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INCLUSIVE STANDARD MODEL HIGGS SEARCHES WITH ATLAS Francesco Polci LAL Orsay On behalf of the ATLAS collaboration. SUSY08 – Seoul (Korea). THE HIGGS BOSON AT THE LHC. PRODUCTION. DECAY. gg fusion process is the more abundant, followed by the Vector Boson Fusion process.

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Inclusive standard model higgs searches with atlas francesco polci lal orsay

INCLUSIVE STANDARD MODEL

HIGGS SEARCHES

WITH ATLAS

Francesco Polci

LAL Orsay

On behalf of the ATLAS collaboration.

SUSY08 – Seoul (Korea)


Inclusive standard model higgs searches with atlas francesco polci lal orsay

THE HIGGS BOSON AT THE LHC

PRODUCTION

DECAY

gg fusionprocess is the more abundant, followed by theVector Boson Fusionprocess.

This talk is based on the results of the ATLAS CSC NOTE: update on the analysis techniques and discovery potentials.

Almost ready for publication.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

H→gg

g

H0

g

  • Important channel in the low mass region.

  • It gives the best mass resolution thanks to excellent electromagnetic energy resolution

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Inclusive standard model higgs searches with atlas francesco polci lal orsay

SELECTION

e

ATLAS

preliminary

pT (Gev/c)

pT (Gev/c)

e

ATLAS

preliminary

  • Trigger: at least 2 isolated photons, with pT >20 GeV/c each

  • e (respect to offline) = (93.6 ± 0.4)%

  • Identification cut exploiting the shower shape.

  • Fiducial cut: 0< |h| < 1.37 & 1.52 < |h| < 2.37.

  • Isolation cut: SpT < 4 GeV/c, considering all tracks with

  • pT >1GeV/c in aDR=0.3 cone around the

  • electromagnetic cluster.

  • Momentum cut: pT>25GeV/c and pT>40GeV/c for the two

  • most energetic photons.

Trigger efficiency for single photons

Selection efficiency:

e = 36.0 % (32.2% with pileup 1033cm-2s-1)

Within a mass window MH +/- 1.4GeV:

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

PRIMARY VERTEX RECONSTRUCTION

Photon 2

Photon 1

Middle

Strips

Calorimeter only

Calorimeter and tracker

Z1 Ztrue O Z2

preliminary

preliminary

zrec-ztrue (mm)

zrec-ztrue (mm)

The primary vertex identification is crucial for a precise measurement of M(gg).

Tracker and calorimeter informations are combined:

  • The calorimeter measure the g flight direction using longitudinal segmentation and pre-shower strips → vertex position accuracy is 19mm (17mm when using conversions).

  • Combining with the tracker information we get ~0.1 mm

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

CONVERSIONS

ATLAS

preliminary

With pileup!

  • Converted photons are an important issue: ~50% of the events have at least one converted photon!

    • energy reconstruction is degraded: ad hoc calibration required.

    • conversion vertex can be used to compute the direction

  • Even conversions with only one reconstructed track are taken into account.

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

BACKGROUNDS

Example: NLO g-jet processes

Within a mass window MH +/- 1.4GeV:

  • Background is evaluated with NLO simulations.

  • It will be measured fromdata sidebands.

Strategy for jet rejection:

  • Longitudinal segmentation of the calorimeter.

  • Fine segmentation of the first layer (h-strips) => good p 0 rejection.

  • Isolationof the electromagneticcluster.

  • Isolationbased on tracks reconstructed by the inner detector.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

INVARIANT MASSES DISTRIBUTIONS

From associated production, high levels of purity for signal achieved thanks to additional requirements.

ZH→ llgg

ATLAS preliminary

H+1jet

ATLAS preliminary

WH → lvgg

ATLAS preliminary

H+2jet

ATLAS preliminary

ATLAS preliminary

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

SIGNIFICANCE

g

q*

In Higgs rest frame

Higgs

g

  • Fit and likelihood ratio are used for setting discovery potential and handle systematics.

  • Different fit based approaches for the evaluation of the significance:

  • 1- fit only the mass distribution;

  • 2- simultaneous fit to the mass, PT(gg) and cosq* distributions

  • The two approaches are performed also in the Higgs mass floating configuration.

  • Use of categories with different resolutions based on , jet multiplicity and presence of conversion improves the significance.

  • PT(gg) it has softer spectrum and less pronounced rise at low values for background respect to the signal.

  • cosq* the photon decay angle in the H rest frame with respect to the H flight direction in the lab rest frame has an enhanced distribution for the background for collinear photons while is flat for the signal.

ATLAS preliminary

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Inclusive standard model higgs searches with atlas francesco polci lal orsay

H → ZZ(*) → 4l

It is the “golden channel”!

  • Observation of a clear peak on top of a smooth background!

  • Clear signature

  • Wide range of masses explored

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

EVENTS SELECTION

  • Trigger: - single isolated m (e) with pT >20 (25) GeV/c;

  • - two m (e) with pT >10 (15) GeV/c.

  • Kinematic: - 2 pairs of same flavor opposite charge lept.

