Baryon production in ALEPH

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

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BaryonProduction

inALEPH

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aPresentedonbehalfoftheALEPHCollaborationbyU.Becker1Institutf¨urHochenergiephysikderUniversit¨atHeidelbergHeidelberg,GermanyAbstractSeveralrecentresultsoftheALEPHCollaborationcoveringdi erentaspectsofbaryonproductionontheZresonancearepresented.Inparticularproductionratesofhyperons,thefullkinematicalreconstructionoftheΛb,observationofΞbinitssemileptonicdecay,andthemeasurementsofthepolarizationofΛandΛbbaryonsarediscussed.

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

1Introduction

Baryonproductioninquarkfragmentationisstillpoorlyunderstood.Theexistingmodelsreproducetheavailabledataquitewell.Todistinguishbetweenthesemodelsandtocometoamorefundamentalunderstandingoftheproductionmechanismofbaryonsmoreexperimentalinputisneeded.ResultsoftheALEPHCollaborationonhyperonandb–baryonproductioninZdecaysarereportedhere.Wheneverpossible,notonlytheoverallproductionratesbutalsothemomentumspectraofbaryonshavebeeninvestigatedandcomparedtoMonteCarlopredictions.

Whilethenumberofproducedhyperonsisquitelargetheproductionrateofb–baryonsislow.OnlyfewofthemhavebeenobservedbyLEPexperimentssofar.Thefullrecon-structionandmassdeterminationoftheΛbandtheobservationoftheΞbthroughΞ–leptoncorrelationswillbedescribed.

ZdecaysprovidemanypossibilitiestotesttheStandardModel.QuarksproducedinZdecaysareexpectedtoobtainastronglongitudinalpolarizationduetoparityviolationintheelectroweakinteraction.Whilemesonsalwayscascadedowntospinzeropseudoscalarstatesretainingnopolarizationinformation,baryonsmightpreservealargefractionoftheinitialquarkpolarization.RecentmeasurementsoftheΛandΛbpolarizationsarepresentedhere.

ThedatasampleusedforthesestudieshasbeentakenwiththedetectorALEPHattheLEPstorageringatacenterofmassenergyofabout91GeV.Thedetectoranditsperformancearedescribedindetailelsewhere[1].2Hyperonproduction

ALEPHhasstudiedrecentlytheinclusiveproductionratesofseveraloctetanddecuplethyperons:Σ±(1385),Ξ ,Ξ0(1530)and [2].Exceptforthe alsotheenergyspectrahavebeenmeasured.Thefourhyperonshavebeenreconstructedintheirdecays:

Σ±(1385)

Ξ

Ξ0(1530)

→→→→Λπ±Λπ Ξ π+

ΛK

wheretheΛcandidatesaretaggedinthedecaychannelΛ→pπ .TheresultingmassdistributionsareshowninFig.1a-d.

Exceptforthe (duetolowstatistics)theenergyspectrahavebeendeterminedandcomparedwithMonteCarlopredictionsfromJETSET7.3[6]andUCLA[7](Fig.2).TheagreementisquitereasonablewithslightpreferencesforJETSET.

ThemeanmultiplicitiesperZdecayhavethenbeendeterminedusingJETSETtoextrapolateintotheinaccessiblelowenergyregion.

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

Figure1:(a)ThesignalforΞ →Λπ .(b)Thesignalfor →ΛK .(c)ThemassspectrumforΣ±(1385)→Λπ±, ttoabackgroundshape,aΞ contribution,plusaBreit–Wignerresonance.Alsoshownisthebackground–subtractedspectrum.(d)Themass–di erencespectrumforΞ0(1530)→Ξ π+candidates, ttothebackgroundshapeplusaBreit–Wignerresonanceconvolvedwithagaussian.

Thepreliminaryresultsare:

NΞ + NΞ¯+

NΞ0(1530) + NΞ¯0(1530)0

NΣ±(1385) + NΣ(1385)± ¯ N + N ¯+ ====0.0285±0.0007±0.00200.0072±0.0004±0.00060.065±0.004±0.0090.0010±0.0002±0.0002

WhileourmeasurementoftheΣ±(1385)rateisingoodagreementwiththeMonteCarlopredictionitisneverthelessabout1.7timeshigherthanthecorrespondingmeasurementsfromDELPHIandOPAL[3],[4],[5].Fig.3givesanoverviewaboutthehyperonproductionratesatLEP[8].

