Measurement of the top quark production cross section and search for the SM Higgs boson in dilepton final states with the CMS detector at the LHC pp collider at √s= 7 TeV

Defensa de la tesis para optar al grado de Doctor presentada porPatricia Lobelle Pardo

Dirigida por Dr. Javier Cuevas Maestro

Santander, 21 de Julio de 2011

• Standard Model , top, Higgs• LHC and CMS• Object reconstruction• Top quark production cross section• Search for SM Higgs boson• Conclusions

Outline

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Standard Model

• Origin of mass /origin of electroweak symmetry breaking Higgs?• unification of forces• fundamental symmetry of forces and matter• where is antimatter• unification of quantum physics and general relativity • number of space/time dimensions • what is dark matter • what is dark energy

Matter composed of fermions (quarks, leptons)Interacting via force carriers (bosons)

Succesfully explains experimental data to date

… still many open questions

Top/Higgs boson at 7 TeV

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Tevatron LHC

Ttbar dilepton candidate

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Introduction

• Study of processes containing 2 leptons (electrons/muons) + MET + 0/2 jets

- ttbar production cross section

- Search for SM Higgs H WW

• 2 lepton physics is an important part of the CMS analysis program SM processes and new physics produce dilepton final states

In this thesis: Background for many new physics searches

• Clear signature, easier to observe at an earlier stage…

• Understand the SM backgrounds before measuring any signal of new physics

One of the most sensitive channels at LHC

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Introduction• Thesis focuses on measurements done with 2010 data

• Analysis strategies defined before the data taking, result of many detailed studies, improvements in the detector knowledge… and adapted to different Ecm scenarios planned for the LHC startup

14 TeV

10 TeV

LHC and CMS

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LHC

• Proton-proton collider at CERN• Design √s = 14 TeV

Several experiments ATLAS, CMS, LHCb, ALICE

• Proton-proton collisions at √s = 7 TeVFrom 30th March – 6th November 2010(initial tests & physics at √s =0.9,2.36 TeV by end 2009)

• Pb-Pb collisions at 2.76 TeV/nucleon8th November – 16th Dec 2010

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CMS

• Top/Higgs analysis require excellent performance ofthe whole detector to efficiently reconstruct

– Muons– Electrons– Jets– Missing Transverse Energy (MET)– b quark jet tagging

Tracker: all-silicon, large solid angle coverage |h|<2.4, excellent position and momentum resolution

ECAL: homogeneous, crystal (PbWO4) calorimeter, highly segmented, excelent energy resolution

HCAL: scintillators/brass, |h|<5

Muon system: DT, RPC,CSC

Object definition

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Muons

• Muon Reconstruction : – Standalone : information from muon system only– Global: information from muon system & tracker

– Global muons: starting from standalone muon, search for compatible tracks in the tracker system

– Tracker muons: starting from all the tracks in the tracker, associate to segments in the muon system

• Muon Identification: Quality requirements on reconstructed muons Quality of the track fit, number of good reconstructed hits in tracker and muon chambers…- Prompt muons are produced close to the PV cut on impact parameter (IP)

• Muon Isolation:Sum of tracks pt, ECAL hits ET, HCAL hits ET in a cone around muon required to be smaller than a given threshold

Iso = (SumPt_tracks + SumET_ECAL + SumET_HCAL)/pt

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Electrons

• Reconstruction —Energy depositions in ECAL matched to tracks in tracker

• Identification : select prompt electrons and reject fakes — Based on shower properties, track-cluster matching, H/E …

• Two approaches for electron seeding:- ECAL driven: Starting from ECAL superclusters and search for compatible hits in the tracker inner layers- Tracker driven: Use all tracks as starting point

Rejection of electrons from conversions: - Impact parameter- Missing hits- Partner conversion track Dist (distance of closest approach of both trackes in phi plane), Dcot(q)

• Isolation:Sum of tracks pT, ECAL hits ET, HCAL hits ET in a cone around electron required to be smaller than a given threshold

Iso = (SumPt_tracks + SumET_ECAL + SumET_HCAL)/pt

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Jets

• Experimental signatures of quarks and gluons

• Jets are corrected for non-linearity and inhomogeneity of calorimeter response JES most important uncertainty related to jets

— Calorimeter Jets (CaloJet): From energy depositions in HCAL & ECAL

— Jet Plus Tracks (JPT): CaloJets corrected by momentum of charged tracks in tracker

—Particle Flow Jets(PF): Information from all sub-detector used to reconstruct all particles in the event

—Track Jets (TJ): from tracks only

• Hadrons detected as clusters of energies in calorimeter, gathered together by Jet Algorithms to deduce the four momentum of the parent parton—Anti kT with a cone R=0.5 standard algorithm in CMS

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b-tagging

• Several algorithms discriminants sensitive to the flavour content of the jet

• IP-based— Track Counting based on track IP

- 2nd highest IP significance in jet: High Efficiency - 3rd highest high purity

— Jet Probability: probability that the given set of tracks come from P.V.

