cdf higgs results winter 2012 michelle stancari fermi national accelerator laboratory on behalf of...
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CDF Higgs ResultsWinter 2012
Michelle Stancari
Fermi National Accelerator Laboratory
On behalf of the CDF collaboration
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The Tevatron
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proton-antiproton collider at √s = 1.96 TeV Two multi-purpose detectors: CDF &
DØ Antiproton Accumulation rate: ~25x1010 /hr Initial luminosity record: 4.31 x1032 cm-2s-1
Record week: 85 pb-1
Run II: 2001-2011 12 fb-1delivered 10 fb-1recorded
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The Tevatron – 25+ years
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proton-antiproton collider at √s = 1.96 TeV Two multi-purpose detectors: CDF &
DØ Antiproton Accumulation rate: ~25x1010 /hr Initial luminosity record: 4.31 x1032cm-2s-1
Record week: 85 pb-1
Run II: 2001-2011 12 fb-1delivered 10 fb-1recorded
THANK YOU!
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CDF II DetectorMuon
Chambers
Calorimeters
Wire Chamber (COT)
1.4 T Superconducting Solenoid
Silicon Tracker
z x
y
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5Hbb HWW
SM Higgs decay and production modes
135 GeV/c2
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Latest W mass results
New measurements from CDF and D0 mW = 80385 +/- 15 MeV/c2 ( World Average – Mar 2012) Updated SM indirect fit gives mH < 152 GeV/c2 at 95% C.L.
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Tevatron Combination Summer 2011
Expected limit <=1.3*SM from 100-185 GeV/c2
Tevatron Exclusion: 100-108 & 156-177 GeV/c2
Broad 1σ excess between 125-155 GeV/c2 compatible with signal plus background and background-only hypotheses
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Strength of Tevatron is H®bb
Single experiment sensitivity Feb 2012, MH=125 GeV
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CDF, D0 Atlas, CMS
H®gg 10-13*SM
1.5-2*SM
H®WW ~3.5*SM 1-2*SM
H®bb ~2*SM ~3.5*SM
arXiv:1202.4195
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CDF Sensitivity Projections
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Winter 2012
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CDF Higgs Search
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Four channels contribute almost equally in the interesting region - need to improve all 4!
ZH®llbb WH®lnbb ZH®nnbb H®WW®lnln Remaining channels
have a combined weight of ~10%
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How to find a needle in a haystack
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Potential Higgs signal is TINY and buried under more common SM processes with same final states
Maximize signal acceptance
Model all signal and background processes well
Use multivariate analysis (MVA) to exploit all kinematic differences
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How to find a needle in a haystack
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Potential Higgs signal is TINY and buried under more common SM processes with same final states
Maximize signal acceptance
Model all signal and background processes well
Use multivariate analysis (MVA) to exploit all kinematic differencesExpect 167 SM Higgs events (reconstructed and selected) and ~200,000 events from SM backgrounds for mH=125 GeV/c2
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“Low Mass” Searches
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ZH®nnbb
Maximize lepton reconstruction and selection efficiencies
Maximize efficiency for tagging b-quark jets Optimize dijet mass resolution
Select:
Strategy:
WH®lnbb
0,1,2 leptons and/or missing Et
Two high Et jets
ZH®llbb
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In pictures . . .
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Loose event selection: 1 high-pt lepton, MET, and 2 jets
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In pictures . . .
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Loose event selection plus one tightly tagged b-quark jet
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In pictures . . .
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Loose event selection plus two tightly tagged b-quark jets
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Improvements Since Summer 2011
25% more luminosity Most recent data Use every last pb-1 of data with component
specific quality requirements New multivariate b-tagger optimized for H ®bb jets ~20% more acceptance
Additional triggers and leptons Improved dijet invariant mass resolution Improved MVA Improved modeling
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Improvements Since Summer 2011
25% more luminosity Most recent data Use every last pb-1 of data with component
specific quality requirements New multivariate b-tagger optimized for H ®bb jets ~20% more acceptance
Additional triggers and leptons Improved dijet invariant mass resolution Improved MVA Improved modeling
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More events
Signal vs. background separation
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Road to improved b-tagging
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In 2010, CDF had 5 b-tagging algorithms with different strengths, weaknesses and applications
b-ness
Roma
SecVtx (cut based)
JetProb
KIT
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Road to improved b-tagging
Can we unify the strengths of each of these into a single Higgs-optimized network? Study of tagger performance says that we . . .
