1. HIGGS BOSON SEARCHES AT THE TEVATRON
Bárbara Álvarez González
On behalf of the CDF and D0 Collaborations
LISHEP 2009, Rio de Janeiro, January 19th

2. V (Φ) = μ2Φ†Φ + λ(Φ†Φ)2 ; μ2<0 λ>0
OUTLINE
Fermilab and Tevatron: Introduction.
CDF and D0 Detectors.
Standard Model (SM) Higgs.
SM Analyses Results.
Beyond Standard Model.
Beyond SM Analyses Results.
Conclusions.
“MEXICAN HAT”
The potential energy of the higgs field has a mexican hat shape.
V (Φ) = μ2Φ†Φ + λ(Φ†Φ)2 ; μ2<0 λ>0
Bárbara Álvarez- U. de Oviedo

3. p p FERMILAB TEVATRON particle accelerator and collider.
Chicago
CDF D0
TEVATRON
MAIN INJECTOR
Fermilab is home to the Tevatron, the world's highest-energy
particle accelerator and collider.
proton-antiproton collisions at √s = 1.96 TeV.
Tevatron is performing really well:
6-8 fb-1 expected by the end of 2009.
At a center of mass energy of 1.96 GeV p p
Bárbara Álvarez- U. de Oviedo

4. LUMINOSITY AND DATA TAKING
Total delivered = 5.5 fb-1
PER EXPERIMENT
Total Acquired = 4.58 fb-1
DELIVERED
The best initial luminosity:
3.61×1032 cm-2 s-1
ACQUIRED
Data Taking Average
Efficiency ~82%
Bárbara Álvarez- U. de Oviedo

5. DETECTORS AT FERMILAB Data analyses with 2 multipurpose
detectors, CDF and D0:
Silicon vertex detectors for b-tagging.
Charged particle tracker in magnetic field.
EM+HAD calorimeter for e,mu id.
Muon detectors for muon id.
Bárbara Álvarez- U. de Oviedo

6. HIGGS BOSON
The search for the Higgs boson
is one of the most active areas
of research at the Tevatron.
The Higgs boson is the only SM particle that has not been observed.
It requires an exceptionally large amount of energy and beam luminosity to create and observe at high energy colliders due to small cross sections and large backgrounds.
Higgs boson is the remaining particle of the SM. It needs a large amount of energy and luminosity to be observed and create.
LHC is going to double the Tevatron center of mass energy.
One of the LHC goals is to discover the higgs boson.
Only ~40 Higgs events are expected to be produced per fb-1 at each experiment in detectable channels
Bárbara Álvarez- U. de Oviedo

7. Indirect EWK constraints: mH < 154 GeV/c2
In the standard model (SM), the Higgs boson is crucial
to the understanding of electroweak symmetry breaking
and the mass generation of electroweak gauge bosons and fermions.
Higher order diagrams involving Higgs Bosons enter as corrections
to SM predictions for EWK processes
Direct searches at LEP:
mH > GeV/c2 at 95% C.L.
Indirect EWK constraints:
mH < 154 GeV/c2
The figure on the left shows the Delta-chi2 curve derived from high-Q2 precision electroweak measurements, performed at LEP and by SLD, CDF, and D0, as a function of the Higgs-boson mass, assuming the Standard Model to be the correct theory of nature. The preferred value for its mass, corresponding to the minimum of the curve, is at 84 GeV, with an experimental uncertainty of +34 and -26 GeV (at 68 percent confidence level derived from Delta chi2 = 1 for the black line, thus not taking the theoretical uncertainty shown as the blue band into account). This result is only little affected by the low-Q2 results such as the NuTeV measurement discussed above.
While this is not a proof that the Standard-Model Higgs boson actually exists, it does serve as a guideline in what mass range to look for it. The precision electroweak measurements tell us that the mass of the Standard-Model Higgs boson is lower than about 154 GeV (one-sided 95 percent confidence level upper limit derived from Delta chi2 = 2.7 for the blue band, thus including both the experimental and the theoretical uncertainty). This limit increases to 185 GeV when including the LEP-2 direct search limit of 114 GeV shown in yellow (see below).
Bárbara Álvarez- U. de Oviedo

