Measurement of the Cross Section for Open-Beauty Production in Photon-Photon Collisions at

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

EUROPEANORGANIZATIONFORNUCLEARRESEARCH

CERN-PH-EP/2005-007February23,2005

Measurementofthe

CrossSectionforOpen-BeautyProduction

inPhoton-PhotonCollisionsatLEP

arXiv:hep-ex/0507041v1 9 Jul 2005

TheL3Collaboration

Abstract

Thecrosssectionforopen-beautyproductioninphoton-photoncollisionsismea-suredusingthewholehigh-energyandhigh-luminositydatasamplecollectedbytheL3detectoratLEP.Thiscorrespondsto627pb 1ofintegratedluminosityforelectron-positroncentre-of-massenergiesfrom189GeVto209GeV.Eventscon-tainingbquarksareidenti edthroughtheirsemi-leptonicdecayintoelectronsor

¯crosssectionismeasuredwithinour ducialvolumemuons.Thee+e →e+e bbX

andthenextrapolatedtothefullphasespace.Theseresultsarefoundtobeinsig-ni cantexcesswithrespecttoMonteCarlopredictionsandnext-to-leadingorderQCDcalculations.

SubmittedtoPhys.Lett.B

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

1Introduction

TheproductionofbquarksthroughhardprocessesconstitutesauniqueenvironmentforthestudyofperturbativeQCD,asthemassofthebquark,mb,largelyexceedsthetypicalnon-perturbativescaleofhadronicinteractions.High-energyhadroncollidersarecopioussourcesofbquarksandthereforeextensiveexperimentalstudiesandQCDcalculationshavebeenperformed.Muchdebatehastakenplaceontheapparentdisagreementbetweenthemeasuredcrosssectionforb-quarkproductioninp¯pcollisionsattheTevatron[1]andthenext-to-leadingorder(NLO)QCDcalculations[2].The rstmeasurementsofopenbeautyproductionine±pcollisionsatHERAwerefoundtobemarkedlyhigherthanNLOQCDpredictions[3].Somemorerecentmeasurementswereinbetteragreement[4],whileothersstillshowedanexcess[5,6].Acomparisonofthesedi erentmeasurementswithNLOQCDpredictionsisshowninReference6.

Photon-photoncollisionsate+e colliders√alsogiveaccesstothehardproductionofbquarks.TheLEPe+e centre-of-massenergy,

s=189 202GeV[8].The

resultswerefoundtobeinexcessoftheQCDpredictionbyafactorofthree.Sincethese rst ndings,compatiblepreliminaryresultswereobtainedbyotherLEPcollaborations[9].InthisLetter,weextendourmeasurementtothewholehigh-energyandhigh-luminositydatasample√collectedatLEPwiththeL3detector[10],correspondingto627pb 1at

1)

Throughoutthisletter,theterm‘electron‘standsforbothelectronsandpositrons.

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

andKORALW[17]fore+e →W+W .

TheL3detectorissimulatedusingtheGEANT[18]andGHEISHA[19]packages.MonteCarloeventsarethenreconstructedinthesamewayasthedata.Time-dependentdetectorine ciencies,asmonitoredduringthedata-takingperiod,areincludedinthesimulations.

3EventSelection

¯processisunchangedwithTheselectionofeventsoriginatingfromthee+e →e+e bbX

respecttoReference8.Hadronsproducedinphoton-photoncollisionsareselectedbymeansofthreecriteria.First,atleast vechargedtracksarerequired,thussuppressingbackground

+

fromthee+e →e+e τ+τ ande+e →τ√τprocesses.Second,thevisibleenergyoftheevent,Evis,isrequiredtosatisfyEvis<

sinthelow-anglecalorimeter,

coveringapolaranglefrom1.4 to3.7 .Thiscriteriacorrespondstoretainingquasi-realphotonswith Q2 0.015GeV2.

