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Standard Model Higgs Searches at LHC. Suyong Choi Korea U. SM Higgs Production and Decay. SM Higgs Production Cross Sections at 7 TeV. SM Higgs Production Cross Sections at 14 TeV. Branching Fractions. SM Higgs . Sensitivity depends on Backgrounds Mass resolution.

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sm higgs
SM Higgs
  • Sensitivity depends on
    • Backgrounds
    • Mass resolution

More info in https://twiki.cern.ch/twiki/bin/view/LHCPhysics/CrossSections

sm higgs search c hannels
SM Higgs Search Channels
    • – good mass resolution
    • - clean, good mass resolution
    • - not clean, worse mass resolution
    • – statistics
    • – statistics, clean
    • - clean, good mass resolution
  • Overall, they are very complicated analyses
slide11

Boosted selection

1 jet pT>150 GeV

VBF selection

slide16

Event categories divided into

    • 2 classes where the smallest of two photons is less or greater than 0.94
    • 2 classes where the largest is in endcap or barrel
    • Total of 2x2=4 classes

Mass resolution for

slide17

Excess:

SM signal x5

consistency
Consistency
  • P-value - Probability that background to produce fluctuation as large as observed

2.3

@123.5 GeV

upper limit
Upper Limit

Data disfavors Higgs in 127 – 131 GeV @ 95% CL

atlas
ATLAS

Mass resolution

1.7 GeV

atlas1
ATLAS
  • 114 – 115, 135-136 GeVexcluded @ 95% CL
slide24

ZZ selection

    • A second lepton pair:
    • for 4e, 4
    • Two sets of cuts for low-mass and high-mass Higgs
  • Signal efficiencies
slide26

Backgrounds

    • Reducible - , ,
    • Irreducible -
    • All derived from data

72 observed

exepected

Theory:

low mass region
low mass region
  • 13 events observed
  • expected
  • No significant excess
limits from
Limits from

340~465 GeV

180~305 GeV

134~158 GeV

expected exclusion: 130-160 GeV, 182-420 GeV

atlas2
ATLAS

71 events observed

629 events expected

Below 180 GeV,

8 events observed

9.31.5 events expected

2e2μ events (m=123.6 GeV, m=124.3 GeV), one 4μ event(m=124.6 GeV)

atlas4
ATLAS

135 – 156 GeV

excluded

181-234 GeV

excluded

255-415 GeV

excluded

further selections
Further selections
  • mass-dependent selection
    • , , ,
yields after signal selection
Yields after signal selection
  • Experimental uncertainties only
  • Signal efficiency uncertainty ~ 20%
  • Background uncertainty in signal region ~ 15%
limits
Limits

129-270 GeV Excluded @ 95%CL

127-270 GeV expected exclusion

atlas7
ATLAS
  • 2.05 fb-1

110 events observed

9110 expected

If Higgs of certain mH existed

slide40

145 – 206 GeV excluded @ 95% CL

    • Excpected exclusion: 134 – 200
slide42

Dilepton trigger

  • Veto events with 3rd lepton
  • Cuts to reject Fake Missing ET
  • Final selection
    • MET cut – mass dependent
    • MT
backgrounds
Backgrounds
  • MET modelingusing events
    • reweighting according to n-jets, boson pT
    • Less reliance on MC simulation
  • Data driven methods to estimate non-resonant backgrounds
    • Top pair, single top, WW, W+jets,
limits1
Limits

270-440 GeV excluded at 95% CL

slide49

Global p-value 1.9 with LEE in 110~145 GeV

0.6 with LEE in 110~600 GeV

consistency with background only hypothesis
Consistency with Background only hypothesis
  • 3.6 excess
    • : 2.8
    • ZZ*: 2.1
    • WW*: 1.4
  • With LEE
    • 3.6→2.3
    • 7% to observe excess in
    • ~30% to observe excess in ZZ
  • SM expectationis 2.4 for 126 GeV Higgs

1.9x10-4

combined atlas sm higgs exclusion limits
Combined ATLAS SM Higgs Exclusion Limits

95% exclusion limits:

112.7 - 115.5 GeV

131 – 237 GeV

251 – 453 GeV

Expected 95%CL

exclusion:

