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Search for the radion using the ATLAS detector

G. Azuelos, D. Cavalli, H. Przysiezniak, L. Vacavant

To cite this version:

G. Azuelos, D. Cavalli, H. Przysiezniak, L. Vacavant. Search for the radion using the ATLAS detector.

EPJdirectC - Articles, 2002, 4, pp.1-13. �in2p3-00014135�

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Search for the Radion using the ATLAS Detector

G.Azuelos 1

,D. Cavalli 2

,H. Przysiezniak 3

andL.Vacavant 4

1

LaboratoireRJALevesque,UniversitedeMontreal,Montreal,Quebec,H3C3J7,CANADA

2

DipartimentodiFisica,UniversitadiMilanoeINFNSezionediMilano,I-20133Milano,ITALY

3

LaboratoiredePhysiquedesParticules,IN2P3-CNRS,F-74019Annecy-le-Vieux,FRANCE

4

Lawrence BerkeleyNationalLaboratory,PhysicsDivision,BerkeleyCA94720-8167,USA

Received:date/Revisedversion:date

Abstract. Thepossibilityofobserving theradion usingthe ATLASdetectorattheLHCis investigated.

Studies on searches for the Standard Model Higgs with the ATLAS detector are re-interpreted to ob-

tainlimitsonradiondecay to and ZZ ()

. Theobservabilityof radiondecaysinto Higgspairs, which

subsequentlydecayinto+b

bor+b

bisthenestimated.

PACS. XX.XX.XX NoPACScodegiven

1Introduction

ThehierarchybetweentheelectroweakandPlanckscales

is oneof theprincipal puzzlesin modelsof unicationof

the interactions. The postulate of the existence of com-

pactiedlargeextradimensionsallowsforaPlanckscale

intheTeVrange,buttransferstheproblemtotheunnatu-

ralsizeoftheextradimension(s).InthemodelofRandall

andSundrum(RS)[1],two4-dsurfaces(branes)bounda

sliceof 5-dspace-time. TheSM elds areassumed to be

locatedononeofthebranes(theTeVbrane),whilegrav-

itylies in thebulk.The fthdimension isnot large,but

themetricisnon-factorizable,allowingforaresolutionof

thehierarchyproblem,givenanappropriatewarpfactor.

Thetheoryadmitstwotypesoffour-dimensionalmassless

excitations:theusualgravitonandagraviscalar,themod-

ulusorradion().Inordertostabilizethesizeoftheextra

dimensionwithoutnetuning ofparameters,Goldberger

and Wise [2] have proposed a mechanism by which the

radion acquiresa mass,expected to besmaller than the

J=2Kaluza-Kleinexcitations.Thepresenceoftheradion

is one of the important phenomenological consequences

of these theoriesof warped extradimensions[3{12].The

study of this scalartherefore constitutes acrucial probe

ofthemodel.

1.1Radion branchingratiosandwidth

The radioncouplingsto fermions and bosonsare similar

tothoseoftheStandardModel(SM)Higgs[3];onlytheir

relativestrengthschange.Theyareexpressedasafunction

ofthree parameters:thephysical massoftheradionm

,

thevacuum expectation valueofthe radionorscale,

,

Inthefollowingstudy,itisassumedthattheSMHiggs

hasbeendiscoveredandthatitsmasshasbeenmeasured.

The branching ratios of the radion are calculated using

thoseoftheSMHiggsascalculatedinHDECAY[15],and

using the ratio of the radion to Higgs branching ratios

given by [3]. For the mixing scenarios considered here,

( = 0 and = 1=6), the branching ratios of the light

HiggsareessentiallySM-like[14].

Figure1showstheprincipalbranchingratiosasafunc-

tionofscalarmassfordecaysoftheSMHiggs(topplots)

and ofthe radionwhenm

h

=125GeVand

=1 TeV,

for = 0 when there is no -h mixing (middle plots),

andfor=1=6whenandhareheavilymixed (bottom

plots).Thesegurespresentthefollowingfeatures:

{ BR ( !gg)is greatlyenhancedwith respectto the

Higgs and is close to unity for m

> 500 GeV and

=1=6,

{ theradiondecaysinto twoSMHiggsform

2m

h ,

{ BR (! )is enhancedfor =1=6andm

600

GeV,

{ for = 1=6 an interference is observed, producing a

strongsuppressionofdecaystovectorbosonsatapar-

ticularmassoftheradion.