Abouttwomillionphoton-photoneventsareselectedbythesecuts,withabackgroundcontaminationof0.1%.Eventsarefurtheranalysediftheyhaveanidenti edelectronormuon.Electronsareidenti edasclustersintheelectromagneticcalorimeterinthepolarangularrange|cosθ|<0.725withenergyabove2GeV.Theyshouldmatchatrackandhaveashowerpro lecompatiblewiththatexpectedforanelectromagneticshower.TheratioEt/ptbetweentheprojectionoftheclusterenergyontheplanetransversetothebeamsandthetransversemomentumofthetrackisrequiredtobecompatiblewithunity.Electronsduetophotonconversionaresuppressedbyrequiringthedistanceofclosestapproach,inthetransverseplane,ofthetracktothemeane+e collisionpointinthetransverseplanetobelessthan0.5mmandtheinvariantmassoftheelectroncandidateandoftheclosesttrack,consideredasanelectron,tobegreaterthan0.1GeV.

1

√Thesecutsselect82eventswithelectroncandidatesinthe217pbofdatacollectedats=189 202GeV[8]giveatotalof219eventswithanexpectedbackgroundof2.0%fromthee+e →q¯qande+e →τ+τ processesandasignale ciencyof1.3%.

Muoncandidatesareselectedfromtracksinthemuonspectrometerintherange|cosθ|<0.8.Aminimalmuonmomentumof2GeVisrequiredtoensurethemuonsreachthespec-trometerafterhavingcrossedthecalorimeters.Thebackgroundfrom√annihilationprocessesissuppressedbyrequiringthemuonmomentumtobelessthan0.1

209GeV.Includingthe269eventspreviouslyselectedat

√

s=202

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

Figure2presentstheWvisspectraoftheselectedeventsfortheelectronandmuonsamples.

4Results

¯processisdeterminedfromthedistributionoftheThecrosssectionforthee+e →e+e bbX

transversemomentumoftheleptonwithrespecttothenearestjet,Pt.Asaconsequenceofthelargevalueofmb,thedistributionofthisvariableisenhancedforhighvaluesascomparedtothebackground.ThejetsarereconstructedusingtheJADEalgorithm[20]withycut=0.1.Theidenti edleptonisnotincludedinthejet.Figure3presentstheobserveddistributionsofPtforelectronsandmuons.

Thedatadistributionsare ttedusingtheleast-squaresmethodtothesumoffourcontribu-tions,whoseshapesare xedbyMonteCarlosimulations.The rstdescribesthebackgroundfromannihilationprocessesandthee+e →e+e τ+τ reaction.Itsnormalisation,Nbkg,is xedtotheMonteCarlopredictionslistedinTable1.Thethreeothercontributionsarethosefrombquarks,cquarksandlighter avours.Theirnormalisations,Nbb¯,Nc¯candNuds,re-spectively,arethefreeparametersofthe t.Theresultsofthe tsaregiveninTable1:ab-quarkfractionof46.2±5.1%isobservedforelectronsand41.2±3.8%formuons,wheretheuncertaintiesarestatistical.Theχ2perdegreeoffreedomofthe tsisacceptable,withvaluesof13.7/6forelectronsand6.4/6formuons.Acorrelationcoe cientofabout75%betweenNbb¯andNc¯cisobserved.Theresultsofthe tsarealsographicallyshowninFigure3.Figure4presentsthedistributionsoftheleptonmomentum,transversemomentumandcosineofpolarangle.

Themeasuredfractionsofbquarkscorrespondtoobservedcrosssectionsfortheluminosity-√averagedcentre-of-massenergy

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

theeventgenerationisestimatedbyrepeating tsbyusingMonteCarloeventsgeneratedwithmassivematrixelements.Thetriggere ciencyisdeterminedfromthedatathemselvesandfoundtobe(95.6±2.0)%,thisuncertaintyisalsopropagatedtothe nalresults.ThelimitedMonteCarlostatisticshasasmallimpactonthetotalsystematicuncertainty.Inthe ts,thesignaleventsareproducedintwoseparatesamplesforthedirectandresolvedprocessesandthencombinedina1:1ratio[7].Systematicuncertaintiesonthispredictionareestimatedbyrepeatingthe tswithratiosof1:2and2:1.

