Summary

Searches for SM Higgs with Searches for SM Higgs with ATLAS experiment at LHCATLAS experiment at LHC

E. Meoni (Università della Calabria – INFN)On behalf of the ATLAS Collaboration

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SummarySummaryThe ATLAS detector at LHC

The SM Higgs physics: - the experimental limits on the Higgs mass - the production and the decay at LHC

Study of the main channels: - H - H (in VBF production) - H ZZ 4l - H WW

Significance

Conclusions

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The ATLAS Detector at LHCThe ATLAS Detector at LHC

•Solenoid in the inner cavity

•Inner Detector Si pixels + TRT for particle identification Solenoid B = 2T R=12-18m, Z=60-580m

σ/pT~5x10-4pT⊕0.01

•3 air-core toroids

•Muon SpectrometerOptimal performance also standaloneσ/pT~2%@50GeV÷10%@1TeV ( ID+MS)

The LHC will collide protons at √s = 14 TeV(at the start-up √s = 10 TeV)

24 m

45 m

Nominal LHC parameter

Beam Energy (TeV) 7.0

Luminosity (cm2s-1) 1034

Bunch Crossing (ns) 25

Number of particles per bunch

1.15 1011

Number of bunches per beam

2808

Dipole field (Tesla) 8.33ATLAS is one of the two general-purpose LHCexperiments mainly aimed to study the origin ofthe electroweak symmetry breaking•Electromagnetic calorimeter

Pb-liquid Argon σ/E~10%/√E⊕0.007

•Hadronic calorimeter Fe-scint + Cu-liquid Argon(10λ) σ/E~50%/√E⊕0.03

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Experimental limits to Higgs Experimental limits to Higgs massmass

An indirect constraint is obtained performinga fit with the electroweak parameters measured with high precision at LEP, Tevatron and SLAC:

MH<154 GeV/c2 (95% C.L.)

From the direct research at LEP2 (result obtained by combining the data of the four LEP experiments):

MH>114.4 GeV/c2 (95% C.L.)

http://lepewwg.web.cern.ch/LEPEWWG/

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The SM Higgs production at The SM Higgs production at LHCLHC

A. Djouadi Phys.Rept.457:1-216

gluon-gluon Fusion

Vector Boson Fusion(VBF)

Higgs-strahlung

Associated Production

gluon-gluon Fusion is the dominant process, Vector Boson Fusion (VBF) is ~10% of the total cross-section for MH<2MZ, but it has a clear signature in the detector due to the presence of two high pT jets in the forward region.

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The SM Higgs decay at LHCThe SM Higgs decay at LHCFor MH<130 GeV, :H (mainly in gg prod, studied also VBF and ass. prod. Golden channel in this region)H (in VBF prod) (dominant decay mode, large QCD background, studied in associated prod., complex final state)For MH>130 GeV:

H WW(*) ll (in gg or VBF prod)H WW(*) lhad (in VBF prod)H ZZ(*) 4 lep (in gg prod. Golden channel for MH>2MZ)

A. Djouadi Phys.Rept.457:1-216

What is the latest ATLAS framework to the Higgs studies?• New Monte Carlo generators also for background (MC@NLO, ALPGEN, HERWIG, PYTHIA, ...)• Detailed GEANT4-based simulation of the ATLAS detector • Detailed event reconstruction (based also on test beam results).

bb H

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H H Background : – Irreducible: pp , pp +jets– Reducible: pp +jets, pp jets , Drell-Yan

Search for two isolated high pT photons

Experimental requirements:– Excellent electromagnetic energy resolution;– High photon efficiency and strong 0/jet rejection (10-3÷10-4 to get σj+σjj<< σ)– Good -vertex reconstruction -Good efficiency reconstruction of converted photons (~57% of the events with at least one converted photon)

It will be measured from data sidebands

Preliminary

Inclusive H+ 2j (VBF mainly) WH)H,( tt

Preliminary

Preliminary

Preliminary

H+ ET

miss+1lept

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The distinctive VBF signature: 1. Two forward “tag” jets (large separation with high pT) with large Mjj,2. No jet activity in the central region (due to no colour flow between tag jets)

Typical selection for VBF channels: 2 high pT jets with large separation, Mjj>0.5-1 TeV, Higgs decay products between tag jets in central, jet veto in the central region.

Experimental issues for VBF channels: good efficiency reconstruction for forward jet; robustness of the jet veto with respect to radiation in the underlying event and to pile-up.

VBF H VBF H (1)(1)

distribution for the highest pT jet

Efficiency of central jet veto at different pileup level

Preliminary

Preliminary

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VBF H VBF H (2)(2)The 3 topologies : qqH → qq , → l-l, l-had, had-had

Dominant backgrounds :

Specific issue of the channel:very good jet identification to have a low rate of fake from jets. Good ET

miss measurement for mass resolution of tau-pairs Analysis:The H mass can be reconstructed using the collinear approximation: mass neglected and assumed the direction coinciding with thevisible decay products of the .

Data-driven control samples are being explored for many backgrounds

tttt

ll

jets, jets,W

jets, Z jets, Z

HiggsZ bkgQCD bkg

Preliminary

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The “golden channel”: observation of a narrow peak on a continuous background.

Backgrounds:

Experimental issues: safe lepton identification and very good energy measurement

H H ZZ ZZ 4l4l

Estimate background from sidebands

Preliminary

masses low at X Z ZW, , ,bZb :Reducible–

dominant *ZZqq/gg :eIrreducibl– )(

tt

Selection: Search for isolated muon and electron pairs with opposite charge, cuts on isolation variables, lepton track impact parameter (to reject reducible bkg), cuts on MZ,, reconstruction of M4l

Track isolation

Impact parameter

Preliminary

Preliminary

Preliminary

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H H WW WW Studied topologies: H + 0 jets lνlν (dominated by gluon fusion) H + 2 jets lνlν or H + 2 jets lνqq (dominated by VBF)

Main backgrounds:

For H WW ll : - no mass peak, use transverse mass (MT) - select events with exactly 2 isolated leptons and ET

miss and jet veto - likelihood fit using MT, the transverse lepton opening angle ll and transverse momentum pT

WW

Need precise knowledge of backgroundDevelop data driven methods

Results obtained with the ATLAS fast simulation code (2004)

The new results obtained with the latest detailed simulation studies(including significance) will beavailable soon

H + 2 jets eνν (VBF channel) (mH=120 GeV)

,W WW, ttt

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Higgs SignificanceHiggs Significance

Combined significance results are in preparation (after finalizing WW analysis).They will be availablebefore the end of the year

Combining variousanalysis strategies(inclusive,H+2j, etc)

Without systematic uncertainties

VBF H H

H ZZ 4lPreliminary

PreliminaryPreliminary

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ConclusionsConclusionsThe Large Hadron Collider will start collisions in the 2009 Spring

ATLAS is well set up to explore the existence of a Standard Model Higgs boson

Many SM Higgs channels have been studied in detail with new MC generators, detailed apparatus simulation and reconstruction codes

The full Standard Model mass range can be covered and a good sensitivity could be achieved with few fb-1

(with a detector well calibrated and after background understanding)