Theradionhasaverynarrownaturalwidth.Figure2

showsthe totalwidth as afunction of mass, for theSM

Higgs and forthe radionwith =0and 1/6,for

=1

TeV.Thewidthisinverselyproportionaltothesquareof

.

Theaimofthepresentstudyistoinvestigatethepos-

sibilityofobservingaRSradionwiththeATLASdetector

throughthefollowingdecays:!,!ZZ ()

!4`,

! hh ! b

b and ! hh ! b

b +

. Only the di-

SN-ATLAS-2002-019 26 September 2002

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10

-4

10

-3

10

-2

10

-1

1

50 100 150 200 10

-4

10

-3

10

-2

10

-1

1

200 400 600 800 1000

10

-4

10

-3

10

-2

10

-1

1

50 100 150 200 10

-4

10

-3

10

-2

10

-1

1

200 400 600 800 1000

10

-4

10

-3

10

-2

10

-1

1

50 100 150 200 10

-4

10

-3

10

-2

10

-1

1

200 400 600 800 1000

Fig. 1. Branching ratios of the SM Higgs (top), and of the

radionwhen=0(middle)and=1=6(bottom)asafunction

oftheirmass,for

=1TeV.TheHiggsmassinthefourlower

plotsissettom

h

=125GeV.

10 -2 10 -1 1 10 10 2 10 3

100 200 300 400 500 600 700 800 900 1000

Fig. 2. Width oftheSM Higgs andof theradion with =0

and1/6bothfor

=1TeV.

it is the main process at LHC and it benets from the

enhancementofthecouplinggg.

Othersignals,observableinSMHiggsdecay[16],such

asH !WW !``, shouldbeobservablein thecorre-

spondingradiondecaychannel[11].Themassreconstruc-

tionis diÆcultin thiscase, butobservationof thesignal

2! and !ZZ

()

!4`

Forthe(m

<160GeV)andZZ ()

(m

>100GeV )

decay channels, the radion signal signicance is deter-

minedfromtheSMHiggsresultsexpectedforATLAS[16],

for100fb 1

(one yearathigh luminosity10 34

cm 2

s 1

).

EvaluatingthesignalsignicanceasS=

p

B,whereS and

B are the numbers of signal and background events re-

spectively, theratiooftheradionsignalsignicanceover

that oftheSMHiggs,isgivenby[3]:

S=

p

B(;;ZZ)

S=

p

B(h;;ZZ)

=

!gg

BR (!;ZZ)

h!gg

BR (h!;ZZ) v

u

u

t

;ZZ

h

;ZZ

Accountingforexperimentalresolution,theZZresonance

width is given by ZZ

h;

= q

( h;

tot

=2:36) 2

+(0:02m

h;

) 2

,

where h;

tot

is thetotalintrinsic width of theHiggs or

resonance. In the energy range considered, the reso-

nance width is essentially independent of the negligible

intrinsic width of the resonance,

h;

= 0:10 p

m

h;

+

0:005m

h;

,andinthiscase,thelastfactorissimplyunity.

UsingtheATLAS SMHiggssignalsignicanceresults[16],

theradionsignalsignicanceisdeterminedandshownin

Figure 3asa function ofthe massof the radion,forthe

channel(top)andfortheZZ ()

channel(bottom),for

=1,10TeV,=0,1/6,andforanintegratedluminosity

of100fb 1

.

10

-2

10

-1

1 10

60 80 100 120 140 160 180 200

10

-2

10

-1

1 10 10

2

100 200 300 400 500 600 700 800 900

Fig. 3. Signalsignicance versusthe massof theradion, for

the channel(top)and forthe ZZ ()

channel(bottom). In

both plots, the values for

=1,10 TeV and =0,1/6 are

shown,foranintegratedluminosityof100fb 1

.

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3Radion production at the LHC

FromtheSMHiggsproductioncrosssection,evaluatedat

nextto leadingorder in [17],the crosssectionfor radion

productionwasestimatedaccordingto:

(gg!)=(gg!h) ()

(h)

BR (!gg)

BR (h!gg)

where isthetotalwidthoftheradionorHiggsreso-

nance,showninFig.2andthebranchingratiostoapair

ofgluonsareshownifFig.1.

Inthefollowingsections,forthepurposeofestimating

thelimitsof observation ofradiondecayto apairofSM

Higgsbosons,tworeferencevaluesaretakenforthemass

oftheradion:300GeVand600GeV.Theproductioncross

sectionsinthese casesare58pband8pbrespectively.