• SV— Simple Secondary Vertex: based on reconstruction of a secondaryVertex in a jet. -Discriminator is flight distance significance.

• Exploit properties of b-quarks:- Lifetime: t ~1.5 ps decay length ~1.8mm (at 20GeV)Secondary decay vertex, displaced tracks with large IP

Tagging efficiencies measured in data and MC data/MC scale factors consistent with 1 within 10-20%

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Missing ET

• Imbalanced transverse energy in the event Signature of only weakly or non-interacting particles

— CaloMET: depositions in ECAL+ HCAL

— tcMET: expected energy depositions of good tracks of charged hadrons replaced with their corresponding momentum

— PFMET: computed from the list of individually reconstructed particles in the event, combining all subdetector information

—Caused by real undetected particles, mis-measured jets, detector effects…

Crucial object for many measurements needs to be very well understood

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Reached instantaneous luminosity peak 2e32 Hz/cm2

2010 LHC Run • 47 pb-1 of pp collision data at 7 TeV delivered by the LHC 43 pb-1 recorded by CMS

• Average fraction of operational channels per CMS sub-system>99%• Overall data taking efficiencies ~92%

Excellent detector performance Many SM measurements

• Only highest quality data used for physics analyses ~ 85%

• Full 2010 data sample: 36 pb-1

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SM re-discovery at 7 TeV

Top cross section measurement

• Motivation• Top production and decay• Top dilepton signal topology and event selection• Measurement with 3 pb-1

• Measurement with 36 pb-1• Yields• Lepton efficiency determination• Data-driven background estimation• Systematic uncertainties• Cross-section

Top Quark Physics

• Special role in the EWK sector and in QCD— Heaviest known elementary particle—Top and W masses constrain Higgs mass—Short lifetime: unique window on bare quarks

A tool for precise SM studies

• Special role in various SM extensions through EWSB— New physics might be preferentially coupled to top—Non-standard couplings between top and gauge bosons—New particles can produce/decay to tops

Sensitive probe to new physics

• A major source of background for many searches (Higgs, SUSY…)

• A tool to understand/calibrate the detector, all sub-detectors involved

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Top quark production

• Top quark pairs produced in strong interaction via• gluon-gluon fusion dominant mode at LHC• Quark- antiquark annihilation

• Single top quarks produced via EWK interaction: t-channel, s-channel, tW-channel

s-channel t-channel tW-channel

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Top quark decay

BR(tWb)≈100%

In SM top decays almost exclusively to W and b

o Dilepton channel o Fully hadronic channel

o Lepton+jets channel

Different signatures according to the W decay:

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Top dilepton topology

Main backgrounds: — Drell-Yan no genuine MET— Diboson: WW,WZ,ZZ small hadronic activity— W+jets one isolated lepton— Single top –tW one b-jet

• 2 prompt leptons from W decay: high pT, opposite charged, isolated• MET from the undetected neutrinos• 2 b-jets

tt DY W+jets WW tW

s(pb) 157.5 3048 31314 42.9 10.6

Most physics objects are used

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MuonspT>20 GeV, |h|<2.4 -ID: globalMuon, trackerMuon, IP<0.02 cm, Nhits>10, chi2<10- Iso<0.15

ElectronspT>20 GeV, |h|<2.5 EcalDriven, eID 90% efficient for e from Ws , IP<0.04 cm , Iso <0.15

Trigger

Jet selection

MET selection b-tag selection

muon and electron triggers

Lepton pair selection

At least 2 PFJets with corrected pT>30 GeV, |h|<2.5,Away from selected leptons

Z veto

Analysis flow

|Mll-Mz|>15ee/mm

At least 1 b-tagged jetTrack Counting High Efficiency discriminatorLoose point: 1.7

PFMET> 30 GeVee/mm

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Top cross section with 3 pb-1

First cross section measurement already possible with only 3 pb-1

• Full selection applied: — 2 leptons, Z-bosonVeto:|M(ll)-M(Z)|>15 GeV— MET >30 (20) GeV in ee,mm, (em)— N(jets)≥2

