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)