INCLUSIVE STANDARD MODEL HIGGS SEARCHES HIGGS SEARCHES WITH ATLAS Francesco Polci LAL Orsay On behalf of the ATLAS collaboration. SUSY08 – Seoul (Korea)

INCLUSIVE STANDARD MODELINCLUSIVE STANDARD MODEL HIGGS SEARCHESHIGGS SEARCHES

WITH ATLASWITH ATLAS

Francesco PolciFrancesco PolciLAL OrsayLAL Orsay

On behalf of the ATLAS collaboration.On behalf of the ATLAS collaboration.

SUSY08 – Seoul (Korea)SUSY08 – Seoul (Korea)

THE HIGGS BOSON AT THE LHCTHE HIGGS BOSON AT THE LHC

PRODUCTION

This talk is based on the results of the ATLAS CSC NOTE: update on

the analysis techniques and discovery potentials.

Almost ready for publication.

DECAY

gg fusion process is the more abundant, followed by the Vector

Boson Fusion process.

Francesco Polci SUSY08 – Seoul 2

• Important channel in the low mass region.

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

H0


HH→→

ATLASpreliminary

pT (Gev/c)

ATLASpreliminary

pT (Gev/c)

Trigger efficiency for single photons

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

(respect to offline) = (93.6 ± 0.4)%

• Identification cut exploiting the shower shape.

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

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

pT >1GeV/c in a R=0.3 cone around the

electromagnetic cluster.

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

most energetic photons.

Selection efficiency: = 36.0 % (32.2% with pileup 1033cm-2s-1)

Within a mass window MH +/- 1.4GeV:


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

Tracker and calorimeter informations are combined:• The calorimeter measure the 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 Z1 Ztrue O Z2

Strips

Middle

Photon 1 Photon 2

PRIMARY VERTEX RECONSTRUCTIONPRIMARY VERTEX RECONSTRUCTION

preliminary preliminary

Calorimeter only Calorimeter and tracker

zrec-ztrue (mm) zrec-ztrue (mm)


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.

CONVERSIONSCONVERSIONS

preliminary

ATLAS


With pileup!

• Background is evaluated with NLO simulations.• It will be measured from data sidebands.

Example: NLO -jet processes

BACKGROUNDSBACKGROUNDS

Within a mass window MH +/- 1.4GeV:

Strategy for jet rejection:• Longitudinal segmentation of the calorimeter. • Fine segmentation of the first layer (-strips) => good 0

rejection.• Isolation of the electromagnetic cluster. • Isolation based on tracks reconstructed by the inner detector.


INVARIANT MASSES DISTRIBUTIONSINVARIANT MASSES DISTRIBUTIONS

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

ATLAS preliminary

WH WH → → lvlv

ZH→ ZH→ llll

H+1jetH+1jet

H+2jetH+2jet


ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

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() and cos* 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.

SIGNIFICANCESIGNIFICANCE

Higgs

*In Higgs rest

frame

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

• cos* 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

H H →→ ZZ(*) ZZ(*) →→ 4l 4l

It is the “golden channel”!

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

• Clear signature

• Wide range of masses explored


muons

EVENTS SELECTION EVENTS SELECTION

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

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

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

- calorimeter identification

- |Mll1-MZ|<M12 and Mll2>M34

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

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

- tracker: pT/pT < 0.15

• Vertexing cut on maximum lepton impact parameter:

d0/d0<3.5 (6.0 ) for (e)

electrons

ATLASpreliminary

ATLASpreliminary


Backgrounds are (||<2.7, pT>5 GeV ):• qq,gg ZZ()4l (l=e,m,t) => 56 fb (NLO);

• qq Zbb4l => 810 fb (NLO );

• qq Zbb3l => 1.3 pb (NLO );

• qq,ggtt => 833 pb (NLO);

• qq,gg WZ3l => 386 fb (NLO);

• Z2l+X => 4030 pb (NNLO)

•Zee mass peak is affected electron bremsstrahlung.

BACKGROUNDSBACKGROUNDS

Background will be estimated in sidebands low systematic uncertainties

Mass resolution @ MZ 1.8 GeV

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


ATLAS preliminary

ISOLATIONISOLATIONIMPACT PARAMETERIMPACT PARAMETER

Lepton from b-quark decay do not point

towards primary vertex

Reducible backgrounds have activity around leptons from b-decay

Transverse impact parameter significance in signal and reducible background events.

Normalized calorimetric and track isolation (R=0.2) for the signal (mH = 130) and the Zbb and tt backgrounds in the 4 channel.

ATLAS preliminary


ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

Calorimetric isolation Track isolation

H4 H4e

Mass resolution as a function of the Higgs mass. Open circles denote the resolution Z mass constraint improves resolution.

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

EFFICIENCY & RESOLUTIONEFFICIENCY & RESOLUTION


ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

H4 H4e

Z on shell Z off shell

INVARIANT MASS DISTRIBUTIONSINVARIANT MASS DISTRIBUTIONS


ATLAS preliminary

ATLAS preliminaryATLAS preliminary

ATLAS preliminary


Preliminary

ATLAS preliminary

Poisson probability

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

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


ATLAS preliminary

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

•Signature is 2, 2e, e + ETmiss.

•Three channels:

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

H H →→ WW WW

- H→WW → e v v (H+0jet)

- H → WW → e v v

- H → WW → l v q q (only for MH=300 GeV)

VBF (H+2jet)


• High backgrounds: WW, Wt, ttbar, Z2l, bb,cc,QCD multijet

SELECTION AND BACKGROUNDS SELECTION AND BACKGROUNDS • No mass peak use transverse mass.• Reconstruction:

– Trigger : single or double lepton selection 120i 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.

• Fit the transverse mass and the transverse momentum of the candidates in two bins of the dilepton opening angle 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.

EVENTS SELECTION: H+ 0jetsEVENTS SELECTION: H+ 0jets

Signal region Control region

preliminary

The transverse mass distribution in the region <1.575 and pT

WW>20 GeV.


preliminary

ATLAS preliminary

Two distinct signatures:1.Two forward “tag” jets (largeseparation with high-pT) with large Mjj 2.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 ET jets in opposite hemispheres, with ET>~40 GeV, jj>~4, Mjj>500-1000 GeV.• Higgs decay products between tag jets in • No additional jet activity in the event: central jet veto.

EVENTS SELECTION FOR VBFEVENTS SELECTION FOR VBF


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

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


preliminaryPreliminary


preliminary

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.

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

COMBINATION OF THE CHANNELSCOMBINATION OF THE CHANNELS

Zooming…

ATLAS IS WELL READY ATLAS IS WELL READY TO CATCH THE HIGGS!TO CATCH THE HIGGS!


ATLAS preliminary

ATLAS preliminary

ATLAS preliminary

Documents

INCLUSIVE STANDARD MODEL HIGGS SEARCHES HIGGS SEARCHES WITH ATLAS Francesco Polci LAL Orsay On behalf of the ATLAS collaboration. SUSY08 – Seoul (Korea)