Download - Run II Standard Model Higgs Searches at DØ
Run II Standard Model Higgs Searches Run II Standard Model Higgs Searches at DØat DØ
Gavin DaviesGavin Davies
Imperial College LondonImperial College London
On behalf of the DØ CollaborationOn behalf of the DØ Collaboration
SUSY2004Epochal Tsukuba, Tsukuba, Japan
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Main Injector & Recycler
Tevatron
Chicago
p source
Booster
p
p
p p
1.96 TeV
CDF
DØ
OutlineOutline
36×36 bunches396 ns bunch crossing
Thanks to all my DØ colleagues
Introduction TevatronTevatron DØ detector Higgs production /
sensitivity Results
Wbb / WH production (Z+b) / (Z+j) ratio HWW l+l- searches(Non-SM results – see A. Turcot)
Conclusion
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Tevatron Performance - ITevatron Performance - I
79% Average Efficiency
89% Current Efficiency
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Tevatron Performance - IITevatron Performance - II
05/09/04
FY04 integrated luminosity
Measured Lumi
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DDØØ detector detector
New (tracking in B field)New (tracking in B field) Silicon trackerSilicon tracker Fibre trackerFibre tracker PreshowersPreshowers SolenoidSolenoid
UpgradedUpgraded Muon systemMuon system Calorimeter Calorimeter DAQ/ triggerDAQ/ trigger
All needed for Higgs searches
“Build on existing strengths by improving the tracking and triggering”
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SM Higgs productionSM Higgs production
Cross sections small: 0.1 - 1 pb Cross sections small: 0.1 - 1 pb
MMH H 135 GeV: decay to bb135 GeV: decay to bb gg gg H: QCD background too large H: QCD background too large HW & HZ associated production HW & HZ associated production
lower (but still large!) bkgd.lower (but still large!) bkgd. Best channels: Best channels:
WHWH→l→lννbb, ZH→bb, ZH→ννννbb, ZH→bb, ZH→llllbbbb
MMH H >135 GeV: decay to WW>135 GeV: decay to WW gg gg H H WW WW((**))ll++ll-- BR smallBR small
H bb
H WW(*)
Dominant decay modes
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SM Higgs sensitivitySM Higgs sensitivity
Global fit – light HiggsGlobal fit – light Higgs
MMH H = 117 GeV= 117 GeV
MMHH < 251 GeV at 95% C.L < 251 GeV at 95% C.L..
New CDF+DØ in 2003 More detailed simulation
More refined analysis
Improved sensitivityImproved sensitivity
No systematics
LEP limit from direct LEP limit from direct searchessearches
MMHH > 114.4 GeV at 95% C.L. > 114.4 GeV at 95% C.L.
+67-45
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The ChallengeThe Challenge
Higgs
Cross Sections (fb)
QCD & EWK cross sections much QCD & EWK cross sections much largerlarger
Needle in the haystackNeedle in the haystack……
Good understanding of detector, trigger..Good understanding of detector, trigger..
Good understanding of SM backgroundsGood understanding of SM backgrounds Use data & MC to estimate themUse data & MC to estimate them Useful: new enhanced LO MC’s Useful: new enhanced LO MC’s
(Alpgen)(Alpgen)
Study background processesStudy background processes Likelihood methods, NN’s require excellent Likelihood methods, NN’s require excellent
modeling of backgroundsmodeling of backgrounds
Look at channels beyond SM (SUSY) with Look at channels beyond SM (SUSY) with enhanced cross section (A. Turcot)enhanced cross section (A. Turcot)
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b-jet taggingb-jet tagging Essential for HEssential for Hbb searchesbb searches
Use track impact parameter (IP) Use track impact parameter (IP) measurements or secondary measurements or secondary vertex reconstructionvertex reconstruction
Several algorithms availableSeveral algorithms available
Performance being improvedPerformance being improved
IP
Can b-tag up to |η| < 2.5
DØ RunII Preliminary
b-tagging efficiency vs light quark mis-tag rate
(Also have on-line b-tagging)
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W(W(ee)bb associated production - I)bb associated production - I Background to WH searchBackground to WH search Event selectionEvent selection
Central isolated eCentral isolated e p pT T > 20 GeV> 20 GeV Missing EMissing ETT > 25 GeV > 25 GeV ≥ ≥ two jets: Etwo jets: ETT > 20 GeV, | > 20 GeV, || < 2.5| < 2.5
2587 events (L2587 events (Lintint=174 pb=174 pb-1-1))
Good understanding of data
• MC: Alpgen & Pythia + detailed detector
response• Cross sections normalized to MCFM
NLO calculations
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W(W(ee)bb associated production -II)bb associated production -II
Observe Observe 8 events8 events, expect , expect 8.3±2.28.3±2.2 Bkgd. dominated by top eventsBkgd. dominated by top events
Require jets to be b-taggedRequire jets to be b-tagged Use various b-tag algorithmsUse various b-tag algorithms
All yield consistentAll yield consistent resultsresults
Good agreement between data and MC
At least one b-tag
At least two b-tags
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W(W(ee)bb associated production -III)bb associated production -III Exactly 2 jets: suppress top Exactly 2 jets: suppress top
production production Observe Observe 2 events2 events, expect , expect
2.5±0.52.5±0.5 Sample compositionSample compositionWbb Wc(c) Wjj tt+t Others
1.4±0.4
0.3±0.1
0.10±0.03
0.6±0.2
0.10±0.03
115 GeV
Set limit on production ofSet limit on production of
(Wbb) < 20.3 pb(Wbb) < 20.3 pb (95%c.l.)(95%c.l.)