124.6 – 520 GeV

99% exclusion limits:

131 – 230 GeV

260 – 437 GeV

summary and outlook
Summary and Outlook
  • Atlas and CMS data narrowed the allowed mass range for SM Higgs
    • ATLAS : 115.5 – 131 GeV
    • CMS : 114 – 127 GeV
  • 20 fb-1 more data per experiment in 2012 allows 5 observation per experiment at mH=125 GeV
dataset
Dataset

Lumi

Uncertainty

4.5%

Good data up to 4.7 fb-1 used in the updated analyses

slide68

Background modeling

    • MC simulation of background was not used for background estimation, but in agreement with data
    • 30% non-prompt photons
    • 5th order Bernstein polynomial fitted to the
      • Maximize sensitivity
slide69

Signal

    • in 5 GeV steps (9 mass points)
    • POWHEG NLO + PYTHIA
    • Higgs reweighted to NNLL+NLO
      • Using HqT program
  • Fine corrections to photon energies
    • Intercalibration
    • Transparency corrections
    • Improves resolutions by 10%
slide70

Diphoton trigger

    • Asymmetric ET thresholds
    • complementary photon quality selections
    • 100% trigger efficiency
  • Photon energy corrected for conversions upstream of Electromagnetic calorimeter
    • Boosted decision tree regression trained on MC samples
slide71

Vertex location

    • Mean number of pp interactions ~ 9.5
    • RMS spread in beam direction ~ 6 cm
    • 10mm accuracy in vertex location ensures that energy resolution is not spoiled
  • Identifying the correct vertex
    • Kinematic properties of tracks emerging from the vertex and their correlation with diphoton kinematics
      • Sum of track , momentum balance
    • Converted tracks point to vertex
  • 3% gain in efficiency
slide72

Photon kinematic selection

    • ,
    • , excl. barrel-endcap transition
  • Backgrounds
    • Irreducible
    • Fakes: , dijet
slide73

Photon isolation

    • Energies in Ecal and Hcal – affected by pile up
      • Estimate effect of pileup in the event by average energy density away from jets
    • charged tracks around the photon candidate – fake vertex allows non-isolated photon to appear isolated
      • Calculate track isolation w.r.t. vertex that maximizes it
  • Photon quality
    • H/E
    • Transverse width of a photon shower
    • Electron track veto (E/p)
slide74

Dividing photon candidates

    • Different S/B for photons of different criteria
    • Barrel vsEndcap
      • Barrel photon has less QCD background
      • Energy in a 3x3 crystals around highest energy / supercluster energy
      • Photons with large have less probability to have converted
slide75

Photon ID efficiencies

    • Measured using , excluding track veto eff.
slide77

3 channels – 4e, 4, 2e2

  • Covers 110 – 600 GeV
  • Used 4.7 fb-1
  • Triggers
    • Dilepton triggers with asymmetric thresholds of pT>8, 17 GeV
slide78

Offline

    • Electrons pT>7 GeV, , (90% for
    • MuonspT>5 GeV, , 98% efficient
    • Small impact parameter significance<4
    • Z1: lepton pair with mass closest to mZand
slide79

2 leptons + MET

    • ee, e, 
    • 1 or 2 high pT leptons in the trigger
      • 97~99% efficiency for signal of mH=160 GeV
    • 0, 1, 2 jet categories considered
offline selection
Offline Selection
  • Offline
    • Lepton pT 20 GeV, 10(15) GeV for e(ee,), Consistent with coming from Vertex
    • Jets ,
    • Projected missing ET>20(40) e(ee,)
    • Azimuthal opening angle dilepton-leading jet < 165 degrees (ee,)
    • Dilepton mass cut
      • Remove low mass resonances, Z
    • Reject events where jets tagged with soft leptons or large impact parameter tracks
      • Remove top events
    • Reject events with 3rd isolated lepton
      • Remove ZZ, WZ
    • Identify converted photons to reject
background estimation
Background estimation
  • Mostly data driven
    • Apply antiselection, then extrapolate to signal region
    • W+jets, QCD multijets
    • ,
    • – select events
    • Statistics of control sample limits background estimate error
slide85

1 jet

0 jet

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