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