6DiscussionandConclusions

Theb-productioncrosssectionsmeasuredinthephasespaceoftheselectedleptonsarecom-paredwiththepredictionsobtainedwiththeCASCADEMonteCarloprogram[21].Thisgeneratoremploysabackward-evolvingpartoncascadebasedontheCCFM[22]equation.Themostimportantdi erenceascomparedtoNLOQCDcalculationsistheuseofanunintegratedgluondensityfunctiontakingexplicitlyintoaccountthetransversemomentumdistributionofinitialstategluonsinhardscatteringprocesses.InNLOQCD,allinitialstatepartonshavevanishingtransversemomentum.CASCADEwasshown[23]togiveaconsistentdescriptionofb-quarkproductionattheTevatron,whereasH1electro-productiondatawasfoundtobeinexcessbyafactorof2.6.BetteragreementwasfoundwithZEUSelectro-productiondata.Thecomparisonofmeasurementsandexpectationsintheactualphasespaceoftheselectedleptonshastheadvantageofprovidinganassessmentoftheagreementbeforeanyextrapolationisperformed.Summingstatisticalandsystematicuncertaintiesinquadrature,one nds:¯observed=0.41±0.11pbσ(e+e →e+e bbX)electrons¯observed=0.56±0.14pbσ(e+e →e+e bbX)muons

¯CASCADE=0.11±0.02pbσ(e+e →e+e bbX)electrons¯CASCADE=0.14±0.02pbσ(e+e →e+e bbX)muons

wheretheuncertaintyontheCASCADEpredictionscorrespondstoavariationofmbinthe

range4.75±0.25GeV[24].Adisagreementofaboutthreestandarddeviationsisobservedforboth avoursofthe nal-stateleptons.Thisdisagreementismostlyduetotheoverallnormalisationofthesampleratherthantoadi erenceinshapeofthemostrelevantkinematicvariables,asalsoshowninFigures3and4.

Thetotalcrosssectionforopen-beautyproductioninphoton-photoncollisionsisdeterminedbyanextrapolationoftheobservedcrosssectiontothefullphasespaceoftheprocessandbycorrectingforthesemi-leptonicbranchingratioofbquarks.TheextrapolationfactorsaredeterminedwiththePYTHIAMonteCarloprogram,andsimilarresultsareobtainediftheCASCADEMonteCarloisused.Theirdi erence,whichamountsto3%,isconsideredasanadditionalsystematicuncertainty.Theexperimentaluncertaintiesonthesemi-leptonicbranchingratioofbquarks[25]isalsopropagatedtothemeasurement.Theresultsfortheelectronandmuon nalstatesread:

¯totalσ(e+e →e+e bbX)electrons=12.6±2.4±2.3pb¯total=13.0±2.4±2.3pb,σ(e+e →e+e bbX)muons

wherethe rstuncertaintyisstatisticalandthesecondsystematic.Theseresultsareinperfect

agreementwitheachotherandtheircombinationgives:

¯total=12.8±1.7±2.3pb,σ(e+e →e+e bbX)

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

where,again,the rstuncertaintyisstatisticalandthesecondsystematic.Thisresultisinagreementwithourpreviousmeasurementperformedwithjustasubsetofthedatainvestigatedhere[8]andhasanimprovedprecision.Asacrosscheck,thevaluesofNc¯cfoundbythe tareusedtoextractthetotalcrosssectionfortheproductionofopencharmattheluminosity-averagedcentre-of-massenergy√

s =194GeV,wheretheuncertaintiesarestatisticalonly.

Anadditionalcrosscheckshowedthatvaluesoftheopen-beautycrosssectiondeterminedwiththe tprocedurediscussedaboveorwithacountingmethod[8]arecompatible.Inthelattercaseexperimentalcriteriawerechosentooptimisethecharmcrosssectionmeasurementyieldingaresultessentiallyuncorrelatedwiththeb-quarkproductionrate.

Thetotalcrosssectionforopen-beautyproductioniscomparedinFigure5toNLOQCDcalculations[7].Thedashedlinecorrespondstothedirectprocesswhilethesolidlineshowsthepredictionforthesumofdirectandresolvedprocesses.Thecrosssectiondependsonmb,whichisvariedbetween4.5GeVand5.0GeV.Thethresholdforopen-beautyproductionissetat10.6GeV.ThetheorypredictionfortheresolvedprocessiscalculatedwiththeGRVpartondensityfunction[26].ThesameresultsareobtainediftheDrees-Grassiepartondensityfunction[27]isused.Forcompleteness,Figure5alsocomparesthecrosssectionsforopen-charmproduction√measuredinReferences8and28withthecorrespondingpredictions.For

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

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The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

