-pT leptonWbb(e or µ)Large missing ET
2 or 3 energetic jets
At least one b-tagged jet Background : W/Z+jets, tt,
single top, diboson, non-W
QCD

Dijet mass
(3 jet sample)

2 jets – Wbb dominates.
3 jets – tt dominates.
tt
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WH → ℓνbb (2)Use multivariate discriminant toseparate signal from backgroundDØ : Neural Network (NN)
CDF : 2 analyses employing NN(2 jets) and ME (2 and 3 jets)
 Optimized for b-tag categories

Neural Nets
(2 jets, double tag)

Matrix Element(2 jets, double tags)

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WH → ℓνbb (3)Upper limits on cross section × branching ratio

NN, 2 jets

MH = 115 GeV/c2

Expected limit

Observed limit

Luminosity

CDF (NN, 2 jets)

4.0 × σSM

5.3 × σSM

4.3 fb-1

CDF (ME, full)

4.1 × σSM

6.6 × σSM

4.3 fb-1

CDF (ME, 3 jets)

18 × σSM

11 × σSM

4.3 fb-1

DØ (NN)

5.1 × σSM

6.9 × σSM

5.0 fb-1
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ZH → ℓℓbb
Clean, but low event rate
Base selection
Z candidates reconstructedfrom e+e− and µ+µ− pairs
2 or more energetic jets
At least one b-tagged jet Background• Z+jets(bb, cc, light flavor)• tt, WZ, ZZ, QCD fakeDijet mass

Multivariate discriminant analysis
CDF : 2D Neural networkDØ : Boosted decision tree

2D NN15

ZH → ℓℓbb (2)We observed no significant excess over the background.
Set upper limit on the cross section × branching ratio

For MH = 115 GeV/c2 w/ 4.1 fb-1
Expected limit : 6.8 × σSM
Observed limit : 5.9 × σSM

For MH = 115 GeV/c2 w/ 4.2 fb-1
Expected limit : 8.0 × σSM
Observed limit : 9.1 × σSM
16τ+τ−qq final state
The following 5 processes are
involved in ττqq final state.
ZH, Z→τ+τ−, H→qq
(AssociatedHZ, H→τ+τ−, Z→qq
production)
HW, H→τ+τ−, W→qq
qq→Hqq, H→τ+τ−(Vector boson fusion)
gg→H, H→τ+τ−, additional 2jets(Gluon fusion)

Signal separation from bkgd isperformed with BDT.
 Background : tt, W/Z+jets,multijetsττ decay identification is essential.
Leptonic + hadronice.g.) τ+τ− → (µ+ ν¯ τνµ) + (π−π0ντ)

1 or 3 charged particles in anarrow cone.
NN is used to identify taus
decaying to hadrons.

For MH = 115 GeV/c2 w/ 4.9 fb-1
Expected limit : 15.9 × σSM
Observed limit : 27.0 × σSM

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H → WW*Sensitive to high mass Higgs (MH > 140 GeV/c2)
Gluon fusion is the dominant
production process.
gg → H → WW* → ℓℓννSignal x 10

Some more contributions from
WH/ZH and VBFqq → WH → WWW*
qq → ZH → ZWW*
qq → qqH → qqWW*

Dilepton opening angle∆Rℓℓ= √(∆ηℓℓ)2 + (∆ϕℓℓ)2Base selection
High pT opposite-sign dilepton
Large missing ET Background : tt, Drell-Yan,
diboson, W+jets, Wγ

Dileptons from Higgs decay tend togo in the same direction.
- Different from SM backgrounds
−νℓ−
W−H

ν

W+ℓ+18

H → WW* (2)NN is used to distinguish signal from background.
Separately trained for different final states
CDF : by number of jets (0 jet, 1 jet, 2 or more jets)DØ : by dilepton flavor (ee, µµ, eµ)

DataZ+jets
W+jetsMultijet
TopDiboson
Signal(× 10)

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H → WW* (3)More samples to increase sensitivity.
Low mass (Mℓℓ < 16GeV) region
Same-sign dilepton + jets (WH → WWW*, ZH → ZWW*) Different background composition⇒ Separate NN training20

H → WW* (4)Expected and observed limits versus Higgs mass for all dilepton
channel combination

For MH = 165 GeV/c2 w/ 4.8 fb-1
Expected limit : 1.21 × σSM
Observed limit : 1.23 × σSM

For MH = 165 GeV/c2 w/ 5.4 fb-1
Expected limit : 1.36 × σSM
Observed limit : 1.55 × σSM

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CDF SM Higgs Combination
CDF combined results with L = 2.0 - 4.8 fb-1
Included channels
WH → ℓνbb (4.3 fb-1)
VH → E/ T + bb (3.6 fb-1)
ZH → ℓℓbb (4.1 fb-1)
VH, VBF, ggH→ 2 jets + ττ (2.0 fb-1)
VH → 2 jets + bb (2.0 fb-1)
ggH → WW* → ℓνℓν (4.8 fb-1)
VH → VWW* (4.8 fb-1)
For MH = 115 GeV/c2Expected limit : 2.38 × σSM
Observed limit : 3.12 × σSM
For MH = 165 GeV/c2
Expected limit : 1.19 × σSM
Observed limit : 1.18 × σSM

