1 Viktor Veszprémi (Purdue University, CDF Collaboration) SUSY 2005, Durham Search for the SM Higgs Boson at the CDF Experiment Search for the SM Higgs

1

Viktor Veszprémi(Purdue University, CDF Collaboration)

SUSY 2005, Durham

Search for the SM Higgs Bosonat the CDF Experiment

Search for the SM Higgs Bosonat the CDF Experiment

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Gluon fusion:

• Large cross section• But no search potential below Mh=~135 GeV

• Indistinctive signature of a pair of b-jets• Signal swamps in QCD multi-jet processes

“Higgsstrahlung” (W/Z associated production):

• Smaller cross section• Good search potential both at lower and higher Higgs mass• Easier to trigger on the decay products of the W/Z bosons

Viktor Veszprémi, SUSY 2005, Durham

Standard Model Higgs ProductionStandard Model Higgs ProductionStandard Model Higgs ProductionStandard Model Higgs Production

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mH<114.4 GeV Excluded at 95 % C.L.

EWK fits prefer light SM Higgs < ~260 GeV

Exclu

ded

Exclu

ded

Mh (GeV)

Exluced by LEP

H->bb dominated region:2 analyses with results1 analysis in progress

H->WW dominated region:2 analysis result

Viktor Veszprémi, SUSY 2005, Durham

Standard Model Higgs Decay Standard Model Higgs Decay ModesModes

Standard Model Higgs Decay Standard Model Higgs Decay ModesModes

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• Charged lepton final states:

• Low Higgs mass scenario

• Requires good b-jet selection, lepton identification, Missing Et reconstruction, di-jet invariant mass reconstruction

• High Higgs mass scenario

• Requires efficient lepton identification

• Final state with no charged leptons and low Higgs mass:

• Requires good b-jet selection, Missing Et reconstruction, di-jet invariant mass reconstructionViktor Veszprémi, SUSY 2005, Durham, England

CDF SM Higgs AnalysesCDF SM Higgs Analyses

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Tagging B-jetsTagging B-jets

Tagging jets with the SecVtx algorithm is a powerful way of selecting b-jets

Jet is tagged:• decay length Lxy/Lxy > 7• Lxy>0 identifies real heavy flavor jet

Tagged jet

Prim. vertex

2nd vertexLxy>0

• Currently using the tight tagger

• Development of a “forward b-jet tagging” is in progress to increase acceptance

• Mistag rate is ~1%

Viktor Veszprémi, SUSY 2005, Durham, England

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• Reconstuction of Z decaying to b-jets– Measure jet energy scale and

resolution– Provides a tool for

investigating b-jet specific jet energy corrections

• Looking for Z in double tagged events with – no additional jets above 10

GeV– Jets are back-to-back

topology

• 3394±515 Zbb events were found in a sample of 85,784 double-tagged events. (333pb-1)

• We can observe the Z peak

Reconstruction of Zbb decays in CDFReconstruction of Zbb decays in CDF

Viktor Veszprémi, SUSY 2005, Durham , England

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WWHgg

W+ e+

W- e-

Event Selection:

• exactly two identified leptons (veto WZ, ZZ)

• no jets with Et > 15 GeV and || < 2.5 (veto top-pairs)

• > 25 GeV

• If < 50 GeV: (veto Z/)

• not 76 < Mee/uu < 106 unless Missing ET significance larger than 3 (veto WZ,ZZ,Z/)

• leptons having opposite charge signs (veto WZ)

• di-lepton invariant mass Mll < half the Higgs mass. (discriminates from WW, WZ, ZZ)

2~ h

ll

MM

Spin of Higgs is 0

W spins are anti-parallel

Leptons from W’s arealigned

Di-lepton mass isconstrained


20),( leptonclosestETTE

TE

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WWHgg

(SM) sourceExpectation

(Mh=180 GeV)

WW 6.49 +/- 0.76WZ 0.18 +/- 0.02ZZ 0.06 +/- 0.01

Drell-Yan ee 0.87 +/- 0.44DY mumu 0.43 +/- 0.19DY tautau 0.03 +/- 0.01

ttbar 0.02 +/- 0.01fakes 0.81 +/- 0.25

total bg 8.90 +/- 0.98HWW 0.17 +/- 0.02data 8

Small di-lepton invariant mass

Leptons originating from the signal process tend to be back-to-back in the transverse plain


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WWHgg

tlltC EMpM 22

Clustered mass:

The 95% C.L. effective cross-section limit of the Higgs boson


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• Like-sign di-lepton selection (“1”=more, “2”=less energetic)

• pT,1>20 GeV, pT,2>6 GeV• reject conversions, cosmics, Zleptons • • Signal region: pT,2>16 GeV, for mH<160 GeV• Harden pT,2 cut to 18 GeV for mH>160 GeV

Category Events in 193.5 pb-1

Conversions 0.61 0.61

Fake Leptons 0.12 0.01

Other sources* 0.22 0.10

Total background 0.95 0.64

Data 0

*Other backgrounds: Di-boson, top, WqqSM WH signal: 0.03 events (mH=160 GeV)


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Results


Acceptance includes W branching fractions

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bbHW Signature:

• isolated lepton with ET > 20 GeV or pt > 20 GeV in ||<1.0

• Missing ET > 20 GeV

• 2 taggable jets (require 1 or 2 tags)

Below is a double-tagged WH event

Applied selection:

• Events with an additional lepton with sign opposite to the primary is removed (Z and conversion removal)

