Baryon production in ALEPH
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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
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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
1.25
0.5
1
0.75
0.5
0.25
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.
References
[1]D.Decampetal.,ALEPHCollaboration,Nucl.Instrum.Meth.A294(1990)121
D.Buskulicetal.,ALEPHCollaboration,Nucl.Insttrum.Meth.A360(1995)99
[2]D.Buskulicetal.,ALEPHCollaboration,HyperonproductioninZdecays,contributed
papertoEPS–HEP95,Brussel,Belgium
[3]P.D.Actonetal.,OPALCollaboration,Phys.Lett.B291(1992)503
[4]P.Abreuetal.,DELPHICollaboration,CERN-PPE/96-12,SubmittedtoZ.Phys.C
[5]P.D.Actonetal.,OPALCollaboration,ProceedingsoftheAugust1994meetingofthe
APSDivisonofParticlesandFields,Albuquerque,N.M.;OPALPhysicsNotePN138
[6]M.BengtsonandT.Sj¨ostrand,Phys.Lett.B185(1987)435;T.Sj¨ostrand,CERN–
TH.6488/92(1992)
[7]S.B.ChunandC.D.Buchanan,Phys.Lett.B308(1993)153.Curvesobtainedfrom
C.D.Buchananbyprivatecommunication.
[8]NumberstakenfromP.V.ChliapnikovandV.A.Uvarov,Phys.Lett.B345(1995)313
[9]P.Abreuetal.,DELPHICollaboration,contributedpapertoEPS–HEP,Brus-
sel,Belgium
[10]D.Buskulicetal.,ALEPHCollaboration,Phys.Lett.B357(1995)685
[11]D.Buskulicetal.,ALEPHCollaboration,CERN-PPE/96-28,submittedtoPhys.
Lett.B
[12]P.Abreuetal.,DELPHICollaboration,Phys.Lett.B374(1996)351
[13]G.Bauer,talkgivenatthe2ndRencontresduVietnam,HoChiMinhCity,October
21-28,1995,CDF/PUB/BOTTOM/PUBLIC/3562
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
[14]ParticleDataGroup.Phys.Rev.D50(1994)1173
[15]D.Buskulicetal.,ALEPHCollaboration,CERN-PPE/96-81,submittedtoPhys.
Lett.B
[16]P.Abreuetal.,DELPHICollaboration,Z.Phys.C68(1995)541
[17]J.K¨orner,A.Pilaftsis,andM.M.Tung,Z.Phys.C63(1994)575
[18]F.Close,J.K¨orner,R.J.N.Phillips,andD.J.Summers,J.Phys.G18(1992)1716
[19]A.F.FalkandM.E.Peskin,Phys.Rev.D49(1994)3320
[20]G.GustafsonandJ.H¨akkinen,Phys.Lett.B303(1993)350
[21]D.Buskulicetal.,ALEPHCollaboration,Phys.Lett.B374(1996)319
[22]J.K¨ornerandM.Kr¨amer,Phys.Lett.B275(1992)495
T.MannelandG.A.Schuler,Phys.Lett.B279(1992)194
G.AltarelliandB.Mele,Phys.Lett.B299(1993)345
J.F.Amundson,J.L.Rosner,M.Worah,andM.B.WisePhys.Rev.D47(1993)1260
[23]M.GronauandS.Wakaizumi,Phys.Rev.Lett.68(1991)1814
[24]J.F.Amundson,J.L.Rosner,M.Worah,andM.B.Wise,Phys.Rev.D47(1993)1260
[25]D.Buskulicetal.,ALEPHCollaboration,Phys.Lett.B365(1996)437
[26]G.BonviciniandL.Randall,Phys.Rev.Lett.73(1994)392
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