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Signature

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Event Selection

Background Estimation

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H→ZZ→llqqone lepton pair + 2 jets

Main Background:Z+jets productionttbar/di-boson ‣ non-negligible in “tagged” case

200 GeV ≤ mH ≤ 600 GeV ‣ Optimized: high/low mass sub-region(mH≥300GeV) ‣ Binned: “tagged” (2 b-tags) & “un-tagged” (0/1 b-tags)

Same-flavour pair of isolated high pT leptons: ‣ pT > 20 GeV, 76 GeV < mll < 106 GeVVeto on 3rd lepton.At least 2 jets: ‣ pT > 25GeV, |η|<2.5, with 70< mjj <105 GeVMET<50 GeV“High-mass”: pT(jets) > 50 GeV, Δφll<π/2, Δφjj<π/2Final discriminant:lljj invariant mass (mjj scaled to mZ)

Normalize Z+jets using the sidebands of mjj: ‣ uncertainty <10% for un-tagged ‣ uncertainty ~20% for tagged

Verify top expectation using sidebands of mll: ‣ uncertainty 10% from theory

Di-boson backgrounds from MC: ‣ uncertainty ~10%

Data-Driven QCD background: ‣ small contributions ‣ ee : QCD background templates ‣ μμ: 2D sideband using isolation and invariant mass of the pair

Good sensitivity in a wide mass range.Expected limit:

2.7 - 9 ×σSM.Observed Limit :

1.7 x σSM at mH = 360 GeV.

un-tagged

tagged

Sidebands of mjj(40,70 GeV) & (105, 150 GeV)

Sidebands of mll(60,76 GeV) & (106, 150 GeV)

Background Control Region: Signature

Search Window

Event Selection

Background Estimation

H→ZZ→llννone lepton pair + MET

Main backgrounds:top and di-boson productionSignal contributions from H→WW→lvlvOrthogonal to H→WW→lvlv search

200 GeV ≤ mH ≤ 600 GeV

Same-flavour opposite-sign pair of isolated high pT leptons.Veto on 3rd lepton & b-jet.MET > 66 GeV (“High-mass” MET > 82 GeV)Δφ(Leading Jet,MET)>0.3, for “High-mass” also Δφ(Z,MET)>1→1<Δφll<2.64 (“High-mass” Δφll<2.25).Final discriminant is the transverse mass:

Di-boson backgrounds from MC ‣ uncertainty ~10%

Top background ‣ MC expectation verified with data

Data-driven W+jets background ‣ taken for same-sign lepton pairs

Data-driven QCD background ‣ generally small contributions

m2T ⌘

qm2

Z + |~P llT |2 +

qm2

Z + |~PmissT |2

�2�h~P llT + ~Pmiss

T

i2 Sensitive in a wide mass range.Expected limit :

1.2 - 7 ×σSMExclude SM Higgs :

340 < mH< 460 GeV

Standard Model H→ZZ(*)→llqq,llνν,llll Searches with ATLAS at LHCRyan R. Rios, Southern Methodist University .

on behalf of the ATLAS CollaborationOverview:The latest results on the direct experimental search for a Standard Model Higgs boson decaying to a pair of Z bosons are presented.Three distinct final state modes are considered: H→ZZ(*)→llll, H→ZZ→llνν, and H→ZZ→llqq, where l=e,μ. These modes are characterized by low branching ratios with small background contributions. Additionally, the presence of at least one on-shell Z boson ensures clean final states.A dataset of more than 1fb-1 of pp collisions at √s=7 TeV collected by the ATLAS detector at the CERN LHC during the on-going 2011 run is used.

Signature

Search Window

Event Selection

Background Estimation

H→ZZ(*)→lllltwo leptons pairs

Main Backgrounds:ZZ→4lZ+jets/ttbar ‣ suppressed with isolation/impact parameter

110 GeV ≤ mH ≤ 600 GeV

Two same-flavour/opposite-sign isolated lepton (e/μ) pairs.Lepton pair mass cuts : |m12-mZ| < 15 GeV, m34 < 115GeV and m34>mThreshold[m4l].