8. SM HIGGS CROSS SECTIONS AND BR
High mass Higgs region (mH>135 GeV/c2):
H → WW dominant decay.
Gluon fusion production search (gg → H).
Low mass Higgs region (mH<135 GeV/c2):
H → bb dominant decay.
Search for associated W/Z production.
We divide the searches in low and high mass region.
Low mass is with mh<135 Gev and associated production with Z/W, high mass mh>135GeV and decay to WW.
Bárbara Álvarez- U. de Oviedo

9. ANALYSIS STRUCTURE RESULTSB K G E S T I M A O N
DATA & MC
SAMPLES
EVENT SELECTION
SYSTEMATIC
UNCERTAINTIES
MULTIVARIATE
TECHNIQUES
Neural
Networks
(NN)
2 1012
Matrix
Element
(ME)
Boosted
Decision
Trees
(BDT)
109
RESULTS
Data and MC samples: we take the samples needed for each analysis.
Event selection: we select the events depending on the final state that we are looking for.
Bkg estimation: requires really hard work to remove all the event that are background and contanimate our signal and understand all the main bkgs.
Because we have really small signal compare with the huge bkgs, we can not use counting experiments, we apply multivariate techquines as ME, BDT, NN... to distinguish signal from bkg.
We study all significant systematics and …
Results: observing no significant excess in the data we place 95 % c.l. Upper limits.
2 105
2 104
THE CHALLENGE IS TO DISTINGUISH A SMALL
SIGNAL FROM ALL THE HUGE BACKGROUNDS
2 102
102 1
Bárbara Álvarez- U. de Oviedo

10. ANALYSIS TOOLS Multiple b-quark tagging algorithms:
SecVtx algorithm
Multiple b-quark tagging algorithms:
DØ: NN tagger based on b-lifetime information,
with multiple operating points.
CDF: Secondary Vertex and Jet Probability
algorithms. Additional NN flavor separators.
Maximize lepton acceptance:
Separate into different lepton categories based on S/B.
Bárbara Álvarez- U. de Oviedo

11. / μ
WH → lvbb
Low Mass
Dedicated trigger: High pT leptons, and MET+jets.
Same selection.
Same background treatment.
Different analysis techniques
In CDF, 2 WH analysis are performed
ME + BDT NN
These two results are combined to have the final WH → lvbb limit (next page).
ME+BDT
Neural Network
One analysis uses the BDT techquine with ME variables as input in the BDT.
The other uses NN with 6 studied input variables.
Then we combine all this, after the job of matching the event selection, in a new result
Into a new NN callled NEAT...
Bárbara Álvarez- U. de Oviedo

12. Parton Distribution Functions lepton and jet 4-vectors
Transfer Function
EVENT PROBABILITY DENSITIES:
P(pl,pj1,pj2) = 1/σ ∫ dρj1dρj2dpν ∑Φ4|M(pi)2| f(q1)f(q2)/|q1||q2| Wjet (Ejet,Epart)
Parton Distribution Functions
Phase Space Factor
Matrix Elements
Inputs:
lepton and jet 4-vectors
– no other information
needed!
Backgrounds peak to zero and signal peaks higher
Single tag region
WHx25
Event Probablity Discriminant
(EPD) = Ps / Ps + Pbkg
Bárbara Álvarez- U. de Oviedo

13. WH → lvbb Low Mass For Higgs searches, the most sensitive
This is the most sensitive production channel at the Tevatron for a mH < ~135 GeV/c2
Low Mass
Analysis
Lumi (fb-1)
Exp. / SM
Obs. / SM
D0 (115 GeV/c2)
1.7
8.9
10.9
CDF (115 GeV/c2)
2.7
4.8
5.6
Combine ME+BDT & NN
Selected using a Neural Network
For Higgs searches, the most sensitive
production channel at the Tevatron for
a mH below ~ 135 GeV/c2 is
the associated production of a Higgs
boson with a W boson.
Neuro-Evolution of Augmenting Topologies
Bárbara Álvarez- U. de Oviedo