1III.PhysikalischesInstitut,RWTH,D-52056Aachen,Germany§

2NationalInstituteforHighEnergyPhysics,NIKHEF,andUniversityofAmsterdam,NL-1009DBAmsterdam,TheNetherlands

3UniversityofMichigan,AnnArbor,MI48109,USA

4Laboratoired’Annecy-le-VieuxdePhysiquedesParticules,LAPP,IN2P3-CNRS,BP110,F-74941Annecy-le-VieuxCEDEX,France

5InstituteofPhysics,UniversityofBasel,CH-4056Basel,Switzerland6LouisianaStateUniversity,BatonRouge,LA70803,USA

7InstituteofHighEnergyPhysics,IHEP,100039Beijing,China△

8UniversityofBolognaandINFN-SezionediBologna,I-40126Bologna,Italy9TataInstituteofFundamentalResearch,Mumbai(Bombay)400005,India10NortheasternUniversity,Boston,MA02115,USA

11InstituteofAtomicPhysicsandUniversityofBucharest,R-76900Bucharest,Romania

12CentralResearchInstituteforPhysicsoftheHungarianAcademyofSciences,H-1525Budapest114,Hungary 13MassachusettsInstituteofTechnology,Cambridge,MA02139,USA14PanjabUniversity,Chandigarh160014,India15KLTE-ATOMKI,H-4010Debrecen,Hungary¶

16DepartmentofExperimentalPhysics,UniversityCollegeDublin,Bel eld,Dublin4,Ireland17INFNSezionediFirenzeandUniversityofFlorence,I-50125Florence,Italy

18EuropeanLaboratoryforParticlePhysics,CERN,CH-1211Geneva23,Switzerland19WorldLaboratory,FBLJAProject,CH-1211Geneva23,Switzerland20UniversityofGeneva,CH-1211Geneva4,Switzerland21UniversityofHamburg,D-22761Hamburg,Germany

22ChineseUniversityofScienceandTechnology,USTC,Hefei,Anhui230029,China△23UniversityofLausanne,CH-1015Lausanne,Switzerland24InstitutdePhysiqueNucl´eairedeLyon,IN2P3-CNRS,Universit´eClaudeBernard,F-69622Villeurbanne,France25CentrodeInvestigacionesEnerg´eticas,MedioambientalesyTecnol´ogicas,CIEMAT,E-28040Madrid,Spain 26FloridaInstituteofTechnology,Melbourne,FL32901,USA27INFN-SezionediMilano,I-20133Milan,Italy

28InstituteofTheoreticalandExperimentalPhysics,ITEP,Moscow,Russia29INFN-SezionediNapoliandUniversityofNaples,I-80125Naples,Italy30DepartmentofPhysics,UniversityofCyprus,Nicosia,Cyprus

31RadboudUniversityandNIKHEF,NL-6525EDNijmegen,TheNetherlands32CaliforniaInstituteofTechnology,Pasadena,CA91125,USA33INFN-SezionediPerugiaandUniversit`aDegliStudidiPerugia,I-06100Perugia,Italy34NuclearPhysicsInstitute,St.Petersburg,Russia

35CarnegieMellonUniversity,Pittsburgh,PA15213,USA

36INFN-SezionediNapoliandUniversityofPotenza,I-85100Potenza,Italy37PrincetonUniversity,Princeton,NJ08544,USA38UniversityofCaliforna,Riverside,CA92521,USA

39INFN-SezionediRomaandUniversityofRome,“LaSapienza”,I-00185Rome,Italy40UniversityandINFN,Salerno,I-84100Salerno,Italy41UniversityofCalifornia,SanDiego,CA92093,USA

42BulgarianAcademyofSciences,CentralLab.ofMechatronicsandInstrumentation,BU-1113So a,Bulgaria43TheCenterforHighEnergyPhysics,KyungpookNationalUniversity,702-701Taegu,RepublicofKorea44NationalCentralUniversity,Chung-Li,Taiwan,China

45DepartmentofPhysics,NationalTsingHuaUniversity,Taiwan,China46PurdueUniversity,WestLafayette,IN47907,USA

47PaulScherrerInstitut,PSI,CH-5232Villigen,Switzerland48DESY,D-15738Zeuthen,Germany49Eidgen¨ossischeTechnischeHochschule,ETHZ¨urich,CH-8093Z¨urich,Switzerland§SupportedbytheGermanBundesministeriumf¨urBildung,Wissenschaft,ForschungundTechnologie. SupportedbytheHungarianOTKAfundundercontractnumbersT019181,F023259andT037350.¶AlsosupportedbytheHungarianOTKAfundundercontractnumberT026178. SupportedalsobytheComisi´onInterministerialdeCienciayTecnolog´ a.