DY and fake lepton backgrounds estimated from data

11 candidates ( 3 ee, 3 mm, 5 em) observed in data

o Signal selectionLepton selection: 4.4%Energy Scale: 3.7%Theoretical : 2.8%

Backgrounds 11%Luminosity 11%

6.4%

Systematic uncertainties on xs

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First top cross section measurement at LHCσ(pp → t¯t) = 194 ± 72(stat.) ± 24(syst.) ± 21(lumi.) pb

Consistent with NLO prediction of 157.5 (+23.2 −24.4) pb for a top quark mass of mt = 172.5 GeV/c2

Top cross section with 3 pb-1

b-quark content of the selected samplestudied consistent with ttbar production

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mm

ee

Expected and observed events 36 pb-1

em

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Data/MC comparison

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Lepton efficiencies

o Lepton efficiencies ( Reco, ID, ISO, trigger) measured using Tag and Probe method

Efficiencies estimated in data and MC Scale factors extracted

• An unbiased and pure sample of leptons is selected using Z ll events— One lepton is required to pass a tight selection ( “tag” )— The other lepton (“probe”) needs to satisfy only loose selection— Tag & probe pairs required to be consistent with Z to ensure the purity of

the probe sample

• Probe leptons used to measure the efficiencies required to pass the cuts of a given selection whose efficiency has to be measured

-Muon RECO/ID/ISO efficiencies ~99% - Electron RECO/ID/ISO ~ 99% /85-95%/98%-Trigger efficiencies > 97%, (99%) mm (ee/em)

SFee= 0.923 ± 0.018, SFmm = 0.967 ± 0.013 , SFem = 0.947 ± 0.011

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Background estimation: Drell-Yan

Non-DY contribution subtracted using emu events and correcting for differences between e and m

76<mll<106

• Off-peak DY events are one of the main sources of background Come from mismeasured MET from jets/leptons

Events outside the control region can be estimatedfrom events inside in data, correcting by a scale factor

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Data-driven estimatemm: 2.6 ± 1.2 (stats) ± 1.3 (sys.)ee: 0.7 ± 0.6 (stats) ± 0.3 (sys.)

Systematic uncertainty comes fromthe variation of Rout/in with the MET cut 50%

Background estimation: Drell-Yan

ee >= 2 jets MET>30 >=1BTAG

DY MC 35.8±1.2 1.7±0.2 0.48±0.1

DY data-driven 34.2±2.4 2.8±0.9 0.66±0.6

Rout/in 0.118±0.004 0.12±0.02 0.10±0.04

mm >= 2 jets MET>30 >=1BTAG

DY MC 43.6±1.4 3.6±0.4 1.1±0.2

DY data-driven 49.2±3.0 8.1±2 2.6±1.2

Rout/in 0.121±0.004 0.23±0.02 0.18±0.04

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• Events with “fake” leptons (W+Jets, QCD and semi-leptonic tt events where a lepton does not come from the decay of a W) constitute a background difficult to predict

• Estimates of such events are based on weighted counting of leptons failing tight selections but passing looser fakeable object (FO) selections.

• Tight-to-Loose (TL) ratio probability of a fakeable object (FO) to pass the full analysis selection. Computed in a QCD-dominated sample:

To estimate bkgs with fake leptons TL ratio applied to sample with-2 leptons passing numerator selections ( dominated by real)-1 lepton passing numerator selection, other failing (combination of all)-2 leptons failing numerator selection (dominated by QCD)

Background estimation: Fake leptons

Events Simulation Data

mumu 0.07±0.02 0.07±0.07±0.02

ee 0.35±0.04 0.13±0.10±0.07

emu 0.99±0.42 0.68±0.21±0.34

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• One of the main sources of systematic uncertainty affects pt of the jets MET

Jet Energy Scale uncertainties ~ 3-4% for jets with pt>30 taken from data base in pt,eta bins On top of that : - 1.5% to account for differences in software and calibrations - b-jet scale: 2% jets with (50<pt<200 GeV and eta<2.0), 3% otherwise

Pt of the jets shifted by this value and also propagated to the MET

Unclustered MET (10%) 1% effect

Total uncertainty:3.8% ee/mm2.8% emu

Systematics: Jet Energy Scale

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Systematics: b-taggingEfficiencies and mistag rates associated to the working point used (1.7) are shifted by their relative uncertainties to obtain new b-tag discriminator cuts.

