Standard model higgs searches at lhc
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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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Standard Model Higgs Searches at LHC

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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

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


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


  • SM Higgs searches at CMS and ATLAS


    CMS SM Higgs Channels


    ATLAS SM Higgs Channels


    Boosted selection

    1 jet pT>150 GeV

    VBF selection


    • W/Z+H


    • 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


    Excess:

    SM signal x5


    Consistency

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

    2.3

    @123.5 GeV


    Upper Limit

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


    ATLAS

    Mass resolution

    1.7 GeV


    ATLAS

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


    • ZZ selection

      • A second lepton pair:

      • for 4e, 4

      • Two sets of cuts for low-mass and high-mass Higgs

    • Signal efficiencies


    • Higgs mass resolutions


    • Backgrounds

      • Reducible - , ,

      • Irreducible -

      • All derived from data

    72 observed

    exepected

    Theory:


    low mass region

    • 13 events observed

    • expected

    • No significant excess


    Limits from

    340~465 GeV

    180~305 GeV

    134~158 GeV

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


    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)


    ATLAS


    ATLAS

    135 – 156 GeV

    excluded

    181-234 GeV

    excluded

    255-415 GeV

    excluded


    Further selections

    • mass-dependent selection

      • , , ,


    Yields after signal selection

    • Experimental uncertainties only

    • Signal efficiency uncertainty ~ 20%

    • Background uncertainty in signal region ~ 15%


    Limits

    129-270 GeV Excluded @ 95%CL

    127-270 GeV expected exclusion


    ATLAS


    ATLAS


    ATLAS

    • 2.05 fb-1

    110 events observed

    9110 expected

    If Higgs of certain mH existed


    • 145 – 206 GeV excluded @ 95% CL

      • Excpected exclusion: 134 – 200


    • Dilepton trigger

    • Veto events with 3rd lepton

    • Cuts to reject Fake Missing ET

    • Final selection

      • MET cut – mass dependent

      • MT


    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,


    Limits

    270-440 GeV excluded at 95% CL


    CMS Combination


    Expected exclusion: 117 – 543 GeV


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

    0.6 with LEE in 110~600 GeV


    CMS Combined Higgs Exclusion Limits


    Atlas Combination results


    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

    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

    • 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


    backup


    Dataset

    Lumi

    Uncertainty

    4.5%

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


    Backgrounds


    WW Selection event yields


    • 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


    • 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%


    • 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


    • 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


    • Photon kinematic selection

      • ,

      • , excl. barrel-endcap transition

    • Backgrounds

      • Irreducible

      • Fakes: , dijet


    • 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)


    • 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


    • Photon ID efficiencies

      • Measured using , excluding track veto eff.


    Systematic Uncertainties in


    • 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


    • 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


    • 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

      • 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

    • Mostly data driven

      • Apply antiselection, then extrapolate to signal region

      • W+jets, QCD multijets

      • ,

      • – select events

      • Statistics of control sample limits background estimate error


    WW+0 jet baseline selection


    WW+1-jet baseline selection


    1 jet

    0 jet


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