CDF
Searches for the Standard Model Higgs at the
Tevatronpresented by
Per Jonsson
Imperial College London
On behalf of the CDF and DØ Collaborations
Moriond QCD 2006
La Thuile, Aosta Valley, Italy, March 18-25, 2006
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Per Jonsson - Imperial College LondonMoriond QCD 2006 2Per Jonsson - Imperial College London
Outline• Introduction
• Higgs bb (low mass)
– ZH bb– WH lbb
• Higgs WW (high mass)
– WH WWW– H WW
• Limits:– combination
Main Injector & Recycler
Tevatron
Chicago
p source
Booster
p
p
p p 1.96 TeV
CDFDØ
36×36 bunches396 ns bunch crossing
Thanks to all colleagues at the Tevatron for their contributions to this talk
New results!
New result!
New result!
Only new results since last year’s Moriond are shown
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Tevatron Performance
80-85% Average Efficiency
Currently in shutdown:• Silicon and Trigger upgrades
Run I
4-8 fb-1 expected by end of 2009
This talk:261-950pb-1
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The DØ and CDF detectors
The Standard Model Higgs Boson is:• Key part of Electro-Weak symmetry breaking• A scalar with the Higgs mass as the only free parameter
DØ and CDF constrain the SM Higgs boson mass:• Indirectly: Top and W mass measurements• Directly: Search for Higgs boson production
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EW constraints on HiggsmH constrained in the Standard Model
mH =89 +45 -30 GeV
Direct searches at LEP2:mH>114.4 GeV @95%CL
mH<175 GeV @95%CL(< 207 GeV if LEP2 limit incl.) A light Higgs is favored
68% CL
[GeV]
[GeV
]
LEPEWWG 18/03/06
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SM Higgs Production• Production cross sections are small: 0.1-1
pb depending on mH:~1 in 1012 pp events is a Higgs
• MH< 135 GeV: decay to bb– gg H bb overwhelmed by QCD
background • searches can be performed in W/Z
associated production with lower background
• Best channels:– WHlbb, ZHbb
• MH > 135 GeV: decay to WW– gg H WW(*) l+l-– WH WWW(*) final states can be explored
We probably have a Higgs in our data already!
H bb
H WW(*)
Dominant decay modes
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Backgrounds and Tools
• Lepton identification• Missing transverse energy
Measurements also rely on:• Jet reconstruction• B-tagging:
– Use track impact parameter (IP) measurements or secondary vertex reconstruction
IP
Both detectors are delivering the excellent performance needed
Understanding and modeling of background is critical:
• Especially so for advanced analysis techniques
• Electroweak backgrounds (W, Z, WZ, WW, Top):
– Monte Carlo distributions normalized to (N)NLO cross sections
• QCD and instrumental background taken from real data control samples
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Z/ ee + jets• Comparison of jet multiplicities and
distributions in data and MC:
– Pythia 6.319 vs. Sherpa 1.0.6 – Sherpa : Matrix Element + Parton
showers, using CKKW algorithm
• Event selection includes:– Electron pT > 25 GeV, – 70 GeV < Mee < 120 GeV
New result!
Sherpa agrees well with dataupto njet=4
L=~950 pb-1L=~950 pb-1
For details see Gavin’stalk yesterday.
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ZH bb
• Event selection includes:– ≥ 1 tagged b-jets– Two jets with ET > 60/25 GeV– ET
miss > 70 GeV
• Backgrounds:– W/Z + heavy flavour jets– QCD– Di-bosons– Mis-tagged b-jets– Top pairs
• MH = 120 GeV, 80 < mjj < 120 GeV
– 6 events observed– 4.36 +/- 1.02 predicted
– pb
ET1
= 100.3 GeVET
2 = 54.7 GeVET
miss = 144.8 GeVMjj = 82.1 GeV
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New result!