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

Figure2:ThemeasuredxEdistributionforΣ±(1385),Ξ ,andΞ0(1530)(withantiparticlesincluded),comparedwithpredictionsfromtheUCLAandJETSET7.3models.

Figure3:ProductionratesofhyperonsatLEPtogetherwithpredictionsofJETSET,HER-WIGandUCLAmodels.Linesaredrawntoguidetheeye.

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

3b–baryons

Thesituationinthesectorofheavybaryonscontainingab–quarkisquitedi erent.Duetotheirlowproductionratescombinedwithsmallbranchingratios(intheorderofafew%)andlongdecaychainsonlyveryfewb–baryonshavebeenobservedatLEPsofar,namelytheΛb,Ξb(whichwillbepresentedhere)andΣb,Σ

bbytheDELPHICollaboration

[9].3.1TheΛb TheΛbhasbeenobservedindirectlysinceseveralyearsinΛl andΛ+clcorrelations(where

l=muonorelectron).Itsproductionratehasbeendeterminedbye.g.ALEPHtobeBr(b→Λb)·Br(Λb→Λ+¯X)=(0.76±0.15±0.13)10 3[10]butitsmassremainedonlyclν

poorlyknown.NowtheΛbhasbeenfullyreconstructedinitsdecaychannelΛb→Λ+cπ

¯0withΛ+c→pKπ,pKorΛπππ.Severalcutshavebeenappliedtoreducethebackground.

TheresultisshowninFig.4

Thebackgroundhasbeenestimatedtobe0.38±0.05entriesleavinguswithastatisti-calsigni canceoftheobservedpeakatthe3.3σlevel,nottakingintoaccountthemassclusteringofthefourevents.TheΛbmasshasbeenfoundtobe(5614±21(stat.)±4(sys.))MeV/c2[11].ThisisingoodagreementwiththerecentmeasurementsofDEL-PHI(5668±16±8)MeV/c2[12]andCDF(5623±5±4)MeV/c2[13]andthePDG94value(5641±50)MeV/c2[14].

Figure4:(a)Λcπinvariantmassdistributionsfortheright–signcombinationand(b)forthewrong–signcombinations.(c)Λbinvariantmassesforthefourselectedcandidates.Thedottedlinesindicatethe±1σvaluesaroundtheALEPHaveragemeasurement.

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

3.2TheΞb

AsearchforΞ l correlations(againlstandsformuonorelectron)inthesamehemisphereofthedetectorhasbeenmadeto ndevidenceforΞbproduction.AftercorrectingforotherpossiblesourcesofΞ l pairsase.g.ΛbBoraccidentalsanexcessof22.5±5.7like–signcombinationswithrespecttoopposite–signcombinationshasbeenobserved.ItisthereforeinterpretedasfromthesemileptonicdecayoftheΞbandcanbetranslatedintothefollowingproductofbranchingfractions:

Br(b→Ξb)·Br(Ξb→Ξ l ν¯X)=(5.3±1.3±0.7)10 4

perleptonspecies[15].ThisvalueisingoodagreementwiththeDELPHIresult[16].4Polarization

PolarizationoftheprimaryquarkscomingfromZdecaysisduetotheparityviolatinginequalityoftheZcouplingtorightandlefthandedfermions.Ignoringmasse ectsandgluonradiation(whichmaydepolarizethequarksbyabout3%[17])theexpectedpolarizationcanbewrittenas 2gvgaq=PL

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

beendeterminedinthedatabymeasuringtheΛforward–backwardasymmetryandthe

¯pairs.Thedi erencesbetweendataandMChavebeencorrelationofback–to–backΛΛ

usedascorrectionfactorsforallpossiblesourcesofpolarizedΛ’s(see

[21]fordetails).ThenthefractionofΛbaryonscomingfrome.g.Λb,Σetc.hasbeencorrected,andtheexpectedpolarizations[20]havebeenaddedup.TheALEPHmeasurementisshowninFig.5togetherwiththeexpectation.ForΛ’swithz=p(Λ)/PBeam>0.3thepolarization

ΛhasbeendeterminedtobePL= 0.32±0.07[21],ingoodagreementwiththeJETSET

predictionof 0.39±0.08.