4!hh!b

b

As was mentioned in Section 1, the radion, similarly to

the heavy Higgs of the Minimal SuperSymmetric Model

(MSSM), candecay into Higgspairs withrelativelyhigh

branchingratio(seeFigure1).Thespecicdecaychannel

!hh!b

b oersaninterestingsignature,with two

high-p

T

isolatedphotonsandtwob-jets.Thebackground

rateisexpectedtobeverylowfortherelevantmassregion

m

h

>115GeVandm

>2m

h

.Inaddition,triggeringon

such eventsis easy andthe diphotonmass providesvery

goodkinematicalconstraintsforthereconstructionofm

.

The decay hh ! b

b was previously studied in the

contextoftheMSSMHiggs[18],althoughthemassranges

investigated were lower. The approach and the selection

usedin thisstudyareverysimilar.

4.1Signal

Signal events were generated with PYTHIA 6.158 [19].

The heavyHiggs H 0

production process viagluon-gluon

fusion(in theframework ofthe Minimal1-Higgsdoublet

Standard Model) was used to produce the radion. The

mass and the width of the H 0

were changed to reect

those oftheradionwhilethelightHiggsmasswasset to

m

h

=125GeV.

AsshowninFigure 2,thetotalwidthoftheradionis

afactorof10(100)smallerfor =0(1/6)thanthatofthe

Higgs,suchthatitiscompletelynegligiblewithrespectto

thereconstructedmassresolutions.

Twosamples of100kevents each were generated, for

m

=300GeVandform

=600GeV.

4.2Background

The backgroundsfor this channel are b

b (irreducible),

cc,bj,cjandjj(reduciblewithb-tagging).The

eventsweregeneratedwithPYTHIA6.158.Intheregion

processesaretheBorndiagramqq!andtheboxdia-

gramgg!.Theratesarethereforeverylow.However

large uncertaintiesapply to these backgroundssincethe

jets arise only from initial state radiation and not from

thehard-scattering.Generatingabackgroundsampleofa

sensible size turns outto be veryCPU time consuming,

andsomecutshad tobeappliedat theeventgeneration:

the sample was generated in seven dierent bins of p^

? ,

thetransversemomentumdenedintherestframeofthe

hard interaction. For each bin, ten million events were

generated.

Singlephoton production in the hard process j, ac-

companied by QCD orQEDradiation, and where either

the photon orjet is misidentied represents another re-

ducible background.This processwasstudied inthecon-

textoftheSMH!channel,andwasfoundtoincrease

thetotalbackgroundbyafactoroftwo.Inthecontextof

the radionwhere the backgroundsare negligible, this is

thereforenotexpectedtoaectthenalresults.

4.3Detectorsimulation

Thedetectoreects onthesignalandbackgroundevents

are simulated with a fast Monte-Carlo code, ATLFAST

2.53[20],basedonparametrizeddetectorresponse.While

mostparametersarechosenfromthestandardATLFAST

lowluminosityconditions(10 33

cm 2

s 1

),afewimprove-

mentswereapplied forthisstudy:

{ jets were recalibrated using a detailed parameteriza-

tion,

{ the photon reconstruction eÆciency was assumed to

be80%,

{ ap

T

-dependent b-tagging parameterizationwasused

with an average eÆciency of

b

=60% and rejection

factorsofapproximately93for light-quarkjetsand 7

forc-jets,respectively[16].

4.4Selection

Toextract thesignal,twoisolatedphotonswith p

T

>20

GeV and jj < 2:5, and two jets with p

T

> 15 GeV,

jj < 2:5 were required. At least one of the jets had

to be tagged asa b-jet. The diphoton and the dijet in-

variantmasses werethen formed. Figure 4showsthe re-

constructed invariant masses for m

= 300GeV, = 0

and

= 1 TeV. Two mass window cuts were applied:

m

=m

h

2GeVandm

bj

=m

h

20GeV.

Thephotonsandjetsfulllingtheserequirementswere

thencombinedtoformthem

bj

invariantmassasshown

inFigure5.Themassresolutionimprovesto5GeVwhen

constrainingthereconstructedmassesm

bj andm

tothe

lightHiggs massm

h

, asshownon theright-handplotof

Figure 5. The signal acceptances after the various cuts

describedabovearegiveninTable1.