L=261 pb-1L=261 pb-1
1 b-tag 2 b-tags
• Improved event selection includes:
– Two acoplanar jets with:
• ET > 20 GeV
– ETmiss > 50 GeV
– Sum of scalar jet ET < 240 GeV
• Separate analysis for single and double b-tagged events:
– Increased statistics
• mH = 115 GeV, 75 < mjj < 125 GeV:– 11 events observed– 9.4 +/- 1.8 predicted– pb
• Same analysis for WH lbb with missed lepton improves the WH limit
ZH bb
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• Muon and electron channel combined:
– 1 or 2 tagged b-jets
– electron or muon with pT > 20 GeV
– ETmiss > 20 GeV
Per Jonsson - Imperial College London
• Main backgrounds:• W + jets• Top pairs• di-bosons (WW, WZ etc.)
e/
b jet
b jet
WHlbb , (l=e,
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• New muon analysis• Re-optimized electron
analysis• Separate single and
double b-tag analysis
• Event selection includes:– Central isolated e/
• pT > 20 GeV– Missing ET > 25 GeV– ≥ two jets:
• ET > 20 GeV, |η| < 2.5– One or two tagged b-jets
• Limit from combined channels:– mH = 115 GeV, 85 < mjj < 140
GeV:• 6 (32 ST) events observed• 9.3 (45.1 ST) predicted• pb
New result!
WHlbb, (l=e,
Per Jonsson - Imperial College London
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• Event selection includes:– Two isolated like signed
leptons (ee,e– pT > 15 GeV
– ETmiss > 20 GeV
• mH=115 GeV)pb
WHWWW(*)llqq, (l=e,
Per Jonsson - Imperial College London
Mainly di-bosons (WZ)
CDF 194 pb-1
L=363 pb-1
observed expected
ee 1 0.70 +/- 0.08
e 3 4.32 +/- 0.23
2 3.72 +/- 0.75
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• Higgs mass reconstruction not possible due to two neutrinos
• Employ spin correlations to suppress the background– (ll) variable is particularly useful– Charged leptons from Higgs are collinear
• Event selection includes:– Two leptons (e/
• pT > 20/10 GeV– Missing ET greater than
• MH/4– Di-lepton invariant mass
• > 16 GeV• < MH/2-5 GeV
• ll distribution fitted to extract 95% CL limit
• SM cross section still far away, but 4th generation models getting closer
mH= 130 GeV
mH= 160 GeV
W+ e+
W- e-
HWW(*)l+l-, (l=e,
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• L= 950 pb-1
• ee and echannels
• Event selection includes:– Isolated e/
• pT(e1, e2) > 15, 10 GeV• pT(e/1) > 15 GeV,• pT(e/2) > 10 GeV
– Missing ET greater than• 20 GeV (ee, e)
– Veto on• Z resonance• Energetic jets
• MH = 120 GeV:– 31 events observed– 32.7 +/- 2.3 (stat)
predicted– Bkg systematic
uncertainty: 15%– pb
Per Jonsson - Imperial College London
New result!
ee
e
After cuts
160 GeV Higgs (x 10)
HWW(*)l+l- (l=e,
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Currently a factor 15 away from mH =115 GeV
• With L= 2 fb-1, both experiments, NN b-tagging, NN analyses, track-cal jets, increased acceptance, new channels, full cross efficiency, and reduced systematics:– Cross section factor = 1.2
New result!
Combined limit• New combined limit from all SM Higgs
search channels!
• 14 orthogonal search channels (incl. single and double-tag analyses and
WH lbb w. missed lepton)
• Full account taken of systematic uncertainties
• High mass region benefits from HWW analyses
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Limits
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Summary
Many results already and more with increased stats are coming soon!
• The Tevatron and the experiments are performing well
• A wide range of Standard Model Higgs searches have been performed by both CDF and D0 with up to 1 fb-1 RunII data:– No deviation from the SM background
expectation observed
• Work underway to improve sensitivity:– First combination of all channels from D0
• Very exciting prospects for the future:
– Sensitivity to m H > 114 GeV starts at ~2 fb-1
– Exclusion up to 180 GeV possible with 8 fb-
1
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Back-up slides
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SM Higgs Sensitivity
• New Higgs sensitivity study from CDF + DØ in 2003:
Statistical power onlySystematics not included
Improved sensitivity from refined analysis and detailed simulation
No systematics
The SM Higgs is a challenge, understanding of bkgs critical!
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Major Improvements
• 6x more lumi (2 fb-1 ): 2.4
• We need both CDF and D0: 1.4• Multivariate analysis: 1.7• NN b-tagging: 1.35• Di-jet mass resolution: 1.2• Additional search channels: 1.15
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