ATLAS Physics Potential II

Borut KersevanJozef Stefan Inst.Univ. of Ljubljana

ATLAS Physics Potential:• Standard Model• Higgs & Susy• BSM: Susy & Exotics

On behalf of the ATLAS collaboration

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• The experimental observation of one (or several) Higgs bosons will be fundamental to understand the mechanism of electroweak symmetry-breaking and may probe physics beyond the SM..

• LHC offers the potential for such a discovery.

• There is a very rich variety of search channels for the discovery of the SM Higgs and even more for the non-SM Higgs bosons. An overview of the most relevant channels will be given.

• It must be stated from the beginning that the “all hadronic” states are impossible to separate from the background and very difficult to be triggered.

• Very good understanding of the detector is mandatory !

Main points

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SM Higgs Searches (benchmark analyses)•HZZ*•HWW*•H•Vector Boson Fusion•ttH(Hbb)Additional MSSM Higgs Searches Example: H/Aτ+τ- , H/Aμ+μ-

Higgs propertiesMass, couplings, …Other scenaria: Little Higgs

Higgs@LHC

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SM Higgs at the TeV scale Many theoretical arguments predict a Higgs mass at the TeV scale: WW scattering violates unitarity if only Z/ are exchanged

For Higgs to be able to restore it at any s: GFm2H ~< O(1)

Triviality bound: Scalar sector is a 4 theory

Energy cut off C where SM is not trivial C~1016(3)GeV mH<200(1000) GeV

Vacuum stability bound: Fermionic contributions could lead to

negative self coupling for too small Vacuum not a minimum anymore C~103(16)GeV mH > 70 (130) GeV

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Current SM Higgs Limits

LEP direct searchmH>114.4 GeV @95% CLTevatron Direct search LEP, SLD, Tevatron e/w fit mH<182 GeV

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Higgs @ LHC: production Gluon Gluon fusion:

Dominant production mode NLO correction important

K = 1.7 Main contribution is gluon radiation

many events with at least one jet

NNLO cross section known Sig(NNLO)/Sig(NLO) = 1.3

Vector Boson Fusion: small K factor ~ 1.1

Small jet multiplicity in final state No color exchange between quarks

large energetic jets at small pT

Low hadronic activity in central region from hard event

a part from Higgs decay Production with Gauge boson:

Known NNLO for QCD and EW corrections

Production with heavy quarks: More complicated final state More than 10 diagrams, known at NLO

Typical uncertainties on cross-sections

• gg 10-20 % (NNLO)• VBF ~ 5% (NLO)• WH,ZH ~< 5% (NNLO)• ttH 10-20 % (NLO)

(A.Djouadi)

q

q

g

g

H

g

g

t

tH

H

q

q

W

W

H

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Higgs decay channels

Uncertainties on branching ratios:few % (NLO)

Inclusive search channels:H → ZZ for mH ≥ 130 GeV

→ 4 lH →WW for mH ≥ 145 GeV

→ lνlν H → γγ for mH ≤ 150 GeV

Exclusive search channels:VBF H →WW for mH ≥ 115 GeV

VBF H →ττ for mH ≤ 150 GeV

ttH, H→bb for mH ≤ 135 GeV

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SM Higgs Search H→ZZ(ZZ*)→l+l-l+l-

• ZZ(*)→4l is very clean• (also → lljj, llνν are studied)• All H decay products are reconstructed• Very sensitive for mH>130 GeV• Golden channel for mH>2mZ

Signature:two opposite sign pair of leptonscoming from the primary vertexcompatible with Z mass (at least 1 couple)

ATLAS

Exploits the excellent e/μ identification and momentum resolution of the detectors