  • - calorimeter identification

  • - |Mll1-MZ|<DM12and Mll2>M34

  • Fiducial cut: 0< |h| < 1.37 & 1.52 < |h| < 2.37.

  • Isolation cut: - Calorimeter: SET/pT < 0.23

  • - tracker:SpT/pT < 0.15

  • Vertexing cut on maximum lepton impact parameter:

  • d0/sd0<3.5 (6.0 ) for m (e)

Trigger efficiency

muons

ATLAS

preliminary

electrons

ATLAS

preliminary

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

BACKGROUNDS

Backgrounds are (|h|<2.7, pT>5 GeV ):

  • qq,gg  ZZ()4l (l=e,m,t) => 56 fb (NLO);

  • qq  Zbb4l => 810 fb (NLO );

  • qq  Zbb3l => 1.3 pb (NLO );

  • qq,ggtt => 833 pb (NLO);

  • qq,gg WZ3l => 386 fb (NLO);

  • Z2l+X => 4030 pb (NNLO)

Background will be estimated in sidebands low systematic uncertainties

ATLAS preliminary

  • Look to the Z with first data to understand lepton reconstruction and detectors response.

Mass resolution @ MZ 1.8 GeV

  • Zee mass peak is affected electron bremsstrahlung.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

ISOLATION & IMPACT PARAMETER

Calorimetric isolation

Track isolation

Reducible backgrounds have activity around leptons from b-decay

ATLAS preliminary

ATLAS preliminary

Normalized calorimetricand trackisolation (DR=0.2) for the signal (mH = 130) and the Zbb and tt backgrounds in the 4m channel.

H4m

H4e

Lepton from b-quark decay do not point towards primary vertex

ATLAS preliminary

ATLAS preliminary

Transverse impact parameter significance in signal and reducible background events.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

EFFICIENCY & RESOLUTION

Z on shell

Z off shell

ATLAS preliminary

ATLAS preliminary

Selection efficiency as a function of the Higgs mass, for each of the three decay channels.

H4m

H4e

ATLAS preliminary

ATLAS preliminary

Mass resolutionas a functionof the Higgs mass. Open circles denote the resolution Z mass constraint improves resolution.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

INVARIANT MASS DISTRIBUTIONS

ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

SIGNIFICANCE

  • Significance estimations with number counting and performing a full range fit are consistent.

  • Other approaches (background only sideband fit, two dimensional fit on Mlland MZ* with Higgs mass floated) are also explored.

Poisson probability

Preliminary

ATLAS preliminary

ATLAS preliminary

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

H → WW(*)

  • Interesting for 2MW<MH<2MZ where all other decay modes are suppressed.

  • Signature is 2m, 2e, em+ ETmiss.

  • Three channels:

  • - H→WW → e v m v (H+0jet)

  • - H → WW → e v m v

  • - H → WW → l v q q

  • (only for MH=300 GeV)

VBF (H+2jet)

  • Measure spin and CP properties possible for heavy H →WW →lvqq

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

SELECTION AND BACKGROUNDS

  • No mass peak  use transverse mass.

  • Reconstruction:

    • Trigger : single or double lepton selection 1m20i or 1e25i;

    • Offline: select events with exactly two isolated (tracking and calorimeter) opposite sign primary leptons and ETmiss.

    • Specific reconstructions for different channels

The challenge: precise knowledge of the backgrounds.

High backgrounds: WW, Wt, ttbar, Z2l, bb,cc,QCD multijet

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

EVENTS SELECTION: H+ 0jets

  • Fit the transverse mass and the transverse momentum of the candidates in two bins of the dilepton opening angle Df in the transverse plane;

  • Account for the ratio of the background in the two regions

  • Extract the signal and background mixture in the signal region.

preliminary

The transverse mass distribution in the regionDf<1.575 and pTWW>20 GeV.

Signal region

Control region

preliminary

ATLAS preliminary

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

EVENTS SELECTION FOR VBF

  • Two distinct signatures:

  • Two forward “tag” jets (large h separation with high-pT) with large Mjj

  • No jet activity in the central region (between the two tag jets): jet veto

  • Typical cuts require:

  • Tag jets are assumed to be the highest ETjets in opposite hemispheres, with ET>~40 GeV, Dhjj>~4, Mjj>500-1000 GeV.

  • Higgs decay products between tag jets in h

  • No additional jet activity in the event: central jet veto.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

SIGNIFICANCE

preliminary

preliminary

Preliminary

Expected significance from the H→WW+0j, WW→enmn channel for an integrated luminosity L= 10 fb-1.

Expected significance from the H→WW+2j, WW→enmn channel for an integrated luminosity L= 10 fb-1.

Francesco Polci

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SUSY08 – Seoul


Inclusive standard model higgs searches with atlas francesco polci lal orsay

COMBINATION OF THE CHANNELS

The median p-value for the SM Higgs Boson exclusion as a function of the Higgs mass for the various channels and for their combination.

ATLAS preliminary

ATLAS preliminary

Zooming…

ATLAS preliminary

The expected luminosity for the exclusion of a Higgs Boson with mass mH @ 95% CL

ATLAS IS WELL READY

TO CATCH THE HIGGS!

Francesco Polci

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SUSY08 – Seoul