recent results in susy higgs searches at dØ
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Recent Results in Susy Higgs Searches at DØ. Fermilab Joint Experimental-Theoretical Seminar Friday, 12 th November 2010. Jonathan Hays On behalf of the DØ Collaboration. Outline. Introduction Searches for Higgs + b-jets tau final states (b ) b-jet final states ( bbb ) - PowerPoint PPT PresentationTRANSCRIPT
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Recent Results in Susy Higgs
Searches at DØJonathan Hays
On behalf of the DØ Collaboration
Fermilab Joint Experimental-Theoretical Seminar
Friday, 12th November 2010
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Wine and Cheese Seminar 2
Outline
Introduction
Searches for Higgs + b-jets
tau final states (b)
b-jet final states (bbb)
Conclusions and Outlook
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Wine and Cheese Seminar 3
Tevatron
5.2 fb-1
4.3 fb-1
Tevatron and the detectors continue to perform very well
~16 fb-1 expected by Oct 2014 Over 9.6 fb-1 delivered
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Wine and Cheese Seminar 4
D-Zero
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Wine and Cheese Seminar 5
Standard Model
Highly successful theory but:
No dark matter candidateNo gravityHierarchy and naturalness
problemsNo unification
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Wine and Cheese Seminar 6
Supersymmetry
Solves naturalness problemLSP = dark matter candidate ?SupergravityGUT unification possible
hep-ph/9709356
Introduce new space-time symmetry between fermions and bosons
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Wine and Cheese Seminar 7
MSSM Higgs Sector2 Higgs doublets
5 physical scalars: 3 neutral: h, H, A
2 charged: H±
A
hH
Totaltan()=30
MSSM
A. Djouadi, hep-ph:0810-2439
tree level two parameters: mA and tanβ
σMSSM ~ 2×Br×tan2β×σSM
Chance of discovery before SM sensitivity!
Radiative corrections large brings in dependence on other model parameters
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Wine and Cheese Seminar 8
MSSM Higgs
→ b → 3b/b
bb→ 4b/bb
Enhancement to “down-type” fermions
BR(φ→bb) ~ 90%BR(φ→) ~ 10%
φ→clean signatures but low BR
bφ→breduced backgrounds added sensitivity at low mA
bφ→bbblarge backgroundhigh BR
φ = (h,H or A)
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Wine and Cheese Seminar 9
MSSM Higgs
→ b → 3b/b
bb→ 4b/bb
Enhancement to “down-type” fermions
BR(φ→bb) ~ 90%BR(φ→) ~ 10%
φ→clean signatures but low BR
bφ→breduced backgrounds added sensitivity at low mA
bφ→bbblarge backgroundhigh BR
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Wine and Cheese Seminar 10
Inclusive Searches
→
φ→
φ→
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Wine and Cheese Seminar 11
Inclusive Searches
http://arxiv.org/abs/1003.3363v3
φ→
Tevatron combination
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Wine and Cheese Seminar 12
Exclusive SearchesPublished results from DØ
bφ→bbφ→bbb
Phys. Rev. Lett. 104, 151801 (2010) Phys. Rev. Lett. 101, 221802 (2008)
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Wine and Cheese Seminar 13
Search StrategyOptimise analysis based on expected limits with full systematics
In absence of significant discrepancy between data and background:Set limits in (almost) model independent waySet limits in benchmark SUSY scenarios
Combine results across channels for particular model choices
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Wine and Cheese Seminar 14
Signal Modelling
Reweighted in pt and eta of spectator b-jet based on MCFM calculation
Important differences in kinematics when moving from LO to NLO
Use 5 flavour number scheme:
Generate gb→bh at LO in PYTHIAAcceptance cuts on the spectator b-jet
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Wine and Cheese Seminar 15
Signal Modelling
Large enhancements to the couplings give large widths
Simulate widths using “narrow” samples and convoluting with Breit-Wigner
Radiative corrections have significant effect
Larger effect for bbb channelsLess significant for bττ
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Wine and Cheese Seminar 16
b-jet identification
Several mature algorithms used:3 main categories:
- Soft-lepton tagging- Impact Parameter based- Secondary Vertex reconstruction
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Wine and Cheese Seminar 17
b-jet identificationMeasure b & c efficiencies on b-jet enriched sample
Fake rate measured on multijet sample
Composition estimated from secondary vertex mass templates
MC and data differences
Data MC
b-tagged samples
Direct tagging Reweightwith TRFs
Tag rate functions (TRF) parameterise efficiencies and fake rates versus pt and eta
NIM A 620, 490 (2010)
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Wine and Cheese Seminar 18
-lepton identification
TRK CALType 1
o
no TRK, but EM sub-cluster
TRK CAL
Type 2
³ 1 TRK
wide CAL cluster
Type 3
Hadronic decays categorised by decay mode
Leptonic decays – single isolated leptons
Neural network (NN) trained for each type to discriminate against jets
Efficiencies measured in clean Z sample
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Wine and Cheese Seminar 19
Searches in tau final states4.3 fb-1 integrated luminosityCollected with single muon trigger
Dominant backgrounds:Z→ + jetstop pairsmulti-jet (QCD + W+jets)
Event selection: Single isolated muon Opposite sign had 1 loose b-tagged jet ( ε ~ 71%)
PreselectionNo b-tag
Complementary to φ→ and bφ→bbb
bφ→bτµτhad
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Wine and Cheese Seminar 20
Searches in tau final statesTrain NNs to discriminate against top and multi-jet backgrounds
Final discriminant = geometric mean of 3 NN outputs
NN b-tagger suppresses Z+jets background
Combine all NNs into single discriminant
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Wine and Cheese Seminar 21
bφ→bμhad limits
Tree level limit
Most stringent limit at low MA
4.3 fb-1 preliminary results
Limits set using “CLs” method
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Wine and Cheese Seminar 22
Searches with b-quarks
5x more dataExtended mass range: 90-300 GeVLarger MC samples
New result with 5.2fb-1 data
Submitted to Phys. Lett. B arxiv.org:1011.1931
Expanded and improved treatment of systematics
- e.g. b-taggingRe-analyzed old 1fb-1 data set
Major improvements since previous 1fb-1 publication
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Wine and Cheese Seminar 23
Searches with b-quarks
Very large multi-jet background
Challenging to model → data driven method
Multijet cross sections not well predicted → float normalisation
b → 3b/b
3 or 4 jets, 3 must be b-tagged
5.2fb-1 collected with jet triggers – making use of lifetime information
Kinematic likelihood (D) used to select best jet pairing, + cut to suppress background
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Wine and Cheese Seminar 24
Background Modelling
MC correctionfactor
2 b-tagdata
3 b-tagbackground
Predict background shape from 2-tagged data with correction from MC
Add plot here...