(WH) x B(H(WH) x B(Hbb) < 12.4 pbbb) < 12.4 pb for Mfor MHH = 115 GeV = 115 GeV (95%c.l.)(95%c.l.)
SourceSource Uncertainty Uncertainty (%)(%)
Jet energy scale
14
Jet ID 7
b-tagging 11
Trigger & e ID 5
EM scale 5
MC simulations 15
Total 2626
•SystematicsSystematics
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Z(Z(ee/ee/)b associated production - I)b associated production - I MotivationMotivation
Background to ZH searchBackground to ZH search Benchmark for SUSY Higgs Benchmark for SUSY Higgs
production via gbproduction via gbbhbh Probes PDF of the b-quarkProbes PDF of the b-quark
Examples of ZQ (Zj) LO diagramsExamples of ZQ (Zj) LO diagrams
Measure cross section ratioMeasure cross section ratio (Z+b)/(Z+b)/(Z+j)(Z+j) Many uncertainties cancelMany uncertainties cancel
Lumi of 184 (ee), 152 (Lumi of 184 (ee), 152 () pb) pb-1-1
Event selectionEvent selection Isolated eIsolated e with p with pT T > 15/20 > 15/20
GeV, |GeV, || < 2.5/2.0| < 2.5/2.0 Z peak for signal, side bands Z peak for signal, side bands
for bkgd. evaluationfor bkgd. evaluation Jet EJet ETT > 20 GeV, | > 20 GeV, || < 2.5| < 2.5 At least one b-tagged jetAt least one b-tagged jet
• MC: Pythia or Alpgen + detailedMC: Pythia or Alpgen + detailed detector responsedetector response
• Cross sections normalized to Cross sections normalized to datadata
• Relative b- and c-quark content Relative b- and c-quark content from MCFM NLO calculationsfrom MCFM NLO calculations
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Z+b/Z+j cross section ratio Z+b/Z+j cross section ratio
0.024 ± 0.005 (stat) (syst)0.024 ± 0.005 (stat) (syst)
Theory: ~0.02 Theory: ~0.02 (hep-ph/0312024)(hep-ph/0312024)
SystematicsSystematics
Z(Z(ee/ee/)b associated production - II)b associated production - II Transverse energy spectrum of b-Transverse energy spectrum of b-
tagged jetstagged jets QCD and mistag bkgd. estimated QCD and mistag bkgd. estimated
from datafrom data MC: Pythia Zb normalized toMC: Pythia Zb normalized to
datadata
+ 0.005– 0.004
SourceSource UncertaintyUncertainty
(%)(%)
Jet tagging 16
Jet energy scale 11
Bkgd. estimation 6
(Z+c)/(Z+b) 3
TotalTotal 2020
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H H WW WW((**) ) l l++ll-- (l = e, (l = e, - I - I Event selection
Isolated e/ pT(e1) > 12 GeV, pT(e2) > 8 GeV pT(e/1) > 12 GeV, pT(e/2) > 8
GeV pT(1) > 20 GeV, pT(2) > 10 GeV
Missing ET> 20 GeV (ee, e); 30 GeV ()
Veto on Z resonance Energetic jets
MC: Pythia + detailed detector response
Rates normalized to NLO cross section values
Lumi. of ~ 180 (ee), 160 (e) and 150 () pb-1
Data vs MC afterpreselection
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H H WW WW((**) ) l l++ll-- (l = e, (l = e, Higgs mass reconstruction not Higgs mass reconstruction not
possible due to two neutrinospossible due to two neutrinos
Spin correlations suppress bkgd.Spin correlations suppress bkgd. (ll) variable is particularly (ll) variable is particularly usefuluseful
Charged leptons from Higgs collinearCharged leptons from Higgs collinear
W+ e+
W- e-
Azimuthal angle between e and (after event pre-selection)
Good agreement between data and MC
Higgs of 160 GeV
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H H WW WW((**) ) l l++ll-- (l = e, (l = e,
Excluded cross section timesBranching Ratio
Signal acceptance is ~ 0.02 – 0.2
(Higgs mass/final state) ee e
Observed 2 2 5
Expected 2.7 ± 0.4 3.1 ± 0.3 5.3 ± 0.6
Number of events after cutsNumber of events after cuts
WW W+jets WZ tt
2.51±0.05
0.34±0.02
0.11±0.01
0.13±0.01
Dominant bkgd. in e sample
DØ Run II Preliminary
Higgs of 160 GeV
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ConclusionsConclusions
SM Higgs searches at DØ underway
Good understanding of backgrounds
First Tevatron Run II Results Limits set on Wbb, WH and HWW(*) production (Z+b) / (Z+j) measured
Many more results to come with improved statistics