0.6

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00.40.30.20.10Figure5:Right:FitofthelongitudinalΛpolarizationtothedecayangledistributionforz>0.3.

Left:ThemeasuredlongitudinalΛpolarizationisshownasasfunctionofztogetherwiththeJETSETpredictionaftermultiplyingbythecorrectionfactor(shownassolidline).ThedashedlinesindicatetheestimateduncertaintyoftheJETSETprediction.TheHERWIGpredictionaftermultiplicationbythecorrectionfactorisshownasdottedline.

4.2TheΛbpolarization

Thereisnoneedtoestimatethe‘primary’Λbfractionsinceb–quarksnearlyalwaysoriginatedirectlyfromZdecays.IthasbeensuggestedforsometimetomeasuretheΛbpolarization(whichisequaltotheb–quarkpolarizationintheheavyquarklimit)initssemileptonicdecay[22].

Apartoftheinterestinthepolarizationitself,weakdecaysofpolarizedb–quarksprovideanotherinterestingaspect.IthasbeenpointedoutbyGronauandWakaizumi[23]that

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

thelefthandednessoftheb–quarkcouplingtothechargedweakcurrenthasneverbeenproven.Theyhaveshownthat,assumingthesamehandednessfortheleptonsinvolvedinthedecay,thechiralitycouldnotbedeterminedinb–mesondecay.Amundsenet.al.[24]suggestedtoresolvethisambiguitythroughaninvestigationofthesemileptonicdecayofpolarizedb–quarkswhichareavailableinΛb’sfromtheZresonance.Whenderivingthepolarizationfromtheleptonspectra,aV+Acouplingwouldresultinachangeofsignofthemeasuredwithrespecttothetruepolarization.ThisisinprincipleanalogoustothedeterminationoftheMichelparameterξinτdecays.

TheALEPHanalysisoftheΛbpolarization[25]hasfollowedasuggestionofBonviciniandRandall[26]whoproposedtostudytheratioyofthemeanenergiesofchargedleptonsandneutrinosfromsemileptonicΛbdecays.Thisprocedureo ersseveraladvantages:In-dependencefromtheΛbfragmentationfunction,greatersensitivitythanthechargedleptonaloneandmorestatisticsthananyexclusivedecay.

ForthispurposeΛπ+l pairshavebeenselected.Theneutrinoenergyiscalculatedfrom√thevisibleamountofenergyinthedetector:Eν=Etot Evis,withEtot=

s.HereMsame(Moppo)istheinvariantmassinthesame(opposite)hemisphere.BiasesduetoselectioncutsandneutrinoenergyreconstructionhavebeencorrectedwithMonteCarlosimulations.

Ry2

ALEPH

Ry = 1.09 ± 0.10

1.5expPΛb = 0.23 0.20+0.24

1

0.5

00. 0.25 0.50 0.75 1.

PΛb

Figure6:ThemethodtoextracttheΛbpolarizationvalue:ComparisonbetweenthemeasuredexpRyvalueandthetheoreticalpredictionRy

Several recent results of the ALEPH Collaboration covering different aspects of baryon production on the Z resonance are presented. In particular production rates of hyperons, the full kinematical reconstruction of the Lambda_b, observation of Xi_b in its

expThepolarizationinthedataisextractedbycomparingRy=ydata/yMC(P=0)(where

yMC(P=0)istheexpectedratiobetweenleptonandneutrinomeanenergywithoutanypolarization)withtheexpectedRycurveandcorrectingfordi erencesbetweenDataandMonteCarlo(Fig.6):

.24+0.08PΛb= 0.23+0 0.20(stat.) 0.07(syst.)

Thisresultcanbecomparedwithanexpectationofabout-0.73estimatedfrom[19]together

( )withtheΣbproductionratesmeasuredbyDELPHI[9].Itisquitelow(butconsistentwith

theexpectation),indicatingadditionaldepolarizatione ectsduringhadronization.

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