Search for weakly decaying (Lambda n)over-bar and Lambda Lambda exotic bound states in central Pb-Pb collisions at root S-NN=2.76 TeV

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REPOSITÓRIO DA PRODUÇÃO CIENTIFICA E INTELECTUAL DA UNICAMP

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https://www.sciencedirect.com/science/article/pii/S0370269315009041

DOI: 10.1016/j.physletb.2015.11.048

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©2016

by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

CEP 13083-970 – Campinas SP

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Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Search

for

weakly

decaying



n and



exotic

bound

states

in

central

Pb–Pb

collisions

at

√

s

_NN

=

2

.

76 TeV

.

ALICE

Collaboration



a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received16July2015

Receivedinrevisedform6November2015 Accepted16November2015

Availableonline28November2015 Editor:L.Rolandi

We present results ofasearch for two hypothetical strange dibaryon states, i.e. the H-dibaryonand thepossible n boundstate.The searchis performedwiththe ALICEdetectorincentral (0–10%)Pb– Pb collisions at√sNN=2.76 TeV, byinvariant mass analysisinthe decaymodes n→d

π

+ and

H-dibaryon→ p

π

−.Noevidencefortheseboundstatesisobserved.Upperlimitsaredeterminedat99% confidencelevelforawiderangeoflifetimesandforthefullrangeofbranchingratios.Theresultsare comparedtothermal,coalescenceandhybridUrQMDmodelexpectations,whichdescribecorrectlythe productionofotherlooselyboundstates,likethedeuteronandthehypertriton.

©2015CERNforthebenefitoftheALICECollaboration.PublishedbyElsevierB.V.Thisisanopen accessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

ParticleproductioninPb–PbcollisionsattheLargeHadron Col-lider(LHC)hasbeenextensivelystudied[1–3].Theobserved pro-duction pattern is rather well described in equilibrium thermal models[4–7].Withinthisapproach,thechemical freeze-out tem-perature Tchem, the volume V and the baryo-chemical potential

μ

B are theonlythreefree parameters.Evenlooselyboundstates suchasthedeuteronandhypertritonandtheiranti-particleshave beenobserved[8–10]andtheirrapiditydensitiesareproperly de-scribed[11–17].Consequentlyother looselyboundstates1 _suchas

theH-dibaryonandthe



n areexpectedtobeproducedwith cor-respondingyields.

ThediscoveryoftheH-dibaryonorthe



n boundstatewould be abreakthrough inhadron spectroscopy asit wouldimplythe existence of a six-quark state and provide crucial information on the



-nucleon and



–



interaction. We consequently have startedthe investigationon the possible existence ofsuch exotic boundstates inpp andPb–Pb collisions atthe LHC.Searches for



-nucleon bound states in the



p and



n channels have been carriedout (seeRefs. [18–20]). The H-dibaryon,whichisa hypo-thetical bound state of uuddss

()

,was firstpredicted by Jaffe usingabagmodelapproach[21].Experimentalsearcheshavebeen undertaken since then, but no evidence for a signal was found (see [22,23] and the references therein). Recently, the STAR Col-laboration investigatedthe



–



interactionthroughthe

measure- _{E-mailaddress:alice-publications@cern.ch.}

1 _{TheexpectedmassesofthesestatesaresomeMeVbelowthesumofthemass}

oftheirconstituents.

ment of



correlations [24]; thisand a theoretical analysis of thesedata[25]didnotrevealasignal.Manytheoretical investiga-tionsofthepossiblestabilityoftheH-dibaryonhavebeencarried out,butpredictingbindingenergiesintheorderofMeVformasses ofaround2 GeV

/

c2 isextremelydifficultandchallenging[26–29]. Ourapproachistosearch forsuchbound statesincentralPb– PbcollisionsatLHCenergies whererapidity densitiescanbewell predictedbythermal[16,17,30]andcoalescence[31] models.The modelpredictionsforrapiditydensitiesoftheseparticlesareused andtestedagainsttheexperimentalresults.