14. ZH → lvbb Event Selection: 2 high pT Main Background:
Analysis
Lumi (fb-1)
Exp. / SM
Obs. / SM
D0, NN & BDT(115 GeV/c2)
2.3
12.3
11.0
CDF, NN (115 GeV/c2)
2.7
9.9
7.1
CDF, ME (120 GeV/c2)
2.0
15.0
14.2
ZH → lvbb
Low Mass
Event Selection: 2 high pT
leptons (ee/μμ) and b-jets.
Main Background:
Z+jets.
Analysis techniques:
D0: NN and BDT discriminants.
CDF:
2D NN analysis: improved dijet mass resolution with METprojection technique.
New ME analysis.
Double tag
CDF, to maximize signal acceptance we relax the lepton and b jet requirements.
To maintain signal efficiency and improve signal discrimination, we employ an additional neural network trained to discriminate event kinematics of $ZH$ compared to the main $Z + jets$ background and the kinematically different $t\bar{t}$ background
Bárbara Álvarez- U. de Oviedo

15. VH → ννbb MET + JETS Low Mass Main background: QCD multijet processes.
SIGNATURE
MET
Main background: QCD multijet processes.
In CDF, a data-driven model for QCD
background removal is used.
Other backgrounds: top, single top,
dibosons, W+jets, Z+jets... +
Low Mass
JETS
3 orthogonal tag categories and 3 jet bin.
D0 analysis has a very close sensitive.
The scenario is a Higgs production associated with a W/Z boson. H decays to bbbar and W/Z to 2 neutrinos, when the lepton from the W scapes to detection or is reconstracted as a jet. The signature is met+jets.
Because for this version of the analysis the jet cuts where relaxed, the main bkg is QCD multijet processes, there is a data-drive QCD model build, NN to remove the not signal like events.
An additinal NN is used to discriminate the Higgs signal from the remaining bkg.
Both experiments have similar sensitivities.
For a Higgs mass of 115 GeV/c2
EXP.
Lumi (fb-1)
Exp. / SM
Obs. / SM D0
2.1
7.0
5.7
CDF
5.6
6.9
Bárbara Álvarez- U. de Oviedo

16. Most sensitive Higgs search at the Tevatron
gg → H → WW(*) → ll'νν (l,l'=e,mu)
Most sensitive Higgs search at the Tevatron
High Mass
Signature: 2 high pT leptons + MET.
Leptons in same directions due to spin correlation.
Different background composition: WW, Drell-Yan, tt...
/ μ+
Final states: e+e-, eμ,
or μ+μ- and 2 ν.
/ μ-
W´s decay leptonically.
There are several production mechanisms besides gluon fusion:
WH → WWW, ZH → WWW, V.B.F H → WW
New dedicated analyses in different 0, 1, 2 jet bins.
Analyses optimized for each jet bin.
Both experiments approaching SM sensitivity
Bárbara Álvarez- U. de Oviedo

17. gg → H → WW(*) → ll'νν (l,l'=e,mu)
CDF, H → WW, 0 jets
Process
# of events
ttbar
0.96 ± 0.19
Drell-Yan
66.88 ± 15.20 WW
± 38.99 WZ
12.17 ± 1.93 ZZ
17.29 ± 2.74
W+jets
83.61 ± 20.09 Wγ
79.15 ± 21.12
Total bkg
± 64.81
gg → H
8.38 ± 1.29
Total Signal
Data
552
ΔΦ between the leptons
is the most discriminating
variable for the 0 jet bin.
WW background distinguish
by the spin correlation
of the leptons.
CDF, H → WW, 0, 1, ≥2 jets
Bárbara Álvarez- U. de Oviedo

18. H → WW(*) → ll'νν, ∫ L = 3.0 fb-1 High Mass EXPERIMENT EXP. Limit/SM
CDF Search for Higgs to WW* Production using
a Combined Matrix Element and NN Technique
NN is used in each of the three di-lepton channels
High Mass
EXPERIMENT
EXP. Limit/SM
mH=165 GeV/c2
OBS. Limit/SM
1.9
2.0
1.6
1.7
Bárbara Álvarez- U. de Oviedo