Need maximum acceptance Can afford an increase in fake rates
Need multiple operating points allows separation of high S/B data (two “tight”
tagged jets) and low S/B data (two “loose” tagged jets) into independent analysis channels
Train with jets from H®bb MC
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Higgs Optimized b Identification Tagger
Multivariate, continuous output
25 input variables (most sensitive inputs to earlier taggers)
Trained with jets from H®bb MC
Validated with ttbar and soft electron samples
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HOBIT
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HOBIT performance
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mistag rate
SecVtx efficiency
HOBIT efficien
cy
~1% 39% 54%
~2% 47% 59%
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HOBIT in WH->lnbb
Significant effort to optimize tagging categories and thresholds for loose/tight HOBIT selections
11% gain in S/√B translates directly into increase in overall search sensitivity
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Tagging Category
S/√B
SecVtx+SecVtx
0.228
SecVtx+JetProb
0.160
SecVtx+Roma
0.103
Single SecVtx
0.146
Sum 0.331
Tagging Category
S/√B
Tight-Tight 0.266
Tight-Loose 0.200
Single Tight
0.143
Loose-Loose
0.053
Single Loose
0.044
Sum 0.369
NEW - HOBIT
OLD – Multiple Taggers
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Jet Resolution Improvements
Bottom quark jets have properties which are very different from standard light flavor quark jets
Specialized jet energy scale corrections focused on bottom quark jets improve our dijet invariant mass and missing transverse energy measurements
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light flavor quark jet bottom quark jet
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Jet Resolution Improvements in ZH->nnbb
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Neural network correlates all jet-related variables and returns most probable jet energy based on bottom quark hypothesis – better signal/background separation
Standard corrections
b-targeted corrections
Signal mass resolution
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MVA improvements
Small MVA improvements in many channels
Example from ZH®llbb to illustrate
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• Many backgrounds processes are present the llbb selection• The individual processes have different kinematics• A single neural net trained to select signal out of a mix of backgrounds can be improved.
YESYES
NO
YES
Tagged
events
Is the event tt-like?
NONOZ+qqlike?
WZ/ZZ like?
Region 1 Region 2 Region 3
Region 4
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MVA improvements in ZH®llbb
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YESYES
NO
YES
Tagged
events
Is the event tt-like?
NONOZ+qqlike?
WZ/ZZ like?
Region 1 Region 2 Region 3
Region 4
tt-like other
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MVA improvements in ZH®llbb
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YESYES
NO
YES
Tagged
events
Is the event tt-like?
NONOZ+qqlike?
WZ/ZZ like?
Region 1 Region 2 Region 3
Region 4
Z+qq - like other
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MVA improvements in ZH®llbb
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YESYES
NO
YES
Tagged
events
Is the event tt-like?
NONOZ+qqlike?
WZ/ZZ like?
Region 1 Region 2 Region 3
Region 4
WZ, ZZ - like ZH - like
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MVA Improvements in ZH®llbb
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No events are discarded, only shuffled Result is a handful of bins with enhanced S/B
WZ, ZZ
Z+qq
tt ZH
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How good is our modeling?
Do we see WZ and ZZ events ? same final state same set of tagged events different MVA optimized for WZ and ZZ events
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well known SM process same background model
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How good is our modeling?
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MVA-based Search
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440 signal events and ~35,000 background
x 3x 16
x 20
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Combined binned likelihood function
Incorporate uncertainties as nuisance parameters
Uncertainties taken on both the shapes and normalizations of signal & background templates
Additional constraints on background model obtained directly from fit!!
From Discriminants to Limits
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Extracting s(WZ+ZZ)
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s(WZ+ZZ)= 4.08 +/- 1.32 pbwith approximate significance of 3.2σ
SM Prediction = 4.4 +/- 0.3 pb
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Maximize lepton reconstruction and selection efficiencies
Separate events into multiple analysis channels (e.g. 2 jets and opposite sign leptons)
Best possible choice of kinematic event variables for separating signal and background
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“High Mass” channel
2 high ET leptons and missing ETSelect:Strategy:
HWWll
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Improvements: 8.2®9. 7 fb-1
MH dependent optimization of neural network inputs
Increased acceptance for low invariant mass dilepton pairs (0.1 < DRll < 0.2)
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HWWll
H®WW signal events are peaked at low DRll because spin 0 Higgs boson anti-correlates the spin of the Ws, favoring a small opening angle of the leptons
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Improved DRll acceptance
Include region 0.1 < DRll < 0.2 special Drell-Yan modeling
(MADGRAPH) new W g modeling (MADGRAPH) cuts to remove J/y and
¡ resonances
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Z/γ*
Control Region
Signal Region
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Validate high mass technique with s(ZZ)
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Same tools and data samples Different neural network optimized for
ZZll
σ(ZZ) = 1.45 pbSM pred: 1.2±0.1 pb
+ 0.60-
0.51
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No channel left behind!
