Prospects for Higgs Searches at the TeVatron run II

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Prospects for Higgs Searches at the TeVatron run II

A. Lucotte

To cite this version:

ArnaudLucotte

onbehalfoftheCDFandDCollaborations

InstitutdesSciences Nucleairede Grenoble 53, av. desMartyrs,38026 Grenoble Cedex

E-mail: [email protected]

Althoughextensivelysoughtthislastdecade,theHiggsbosonstillremainsundetected. Relevantconstraintson HiggsmasscomefromthedirectsearchesperformedatLEP,excludindHiggswithamasslowerthan114GeV =c

2 . TheFermilabTeVatronppcollider,with

p

s=2:0TeVwillgivethehighestavailablecenterofmassenergyuntil theLHCstarts. BothCDFandDexperimentsaretheonlyplaceswheredirectevidenceofaHiggssignalcan takeplace. ThisdocumentreportstheprospectsforHiggsdiscoveryatRun II,basedonprojectionsfromthe TeVatronHiggsWorkingGroupanalyses,anddescribetheprogressmadesincethen.

1 Introduction

TheStandardModel(SM)of particlephysicshas been studied with very high precision over the pastyearsandnosignicantdeviationshavebeen found. Stilltheparticlethatmediateselectroweak symmetrybreaking,theHiggsboson,remains un-detected.

RunI HiggssearchesattheTeVatronwere lumi-nosity limited, with a sensitivity to a standard Higgs production cross-section afactor 50 above that oftheStandardModelprediction. Thenext run oftheTevatronoersreasonsto bemore op-timistic. Basedon amajorupgradeof the accel-erator chain, the design goal is to deliver about 15 fb

1

per experiment of pp collisions by the endof 2007. These performances representa fac-tor more than 100 enhancement with respect to Run I luminosity. CDF and D detectors have undergonemajorupgrades,bothtoadaptfrom in-creasedcolliderperformancesandtoimprove sig-nicantely the physics reach based on new high performance trackingsystems,andextended par-ticleIDcapabilities. Finally,allpossiblenalstates for Higgs searches have been analyzed, and have driven the developpement of specic triggers as wellassoftwaretools.

1.1 The Collider upgrade

Thecolliderupgrade 1

isbasedonthereplacement of the MainRing, the last pre-acceleration stage originally used for the run I,by the Main Injec-tor(MI). Located in atunnel separatefrom that of theTevatron, the MI is anew120-150 GeV/c

rapidprotonsynchrotronmachinewhichimproves signicantlythep-production capability.

Figure1: TeVatronschedulefrom2001to2007. Theleftaxis indicates theintegrated luminosity; Right handaxis reports instantaneousluminosity

Inthe rst period of RunIIa, the use of the MI coupledwithupgradesinthep-cooling and stack-ing

2

, permits the use of more intense particule beamsintheTeVatron. Thenumberofcirculating bunches isincreased from 66(Run I) to 3636 inarstphaseandeventuallyto108140,witha spacing between crossings respectively decreased from4.2sto 396ns andeventually132nsin or-dertokeepthenumberofinteractionspercrossing between1-2. By that time, typical luminosity of 210 32 cm 2 :s 1

lumi-2003.

RunIIbisscheduledtostartattheendof2003or 2004

3

. Improvementsinthemachineareexpected fromtheuseoftherecyclerringaswellastheuse ofanewcoolingfortheTeVatronbeams. The Re-cycler ring

4

, located in the sametunel asof the MainInjector,allowsthere-useofthosep remain-ingintheTevatronattheendofastore,resulting inafactor2enhancementinthenumberofp avail-ableforcollisions. Itrequires theuseofaspecic electron-beamcooling

5

adaptedto high intensity 

p beams. At the same time, the TeVatron ring should benetfrom theuseof anewbeam-beam conpensationsystemusinghighintensity electron-beam, designed toreduce beam emittanceat the interaction points

6

. Combined, those upgrades areexpectedtoachieve510

32 cm 2 :s 1 in lumi-nosityforanextra13fb

1

perexperimentbythe end of 2007. A preliminary schedule in term of luminosityisdiplayedin Fig.1.

Togetherwiththeluminosityupgrade,theincrease in the beamenergy from 0.9 TeV to 1.0 TeV al-lows a 40% enhancement in t



t yields and a20% increasein higgsproductioncross-section.

1.2 The Detectorsupgrade

Both D and CDF detectors have undergone a majorupgradeinpreparationfortheRunII. De-taileddescriptionsoftheupgradeddetectorsmay be found respectively in

7 and

8

. Both of them

Figure2:ViewoftheNewTrackingdevecesintheD0 up-gradedetector.

improved theircapabilities in key elds for higgs searches: Trackrecontruction,momenta measure-ment, soft lepton triggering, secondary vertices tagging,displacedvertextriggeringandjetenergy resolution.

