Measurement of the Cross Section for Open-Beauty Production(3)

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