Thesameanalysisprocedurewasappliedtotheback-

ground sample. Because of the uncertainties concerning

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0 10 20 30 40 50 60

80 100 120 140 160

Mean RMS

125.0 1.859

mγγ (GeV/c2)

Events/1 GeV/30 fb-1

0 5 10 15 20 25 30 35 40

0 50 100 150 200 250

mjj (GeV/c2)

Events/10 GeV/30 fb-1

bj bb

Fig.4.Diphoton(left)anddijet(right)invariantmassdistri-

butions, for m=300GeV, =0, =1TeVand 30fb 1

(threeyearsatlowluminosity10 33

cm 2

s 1

).Theright-hand

plotshowstheimpactofrequiringtwob-taggedjetsinsteadof

one.

0 5 10 15 20 25

200 250 300 350 400

mγγbj (GeV/c2)

Events/8 GeV/30 fb-1

all window:

mh ± 2 mh ± 20

0 5 10 15 20 25 30

200 250 300 350 400

Mean RMS

301.0 10.16 7.589 / 35

Constant 25.47

Mean 300.4

Sigma 5.192

mγγbj (GeV/c2)

Events/4 GeV/30 fb-1

mass constraint

Fig. 5. Reconstructed bj invariant mass distribution, for

m

= 300 GeV, = 0,

= 1 TeV and for 30 fb 1

. The

plotsontheleft showallcombinationsandtheones fullling

themasswindowcuts(cf.text).Thedistributionontheright

isobtainedbyconstrainingthereconstructedmassesm

bj and

m

tothelightHiggsmassm

h

,afterthemasswindowcuts.

Cuts m

=300GeV m

=600GeV

photonskinematics 46% 51%

jetskinematics 36% 28%

b-tagging 76% 78%

m windowcut 83% 85%

mbj windowcut 49% 53%

total 5% 5%

Table1.Acceptanceforthesignal,for=0,

=1TeVand

forthetworadionmassesstudied.Foreachcuttheacceptance

isdenedwithrespecttothepreviousone.

themasswindowcutswereloosenedtokeepeventsfulll-

ing: m

=m

h

30 GeVandm

bj

=m

h

40GeV.The

backgroundlevelremainsneverthelessextremelylow,even

in thisconservativeapproach.

4.5Results

Thenalcandidateeventsare selectedin amasswindow

< m

bj

> 1:5

m

bj

for signal and background.The

m=300GeVm=600GeV

=0;

=1TeV 84:5 7:0

=0;

=10TeV 0:9 0:1

=1=6;

=1TeV 150:9 5:3

=1=6;

=10TeV 1:2 0:1

background 1:4210 4

0

Table 2. Numberofeventsselected for signal and for back-

ground, for m =300 and 600 GeV, for 30 fb 1

and for

mh=125GeV.

m

=300GeVm

=600GeV

=0;

=1TeV 4 (43)

=0;=10TeV (333) N=A

=1=6;=1TeV 2 (57)

=1=6;=10TeV (250) N=A

Table 3. Minimum integrated luminosity (fb 1

) needed for

discovery.N/AmeansthatthesignalisnotaccessibleatLHC.

Integratedluminositieslarger than30fb 1

areinparentheses

sincethefeasibilityoftheanalysisat highluminosity hasnot

beenstudied.

Sincethischannelispracticallybackgroundfree,asig-

naldiscoveryisdened hereasaminimumoftenevents.

The minimum integrated luminosities needed fordiscov-

eryarelistedin Table3.Afewfb 1

areneededfor=0

if

1TeV.

In the special case where = 0, the cross-section is

proportionalto 2

.Alowerlimiton

canthereforebe

derivedfromthisstudy.Itisobtainedusingtheprescrip-

tionof [21]:foraknownmeanbackgroundofzero,there

ismorethan95%chanceofobservingatleast10eventsif

the expected numberof signaleventsis greaterthan 18.

The correspondingreach in

is 2.2 TeV form

=300

GeV and 0.6 TeV for m

= 600GeV for an integrated

luminosityof30fb 1

.

5!hh !b

b +

Thechannel!hh !b

b +

providesanotherpoten-

tiallyinterestingsignalforradiondiscovery,althoughthe

background ishigher andthe reconstructed massresolu-

tionsarepoorerthaninthe!hh!b

bchannel.

Inordertoprovideatrigger,aleptonicdecayofthe

isrequired.Here,onlythecasewhenone decayslepton-

icallyandtheotherhadronicallyisconsidered.Asabove,

the events weregenerated byappropriately adaptingthe

process of MSSM decayof the heavy Higgs H 0

into two

light Higgs bosons (h) in Pythia 6.158 [19]. The eect

of the ATLAS detector on the resolution and eÆciency

of reconstruction of these eventswassimulated withthe

ATLASfastsimulationpackage(ATLFAST2.53).Theef-

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