Fig2. Itispartofanewtrackingsysteminsidea 2Tsuperconductingmagnetandcomposedofa 4-layerSilicon MicrostripTrakerforradii10 cm, and of a 8-super-layer scintillating bers device providing16measurementsbetween21and50cm. Acombinedresolutionof  p T =p T =0:2%on mo-mentameasurementisachievedovertherangejj 2:0,whilea40mresolutioninrand100min rzondisplacedverticesisexpected. New scintillators-basedpre-showerdetectorshavealsobeeninstalled onto the calorimeter crystat walls and will help improveparticlediscrimination; themuonsystem benetsfromanextensionofthescintillators cov-eragebothinthecentralandforwardregion,while theoldforwardmuonchambersarereplacedwith negranularityMiniDrift tubes extendingupto jj2:0. Thecalorimeterremainsunchanged, ex-cept for acompleterevisionof its front end elec-tronics. All electronic systems,DAQand trigger-ingarchitecturearealsore-designedforrun II.

Figure3:LongitudinalviewoftheCDFIIdetector

AviewoftheupgradedCDFIIdetectoris dis-playedin Fig 3. Theupgradeis basedonthe in-stallationofan\integratedtrackingsystem", com-biningaSiliconlayer0aroundthebeampipewith a 5-layer Vertex detector covering up to jj < 2 at very small radii (10cm), and two interme-diate Silicon layers (at radii of 20-28 cm). This system provides p

T

sili-newCentral DriftChamberreplacestheoldRun I CTC,using small drift cellsand fast gasto re-ducedrifttimesbelow100ns,willprovide96 mea-surementsbetween 44 and 132 cm. TheCDF II detectoralso includes animprovedmuon system, with new scintillatorsand ne granularity Muon DriftChambersextendingthepresentcoveragein theforwardregion upto jj2:0. Finally, while thescintillator-basedcentral calorimeter remains unchangedforRun II, thegas calorimeter in the regionjj 1:0is replaced withanew scintillat-ingtile plugcalorimeter,with bothnewEM and hadroncalorimeters.

2 HiggsPhenomenology

2.1 SMHiggs production anddecays

Theproductioncross-sections and decay branch-ingratiosforHiggsbosonareshowninFig.7. The highestcross-sectionmodesarepp!Hviagluon

Figure 4: Standard Higgs production cross-section at p

s=2:0TeV

fusion, and the Higgs-strahlung processes pp ! HWandpp!HZ. TheHiggsbosondecays dom-inantelytothemostmassivekinematicallyallowed nal state. That is H ! b

 b for m

H

<135 GeV with a Branching ratio of 80%, and H ! WW

 form

H

>135GeVwhereoneoftheW maybeo themassshell. Thelowermassanalysiswill thus seeknalstateswithatleasttwob-jets. However QCDb



bproductionwithacross-sectionof100b 9

makethepp!H!b 

b modeimpossibleto

ex-wheretheboson isdecayingintoleptons.

2.2 SupersymmetricHiggsproductionanddecays

SupersymmetricextensionsoftheSMgiveatleast vephysicalHiggsstates: twoneutralscalars de-notedhandH (withm

h <m

H

),asingle pseudo-scalarAand achargedoubletH



. Attree level, strongconstraintsapply tomassspectrathat de-pend upontwoparameters,chosenbyconvention to be m

A

and tan the ratio of the vacuum ex-pectation values for the two higgs doublets. It is worthwileto note that, despite signicant im-pact of the higher order corrections on the mass spectra,mostSUSYmodelspredictalighter (neu-tral) Higgs with a mass below 130 GeV =c

2 . For

Figure 5: SUSY Higgsproduction cross-section for four b'snalstatesHiggs

thelightestneutralhiggs,MSSMphenomenology is similar to the SM. Production modes pp ! W=ZH have cross-sections that only dier by a factorsin

2

( )wrtSMcross-sections. However, the gluonfusion mode pp ! h is enhancedby a factortan

2

. Higgs decaymodesalsodierfrom theSMmodes sincehb

 b;Ab  b andHb  b couplings are function of tan

2

. As a results, the decay modeh!b



bisdominantoverthefullmassrange m

h

<200GeV and low massSM Higgs searches can be easily converted to SUSY searches. An-other consequenceis that thefour b's nal state channelbecomessignicant,speciallyforhigh val-ues of tan as shown in Fig. 8. Charged higgs searches are made possible if m

H < m t m b . In this case indeed, the Top quark decay mode t ! H

+

b can compete with the standard mode t ! W

+

b for both tan  1 and tan  1. Crosssectionsasfunction ofHiggs massisshown inFig.9. Inthismassrange,chargedhiggsbosons decaydominantely toH

+ !