19. ADDITIONAL CHANNELS Four signal processes considered:
Many other SM Higgs searches, not as sensitive but contribute in combination.
Analysis
Lumi (fb-1)
Exp. / SM
Obs. / SM
D0, ttH → lvbbbbqq
2.1
45.3
63.9
D0, H → γγ
2.7
23.2
30.8
CDF, H → ττ
2.0
24.8
30.5
Signature: 1 lepton + MET + 4 jets.
12 channels (4, ≥5 jets) x (1, 2, ≥3 b-tags).
Scalar sum of jets (HT ) to extract signal.
Four signal processes considered:
W(→ qq') H(→ τ+τ-)
Z(→ qq) H(→ τ+τ-)
VBF qHq'→ q' τ+τ-q
gg → H → τ+τ-
Bárbara Álvarez- U. de Oviedo

20. INDIVIDUAL EXPERIMENTS
COMBINATION
100 < mH < 200 GeV/c2
Contributing production processes:
Associated production (W/Z).
Gluon fusion.
Vector boson fusion.
Analyses are conducted with integrated luminosities from 1.1 to 3.0 fb-1
EXPERIMENT
EXP.(OBS)/SM
mH=115 GeV/c2
mH=165 GeV/c2 D0
4.6 (5.3)
1.9 (2.0)
CDF
3.1 (3.7)
1.6 (1.8)
Bayesian and modied frequentist approaches used
We have calculated combined upper limits on the ratio of Higgs boson cross section times the branching ratio to its Standard Model prediction(R95) for Higgs boson masses between 100 and 200
Bárbara Álvarez- U. de Oviedo

21. Tevatron Run II Preliminary, L= 3fb-1
SM TEVATRON HIGH MASS
COMBINATION
Tevatron Run II Preliminary, L= 3fb-1
Combine results from CDF and D0
searches for SM Higgs boson.
Systematics and their correlation
between channels and experiments
taken into account.
Results presented at ICHEP 2008, Philadelphia
Tevatron high mass combination:
We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV/c2.
First direct exclusion since LEP!
We combine results from CDF and DO searches for a standard model Higgs boson in ppbar collisions at the Fermilab Tevatron, at sqrt{s}=1.96 TeV. With 3.0 fb-1 of data analyzed at CDF, and at DO, the 95% C.L. upper limits on Higgs boson production are a factor of 1.2, 1.0 and 1.3 higher than the SM cross section for a Higgs boson mass of m_{H}=$165, 170 and 175 GeV, respectively. We exclude at 95% C.L. a standard model Higgs boson of m_H=170 GeV. Based on simulation, the ratios of the corresponding median expected upper limit to the Standard Model cross section are 1.2, 1.4 and 1.7. Compared to the previous Higgs Tevatron combination, more data and refined analysis techniques have been used. These results extend significantly the individual limits of each experiment and provide new knowledge on the mass of the standard model Higgs boson beyond the LEP direct searches.
RESULT
3 fb-1
EXP.(OBS.)/SM
mH=165 GeV/c2
D0 + CDF
1.2 (1.2)
Bárbara Álvarez- U. de Oviedo

22. We are sensitive to a Higgs
The low mass region is challenging, but in the
next Tevatron combination the expected limit
should fall below:
~3 x SM for mH = 115 GeV/c2
Achieved & Projected
There is still room for improvements:
Increase in acceptance.
Sophisticated analysis tools.
B-tagging.
jet/MET resolutions.
The top of the yellow band corresponds to the Summer 2007 performance expected limit divided by 1.5, and the bottom of the yellow band corresponds to the Summer 2007 performance expected limit divided by This plot is shown for mH=115 GeV/c2
Achieved and projected median expected upper limits on the SM Higgs boson cross section, by date.
We are sensitive to a Higgs
of 160 GeV/c2
Bárbara Álvarez- U. de Oviedo

23. "Beyond the Standard Model"
The Minimal Supersymmetric Standard Model (MSSM) is the minimal extension to the SM that realizes supersymmetry.
In the MSSM, 5 Higgs bosons remain after EW symmetry breaking:
3 neutral: h, H, and A - denoted as Φ.
2 charged: H±.
MSSM
MSSM processes (next pages):
bΦ → bbb
bΦ → bττ
Φ → ττ
In physics, the Standard Model of particle physics is currently the best description of all experimental data.[1] Nevertheless, there are reasons to believe that there are phenomena that are not accurately described by this theory and "Beyond the Standard Model" physics studies possible extensions to the Standard Model that will be probed in up-coming experiments.
At tree level the Higgs sector can be parameterized by tan β, the ratio of the
two Higgs doublet vacuum expectation values, and mA, the mass of the pseudo-scalar Higgs boson A.
Bárbara Álvarez- U. de Oviedo