σ=1.4 GeV

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Irreducible Background:continuum ZZ(*) →4 leptonsReducible Backgrounds:Zbb 4 leptonstt 4 leptonssuppressed by impact parameter and isolation criteria

gg->ZZ is added as 20% of LO qq->ZZ

ZZ NLO k factor depends on m4l

Background control: a) from side bands b) from ZZ 4l / Z 2l

Discovery with less than 10 fb-1 130<mH<160 GeV, 2mZ< mH<

550 GeV

SM Higgs Search H→ZZ(ZZ*)→l+l-l+l-

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SM Higgs Search H→WW→l+νl-ν

• Important channel for 2mW <mH<2mZ

• HWW BR ~ 95% • Exclusive VBF also sensitive in lower mass

regions• Inclusive HWW Using dilepton final state• Signature l+ l- and MET

• no mass peak, have to use transverse mass l+ l- ETmis

• need to determine shape of background

• Leptons anti-correlated• W+, W- opposite spin• Leptons tend to be close

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Backgrounds:tt, tWb : rejected by vetoing the jetsWW,WZ,ZZ: rejected by kinematical cutsi.e.- ETmiss > 50 GeV- jet veto in < 2.5- 30 <pT max<55 GeV- pTmin > 25 GeV

- 12 < mll < 40 GeV Background Controla) Invert cut on l+l- proximityb) Create control samples for

tt,WW,WZFor 1,2 and 10 fb-1 syst err ~19,16

and 11%Discovery may happen within

~1fb-1

SM Higgs Search H→WW→l+νl-ν

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SM Higgs Search H→ γγ

•With H BR 10-3 only:•Still, very important in the mass range mH ≤ 150 GeV•Requires good energy resolution of the EM calo

•Signature: 2 isolated high Et gammas from PV Need of excellent energy resolution•Irreducible Background: Continuum gamma-gamma

•Reducible Backgrounds:•jet-jet and gamma-jet events•Need of Excellent jet rejection factor (> 103 for 80% efficiency) good π0 rejection

5000

Jet rejection

80 %

γ efficiency

σ=1.36 GeV

Mass γγ

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Event Selection Kinematical cuts pT

1>40 GeV, pT2>25 GeV, ||<2.5

Photon identification cuts Photon reconstruction and calibration Photons direction corrected for PV

resbos

ATLASImprove the discovery potential using the shape of kinematical variablesLikelihood ratio method based on kinematical variables of signal and background

SM Higgs Search H→ γγ

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ATLASSignificance change LO -> NLO

Carminati L., Physics at LHC 2006

SM Higgs Search H→ γγ

Impact of kinematical variables not shown in this plot

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SM Higgs search VBF with H→ ττ and H→ WW

• At low Higgs masses the largest sensitivity search channels are found in the vector boson fusion production mode.

• The two jet of the quarks are energetic and distributed in the forward region.

•The Higgs decay products in between..

•Signature:•two tag jets in the forward region•One of the W or τ decay leptonicaly

•Irreducible Background •qq Z/W•Reducible backgrounds•QCD multi-jet, W+jet, Z+jet, g+jet and tt

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Significant background suppression by

Two tag jets in forward region

sn-atlas-2003-024

SM Higgs search VBF with H→ ττ and H→ WW

Forward tagging jets: energetic jets at high

No color flow between initial partons

No jet radiationCentral Jet Veto

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SM Higgs search VBF with H→ WW

mH=120 GeV mH=160 GeV

Signal to background with VBF increases by a factor >3

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SM Higgs search VBF with H→ ττ

Backgrounds : QCD ττ +jets, EW ττ +jets, W+jets, ttSelection:VBF tag jetsτ selectionΜΕΤ reconstruction, Kinematical cutsBackground ControlSide bandsRelaxed cuts

ττ → lνν+jν ATLAS ττ → eνν+μνν

PRELIMINARY

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SM Higgs search ttH (H→ bb) →lνbbbbjj

Signature4b-jets + lepton + 2 jets +MET

Irreducible background: Non resonant ttbbReducible backgrounds:ttZ, ttjj, WWjj

Event Selection:Reconstruction of at least 6 jetsB-tagging of exactly 4 jetsKinematical cutsInvariant Mass of bb from HUse of Likelihood functions• To associate bs from t decays• To discriminate ttbb bkg

Very challenging channelSignificant for very low Higgs masses

Difficult to control the background with the use of the data

mH = 120 GeV, L = 30 fb-1 S/B = 2.8, with LO

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ATLAS Summary for the the SM Higgs discovery

ATLAS uses LO cross-sections in the plot, NLO cross sections ‘in reserve’ATLAS new sensitivity study is ongoing..