+

toH !cs;W b 

b (lowtan ).

Figure6: Top quarkdecaycross-sectionwith onetopdecayingintoachargedHiggsas func-tionofm

H 

3 HiggsSearches

All numbers quoted here are extracted from 10

. Generators used for signal and backgrounds are PYTHIA and ISAJET. Experimental eÆciencies andbackgroundrejectionareestimatedwithSHW 12

, using parametrized resolutions for tracking and calorimetersystemstoperformsimple reconstruc-tion of tracks, jets, vertices and trigger objects. DandCDFnumbers. However,inallthose spe-cic issue detector specic full simulations have beenperformedtocheckthosenumbers.

3.1 LowMass Higgs search (m H <135GeV) Inthe110<m H <135GeV =c 2

range,allpossible nalstateshavebeeninvestigated: pp !WH! lb  b ,pp!ZH!b  bandpp!ZH!l + l b  b. Common selections are based on the reconstruc-tion of theb-quark invariant mass together with leptonicdecaysoftheW/Zboson. Dominant back-groundsto thesechannelsareWb



bandZb  b with b-quarkpairfromgluonradiation,singleTopquark and top quarkpair production. Key parameters forlightmassHiggssearchesarethefollowing: b-tagging: eÆcient b-tagging is mandatory to ensurethe rejection of light quarksand c-quarks comingfromstandardbackgrounds. Present stud-ies based on full simulation indicate that more

than 65%(single)-tag eÆciency for less than 1% contaminationareachievableforbothDandCDF. Thisnumberincludesdisplacedvertextagging (typ-icallymorethanabout50%)aswellassoftleptons tagging(15%forbothande).

Mass reconstruction: Sensitivity to low mass higgsbosonanalysisdependsstronglyonthe reso-lutionachievedintheb-quarkpairinvariantmass. Improvedmassresolutionfrom15%(typicalRunI performance) to 9% resolution makes the signal signicanceincreasebyatleast1. This improve-ment can be made possible with the developpe-mentalgorithmstakingintoaccountchargedtrack measurementsinassociationwithcalorimeter-based jetenergies. AlthoughstatisticallylimitedinRunI, CDFalreadyshoweda30%improvementinjet en-ergyresolution

10

withrespecttoresultsnotusing such energy ow algorithms. Such performances stillremaintobeconrmedwithnewDstudies.

Figure 7: Invariant mass for a m H

= 120 GeV Higgs decayingintob-quarkpair

Z!b 

bselection:theselectionofalargeZ!b  b control sample iscrucial for measuringb-tagging eÆciency,demonstratingtheabilitytoreconstruct b



impact parameter tracks. Such algorithms have beenimplementedin Level2triggersystems,and preliminaryresultsindicatethat ayieldof25,000 Z!b



beventsperfb 1

andperexperimentcould bereached.

Neural-Net(NN)techniques: additional multi-variate analysis using Neural Network have also beenapplied. Thesetechniques,usedwithsuccess byDin thetopquarkmassandall-hadronict

 t analyses,exploittopologicaldierencesinan opti-malwayandareshowntobringafactor30% im-provementwith respectto classiccut-based anal-ysis. Theuse ofsuchtechniquesrelies onagood MCsimulationofsignalandbackgrounds(inboth cross-sectionandshape),andthusnecessitatesprogress in the full (NLO) calculations of Wb



b and Zb  b processes,whichconstitutethemainbackgrounds tolowmassHiggsanalyses.

HiggsMass(GeV=c 2 ) Channel Rate 110 120 130 S 1.9 1.2 1.8 b  b B 8.0 6.5 4.8 S= p B 0.7 0.5 0.3 S 5.0 3.7 2.2 lb  b B 48 48 42 S= p B 0.7 0.5 0.3 S 0.8 0.5 0.3 l + l b  b B 2.5 1.8 1.1 S= p B 0.5 0.4 0.3

Table1: Expected performance in terms of S and Back-groundfor1fb

1

luminosity

Prospects for Run II have been established in 10 and are reported in Table 1 under the assump-tions of a mass resolution of 9%, and b-tagging eÆciencyof 60%. Bringingbackthe mass resolu-tionto15%resultsinadegradationofabout0.1 signicance. NotethatQCDb



bbackgroundtothe pp !ZH! b



b channel istaken to be 50%of thetotalbackgroundandwill beestimated using data. Allthreemodesgivesimilarsensitivity.

