Standard Model Higgs Searches at LHC

Suyong ChoiKorea 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

VBF selection Boosted selection1 jet pT>150 GeV

𝐻→𝜏 τ

𝐻→𝑏𝑏• W/Z+H

𝐻→𝑏𝑏

𝑯→𝜸𝜸

𝐻→𝛾𝛾• 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

𝐻→𝛾𝛾

SM signal x5

Excess:

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 resolution1.7 GeV

ATLAS • 114 – 115, 135-136 GeV excluded @

95% CL

𝑯→𝒁 𝒁∗→𝟒ℓ

𝐻→𝑍 𝑍∗→4 ℓ• ZZ selection– A second lepton

pair: – for 4e, 4– Two sets of cuts for

low-mass and high-mass Higgs

• Signal efficiencies

Channel4e 49% 59%

2e2 61% 71%4 78% 82%

𝐻→𝑍 𝑍∗→4 ℓ• Higgs mass resolutions

Channels4e 2.7 GeV 3.5 GeV

2e2 2.1 GeV 2.8 GeV4 1.6 GeV 2.5 GeV

𝐻→𝑍 𝑍∗→4 ℓ• Backgrounds– Reducible - , , – Irreducible - – All derived from

data72 observed exepected

Theory:

low mass region

Chan-nels

Ex-pected

Ob-served

4e 1.7 32e2 4.5 54 3.3 5

• 13 events ob-served

• expected

• No significant ex-cess

𝐻→𝑍 𝑍∗→4 ℓ

Limits from

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

134~158 GeV 180~305 GeV

340~465 GeV

ATLAS 71 events observed629 events expected

Below 180 GeV,8 events observed9.31.5 events expected

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

ATLAS

ATLAS

135 – 156 GeVexcluded

181-234 GeVexcluded

255-415 GeVexcluded

𝑯→𝑾𝑾 ∗→𝟐ℓ𝟐𝝂

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%CL127-270 GeV expected exclusion

ATLAS

ATLAS

ATLAS • 2.05 fb-1

110 events observed9110 expected If Higgs of certain mH existed

𝐻→𝑊𝑊→2 ℓ2𝜈• 145 – 206 GeV excluded @ 95% CL– Excpected exclusion: 134 – 200

𝑯→𝒁 𝒁∗→𝟐 ℓ𝟐𝝂

𝑯→𝒁 𝒁∗→𝟐 ℓ𝟐𝝂• Dilepton trigger• Veto events with 3rd lepton• Cuts to reject Fake Missing ET• Final selection–MET cut – mass dependent–MT

Backgrounds• MET modeling using

events– reweighting according

to n-jets, boson pT– Less reliance on MC

simulation• Data driven methods

to estimate non-res-onant backgrounds– Top pair, single top,

WW, W+jets,

𝑯→𝒁 𝒁∗→𝟐 ℓ𝟐𝝂

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 Lim-its

ATLAS COMBINATION RE-SULTS

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 Exclu-sion Limits

95% exclusion limits:112.7 - 115.5 GeV131 – 237 GeV251 – 453 GeV

Expected 95%CL exclusion:124.6 – 520 GeV

99% exclusion limits: 131 – 230 GeV260 – 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

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

LumiUncertainty4.5%

Backgrounds

WW Selection event yields

𝐻→𝛾𝛾• 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

𝐻→𝛾𝛾• 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%

𝐻→𝛾𝛾• 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

𝐻→𝛾𝛾• Vertex location

– Mean number of pp interactions ~ 9.5– RMS spread in beam direction ~ 6 cm– 10mm accuracy in vertex location ensures that energy reso-

lution 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

𝐻→𝛾𝛾• Photon kinematic selection– , – , excl. barrel-endcap transition

• Backgrounds– Irreducible – Fakes: , dijet

𝐻→𝛾𝛾• 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)

𝐻→𝛾𝛾• Dividing photon candidates– Different S/B for photons of different cri-

teria– Barrel vs Endcap• Barrel photon has less QCD background

• Energy in a 3x3 crystals around highest en-ergy / supercluster energy• Photons with large have less probability to

have converted

𝐻→𝛾𝛾• Photon ID efficiencies

–Measured using , excluding track veto eff.

Systematic Uncertainties in

𝐻→𝑍 𝑍∗→4 ℓ• 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

𝐻→𝑍 𝑍∗→4 ℓ• Offline– Electrons pT>7 GeV, , (90% for –Muons pT>5 GeV, , 98% efficient– Small impact parameter significance<4– Z1: lepton pair with mass closest to mZ

and

𝑯→𝑾𝑾 ∗→𝟐ℓ𝟐𝝂• 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 com-ing 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 back-

ground estimate error

WW+0 jet baseline selection

WW+1-jet baseline selection

𝑯→𝑾𝑾 ∗→𝟐ℓ𝟐𝝂ee+𝜇𝜇

𝑒𝜇

0 jet 1 jet