SFlight= 1.0 ± 0.25SFb = 1.0 ± 0.1SFc = 1.0 ± 0.2 5% systematic uncertainty

Data-MC scale factors:

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Summary of systematics

• Backgrounds from MC:Ztautau, VV and tW

• Uncertainties on signal selection:

15% uncertainty on the NLO cross section

30% uncertainty on the cross section

Detector effects, JES, btag—38% Ztautau, VV—32% tW

Largest systematic uncertainty Comes from b-tagging

Theoretical systematics estimated using ttbar MC samples produced with different configurations

Lepton selection: T&P + difference between tt and Z

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Cross section measurementN: Number of events observed in dataB: Number of background eventsA: signal acceptanceL: luminosity

Analysis was combined with other dilepton measurements: 1 jet / 2-jets ( without b-tagging)

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Combined cross section

12% precision on top cross section

Measurements of the top cross section in dilepton & lepton+jets channels: s = 158 ± 10 (stats) ± 16 (syst) ± 6 (lumi) pb

Search for the SM Higgs boson

• Current limits• Higgs production and decay• HWW channel• Event selection• Cut-based analysis• BDT analysis• Results and limits

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Higgs limits up to 2010

• Experimental limits LEP : mH > 114.4 GeV/c2 at 95% CL Tevatron : Excluded the mass range of 158 GeV/c2 to 173 GeV/c2 at 95%CL • Indirect constraints

Derived from precise EWK measurements: mH = 98 +58

-37 GeV/c2

(mH < 158 GeV/c2) (mH <185 GeV/c2 if including LEP2 results)

Theoretical limitsFinite and positive Higgs couplings

A light Higgs is favoured by measurements

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SM Higgs production

Gluon-gluon fusion (ggH)

Vector boson fusion (VBF)

In association with W,Z (VH)

In association with tt (ttH)

ggH dominant mode at LHC

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SM Higgs decays

At Low mass ( mH< 2mZ )

• H bb : BR ~0.85 but huge QCD background

• H tt : accessible through VBF

• H gg : very important despite the low BR (~0.002 ) due to the excellent g/jet separation and g resolution

• HWW*2l2n : accesible through gg fusion and VBF, BR~1 at mH ~160 GeV/c2

• HZZ*4l : also performant

For Higher masses HWW* 2l2v and HZZ*4l

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H→WW*→2l2n

Signal: 2 high pT isolated leptons, MET and no hard jets

• Backgrounds:– WW, tt, W+jets, Z+jets, tW, WZ, ZZ…

No mass peak (undetected neutrinos) Needs a good background understanding

Main search channel for range 140<mH<2mZ

-Highest branching ratio for mH >140 GeV/c2: 95% at mH = 160 GeV/c2

Two main discriminants: Angle between leptons in the transverse plane : main variable to reject WW (small opening angle for the signal due to spin correlations)Jet Veto for ttbar reduction

tt DY W+jets WW

s(pb) 157.5 3048 31314 42.9

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Triggermuon and electron triggers

WW preselection

Analysis flow

Higgs selection

Cut-based analysis MVA analysis

- Z veto |Mll-Mz|>15- MET- Jet Veto- Top tagging (b-tag veto)

Lepton pair selection

MuonspT>20 GeV, |h|<2.4 -ID: globalMuon, trackerMuon, IP<0.02 cm, Nhits>10, chi2<10- Iso<0.15

ElectronspT>20 GeV, |h|<2.5 EcalDriven, eID 80% efficient for e from Ws , IP<0.04 cm, Iso <0.10

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Missing ET

• Essential to control Z+jets background and reduce it to an acceptable level

• Projected MET

This variable helps to suppress Ztautau that tend to have MET aligned with one of the leptons

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Jet Veto and top tagging Signal has little hadronic activity main property to reject ttbar background

Events with at least one jet pT>25 GeV, |h|<5 are vetoed

Top tagging soft muon + b-jet tagging - Muons from b-quark decays are not isolated and have softer pt - Look for b-tagged jets (TCHE > 2.1) remaining after Jet Veto

Presence of b-quarks in top events also exploited

- Requiring no additional soft muons and no b-tagged jets ~40% top bkg reduction - Control sample for top background estimation

signal

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Cut-based analysis

Secuential cuts applied on main discriminant variables :— Pt of the highest pt lepton (Ptmax)— Pt of the second lepton (Ptmin)— Dilepton invariant mass (mll)— Angle between the leptons in the transverse plane (dPhi)

Most powerful observable to distinguish the signal from the irreducible WW background

Due to spin correlations, leptonsfrom Higgs decays tend to have smalleropening angles

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Cut-based analysis

-Kinematics change with the Higgs boson mass mass dependent optimization

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BDT analysis

• MVA analysis used to improve sensitivity

• Boosted Decision Trees — Binary classifier— Sample splitted by cutting on the variable that gives the best S-B separation at each step

• Training Sample with known Signal and background composition , selection of a set of discriminant variables…• Resulting function applied to the data sample

Different approaches according to the training procedure, similar performance:

— BDT 3D separate trainings for each background— BDT 1D one single training against all backgrounds together

Mass dependent optimization (130,160,190)

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BDT analysis

Input variables

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BDT 3D

- Main backgrounds: WW, ttbar and Z+jets

- 3 independent trainings using the same set of input variables and then combined into a Single variable

BDT anti WW BDT anti ttbar BDT anti Z

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signal WW

ttbar Z+jets

BDT 3D

Signal-like background-likeX axis: BDT1Y axis: BDT2Z axis: BDT3

Final discriminant variable

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Results BDT/Cut-based

BDT 1DTrained against theSum of all backgrounds

~15% more signalwith BDT

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Background estimation

• DY: in/out method• Fake leptons: Tight to Loose method

Same as for the ttbar cross-section

• WW: Control region defined by looking at dilepton massLarge statistical uncertainty ~50% with the available data sample

• Top- Events with N>=1 are dominated by top eventsremoving anti b-tagging requirement- No events in the b-tagged region estimate taken from simulation with a 100% uncertainty

- Background prediction is then extrapolated to the Higgs signal region by using the efficiency of the additional requirements from simulation

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• Luminosity : 11%• Lepton efficiencies: 1-2% (T&P)• Lepton momentum scale: 1.3% varying pt of leptons• Pile-Up : reweighting MC 0.5%• Jet Veto: 6.9% on the signal• Background estimation:

— WW ~55%— DY ~100%— top ~100%— Fakes ~50%

Systematic uncertainties

Uncertainty on the H→WW signal yield ~ 14% (dominated by uncertainty on the jet veto efficiency and the luminosity determination )

~40% uncertainty on the background

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Exclusion limits

SM extension with 4 generations predict ~9 enhacement in cross section For this model Higgs excluded from 144 to 207 GeV

No excess foundabove SM expectations

Cut-based BDT

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Projections 1 fb-1

With 1 fb−1 at 7 TeV expected exclusion limit from 135 to 450 GeV

With 5 fb−1 of data up to 600 GeV

With 1 fb−1 at 7 TeV 3s-significance in the mass range of mH = 145 − 195 GeV

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Phys.Lett. B 695 (2011) 424

Phys.Lett. B 699(2011) 25-47

Accepted by JHEP

The work described in this thesis contributed to the published analysis:

arXiv:1105.5661

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Conclusions

• The measurement of the tt cross section, as well as the search for the SM Higgs boson in H WW decay mode, in the dilepton final states, for 36 pb-1 of data collected during the 2010 run of the LHC has been presented

• For the final cross-section measurement, several analysis applying slightly different selections were combined: at least 2 jets with at least one b-tagged jet (described in this thesis), at least 2 jets without any b-tagging requirement and the 1-jet analysis. This led to the value : s =168±18(stats.) ±14(sys.) ± 7(lumi.) pb , consistent with the NLO cross section prediction of 158±23 pb

•In this work we searched for the H WW 2l2v final state that is the most promising in a wide range of Higgs boson masses around 160 GeV . The integrated luminosity was not yet large enough to be sensitive to the SM Higgs boson but it was possible to set limits in the context of a SM extension with a fourth generation of fermions, from 144 to 207 GeV

BACKUP

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Data-driven estimatemm: 2.6 ± 1.2 (stats) ± 1.3 (sys.)ee: 0.7 ± 0.6 (stats) ± 0.3 (sys.)

Systematic uncertainty comes fromthe variation of Rout/in with the MET cut 50%

Average between PU and nonPU samples

Background estimation: Drell-Yan

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Muon Fake ratio determination Jet triggers used to select a fake lepton enriched samplesEWK contamination (W,Z) reduced by applying cuts on MET<20, MT<20And Z vetoTo avoid any possible trigger bias -> muons matched in dr<1 to the leading jet are removed

FO definition analysis cuts + relaxed isolation cut

Background estimation: Fake leptons

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Background estimation: Fake leptons

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Lepton efficiencies

RECO ID ISO SF

Muons 0.997± 0.005 0.997± 0.002 0.997± 0.002 0.992 ± 0.005

Electrons 0.994± 0.006 0.961 ± 0.009

• Single lepton RECO, ID, ISO scale factors

mm ee em

0.983± 0.007 1.000± 0.001 0.994± 0.003• Trigger

0.972± 0.006

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Background estimation: Fake leptons

W+jets Semileptonic tt Data

mumu 0 0.07±0.02 0.07±0.07±0.02ee 0 0.35±0.04 0.13±0.10±0.07

emu 0.07±0.07 0.92±0.42 0.68±0.21±0.34

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Systematic uncertainties