2D correction: likelihood vs invariant mass
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Wine and Cheese Seminar 25
Background Modelling:Sample composition
In 3-tag samplebbb ~ 47% bbj ~ 32%bbc ~ 17%ccj ~ 2%
Needed for MC correction factor
Estimated using MC fit to data over several b-tag operating points
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Wine and Cheese Seminar 26
Background Modelling
Validate modelling in a signal poor region
“wrong” jet pair looks like background
Pick lowest likelihood pairing and select D < 0.12
Excellent agreement seen between model and data
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Wine and Cheese Seminar 27
Kinematic Likelihood
Trained on jet-pairings
Two likelihoods: low mass MA < 140 GeV high mass MA ≥ 140 GeV
In each event select pairing with highest LH value
Cut on LH optimised considering expected limits with full systematics
LH > 0.65 for all mass points
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Wine and Cheese Seminar 28
Kinematic Likelihood
Cut Cut
Projection of 2D distributions onto likelihood axis
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Wine and Cheese Seminar 29
Mass distributionsDi-jet invariant mass distribution used as input for the limit setting
D > 0.65, background normalised to data
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Wine and Cheese Seminar 30
Systematics
Background : normalisation included as nuisance parameter
Only consider variations in shape
Signal: dominated by b-tagging (15%-20%) and jet energy scale (2-14%) includes both rate and shape systematics
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Wine and Cheese Seminar 31
Systematics: Fake-rate
An area of major improvement since 1fb-1 result
remeasured on hbb specific samples
Detailed approach to systematics
b-tagging SF SVT Template fit
Sample composition
Fake rate determinationFa
ke ra
teFa
ke ra
te
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Wine and Cheese Seminar 32
Results
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Wine and Cheese Seminar 33
Results
Small excess ~ 2.5σAfter trials factor ~ 2.0 σ
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Wine and Cheese Seminar 34
SUSY Benchmark ScenariosFive additional parameters due to radiative correction
MSUSY (parameterizes squark, gaugino masses)Xt (related to the trilinear coupling At → stop mixing)M2 (gaugino mass term) (Higgs mass parameter)Mgluino (comes in via loops)
Two common benchmarksMax-mixing - Higgs boson mass
mh close to max possible value for a given tan
No-mixing - vanishing mixing in stop sector → small mass for h
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Wine and Cheese Seminar 35
MSSM Scenario Limits
μ>0 suppressed production x BR – only set limits for μ<0
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Wine and Cheese Seminar 36
OutlookStill large potential for improvements:
More data: 5 → 7+ fb-1
Improved b-tagging → 30% (bbb) yield
Improved analysis techniques e.g. Event based discriminants → 15-30% sensitivity
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Wine and Cheese Seminar 37
Outlook: CombinationsCombine within channels – D0 + CDF – can be done in roughly model independent way
Combine across channels – generally requires picking a model
Aim for new D0 combination by Moriond with up to ~7fb-1
Preparations for Tevatron combination also underway
φ→
(φ→) + (bφ→b) + (bφ→bbb)
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Wine and Cheese Seminar 38
Outlook: SM Contributions?
eg P. Draper et al. arXiv:0905.4721v2
Interpret SM limits within MSSM
Real potential to probe large region of MSSM Higgs parameter space
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Wine and Cheese Seminar 39
ConclusionsInteresting time to be doing Higgs searches at the Tevatron!
Large data sets + sensitive analyses = discovery potential!
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Wine and Cheese Seminar 40
Backup slides
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Wine and Cheese Seminar 41
Mass distribution
Background normalised to data-signal
(S+B = D)
3-jet channels
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Wine and Cheese Seminar 42
?
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Wine and Cheese Seminar 43
Limit SettingUse modified frequentist
method “CLs”
Test statistic: negative poisson log likelihood ratio
Pseudo-experiments to extract likelihood distribution for B and S+B hypotheses
iiii
i
idi
bsbpd
ppLi
or
,!
)exp(
Systematics incorporated as Gaussian smearing in pseudo-experiments
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Wine and Cheese Seminar 44
LLR Distributions
Background likeSignal like
CLb CLs+b