In this paper the analysis strategies for the searches of the



n

→

d

π

+ bound state and the H-dibaryon

→ 

p

π

− are pre-sented. Theanalysisfocuseson the



n boundstate because pro-duction ofanti-particlesin the detectormaterial isstrongly sup-pressedandthussecondarycontaminationofthesignalisreduced. FortheH-dibaryonboththe



andtheporiginatefromsecondary vertices whereknock-outbackground islesslikely. No search for the anti-H is performed yet, although it is assumed to be pro-ducedwithequalyieldbutthemeasurementdependsstronglyon the absorption correction.We beginwith a shortintroductionto theALICEdetectoranda descriptionofthe particleidentification techniqueusedtoidentifythedecaydaughtersandreconstruct in-variantmassdistributions.Toassessthepossibleexistenceofthese stateswecomparetheexperimental distributionswiththemodel predictions.

2. Detectorsetupanddatasample

TheALICEdetector[32]isspecificallydesignedtostudy heavy-ion collisions. The central barrelcomprising the two main track-ing detectors, theInner TrackingSystem (ITS) [33] andthe Time

http://dx.doi.org/10.1016/j.physletb.2015.11.048

0370-2693/©2015CERNforthebenefitoftheALICECollaboration.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

Projection Chamber (TPC) [34] is housed in a large solenoidal magnetproviding a 0.5 Tfield. The detector pseudorapidity cov-erageis

|

η

|

≤

0.9over thefullazimuth. Anadditionalpartofthe central barrel are detectors inforward direction used mainly for triggeringandcentralityselection.TheVZEROdetectors,two scin-tillation hodoscopes, are placed on either side of the interaction point andcover the pseudorapidity regions of 2

.

8

<

η

<

5

.

1 and

−

3

.

7

<

η

<

−

1

.

7.Thecentralityselection isbasedonthe sumof the amplitudes measured in both detectors as described in [35] and[36].

TheITSconsistsofsixcylindricallayersofthreedifferenttypes ofsilicondetectors.Theinnermostpartcomprisestwosiliconpixel (SPD) and two silicon drift detector (SDD) layers. The two outer layers are double-sidedsilicon microstrip detectors(SSD). Dueto theprecisespacepoints providedby theITSa highprecision de-terminationofthe collision vertexispossible. Therefore,primary andsecondary particles can be well separated, down to 100 μm precisionatlowtransversemomentum(pT

≈

100 MeV

/

c).

TheTPCis themaintracking detectorofALICEandsurrounds theITS. It has acylindricaldesign witha diameterof

≈

550 cm, aninnerradiusof85 cm,anouterradiusof247 cmandanoverall lengthinthebeamdirectionof

≈

510 cm.The88 m3 gasvolume of the TPC is filled with a mixture of 85.7% Ne, 9.5% CO2 and

4.8% N2. When a charged particle is travelling through the TPC,

it ionizes the gas along its path and electrons are released. Due totheuniformelectricfieldalongthez-axis(paralleltothebeam axisandtothemagneticfield)theelectronsdrifttowardstheend plates, where the electric signals are amplified and detected in 557 568 pads.These data are used to calculate a particle trajec-toryinthemagneticfieldandthusdeterminethetrackrigidity p_z (themomentump oftheparticledividedbyitschargenumber z). TheTPCisalsoused forparticle identificationviatheenergy de-positdE

/

dx measurement(seesection3).

A complete description ofthe performance of the ALICE sub-detectorsinpp,p–PbandPb–Pbcollisionscanbefoundin[37].

The searches carriedout andreportedhere are performedby analysing the data set of Pb–Pb collisions from2011. In the de-scribed analyses we use 19

.

3

×

106 events with a centrality of 0–10%,determined by theaforementioned VZERO detectors from thepreviouslymentionedcampaign.

3. Particleidentification

ThepreciseParticleIDentification(PID)andcontinuoustracking fromvery low pT (100 MeV

/

c) tomoderatelyhighpT (20 GeV

/

c)

is a unique feature of the ALICE detector at the LHC. The PID used in the analysis described in this letter takes advantage of twodifferenttechniques.Theenergydeposit

(

dE

/

dx

)

andrigidity aremeasuredwiththeTPCforeachreconstructedcharged-particle trajectory.Thisallowstheidentificationofallchargedstable parti-cles,fromthelightest(electron)totheheaviestones(anti-alpha). TheenergydepositresolutionoftheTPCincentralPb–Pbcollisions (investigatedhere)isaround7%.Thecorresponding particle sepa-ration poweris demonstrated in Fig. 1. This technique was used inthefollowingtoidentifythedeuterons,protons andpions.The secondmethodmakesuseofspecifictopologiesfromweakdecays, whichresultintypicalV0decaypatterns.Thisisusedhereforthe detectionof the



n bound state andthe two V0 decay patterns ofthe



,namely forthe



identification andthe proton–pion decayvertex.