PRELIMINARY

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F.Gianotti, ICHEP06

• 5σ discovery over all allowed mass range with ≤ 5 fb-1

• More than one channel must be combined for early discovery at low masses (~ 115 GeV)

LHC Summary for the the SM Higgs discovery

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• large loop corrections to masses and couplings

• mainly dependent on t/ t sector • parameters:

Mtop and Xt, MSUSY, M2, , Mgluino

• mass prediction Mh < 133 GeV

(for Mt = 175GeV)

• large loop corrections to masses and couplings

• mainly dependent on t/ t sector • parameters:

Mtop and Xt, MSUSY, M2, , Mgluino

• mass prediction Mh < 133 GeV

(for Mt = 175GeV)

MSSM Higgs Sector MSSM: 2 Higgs doublets 5 physical bosons: h, H, A, H+, H-

phenomenology at Born level described by tan, m A

mass prediction: Mh < MZ

couplings: gMSSM = ξ · gSM

no coupling of A to W/Z

large tan large BR(h,H,A,bb)

MSSM: 2 Higgs doublets 5 physical bosons: h, H, A, H+, H-

phenomenology at Born level described by tan, m A

mass prediction: Mh < MZ

couplings: gMSSM = ξ · gSM

no coupling of A to W/Z

large tan large BR(h,H,A,bb)

ξ t b/ W/Z

h cos/sin -sin/cos sin(-)

H sin/sin cos/cos cos(-)

A cot tan -----

: mixing angle between CP even Higgs bosons (calculable from tanand MA)

for exclusion bounds and discovery potential: fix the 5 parameters inbenchmark scenarios and scan (tan, MA)- plane

for exclusion bounds and discovery potential: fix the 5 parameters inbenchmark scenarios and scan (tan, MA)- plane

~

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current exclusion in (tan,MA)-plane:

LEP excludes low tan and low MA region

note: no exclusion from LEP for Mt larger

~183 GeV

current exclusion in (tan,MA)-plane:

LEP excludes low tan and low MA region

note: no exclusion from LEP for Mt larger

~183 GeV

main questions for LHC/ ATLAS:

Can at least 1 Higgs be discovered in the allowed parameter space?

How many Higgs bosons can be observed ?

Can the SM be discriminated from models with extended Higgs sectors (like MSSM) ?

main questions for LHC/ ATLAS:

Can at least 1 Higgs be discovered in the allowed parameter space?

How many Higgs bosons can be observed ?

Can the SM be discriminated from models with extended Higgs sectors (like MSSM) ?

The (tan, MA)-Plane

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Benchmark Scenarios

1) MHMAX scenario maximal Mh < 133 GeV

2) Nomixing scenario small Mh < 116 GeV

3) Gluophobic scenario Mh < 119 GeV • coupling of h to gluons suppressed • designed to affect discovery via

gg h, h and hZZ 4l4) Small scenario Mh < 123

GeV• coupling of h to b () suppressed

(for large tan and MA 150500GeV)• designed to affect discovery via

VBF, hand tth, hbb

1) MHMAX scenario maximal Mh < 133 GeV

2) Nomixing scenario small Mh < 116 GeV

3) Gluophobic scenario Mh < 119 GeV • coupling of h to gluons suppressed • designed to affect discovery via

gg h, h and hZZ 4l4) Small scenario Mh < 123

GeV• coupling of h to b () suppressed

(for large tan and MA 150500GeV)• designed to affect discovery via

VBF, hand tth, hbb

• 4 CP conserving scenarios considered

• to examplify the discovery potential

• mainly influence on phenomenology of h

masses, coupling and BRs calculated with FeynHiggs (Heinemeyer et

al.)