For m4l < 190 GeV: ‣ additional impact parameter requirements.

Normalize ZZ production to MC expectation ‣ Including both qq/gq→ZZ and gg→ZZ ‣ Integrated luminosity uncertainty 3.7% ‣ Theory uncertainty 15%

Normalize top production to MC expectation ‣ Verified in control region ‣ Theory normalization uncertainty 10%

Normalize Z+jets production using control regions ‣ Z(→ll) + μμ/ee (no isolation/impact parameter requirements) → separate components (heavy flavour, fakes, electroweak) → extrapolated to signal region; uncertainties 20 - 40%

Sensitive for low and high masses.Expected limit :

≥1.05×σSMExclude SM Higgs :

192 < mH < 196 GeV214 < mH < 222 GeV

un-tagged

Candidate event with a Z decay to muon pairs,

recoiling against missing-ET. The muon candidates have transverse momenta of 50 and 126 GeV and a dimuon invariant mass of 94 GeV; the missing ET is measured to be 161 GeV.

Candidate event with 4μ.m12 : 90.6 GeVm34 : 47.5 GeVm4μ : 143.6 GeV

Summary: ‣ H→ZZ(*)→llll covers a large mass range from 110 to 600 GeV. ‣ H→ZZ →llνν and →llqq complement the H→ZZ(*)→llll channel by substantially improving the sensitivity in the high mass region above 200 GeV. ‣ The combined exclusion region of the 3 channels, H→ZZ(*)→llll, llqq and llνν, is:⇢192 < mH < 196 GeV, 214 < mH < 222 GeV, and 340 < mH < 460 GeV.

‣ H→ZZ(*) searches are supplemented by searches in the other decay channels such as H→WW, bb, ττ, γγ. When combined altogether, the excluded mass range extends to:⇢146 < mH < 232 GeV, 256 < mH < 282 GeV, and 296 < mH < 466 GeV.

Background Control Region:

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top eμ

top mll sidebands with b-tags(60, 150 GeV) not in mZ window

η distribution of the lepton candidates surviving selection criteria in data and MC.

pT distributions of the lepton candidates surviving selection criteria in data and MC.

Scatter plot of the invariant mass of leading and sub-leading di-lepton pair candidates (m12 and m34) surviving selection criteria in

data and MC.

4l invariant mass of candidates surviving selection criteria in data and MC.

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Δφll for events with exactly two oppositely charged electrons or muons and after mass window cut.

MET of events with exactly two oppositely charged electrons or muons.

Di-lepton invariant mass distribution of selected

candidates surviving selection criteria in data and MC.

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Relatively low branching ratios, especially for llll.H→ZZ final states: ‣ llqq ‣ llνν ‣ llll

Standard Model Higgs Production σ×BR

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Standard Model Higgs Production Cross Sections

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LHC Delivered

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ATLAS - A Toroidal LHC ApparatusATLAS - A Toroidal LHC Apparatus

Tracking ‣ Pixel, Silicon detector, Transition Radiation Tracker; in a solenoidal magnetic field

Calorimetry‣ Liquid Argon + Scintillators

Muon Spectrometer‣ Cathode Strip Chambers, Monitored Drift Tubes, Resistive Plate Chambers, and Thin Gap Chambers; in a toroidal magnetic field

Trigger‣ A complex trigger system determines the events that are recorded, 3 levels. → Level 1 has a < 2.5 μs decision time

Magnets‣ 2T solenoid and 3 air-core toroids

More than 2.55fb-1 of data collected!Data taking efficiency: ~96%

Tracking ‣ Pixel, Silicon detector, Transition Radiation Tracker; in a solenoidal magnetic field

Calorimetry‣ Liquid Argon + Scintillators

Muon Spectrometer‣ Cathode Strip Chambers, Monitored Drift Tubes, Resistive Plate Chambers, and Thin Gap Chambers; in a toroidal magnetic field

Trigger‣ A complex trigger system determines the events that are recorded, 3 levels. → Level 1 has a < 2.5 μs decision time

Magnets‣ 2T solenoid and 3 air-core toroids