4. Analysis

Thestrategies ofinvestigation forthetwo exoticbound states discussed hereare quite similar. They bothrequire the detection

Fig. 1. TPC dE/dx spectrumfornegativeparticlesinasampleofthreedifferent trig-gertypes(minimumbias,semi-centralandcentral).Thedashedlinesare parametri-sationsoftheBethe–Bloch-formula[38–40]forthedifferentparticlespecies.

ofasecondary vertex,whichinonecaseisapure V0 _andinthe

second adoubleV0 decaypattern.Wediscussthemseparatelyin the following sub-sections.First we describe briefly the common aspectsofbothanalyses.

Thetracksusedintheanalyseshavetofulfilasetofselection criteria to ensure high tracking efficiency and dE

/

dx resolution. Each track was required to have at least 70 of up to 159 clus-tersintheTPCattachedtoit,withthe(ratherloose)requirement, that the

χ

2 _{of the momentum fit is smaller than 5 per cluster.}

Trackswithkinksduetoweakdecaysofkaonsandpionsare re-jected.Toachievefinalprecisiontheacceptedtracksarerefitwhile the trackfinding algorithmisrun inwards,outwardsandinwards again(formoredetailsontheALICEtrackingsee[37]andsection 5 of[41]).

V0 decays are determinedby two (or more) trackswhich are emitted fromasecondary vertexandwhichmightcome closeto each other (the minimum distance is called Distance-of-Closest-Approach DCA) while each of the tracks has a certain minimum distance (DCA of the track to a vertex) to the primary vertex. A powerful selectioncriterion fordetecting proper V0 candidates istherestrictionofthepointingangle,namelytheanglebetween the reconstructedflight-lineandthereconstructedmomentum of the V0 _{particle.More detailsof thesecondary vertex}

reconstruc-tion canbe foundin [3,37,41], wherealsotheclearandeffective identificationof



baryonsisdisplayed usingtheaforementioned technique. The selection criteria, described below, are optimised usinga MonteCarlosetwherethe simulatedexoticboundstates are assumedtoliveaslongasafree



baryon. Thisisa reason-able assumption forall strange dibaryons, which areexpected to livearound2–4

×

10−10_{s[42–44] intheregionsofbinding}

ener-giesinvestigatedhere. 4.1.



n boundstate

In analogyto recenthypertritonmeasurements [8,9] we focus here on the expected two-body decay



n

→

d

π

+. For the data analysisthefollowingstrategyisused:firstdisplaced verticesare identifiedusingITSandTPCinformation.Inasecondstepthe neg-ative trackof the V0 _{candidateis identified asan anti-deuteron}

viatheTPCdE

/

dx information.Iftheseconddaughterisidentified asapion,theinvariantmassofthepairisreconstructed.Both par-ticles arerequiredto liewithin a3 standarddeviations(

σ

)band of theexpected Bethe–Blochlines ofthe corresponding particles.

Table 1

Selectioncriteriaforn analysis.

Selection criterion Value

Track selection criteria

Tracks with kinks rejected

Number of clusters in TPC ncl>70

Track quality χ2_/cluster<5

Acceptance in pseudorapidity |η| <0.9

Acceptance in rapidity |y| <1

V0_{and kinematic selection criteria}

Pointing angle  <0.045 rad

DCA between the V0_{daughters DCA<0.3 cm}

Momentum ptotof the anti-deuteron ptot>0.2 GeV/c

Energy deposit dE/dx anti-deuteron dE/dx>110 (fromFig. 1)

PID cut for daughters ±3σ(TPC)

Fig. 2. Invariant mass distributionfor dπ+ for thePb–Pb datacorrespondingto 19.3×106 _{centralevents.Thearrowindicatesthesumofthemassofthe}

con-stituents(n)oftheassumedboundstate.Asignalfortheboundstateisexpected intheregionbelowthissum.Thedashedlinerepresentsanexponentialfitoutside theexpectedsignalregiontoestimatethebackground.