24. MSSM
bΦ → bbb (multi-b-jets)
This process could be observable in supersymmetric models with high values
of tanβ.
Event signature: at least 3 b-jets.
Data required (trigger): 2 central clusters
with ET>15 GeV, the third jet ET>10 GeV.
Offline: three jets with ET>20 GeV, |η| < 2.
This final state suffers from a large
multijet background.
The dijet mass of the 2 leading jets in the events (m12) used , in triple tag events, to separate Higgs signal from background events.
RESULTS
The production rate of light Higgs bosons in association with b-quarks can be significantly
enhanced in supersymmetric extensions of the standard model. This occurs for large values
of tan , the ratio of the Higgs coupling to down-type versus up-type quarks
In this analysis we search for Higgs decays into b¯b, accompanied by an additional highpT
b, giving an event signature of at least three b-jets. We study the dijet mass spectrum
of the two leading jets in three-jet events with all three jets identified as b-jet candidates using a displaced vertex algorithm
EXP.
LUMI
EXP. (OBS.) pb
mH = 90GeV/c2
mH= 200GeV/c2
CDF
1.9 fb-1
77 (57)
6.1 (4.1) D0
1.6 fb-1
149 (201)
7.7 (6.2)
Bárbara Álvarez- U. de Oviedo

25. MSSM Φ (=H; h;A)+b → b τ τ Φ (=H; h;A) → τ+ τ-
Search requires the tau pairs to decay
into τeτhad, τμτhad, or τeτμ
Backgrounds:
W+jets
Top
Z → τ+ τ-
Others...
Φ (=H; h;A)+b → b τ τ
The final state includes a μ candidate, a τhadronic candidate, and a jet tagged as a b-quark jet.
bΦ → bbb (multi-b-jets)
95% CL exclusion limit in the (mA; tan β) plane
obtained with the current analysis for the mhmax ,
μ=-200 GeV scenario. The exclusion limit
obtained from the LEP experiments is also shown.
The bττ channel offers a much cleaner final state than the bbb channel, giving the two channels similar sensitivities.
Exclusion limits are set at the 95% C.L. for several supersymmetric scenarios.
Bárbara Álvarez- U. de Oviedo

26. H → γγ WH → WWW FERMIOPHOBIC HIGGS
Photon energy resolution better than jets.
Look for peak in di-photon mass.
SM-like search but branching ratio is
enhanced in fermiophobic model.
WH → WWW
Look for same-sign leptons.
Also sensitive to SM at high mass.
At low mass more sensitive if H is
fermiophobic.
The mass resolution of the diphoton channel is extremely good compared to dijet decay modes.
Limits are set on the production cross section and branching fraction using a Bayesian binned likelihood approach.
The fermiophobic Higgs boson has no coupling to the fermions.
Existence of the fermiophobic Higgs could be an indication that the origin
of particle mass would be different for the bosons and the fermions.
Bárbara Álvarez- U. de Oviedo

27. CONCLUSIONS The Higgs boson search is in its most exciting era ever.
Tevatron is running really well.
The Higgs boson search is in its most exciting era ever.
Reaching sensitivity to SM Higgs
over full mass range and beyond SM Higgs boson.
NO evidence for signal found yet.
We exclude at 95% C.L. a SM
Higgs boson of 170 GeV/c2
We are sensitive to a Higgs of 160 GeV/c2
Stay tuned for further improvements!
Bárbara Álvarez- U. de Oviedo

28. THANK YOU!!! Extremely sophisticated analyses.
Tevatron performing really well, almost every week.
THANK YOU!!!
Bárbara Álvarez- U. de Oviedo

29. BACK UP SLIDES
Bárbara Álvarez- U. de Oviedo

30. Only ~40 Higgs events are expected to be produced per fb-1 at each experiment in detectable channels
TOTAL
WH → lvbb
H → WW
ZH → vvbb
ZH → llbb
Bárbara Álvarez- U. de Oviedo