3.2 High massHiggs searches

Highmassrangesearcheshaveusedthefollowing modes: pp !H !W  W! ll 0 , pp !WH ! WW W ! lll3 and pp ! WH ! WW W ! ll 0

jj. Common selection requires two high-p T leptons with accompanying missing energy E

mis T

. Dominantbackgroundscomefromstandardpp! WW , pp !W+jetswith fakeleptons, pp !t

 t and pp !  productions. While 3rd jet veto andcutonthehadronictotalenergyhelprejectt

 t events,transversemassM

T (llE mis T )isusedagainst 's.

Figure 8: Spincorrelation between leptons comingfrom WpairsinHiggseventcomparedtoSMprocesses.

HiggsMass(GeV=c 2 ) Channel Rate 140 160 180 S 0.11 0.15 0.09 l  l + l B 0.73 0.73 0.73 S= p B 0.13 0.18 0.11 S 2.6 1.5 1.0 l + l  B 11 4.4 3.8 S= p B 0.39 0.71 1.9 S 0.34 0.45 0.29 l + l jj B 2.85 0.85 0.85 S= p B 0.37 0.49 0.31

Table 2: Expected performance in terms of S and Back-groundfor1fb

1

luminosity

im-provestandardbackgroundeventsrejection. Com-bining associate pp ! H ! WW  with gluon fusion pp ! H ! WWW  productions lead to about2-3expectedeventsperfb

1 withS= p B= 0:5 0:7 in 150 160GeV =c 2 range. Sensitivity fordierentchannelsarereportedin Table2

11 .

3.3 NeutralSUSY Higgs searches

For lighstest mass Higgs results show that a 5 discoveryis possible with 20fb

1

perexperiment for1<tan<50and80<m

A

<400GeV. For largevaluesof tan CDF searches forthe modes pp!b  b!b  bb 

b with =h;H;A will seek nal stateswithfourb-jets,withatleast3ofthem be-ingtagged. Allpossiblemasscombinationsofjets are computed and the distribution is compared withtheexpectations. 95%CLexclusioncontour inthetan vsm

A

planeareshownin Fig.9.

Figure 9: Neutral higgs exclusion contour in the pp ! hb  b!b  bb 

bchannelforvariousluminosityhypotheses

3.4 Charged Higgssearches

Indirectsearchesis basedonaccurate determina-tionof the pp! t



t cross-section,where theTop quarkdecaysintotheSMmodet!W

+

bwitha Branching Ratioofnearly1. Top quarkselection criteriaarewelldenedin

13

. Anydeviationfrom the SM prediction can be interpreted as the oc-currenceof thecompeting SUSYmodet!H

+ b with H + ! cs (low tan ) orH + !  +   (high tan ). Sensitivityto such eventsare luminosity-dependent. At RunII, about 1,000t



t events per

fb areexpected,makingindirectsearchesmore promisingthandirectsearchesuntil2003. Projec-tionsareshowninFig.10.

Figure10: ChargedHiggsreach atRunIIfromindirect searches

4 Conclusion

Run I searches were strongly luminosity limited. ForRunII,theTeVatronUpgrade isexpectedto provideanintegratedluminosityof15fb

1 bythe end of 2007. Together with thebeam energy in-crease,thisresultsinafactor180inthestatistics availableforHiggsstudywithrespecttoprevious RunIperformances. BothDandCDFdetectors also have undergone major upgradesfor Run II. Newtrackingdeviceshavebeeninstalled in both experiments, includingnew high precisionvertex detectors; Upgrades in muonand calorimeter de-tectorsalsoleadsto signicantprogressin lepton ID and coverage; Revisited triggering should al-low theselectionof Z ! b



b high control samples mandatory to anylowHiggs mass analysis. pre-liminarystudieshavequantiedtheimportanceof keysectorsinHiggssearches,suchasb-tagging,jet energycalibrationand massresolution. Analyses toolsarecontinouslybeingdeveloppedandshould conrmthesensitivityoftheTeVatrontoexclude ordiscoverHiggswithamassbelow180GeV =c

2 .

5 Acknowledgements

months. I would alsoliketo thankJohn Womer-sley, Boaz Klima, JohnHobbs, Ela Barberis and themembersoftheHiggsWorkingGroupfortheir helpinpreparingthetalk.AspecialthanktoElsa forhercontinuingsupport.

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Beam Division seminar5-23-00see onpage http://cosmo.fnal.gov/organizationalchart/ mcginnis/Talks/Talks.htm

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