Toidentifythesecondary vertexthetwo daughtertrackshaveto have a DCA smaller than 0.3 cm. Another condition is that the maximumpointing angleis smaller than 0.045 rad (see descrip-tionabove).Deuteronsarecleanlyidentifiedintherigidityregion of 400 MeV

/

c to 1

.

75 GeV

/

c. To limit contamination from other particlespecies, the dE

/

dx hasto be above110units oftheTPC signal,showninFig. 1.

TheselectioncriteriaaresummarisedinTable 1.Theresulting invariantmassdistribution,reflectingthekinematicrangeof iden-tifieddaughtertracks,isdisplayedinFig. 2.

4.2.H-dibaryon

Thesearch fortheH-dibaryonisperformedinthedecay chan-nel H

→ 

p

π

−,with a mass lying inthe range 2

.

200 GeV

/

c2

_<

mH

<

2

.

231 GeV

/

c2 (see Fig. 3). The analysisstrategy forthe

H-dibaryon is similar as for the



n bound state described above, exceptthathereasecond V0-typedecayparticleisinvolved.

One V0 _{candidate originatingfromthe H-dibaryondecay}

ver-tex has to be identified as a



decaying into a proton and a pion.In additionanother V0 decay patternreconstructed froma proton and a pion is required to be found at the decay vertex of the H-dibaryon. First the invariant mass of the



is recon-structedandthenthecandidatesintheinvariantmasswindowof 1

.

111 GeV

/

c2

_<

_m



<

1

.

120 GeV

/

c2 are combinedwith the

four-vectorsofthe proton andpionat thedecay vertex. A 3

σ

dE

/

dx cut in theTPC is used to identifythe protons and thepions for boththe



candidateandthe V0 topologyattheH-dibaryon de-cayvertex.

Fig. 3. Invariant massdistributionfor pπ−forthePb–Pbdatacorrespondingto 19.3×106 _{centralevents.Theleftarrowindicatesthesumofthemassesofthe}

constituents()ofthepossibleboundstate.Asignalfortheboundstateis ex-pectedintheregionbelowthissum.Forthespeculatedresonantstateasignalis expectedbetweentheandthe p (indicatedbytherightarrow)thresholds. Thedashedlineisanexponentialfittoestimatethebackground.

Table 2

Selectioncriteriausedfor(H-dibaryon)analysis.

Selection criterion Value

Track selection criteria

Tracks with kinks rejected

Number of clusters in TPC ncl>80

Track quality χ2_/cluster<5

Acceptance in pseudorapidity |η| <0.9

Acceptance in rapidity |y| <1

V0_{selection criteria}

DCA V0_{daughters DCA<1 cm}

DCA positive V0_{daughter – H decay vertex DCA>2 cm}

DCA negative V0_{daughter – H decay vertex DCA}

>2 cm Kinematic selection criteria

DCA positive H daughter – primary vertex DCA>2 cm DCA negative H daughter – primary vertex DCA>2 cm

DCA H daughters DCA<1 cm

Pointing angle of H  <0.05 rad

PID cut for daughters ±3σ(TPC)

mass window ±3σ

Tocopewiththehugebackgroundcausedbyprimaryand sec-ondarypionsadditionalselectioncriteriahavetobeapplied.Each trackisrequiredtobeatleast2 cmawayfromtheprimaryvertex andthetrackscombinedtoa V0 arerequiredtohaveaminimum distancebelow1 cm. The pointingangleisrequired tobe below 0.05 rad.Allselection criteriaare summarisedinTable 2.The re-sultinginvariantmassisshowninFig. 3.Theshapeoftheinvariant massdistribution iscausedby thekinematic rangeofthe identi-fieddaughtertracks.

5. Systematicsandabsorptioncorrection

Monte Carlosamples havebeen produced to estimate the ef-ficiency for the detection of the



n bound state and the H-dibaryon.The kinematicaldistributionsofthehypotheticalbound states were generated uniformly in rapidity y and in transverse momentum pT.Inordertodealwiththeunknownlifetime,

differ-entdecaylengthsareinvestigated,rangingfrom4 cmup to3 m. Thelowerlimit isdetermined bythesecondaryvertexfinding ef-ficiency and the upper limit by the requirement that there is a significantprobability fordecaysinsidetheTPC2 (thefinal

accep-2 _{FortheH-dibaryonthereisalsoatheoreticalmaximaldecaylengthcalculated}

tance

×

efficiencydropsdownto1%forthe



n and10−3 _forthe

H-dibaryon).Theshapeoftransversemomentumspectrain heavy-ioncollisions is described well by the blast-waveapproach, with radialflowparameter

