First results on Higgs boson searches (SM + MSSM) and ... · First results on Higgs boson searches (SM + MSSM) and prospects from ATLAS Johannes Elmsheuser Ludwig-Maximilians-Universit

First results on Higgs boson searches (SM+ MSSM) and prospects from ATLAS

Johannes Elmsheuser

Ludwig-Maximilians-Universitat Munchen, Germany

on behalf of the ATLAS Collaboration

01 June 2011

23rd Rencontres de Blois, Particle Physics and Cosmology

ATLAS Detector and Datasets

General purpose detector

Magnets: 2T solenoid 3 air-core toroids EM Calo.: sampling LAr technologyTracking: silicon + transition Hadron Calo.: plastic scintillator (barrel)

radiation tracker LAr technology (endcap)Muon: independent system

Johannes Elmsheuser (LMU Munchen) First ATLAS Higgs results 01/06/2011 2 / 19

Datasets

2010:

Peak Lumi.: L ≈ 2 · 1032 cm−2s−1

Int. Lumi.:∫Ldt ≈ 45 pb−1

2011:

L ≈ 1.2 · 1033 cm−2s−1∫Ldt & 400 pb−1


Search for the Higgs boson

ATLAS measured most of knownStandard Model (SM) processes with∼ 40pb−1 of data collected 2010

SM Higgs boson sensitivity will bereached soon


Higgs Productions at the LHC

Gluon fusion:

• known at NNLO (theo. uncert. O(15%))

Vector boson fusion:

• known at NLO (theo. uncert. O(5%))

• distinctive experimental signature: 2forward jets and rapidity gap

Intermediate to high mH range:

• WW and ZZ most sensitive channels

Low to intermediate mH range:

• γγ: very clean but small BR

• ττ : VBF to reduce BG

• bb: huge QCD BG, some potential inassoc. prod.


Intermediate mH: H → WW (∗) → `ν `ν

• Most sensitive channel in the mH = 130-190 GeV range

• large event yields and clean signature by lack of mass resolution

• Main backgrounds: WW, di-bosons (WZ/ZZ/Wγ), W/Z+Jets, Drell-Yan, top, QCD

Common pre-selection:

• 2 isolated high pT leptons

• suppress low mass resonances with lowerbound cut on m``

• Z0 veto (|m`` −mZ | > 10 GeV)

• E/T

• transverse plane angle ∆φ`` cut

Exclusive analysis in bins of jet multiplicities:

• 0-jets: optimized for gluon-fusion, purest channels, least affected by top BG

• 1-jet: more affected by top BG

• 2-jets: optimized for VBF (tag jets in opposite hemispheres, rapidity gap), small signalexpected


H → WW (∗) → `ν `ν Background estimationData driven techniques to estimate each contribution• Main backgrounds estimated in control regions, extrapolated into signal region• Cross-contamination of different backgrounds in various control regions taken into account

• WW background from side-bands in m`` and mT :

• small mH → high m`` WW dominated

• large mH → low m`` WW dominated

• Top: estimate jet-veto efficiency in enriched sampleswith b-tagging

• W+jets:

• kinematic from control sample with relaxedlepton ID

• lepton fake probability from independent

jet-trigger sample

• Z+jets: background estimated using ABCD method inm`` - MET plane

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H → WW (∗) → `ν `ν Results

• Analysis optimized in each jet-bin and for each mass hypothesis

• final discriminating observable: transverse mass mT =√

(E ``T + EmissT )2 − (P``T + Pmiss

T )2

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PreliminaryATLAS

mH=170 GeV 0-Jets 1-Jet 2-JetsData 3 1 0BG 1.8±0.1 1.2±0.1 0.02±0.01

Higgs 1.26±0.02 0.6±0.01 0.6±0.01

Most sensitive around mH=160 GeV for SM-like Higgs boson


H → ZZ ∗ → 4 leptons

Clean unambiguous signature, low background, robust against pileup but small event yield dueto low Z0 leptonic BR, golden channel to measure mH

Simple selection:

• two pairs of opposite sign and same flavor leptons pT>20 GeV

• consistent with 2 Z0 decays

• isolated leptons without significant impact parameter

for mH ≈ 200 GeV sensitivity comparable with other channels due to clean signature


High mH: H → ZZ → ``(qq/νν)

Much less clean topologies compared to 4` channel, but a factor ∼27 higher BR (for ``qq)More sensitive than 4` for large mH when background from W/Z+jets get smallBackgrounds:

• Z/W+jets, tt control regions from sidebands in mjj and m`` and reversed cuts,backgrounds estimated with MC and normalization checks from control regions

• Background from QCD multi-jets negligible (checked with data-driven techniques withrelaxed lepton-ID)

• Di-boson background (ZZ/WW/WZ) estimated from MC

Analysis optimized for low and high mH :llqq llqq

mH<360 GeV mH>360 GeVData 216 11BG 226 ±28 9.88 ± 1.75

Higgs 0.60±0.12 0.24±0.05(mH=200 GeV) (mH=400 GeV)

llνν llννmH<280 GeV mH>280 GeV

Data 5 5BG 5.76 ± 1.38 3.45 ± 0.86

Higgs 0.19±0.04 0.30 ± 0.06(mH=200 GeV) (mH=400 GeV)

no excess observed in data

Cross section limits at ∼10 x σSMJohannes Elmsheuser (LMU Munchen) First ATLAS Higgs results 01/06/2011 10 / 19

Low mH search: H → γγ

• SM prefers light Higgs Boson, mH<158GeV

• H → γγ has small BR (∼0.2%), butlargest event yield after all cuts at low mH

• Expect 25 events/fb−1 at√s = 7 TeV

(includes all efficiencies)