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Channel Luminosity 95% CL limit MH=125 GeV
H®gg 10.0 fb-1 10.8 x SM
VH®bb+jets
9.45 fb-1 11.0 x SM
ttH®ln+jets 9.4 fb-1 12.4 x SM
H®tt+jets 8.4 fb-1 14.8 x SM
Channel Luminosity 95% CL limit MH=150 GeV
H®ZZ®llll 9.7 fb-1 9.4 x SM
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CDF Sensitivity Projections
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Winter 2012
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CDF Sensitivity Accomplishments!
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Winter 2012
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Results
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Combined Higgs Discriminants
Combine 16 analyses, 93 orthogonal channels
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Combined binned likelihood function
Incorporate uncertainties as nuisance parameters
Uncertainties taken on both the shapes and normalizations of signal & background templates
Additional constraints on background model obtained directly from fit!!
From Discriminants to Limits
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New CDF combination
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Exclude SM Higgs at 95% C.L. : 147 < mH < 175 GeV/c2
Expect to exclude: 100 < mH < 106 GeV/c2 & 154 < mH < 176 GeV/c2
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Compatible with background-only?
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Highest local p-value, 2.6s, is found at mH = 120 GeV/c2
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Is the excess compatible with a SM Higgs?
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Consistent with SM Higgs at 1σ level for mass range between 107 and 142 GeV/c2
Fitted Higgs cross section
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How much did things change?
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Summer 2011
Winter 2012
A ~0.5σ excess in mass range from 115 to 135 GeV/c2 has become a ~2σ excess. How can this happen?
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HWW lnln
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18% additional data Small signal acceptance improvements (0.1 < ΔRll < 0.2) No appreciable change in behavior of limits
Summer 2011
Winter 2012
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ZHnnbb
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21% additional luminosity Small improvements in background rejection Limits show same basic behavior with 0.5 to 1.0σ
increases in significance of excess
Summer 2011
Winter 2012
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WHlnbb
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26% (69%) additional luminosity for 2-jet (3-jet) channels 5-10% level lepton acceptance/trigger efficiency improvements New HOBIT b-tagger equivalent to adding another 20% in
additional luminosity Limits show same basic behavior with 1.0 to 1.5σ increases in
significance of excess
Summer 2011
Winter 2012
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ZHllbb
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23% additional luminosity More gain from HOBIT in this analysis than WH (original tagging not as
sophisticated) 56% of data events in current analysis were not included in previous analysis! 37% sensitivity improvement (4.67® 2.95 at mH=120 GeV/c2)
Summer 2011
Winter 2012
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ZHllbb
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Muon channels See only a slight change in behavior of limits
(~0.5σ)
Summer 2011
Winter 2012
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ZHllbb
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Electron channels Here we observe a significant change
Summer 2011
Winter 2012
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ZHllbb
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ZHllbb channel has . . . lowest backgrounds smallest expected
signal yields (9 events for mH=120 GeV/c2)
Some discriminant bins with large S/B Low probability for
observing events in these bins
A few such events can have substantial effects on observed limits
S = 0.16 events, B= 0.06 events
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ZHllbb
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Examine top 20 events in both channels based on S/B of the discriminant bin in which it’s located
The electron channel contains 12 new candidates within this high score region, while muon channel has 5
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ZHllbb
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To study the effect of high S/B events on our observed limits, we remove our best new and best two new events from the e+e- channel and re-run the limits
Gives one sigma level changes in the limits at 120 GeV/c2
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Global Significance of Excess
Highest local p-value at mH = 120 GeV/c2
mass resolution of searches, dominated by bb at low mass and WW at high mass, is broad
Estimate LEE of 4 for our entire SM search range from 100 to 200 GeV/c2
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SM Higgs Searches
Experiment Local P-value Global P-value
CDF 2.6σ 2.1σ
ATLAS 3.5σ 2.2σ
CMS 3.1σ 2.1σ
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Tevatron strength: Hbb
Combine our three primary low mass search channels WH®lnbb ZH®nnbb ZH®llbb
Allows for a quasi-model independent search for associated Higgs production with Hbb
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Consistent with background-only?
Highest local p-value of 2.9s is found at mH = 135 GeV/c2
6161
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Compatible with SM Higgs?