β

andkinetic freeze-outtemperatureTkin

asin[46].The trueshapeofthe pT spectrum isalsonot known,

thereforeitis estimatedfromthe extrapolationofblast-wavefits todeuterons and3_{He spectraatthe sameenergy[10].Toobtain}

finalefficiencies,theresultingblast-wavedistributionsconstructed fortheexoticboundstatesarenormalisedtounityandconvoluted withthecorrectionfactors(efficiency

×

acceptance).

Typical valuesofthe finalefficiency areofthe orderofa few percentassumingthelifetimeofthefree



.Theuncertaintyinthe shapeofthe pT distributionsisthemainsource ofsystematic

er-ror.Blast-wave fitsof deuteronand3_{He spectra areemployed to}

explore the range of systematic uncertainties. Analyses of these results lead to a systematic uncertainty in the overall yield of around 25%.

Other systematic uncertainties are estimated by varying the cutsdescribed inTable 1andTable 2within thelimitsconsistent withthedetectorresolution.Thecontributionsofthesesystematic uncertainties are typically found to be in the percent range.The combinationof thedifferent sources leads to aglobal systematic uncertaintyofaround 30% forboth analyses,when all uncertain-tiesareaddedinquadrature.

Forthe



n boundstateanalysisthepossibleabsorptionofthe anti-deuteronsandthe bound state itself whencrossing material hastobetakenintoaccount.Forthis,thesameprocedureasused fortheanti-hypertritonanalysis[9]isutilised.Theabsorption cor-rectionrangesfrom3to40%(dependingonthelifetimeofthe



n bound state, which determines the amount of material crossed) withanoveralluncertaintyof7%.

6. Results

No significant signal in the invariant mass distributions has been observed forboth cases,as visible fromFig. 2 and Fig. 3.3

The shapeof theinvariant mass distributionof d

π

+ is ofpurely kinematicorigin,reflectingthemomentumdistributionofthe par-ticles used. The selection criteria listed in Table 1 are tuned to select secondary decays. The secondary anti-deuterons involved in the analysisoriginate mainly from two sources: The first and dominating source are daughters from three-body decays of the anti-hypertriton (3_¯H

→ ¯

dp

¯

π

+ and3_¯H

→ ¯

dn

¯

π

0_{) wherethe other}

decaydaughtersarenotdetected. Theinvariantmassspectrumis obtainedbycombiningthesesanti-deuteronswithpionsgenerated inthecollision.Thesecondsourceisduetopromptanti-deuterons whichareincorrectlylabelledasdisplaced,becausetheyhavesuch lowmomentathattheDCAresolutionofthesetracksisnot suffi-cienttoseparateprimaryfromsecondaryparticles.

Sinceno signalinthe invariantmassdistributions isobserved upper limits are estimated. For the estimation of upper limits for the rapidity density dN

/

d y the method discussed in [47] is utilised.Inparticular,weapplythesoftwarepackageTRolke as im-plemented in ROOT [48]. This method needs as input mass and experimentalwidth(3

σ

)ofthehypotheticalboundstates.The ob-served counts are therefore compared to a smooth background asgivenby an exponential fitoutside thesignal region (as indi-cated by the line in Fig. 2 and Fig. 3). For both candidates



n andH-dibaryonweassumeabindingenergyof1 MeV.Thewidth is determined by the experimental resolution andobtained from

3 _{NotethatahypotheticalH-dibaryonwithamassabovethe}

p thresholdwould notbeobservableinthepresentanalysis.

Fig. 4. Upper limitoftherapiditydensityasfunctionofthedecaylengthshownfor then boundstateintheupperpanelandfortheH-dibaryoninthelowerpanel. Hereabranchingratioof64%wasusedfortheH-dibaryonandabranchingratioof 54%forthen boundstate.Thehorizontal(dashed)linesindicatetheexpectation ofthethermalmodelwithatemperatureof156 MeV.Theverticallineshowsthe lifetimeofthefreebaryon.(Forinterpretationofthereferencestocolourinthis figure,thereaderisreferredtothewebversionofthisarticle.)