Backgrounds:

• irreducible: γγ

• reducible: γ-Jet, Jet-Jet ( handles: massresolution, photon-ID, isolation)

Experimental requirements:

• excellent mass resolution

• precise primary vertex reconstruction

• photon pointing and conversions tracks


H → γγ results

Backgrounds estimated with data-driven techniques:

Exclusion limits:

• mH=127 GeV: 8 x σSM

• mH=116 GeV: 38 x σSM

• Sensitivity close to currentTevatron limits


H → γγ results, 2011 update

2011: 2011:

2010+2011:


SUSY Higgs: h/H/A→ ττ

• MSSM Higgs sector: 5 bosons h, H, A, H+, H− with 2 parameters mA, tanβ at LO• Higgs coupling to b, τ enhanced for high tanβ → Search for: h→ ττ (`τhad3ν,eµ4ν)• Backgrounds: Z → ττ , (Z → ``, W+jets (`h), tt(eµ), QCD)• Use of data-driven techniques to predict QCD and W+jets BG from same sign data.• Main background Z → ττ estimated from MC and shape validated with embedding

technique using data

mττ (visible) after all cuts:

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Others(OSSS)

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τµ + hadτe

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Data Total BG h/H/Atan β = 40

mA=120 GeV`τhad 206 206±35 55.3 ±0.9eµ 70 63±7 15.2±0.3

Limits in (mA, tanβ) plane:

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ATLAS Preliminary

)1

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LEP2Observed (CLs)

Observed (CLs+b, PCL)

Expected (95%CL)

σ 1±σ+ 2

extends Tevatron exclusion region


Light CP-odd Higgs boson in µµ final state

• In NMSSM: additional singlet complex field leads to additionalCP-even and odd Higgs

• For low CP-odd masses (ma1 < 2mb) lightest CP-even Higgs avoidsLEP limits

• Search for direct production in 6-9 GeV and 11-12 GeV mass range,avoiding Υ resonances

Selection:

• two isolated muons with pT>4 GeV

• Likelihood-ratio selection on primaryvertex χ2/ndf and calorimetric isolation,PDFs derived from data

No significant excess in data, limitson σxBR


Combination for SM Higgs boson search (I)

Combination of SM Higgs boson decay channels for mH = 110 - 600 GeV:H → γγ, H → ZZ (∗) → ````, H → ZZ → ``νν, H → ZZ → ``qq,H →WW (∗) → `ν`ν, H →WW → `νqq

Individual limits:

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Limits PCL 95% CL

ATLAS Preliminary

S.M. Expect. Observed

Combination:

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Observed, PCL

Observed, before PCL

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ATLAS Preliminary

mH= 160 - 170 GeV expected exclusion 2.3 x σSM


Combination for SM Higgs boson search II

CLs limits and other experiments:

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LEP exclusion

Observed, PCLExpected, PCL

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SExpected, CL

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4th generation exclusion:(with 4th generation of high massquarks and leptons with SM-likecouplings to the Higgs boson)

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Expected PCL

PCLσ 1± PCLσ+ 2

Observed CLsExpected CLs

CMS exclusionTevatron exclusion

generation modelth

4

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L dt ~ 35 pb∫Excluded

ATLAS Preliminary

Exclusion for m4thH = 140 - 185 GeV


Summary and Future perspectives

• ATLAS detector performs very well, but no hint for Higgs at LHCwith ∼40 pb−1 yet: inclusive and simple cut-based selections

• 1-3 fb−1 are expected in 2011 (∼ 1fb−1 until June) and more in 2012→ ATLAS will be able to make a statement about the SM Higgsboson existence over a large mH range

with <4 fb−1 we could exclude down to LEP limit, but hopefully we willnot do so!


Documentation

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/

H → γγ ATLAS-CONF-2011-071, ATLAS-CONF-2011-025H → ZZ ATLAS-CONF-2011-048, ATLAS-CONF-2011-026H →WW ATLAS-CONF-2011-052, ATLAS-CONF-2011-005h/H/A→ ττ ATLAS-CONF-2011-024CP-odd Higgs boson in µµ ATLAS-CONF-2011-020Charged Higgs ATLAS-CONF-2011-018, ATLAS-CONF-2011-051

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome

SM Higgs Combination ATLAS-HIGG-2011-01-002, CERN-PH-EP-2011-076


https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome

BACKUP

High mH: H → WW → `νqq

• Most sensitive channel whenmH>400-500 GeV

• Cross section limits at ∼10-20 xσSM

SUSY Higgs: H± → τhν

Hadronic tau, hadronic W:

• bτhadνbqq ie. 1 τ -jet, 2b, 2q,2ν

• largely data driven SM estimates

• good agreement so far

Hadronic tau, leptonic W:

• bτhadνb`ν ie. 1 τ -jet, 2b, 1`, 3ν

• largely data driven SM estimates

• good agreement so far

SUSY Higgs: H± → τe/µν

Leptonic τ , hadronic W:

• bτe/µνbqq ie. 1 ` , 2b, 2q, 3ν

• SM estimates from MC only

• Distributions of discriminatingvariables

• fair agreement with SM

• signal alters the distributions

• more luminosity needed todistinguish

Leptonic τ , leptonic W :

• bτe/µνb`ν ie. 2` , 2b, 4ν

• similar plots (not shown)

• exclusion plots for 1 fb−1 MCstudy

Documents

First results on Higgs boson searches (SM + MSSM) and ... · First results on Higgs boson searches (SM + MSSM) and prospects from ATLAS Johannes Elmsheuser Ludwig-Maximilians-Universit