Name MSUSY (GeV)

(GeV) M2 (GeV) Xt (GeV) Mgluino (GeV)

mh-max 1000 200 200 2000 800

no mixing 2000 200 200 0 800

gluophobic 350 300 300 -750 500

small 800 2000 500 -1100 500

suggested by Carena et al.,EPJ C26, 601(2003)

eff. hg - coupling

hbb - coupling

already at LEP

Newly designed for hadron colliders

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MSSM Higgs Production

Higgs sector of the MSSM: physical states h,H,A,H±

Described by two parameters at lowest order: MA, tanbDiscovery of extended Higgs sector leads to physics beyond SM

At high tan associated bbH production is greatly enhanced!

tan = 3 MSSM neutral Higgs production tan=30

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MSSM Higgs search Channels taken into consideration

…and BR to WW,ZZ strongly suppressed

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most important channels: VBF

differences mainly due to Mh

almost entire (tan, MA)-plane covered

most important channels: VBF

differences mainly due to Mh

almost entire (tan, MA)-plane covered

Light Higgs Boson (30 fb-1)

h observable in entire parameter space and for all benchmark scenarios? h observable in entire parameter space and for all benchmark scenarios?

ATLAS (prel.)

ATLAS (prel.) ATLAS (prel.)

See talk of P. Conde for SM Higgs searches with ATLAS

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hole due to reduced BR for H hole due to reduced BR for H

Light Higgs in Small a Scenario (30 fb-1)

Complementarity of search channels almost guaranteesthe discovery of h

Complementarity of search channels almost guaranteesthe discovery of h

ATLAS (prel.)

ATLAS (prel.)

covered by enhanced BR to gauge bosons

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Light Higgs Boson (300 fb-1)

VBF: only 30 fb-1

• also h, hZZ4 leptons, tthbb contribute • large area covered by several channels stable discovery and parameter determination possible

• small area uncovered (Mh = 90 to 100 GeV)

• also h, hZZ4 leptons, tthbb contribute • large area covered by several channels stable discovery and parameter determination possible

• small area uncovered (Mh = 90 to 100 GeV)

ATLAS (prel.)

ATLAS (prel.)

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rec. mass for had.had.

Neutral Heavy Higgs Bosons (H/A)

prod ~ (tan)2; important at large tan new analysis: had. had. BR(H/A) ~ 10 % , rest is bb

prod ~ (tan)2; important at large tan new analysis: had. had. BR(H/A) ~ 10 % , rest is bb

example: bbH/A, H/A

bb H/A bb covers large tanregion

other scenarios similar intermediate tan region

not covered

bb H/A bb covers large tanregion

other scenarios similar intermediate tan region

not covered

New: take running b-quark mass for prod

30fb-1

discovery reach for H/A:

ATLAS (prel.)

ATLAS (prel.)

only very few events remain after cuts (acceptance ~10-3)

LVL1 trigger performance crucial detailed study: >90% LVL1 efficiency for

MA>450GeV via “jet+ET,miss” and “+ ET,miss” triggers with a rate of ~1.4 kHz (within rate limit)

only very few events remain after cuts (acceptance ~10-3)

LVL1 trigger performance crucial detailed study: >90% LVL1 efficiency for

MA>450GeV via “jet+ET,miss” and “+ ET,miss” triggers with a rate of ~1.4 kHz (within rate limit)

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VBF channels, H/A only 30fb-1

300 fb-1

Overall Discovery Potential (300 fb-1)

ATLAS (prel.)

at least one Higgs boson is observable for all parameter points (in all four benchmark scenarios)

in some parts: >1 Higgs bosons observable

distinguish between SM and extended Higgs sector

but: significant area where only h is observable.

basic conclusions independent of mtop

at least one Higgs boson is observable for all parameter points (in all four benchmark scenarios)

in some parts: >1 Higgs bosons observable

distinguish between SM and extended Higgs sector

but: significant area where only h is observable.

basic conclusions independent of mtop

ongoing: including SUSY decay modes to increase areas for heavy

Higgs bosons , e.g. H

3l + ET,miss can SM be discriminated from extended Higgs sector by

parameter determination e.g. via rate measurements?

ongoing: including SUSY decay modes to increase areas for heavy

Higgs bosons , e.g. H

3l + ET,miss can SM be discriminated from extended Higgs sector by

parameter determination e.g. via rate measurements?