Monte Carlo simulations. In addition, the final efficiency which is discussed in section 5 is required. Further, values of branch-ing ratiosoftheassumedboundstatesareneeded. Thesedepend stronglyonthebinding energy.Witha 1 MeVbinding energyfor the



n boundstatethebranchingratiointhed

+

π

+decay chan-nelisexpectedtobe54%[49].Thebranchingratiofora1 MeVor lessboundH-dibaryondecayinginto



p

π

−ispredictedtobe 64%, see[44].

The resulting upperlimits, for99% CL, are showninFig. 4 as a function of the different lifetimes; for the



n bound state in theupperpanelandfortheH-dibaryoninthelowerpanel.These upperlimitsincludesystematicuncertainties. Forthe



n the ab-sorptioncorrectionsarealsoconsideredinthefigure,whichcauses theupperlimitstobeshiftedupwards.

The obtained upper limits can now be compared to model predictions. The rapidity densities dN

/

d y from a thermal model prediction fora chemicalfreeze-out temperatureof,forexample, 156 MeV, are dN

/

d y

=

4

.

06

×

10−2 for the



n bound state and dN

/

d y

=

6

.

03

×

10−3 for the H-dibaryon [16]. These values are indicated withthe (blue) dashed lines in Fig. 4. Forthe investi-gated rangeoflifetimestheupperlimitofthe



n boundstate is atleasta factor20belowthisprediction.Forthe H-dibaryonthe upper limitsdepend morestronglyon the lifetimesince ithas a differentdecaytopology andallfourfinal state trackshavetobe reconstructed.Theupperlimitisafactorof20belowthethermal modelpredictionforthe lifetimeofthefree



andbecomesless stringentathigherlifetimessincethedetectionefficiencybecomes small.Foralifetimeof10−8 s,correspondingtoadecaylengthof 3 m, the differencebetweenmodel andupperlimit reducesto a factortwo.

In order to take the uncertainties in the branching ratio into account, we plotin Fig. 5 theproducts of theupper limit ofthe rapidity density times the branching ratio together with several theorypredictions[16,30,31,50].Thecurvesareobtainedusingthe valueforthe



-lifetimeofFig. 4.

The (red) arrows in the figures indicate the branching ratio fromthetheorypredictions[44,49].Theobtainedupperlimitsare afactorofmorethan5belowalltheorypredictionsfora branch-ingratioofatleast5%forthe



n boundstateandatleast20%for theH-dibaryon.

Fig. 5. Experimentally determinedupperlimit,undertheassumptionofthelifetime ofafree . Intheupperpanel shownfor then boundstateand for the H-dibaryoninthelowerpanel.Itincludes30%systematicuncertaintyforeachparticle and6%correctionforabsorptionwithanuncertaintyof7%forthen boundstate. Thetheorylinesaredrawnfordifferenttheoreticalbranchingratios(BR)inbluefor theequilibriumthermalmodelfrom[16]fortwotemperatures(164 MeVthefull lineand156 MeVthedashedline),ingreenthenon-equilibriumthermalmodel from[30]andinyellowthepredictionsfromahybridUrQMDcalculation[50].The H-dibaryonisalsocomparedwithpredictionsfromcoalescencemodels,wherethe fullredlinevisualisesthepredictionassumingquarkcoalescenceandthedashed redlinecorrespondstohadroncoalescence[31]. (Forinterpretationofthe refer-encestocolourinthisfigurelegend,thereaderisreferredtothewebversionof thisarticle.)

7.Discussion

Thelimitsobtainedon therapiditydensityoftheinvestigated exoticcompound objectsarefound tobemorethan oneorderof magnitudebelowthe expectationsofparticle productionmodels, whenusingarealisticbranchingratioandareasonablelifetime.It hasto be notedthat simultaneously, aclear signal was observed fortheverylooselyboundhypertriton(bindingenergy

<

150 keV) forwhichproductionyieldshavebeenmeasured [9].Theseyields along with those of nuclei A

=

2

,

3

,

4 agree well with the pre-dictionsofthethermalmodeldiscussedabove anddecreasewith eachadditionalbaryonnumberbyroughlyafactor300.Onewould thereforeassumethattheyieldofthe



n,ifsuchparticleexisted, shouldalso be predictedby thismodeland witha value forthe rapidity densityof abouta factor 300higher than the measured hypertritonyield.SimilarconsiderationsholdfortheH-dibaryon.