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DØ SM Higgs Combination
DØ combined results with L = 2.1 - 5.4 fb-1
Included channels
WH → ℓνbb (5.0 fb-1)
XH → ττbb/qqττ (4.9 fb-1)
ZH → ννbb (5.2 fb-1)
ZH → ℓℓbb (4.2 fb-1)
WH → WWW* (3.6 fb-1)
H → WW* (5.4 fb-1)
H → γγ (4.2 fb-1)
ttH → ttbb (2.1 fb-1)
For MH = 115 GeV/c2Expected limit : 2.80 × σSMObserved limit : 4.05 × σSMFor MH = 165 GeV/c2Expected limit : 1.35 × σSM
Observed limit : 1.53 × σSM23

Tevatron SM Higgs Combination
Combined results of CDF and DØ with L = 2.0 – 5.4 fb-1
 Systematics correlation b/w experiments are taken into account.

Expected limit at MH = 115 GeV/c2 : 1.78 × σSMExpected exclusion range at 95% C.L. : 159 – 168 GeV/c224

Tevatron SM Higgs Combination
Combined results of CDF and DØ with L = 2.0 – 5.4 fb-1
 Systematics correlation b/w experiments are taken into account.

Observed (expected) limit at MH = 115 GeV/c2 : 2.70 (1.78) × σSM
Excluded mass range at 95% C.L. : 163 - 166 GeV/c2(Expected exclusion range
arXiv:0911.3930[hep-ex]: 159 – 168 GeV/c2 )as of November 200925

Higgs Bosons Beyond the SM

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MSSM Higgs at the Tevatron
Two-Higgs-doublet fields provide
5 physical Higgs bosons.
3 neutral : φ = h, H, A
2 charged : H± Phenomenology described attree level by tanβ and MA.

Neutral Higgs
Coupling to d-type quarks
enhanced by tanβ ⇒ σ φ ∝ tan2βBr(φ → ττ) ~10%, Br(φ → bb)~90% for low and intermediate
massesCharged Higgs
For (MH+ < Mt – Mb), a top quark
can decay into H±b.

Total

AH

hMSSM: tanβ = 30

Tevatron has sensitivity forsome MSSM scenarios.

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MSSM Neutral Higgs : φ → τ+τ−gg, bb → φ → ττgluon fusion

bb fusion

Tau pairs are identified in τeτµ,τeτhad, and τµτhad.
Background :••Modelindependentlimit

Z → ττ, Z → ee/µµ
Diboson, tt, W + jets

Combined CDF and DØ results
Interpretation to typical
MSSM scenario :Maximal stop mixingµ = −200GeV
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MSSM Neutral Higgs : φb → bbb
gb → φb → bbb

Required 3 b-tagged jets.
Large multijet backgroundSearch for peak in dijet mass
CDF : 1.9 fb-1, DØ : 2.6 fb-1

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MSSM Charged Higgs
Search for H± in top decayst → H±b• H± → cs (for small tanβ)• H± → τν (for large tanβ)τ + lepton

dileptonℓ + jets, 2tagℓ + jets, 1 tag

If H± exists, there would be deviationfrom the SM prediction for the final
states of tt decay.

Dijet mass

Consistent with SM
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Fermiophobic Higgs
In some BSM models, Higgs couplings
to fermions are suppressed.
⇒ Higgs decays to vector bosons aresignificantly increased.
Low mass region : H → γγHigh mass region : H → WW/ZZ

Benchmark scenario
No fermion couplings and SM couplings tovector boson

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Future Prospects

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Luminosity Prospects
Tevatron performance and projection
We expect ~ 9 fb-1 by the end of FY10.
If FY11 budget is approved, we can go up to ~12 fb-1.
12 fb-1 delivered luminosity doubles
the current data set and results inanalysis with 10 fb-1.
We are here

~ 12 fb-1

9.3 fb-1
7.8 fb-1

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SM Higgs Sensitivity Prospects
For MH = 115 GeV/c2

For MH = 160 GeV/c2

Analysis improvements help the sensitivity increase better than 1/sqrt(L).
Expect to reach 115GeV Higgs with 6~10 fb-1

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Conclusions
Tevatron and the collider detectors (CDF and DØ) are performing
very well.
Delivered 7.4 fb-1, Acquired 6.1 fb-1, Analyzed 5.4 fb-1
Expect ~ 9 fb-1 by the end of FY10 We all thank accelerator people for excellent beam !

Higgs searches are in progress in various production and decaychannels.
SM Higgs Boson :Observed (expected) limit at MH = 115GeV/c2 : 2.70 (1.78) × σSM
Tevatron expects to exclude 159 – 168 GeV/c2 at 95% C.L.
Excluded mass range : 163 – 166 GeV/c2

Higgs Bosons Beyond the SM :No sign of discovery yet. But sensitivity is increasing steadily.

Increasing luminosity, analysis improvements, … We can go further !

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Backup Slides

36

Spring 2009 Result37

Multivariate Techniques
Both experiments use advanced multivariate techniques, which combine
information from kinematical, topological and particle identification variables,
to enhance the signal/background discrimination.
Artificial NeuralNetworks (NN)Single variable discriminant

Boosted Decision
Trees (BDT)

Matrix Element (ME)Calculating event probability
integrating the LO matrix elements

Neural network output38