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Relatively easy from H or 4leptons. The channel H can also contribute at low luminosity

(H) only directly accessible for m>200 GeV

Higgs mass measurement

HWW no mass peakHigh mass region: largerWidth, weaker statistical power

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Z polarizationplane angle

Atlas-sn-2003-025

Higgs spin, CP• Observation of ggH or H

excludes spin 1• For MH>200 GeV, study spin/CP

from HZZ4l• Exclusion can be deduced from

and distributions

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• Concentrate on “low” MH

• Series of assumption needed:

• Assume spin 0:• allow to use angular distribution on HWW (most precise measure)• measure .BR in different channels:.BR = (NS+B - <NB>)/L

• Uncertainties: Selection efficiencies Background subtraction Luminosity

• Second step: assume only one Higgs boson

•BR(Hx)/BR(HWW) = x/W

•Reduced number of fitted parameters

smaller errors

Atlas note phys-2003-030Couplings

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• Third step, more assumptions:-No new particles in loop-Light mass H not accessible-Absolute scale not measurable, measure gx/gW

• Express all rates and BR as a function of 5 couplings:

gW,gZ,gtop,gb,g

Examples: (VBF): aWF.gW

2+aZF.gZ2

BR(): (b1.gW2 – b2.gtop

2)/H

Syst uncertainties from exp+theory

Couplings

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Higgs Parameters – Couplings to Weak Bosons sn-atlas-2007-060

VBF H->WW, H->ττ

Δφjj: azimuthal angle of tag jets

Determination of the dominant coupling term

Put limits on anomalous couplings

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SM or extended Higgs Sector ?

BR(h WW) BR(h )

estimate of sensitivity from rate measurements in VBF channels (30 fb-1)

only statistical errors

assume Mh exactly known

potential for discrimination

seems promising

needs further study incl. sys. errors

compare expected measurement of R in MSSM with prediction from SM

=|RMSSM-RSM|exp R =

ATLAS (prel.)

ATLAS (prel.)

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Charged Higgs Bosons

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A “Little” Higgs ?Seeks to solve the radiative instability of the SM Higgs sector

In the “Little Higgs” model, the massless Higgs is generated (in analogy of the pion in QCD) through SSB of a new symmetry

It’s mass is acquired during EWSB. The new symmetry being still approximately valid, the mass is protected and stays small

Breaking SU(5) requires at least one heavy, O(TeV), new particle for each particle contributing to the radiative corrections of the Higgs, which cancel the SM corrections

By construction: the W±H, ZH cancel the weak divergence, a new quark T cancels the

top-quark divergence, the new Higgs triplet cancels the SM Higgs divergence

The new heavy top and gauge bosons decay into their SM partners through associated Higgs production. These and the new Higgs fields could be discovered at ATLAS, studies performed!

As new symmetry one could use SU(5), embedding the unified gauge group (SU(2)U(1))2

Breaking SU(5) by a VEV into SO(5) creates 14 “Goldstone” bosons

Then, the group (SU(2)U(1))2 is broken into SU(2)LU(1)Y, where 4 of the 14 Goldstone bosons are used to create massive longitudinal SM gauge fields (W±

H, ZH, AH) of the broken gauge group

Among the remaining Goldstone bosons one finds a complex scalar doublet (SM Higgs), and a scalar triplet with 5 Higgs bosons: 0 , ± , ±±

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Two other modes: ZH Zh l+l- ZZ l+l-jj l+l-

WH Wh l ZZ ljj l+l-1.5 TeV

1 TeV 18.6 18.6

S= 92S= 92

Mass reco. bias: 1%Mass resolution: 4%~same all modes

cot = 0.5

300 fb-1

300 fb-1

9.2 9.2

S= 31S= 31

S≈21%

B<1%

S≈24%

B<1%

ZHZh l+l-WW l+l-jj l

VH decays to Higgs (mh=200 GeV)

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• If the Standard model Higgs boson exists, it cannot

escape detection at the LHC.

• Discovering the Higgs boson is just the first step, the

next step is to measure its mass and couplings.

• Discovery of enhanced Higgs sector directly prompts to

physics beyond the SM.

… the adventure is about to start

Higgs summary