8. Conclusion

Asearch isreportedfortheexistenceoflooselyboundstrange dibaryons



and



n whose possible existence has been dis-cussed widely in the literature. No signalsare observed. On the otherhand,looselyboundobjectswithbaryonnumberA

=

3 such asthe hypertritonhavebeenmeasured inthesame datasample. Theyields of nuclei[10] andofthe hypertriton[9]are quantita-tivelyunderstoodwithinathermalmodelcalculation.Thepresent analysisprovidesstringentupperlimitsat99%confidencelevelfor theproductionofH-dibaryonand



n boundstate,ingeneral sig-nificantlybelowthe thermalmodel predictions.The upper limits areobtainedfordifferentlifetimes.Thevaluesarewellbelowthe modelpredictions whenrealistic branching ratiosandreasonable lifetimesare assumed. Thus,our resultsdo notsupport the exis-tenceoftheH-dibaryonandthe



n boundstate.

Acknowledgements

WethankS. Beane, M.Petrá ˇn, J. Schaffner-BielichandJ. Stein-heimerforusefulcorrespondence.

The ALICE Collaboration would like to thank all its engineers andtechniciansfortheir invaluablecontributions tothe construc-tionoftheexperimentandtheCERNacceleratorteamsforthe out-standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by all Gridcentres andtheWorldwide LHC ComputingGrid (WLCG) Collaboration. The ALICE Collaboration acknowledges the follow-ing funding agencies for their support in building and running theALICEdetector:StateCommittee ofScience,WorldFederation of Scientists (WFS) andSwiss Fonds Kidagan, Armenia, Conselho Nacional de DesenvolvimentoCientífico e Tecnológico (CNPq), Fi-nanciadorade Estudos eProjetos(FINEP),Fundação de Amparoà PesquisadoEstadodeSãoPaulo(FAPESP);NationalNaturalScience Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) andthe Ministry of Science and Technology of the Peo-ple’sRepublicofChina (MSTC);MinistryofEducationandYouthof theCzech Republic;Danish NaturalScience ResearchCouncil, the Carlsberg Foundation and the Danish National Research Founda-tion;TheEuropeanResearchCouncilundertheEuropean Commu-nity’sSeventhFrameworkProgramme;HelsinkiInstituteofPhysics andtheAcademyofFinland;FrenchCNRS-IN2P3, the‘RegionPays deLoire’,‘Region Alsace’,‘RegionAuvergne’andCEA,France; Ger-manBundesministeriumfurBildung,Wissenschaft,Forschungund Technologie (BMBF)andthe Helmholtz Association; General Sec-retariat for Research and Technology, Ministry of Development, Greece; HungarianOrszagos TudomanyosKutatasi Alappgrammok (OTKA)and NationalOfficeforResearch andTechnology (NKTH); Department of Atomic Energy and Department of Science and TechnologyoftheGovernmentofIndia;IstitutoNazionalediFisica Nucleare (INFN) and Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF);ConsejoNacionaldeCiencayTecnologia(CONACYT), Direc-cionGeneraldeAsuntosdelPersonalAcademico(DGAPA),México, Amerique Latine Formation academique – European Commission (ALFA-EC) and the EPLANET Program (European Particle Physics LatinAmericanNetwork);StichtingvoorFundamenteelOnderzoek der Materie(FOM) andtheNederlandse Organisatievoor Weten-schappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); National Science Centre, Poland; Ministry of Na-tionalEducation/InstituteforAtomicPhysicsandNationalCouncil ofScientificResearchinHigherEducation (CNCSI-UEFISCDI), Roma-nia; MinistryofEducation andScienceofthe RussianFederation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The RussianFoundation forBasicResearch;Ministry ofEducation of Slovakia; Department of Science and Technology, Republic of South Africa; Centro de Investigaciones Energeticas, Medioambi-entales yTecnologicas (CIEMAT),E-Infrastructureshared between EuropeandLatin America(EELA),MinisteriodeEconomíay Com-petitividad (MINECO) of Spain, Xunta de Galicia (Consellería de Educación), Centro de Aplicaciones TecnológicasyDesarrollo Nu-clear(CEADEN),Cubaenergía,Cuba,andIAEA(InternationalAtomic EnergyAgency);SwedishResearchCouncil(VR)andKnutandAlice WallenbergFoundation (KAW);UkraineMinistryofEducationand Science;UnitedKingdomScienceandTechnologyFacilitiesCouncil (STFC);TheUnitedStatesDepartmentofEnergy,theUnitedStates NationalScience Foundation, the State of Texas, and the State of

Ohio; Ministry of Science, Education and Sports of Croatia and Unity throughKnowledge Fund, Croatia; Council ofScientific and IndustrialResearch(CSIR),NewDelhi,India.