Data are most consistent with SM in mass range from 105 < mH < 120 GeV/c2
Behavior at higher mH values is consistent with the expectation from a lower mass Higgs
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Fitted Higgs cross section
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Global Significance of Hbb alone
Highest local p-value is found at mH = 135 GeV/c2
These searches are performed in the mass range between 100 to 150 GeV/c2
Estimate LEE of 2
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Single Channel Searches
Experiment Channel Local P-value
Global P-value
CDF H->bb 2.9σ 2.7σ
ATLAS H->gg 2.8σ 1.5σ
CMS H->gg 3.1σ 1.8σ
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CDF Conclusions
CDF has significantly increased the sensitivity of its Higgs searches by incorporating the full 10 fb-1 dataset and a wide range of analysis improvements
All SM searches combined excess of Higgs-like events observed consistent with SM Higgs production in the mass
range from 107 to 142 GeV/c2. global significance of 2.1σ
Associated Higgs production in the decay mode Hbb excess of Higgs-like events observed, again
consistent with SM Higgs production global significance of 2.7σ 64
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Combined Tevatron ResultsWinter 2012
CDF and DØ Collaborations
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s(WZ+ZZ)= 4.47 +/- 0.64 (stat) +/- 0.73 (syst) pbwith approximate significance of 4.6σ
SM Prediction = 4.4 +/- 0.3 pb
Verify modeling with s(WZ+ZZ)
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Combined CDF and D0 discriminants
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Combined Tevatron SM Higgs Limits
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Exclude SM Higgs at 95% C.L. for 147 < mH < 179 GeV/c2
Expect to exclude 100 < mH < 120 GeV/c2 & 141 < mH < 184 GeV/c2
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Compatible with background-only?
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Compatible with SM Higgs?
Consistent with SM signal plus background hypothesis over Higgs mass range from 110 to 140 GeV/c2
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Global Significance
Highest local p-value is found at mH = 120 GeV/c2
Same LEE of 4 for entire SM search range from 100 to 200 GeV/c2
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SM Higgs Searches
Experiment Local P-value
Global P-value
CDF+D0 2.8σ 2.2σ
ATLAS 3.5σ 2.2σ
CMS 3.1σ 2.1σ
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Tevatron Conclusions
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CDF and D0 have significantly increased the sensitivity of their Higgs searches by incorporating the full 10 fb-1 dataset and a wide range of analysis improvements
We measure s(WZ+ZZ) with a significance of 4.6s and a value compatible with SM
We observe an excess of Higgs-like events consistent with SM Higgs production in the mass range from 115 to 140 GeV/c2.
The global significance of this excess is 2.2σ
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Backup Slides
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Anatomy of a Limit Plot
4. Analysis repeated using different signal templates for each mH between 110 and 200 GeV in 5 GeV steps
1. Upper cross section limit for Higgs production relative to SM prediction
3. Median expected limit (dot-dashed line) and predicted 1σ/2σ (green/yellow bands) excursions from background only pseudo-experiments
2. Observed limit (solid line) from data
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Tevatron/LHC production cross sections
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CDF Wjj
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Tagged samples used for Higgs searches do not contain any sign of abnormalities that were seen previously in pre-tagged region
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CDF Wjj
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Lots of studies to try to understand what’s going on in the pre-tag region
Detailed studies in Z + 1 jet events to understand potential differences in quark and gluon jet energy scales
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CDF Wjj
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Bottom line of these studies is that the JES for gluon jets needs to be shifted by 2σ in MC to match with data
The JES for quark jets is good – not surprising since well constrained by top mass measurements
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CDF Wjj
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In CDF Higgs searches we apply -2σ JES corrections to the gluon jets in our MC samples
In the end, the effect of this is small since there are few gluon jets in our tagged event samples
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CDF Wjj
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With these corrections in place we do not observe mis-modeling in the pre-tag region of our lνjj Higgs search
Caveat is looser cuts are applied than in the “bump” search analysis
No official statement from CDF regarding “bump” at this time
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Change in Limits at mH = 115 GeV/c2
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Summer 2011
Winter 2012
Excess of high S/B events was present in previous analysis
Change is that the lower S/B event region has become more consistent with S+B hypothesis
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Excess at mH = 195 GeV/c2
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Behavior of observed limits driven by small event excesses in the high S/B regions of opposite-sign dilepton 0 and 1 jet channels
Nothing peculiar in the modeling of these distributions
Of course, ATLAS and CMS have ruled out a mH = 195 GeV/c2 SM Higgs based primarily on equivalent searches in H->WW
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Deficit at mH = 165 GeV/c2
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Driven by deficit of events in high S/B region of our opposite-sign, low invariant mass dilepton channel
This is the channel in which we obtain increased acceptance from low ΔRll events
Nothing peculiar in the overall modeling of this distribution and deficit is not spread over a wide mass range
mH = 165
mH = 125
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Signal Injection study
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The figure on right shows the results of a previous study where we injected a mH = 115 GeV/c2 Higgs signal into background-only pseudo-experiments to study the potential effect on our observed limits
Because our neural network discriminants are optimized for separation of signal and background rather than mass reconstruction, we expect to observe (in the presence signal) higher than expected observed limits over a broad mass range