AlsosupportedbyCONICETandUniversidadNacionaldeLaPlata,CC67,1900LaPlata,Argentina.△SupportedbytheNationalNaturalScienceFoundationofChina.

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

Electrons

NbkgNbb¯Nc¯cNuds

χ2/d.o.f.

24.8( xed)172.0±31.0220.5±35.4

.3

0.0+52 0.06.4/6

Table1:Resultsofthe ttothedistributionofthetransversemomentumoftheleptonwithrespecttothenearestjet.The tparametersareconstrainedtobepositive.ThecorrelationbetweenNbb¯andNc¯cis75%.

Uncertaintyoncrosssection(%)

Electrons

10.42.210.08.23.02.01.41.0

18.3

Eventselection

Leptonidenti cationFiducialvolumeJetreconstruction

Massive/masslesscharmTriggere ciency

MonteCarlostatisticsDirect/resolvedratio

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

e

γ

e b¯be+

γ

e+

Direct

e

γg

e

spectatorjetb

γ

¯be+

e+

Singleresolved

Figure1:Dominantdiagramscontributingtoopen-beautyproductioninphoton-photoncolli-sionsatLEP.

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

80

60

Events/5 GeV

40

20

Wvis (GeV)

100

75

Events/5 GeV

50

25

Wvis (GeV)

+ +

Figure2:Visible-massspectrafortheselectedee→eehadronseventscontaining(a)an√electronor(b)amuoncandidateat

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

10

10

Events/100 MeV

1

10

10

Pt (GeV)

10

10

Events/100 MeV

1

10

10

Pt (GeV)

Figure3:Distributionsofthetransversemomentumofa)theelectroncandidateandb)themuoncandidatewithrespecttotheclosestjetforthedataandtheresultsofthe t.Thepointsarethedatawhilethedottedlinerepresentsthebackgroundfromthee+e →q¯q,e+e →τ+τ ,e+e →W+W ande+e →e+e τ+τ processes.Thedashedlinesarethesumofthisback-groundandthelight-quarkcontribution,whilethesolidlinesalsoincludeb-quarkproduction.Thenormalisationfollowsfromthe tdiscussedinthetext.

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

10

10

10

10

Events/1 GeV

10

Events/1 GeV

10

11

10

10

10

10

10

10

Events/1 GeV

10

Events/1 GeV

10

11

10

10

60

150

100

Events/0.2

40

Events/0.2

50

20

cos Θcos Θ

Figure4:Distributionofa)theleptonmomentum,c)itstransversemomentumande)thecosineofitspolarangleforeventscontainingelectronsandb),d)andf)foreventscontainingmuons.Thepointsarethedatawhilethedottedlinerepresentsthebackgroundfromthee+e →q¯q,e+e →τ+τ ,e+e →W+W ande+e →e+e τ+τ processes.Thedashedlinesarethesumofthisbackgroundandthelight-quarkcontribution,whilethesolidlinesalsoincludeb-quarkproduction.Thenormalisationfollowsfromthe tdiscussedinthetext.

The cross section for open-beauty production in photon-photon collisions is measured using the whole high-energy and high-luminosity data sample collected by the L3 detector at LEP. This corresponds to 627/pb of integrated luminosity for electron-positron

σ(ee → eecc,bb X) pb

10

–

+-–

10

+-

10

1

Figure5:Theopen-charm,upper,andopen-beauty,lower,productioncrosssectionsinphoton-photoncollisionsmeasuredwiththeL3detector.Statisticalandsystematicuncertaintiesareaddedinquadrature.Thedashedlinescorrespondtothedirect-processcontributionandthesolidlinesrepresenttheNLOQCDpredictionforthesumofthedirectandsingle-resolvedprocesses.Thee ectsofadi erentchoiceofthevaluesofthequarkmasses,mcandmb,areillustrated.

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