• 2-jet bin contains the signal

• inclusive single / exclusive double tag is required -> single tag search is more sensitive


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bbHW

Background Source Events in 319 pb-

1

W+light flavor 39.3 3.1

54.0 18.4

19.5 6.6

16.8 4.3

Di-boson+Z+- 5.0 1.1

Non-W 16.5 3.2

14.1 2.5

9.6 2.0

Total Background 174.7 26.3

Observed Data 187

bbW ccW cW

tt

bt


Number of background events in the 2-jet bin:

W+h.f. normalization

Signal Top background normalization

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Dijet invariant mass in the inclusive single tag sample

The 95 % C.L. limit set on the effective cross-section of the Higgs boson


bbHW

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Z-> ν ν, H-> b bZ-> ν ν, H-> b b

• Signal has a distinctive topology

• Large missing transverse energy

• two jets (one is b-tagged)

Missing ETb-jet

b-jet

y

x

• This signature proved to be the most sensitivein the Run I and recent analyses at CDF


Basic Selection cuts:

• Jet requirements

• 1st Jet ET > 40 GeV

• 2nd Jet ET > 20 GeV

• No other jet with ET> 15 GeV

• ET > 70 GeV

• Minimum 1 b-tag

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Analysis OutlineAnalysis Outline

We performed a blind analysis

Extended Signal Region

• Veto events with leptons

• Missing ET and 2nd leading jet are not parallel

• Cut optimization is performed in this region based on Monte Carlo simulation before looking at the real data


Control Region 1 – QCD h.f.

• Veto events with identified leptons

• Missing ET and 2nd leading jet are parallel

Control Region 2 – EWK

• Require min. 1 lepton

• Missing ET and 2nd leading jets are not parallel

• Optimized cuts are tested in this region before looking at the real data in the Signal Region

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QCD heavy flavor normalizationQCD heavy flavor normalization

Control Region 1: • QCD multi-jet is the most dominant process because of its large cross-section

• A good description of this background allows us to efficiently reduce it with optimized cuts


2nd jet

Fake Missing ET

1st jet

~180o

A di-jet QCD event:

QCD events also have a particular topology:

• jets are back-to-back

• “Missing ET“ points along the leading jet• Events are simulated by a b-filtered

Pythia Monte Carlo sample which is normalized to data

• Mistags (tagged light flavor jet events) are estimated from Data before tagging

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Cut OptimizationCut Optimization


The following optimized cuts were developed on a benchmark signal MC sample (Mh=120 GeV)

Selection cut ZH 120

288.9 pb-1

Acceptance (%)

Basic Cuts 0.2050.0035 5.92 0.1 0.026

0.205 0.0035 5.920.1 0.027

significance 0.1830.0033 5.230.095 0.031

0.1610.0031 4.680.089 0.037

Di-jet mass cut 0.1260.016 3.640.079 0.062

8.0,1 Tst EJet

THGeVJetET

st 601

• The last selection is on the di-jet mass. A 20 GeV mass window is applied to the data and Monte Carlo around the reconstructed Higgs mass peak

• Performed a counting experiment

BS /

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ResultResult


The top plot : di-jet invariant mass in Control Region 2 after optimization, before the mass-window cut

The bottom plot is the same in the Signal RegionFor the 120 GeV Higgs mass we apply a 80-120 GeV window cut:

SM background prediction: 4.36 1.02 events• QCD : 11.4%• Top : 20.5%• EWK : 18.2%• Light flavor mistag: 50%

Observed: 6 events.With a 95 % C.L. we expect the limit to be for the

Observed limit:

pbbbBRZH 4.16.3)()(

pbbbBRZH 5.4)()(

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Signal Event CandidateSignal Event Candidate


Data events from Signal Region• passing all selection cuts• candidate in the 80-120 GeV mass window

Double-tagged event

Missing ET

144.8 GeV

Di-jet invariant mass = 82 GeV

Leading Jet ET = 100.3 GeV

Second Jet ET = 54.7 GeV

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Summary of Limits set at CDFSummary of Limits set at CDF


• First analysis with 200-300 pb-1 data completed• Nearly 1 fb-1 on CDF tape• Estimate sensitivity:

• 2 fb-1: exclude Higgs up to mass of 120 GeV• 8 fb-1: exclude Higgs up to mass of 135 GeV

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Backup Slides

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FY02FY03

FY04

FY05

Increasing Tevatron LuminosityIncreasing Tevatron Luminosity


Tevatron8 fb-1

Tevatron2 fb-1LE

Pexcl

ud

ed

Future expectations based on the Higgs sensitivity study in 2003.

Below is the instantaneous and integrated luminosities recorded at CDF so far

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Missing ET ReconstructionMissing ET Reconstruction

2nd jet

Fake Missing ET

1st jet

180o

A di-jet QCD event:

QCD events also have a particular topology:

• jets are back-to-back

• Fake missing ET points along the leading jet

Real missing ET – produced by undetected particles

• Neutrinos

Fake missing ET

• Beam/detector effects• A set of clean-up cuts are applied to eliminate beam effects

• Unbalanced, mis-measured jets

• The primary source of ET in QCD events

• Since the cross-section of QCD is large, most of the triggered events are like this

• Makes QCD the main background :

Viktor Veszprémi, SUSY05, Durham, England

Documents

1 Viktor Veszprémi (Purdue University, CDF Collaboration) SUSY 2005, Durham Search for the SM Higgs Boson at the CDF Experiment Search for the SM Higgs