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1

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T. Gunji

125

,

A. Gupta

89

,

R. Gupta

89

,

R. Haake

53

,

Ø. Haaland

18

,

C. Hadjidakis

50

,

M. Haiduc

61

,

H. Hamagaki

125

,

G. Hamar

134

,

L.D. Hanratty

101

,

A. Hansen

79

,

J.W. Harris

135

,

H. Hartmann

42

,

A. Harton

13

,

D. Hatzifotiadou

104

,

S. Hayashi

125

,

S.T. Heckel

52

,

M. Heide

53

,

H. Helstrup

37

,

A. Herghelegiu

77

,

G. Herrera Corral

11

,

B.A. Hess

35

,

K.F. Hetland

37

,

T.E. Hilden

45

,

H. Hillemanns

36

,

B. Hippolyte

54

,

P. Hristov

36

,

M. Huang

18

,

T.J. Humanic

20

,

N. Hussain

44

,

T. Hussain

19

,

D. Hutter

42

,

D.S. Hwang

21

,

R. Ilkaev

98

,

I. Ilkiv

76

,

M. Inaba

126

,

C. Ionita

36

,

M. Ippolitov

75

,

99

,

M. Irfan

19

,

M. Ivanov

96

,

V. Ivanov

84

,

V. Izucheev

111

,

A. Jachołkowski

29

,

P.M. Jacobs

73

,

C. Jahnke

118

,

H.J. Jang

67

,

M.A. Janik

132

,

P.H.S.Y. Jayarathna

120

,

C. Jena

30

,

S. Jena

120

,

R.T. Jimenez Bustamante

62

,

P.G. Jones

101

,

H. Jung

43

,

A. Jusko

101

,

P. Kalinak

58

,

A. Kalweit

36

,

J. Kamin

52

,

J.H. Kang

136

,

V. Kaplin

75

,

S. Kar

130

,

A. Karasu Uysal

68

,

O. Karavichev

55

,

T. Karavicheva

55

,

E. Karpechev

55

,

U. Kebschull

51

,

R. Keidel

137

,

D.L.D. Keijdener

56

,

M. Keil

36

,

K.H. Khan

16

,

M.M. Khan

19

,

P. Khan

100

,

S.A. Khan

130

,

A. Khanzadeev

84

,

Y. Kharlov

111

,

B. Kileng

37

,

B. Kim

136

,

D.W. Kim

67

,

43

,

D.J. Kim

121

,

H. Kim

136

,

J.S. Kim

43

,

M. Kim

43

,

M. Kim

136

,

S. Kim

21

,

T. Kim

136

,

S. Kirsch

42

,

I. Kisel

42

,

S. Kiselev

57

,

A. Kisiel

132

,

G. Kiss

134

,

J.L. Klay

6

,

C. Klein

52

,

J. Klein

92

,

C. Klein-Bösing

53

,

A. Kluge

36

,

M.L. Knichel

92

,

A.G. Knospe

116

,

T. Kobayashi

126

,

C. Kobdaj

113

,

M. Kofarago

36

,

M.K. Köhler

96

,

T. Kollegger

96

,

42

,

A. Kolojvari

129

,

V. Kondratiev

129

,

N. Kondratyeva

75

,

E. Kondratyuk

111

,

A. Konevskikh

55

,

C. Kouzinopoulos

36

,

V. Kovalenko

129

,

M. Kowalski

115

,

36

_,

_{S. Kox}

70

_,

_{G. Koyithatta Meethaleveedu}

47

_,

_{J. Kral}

121

_,

_{I. Králik}

58

_,

_{A. Kravˇcáková}

40

_,

M. Krelina

39

,

M. Kretz

42

,

M. Krivda

58

,

101

,

F. Krizek

82

,

E. Kryshen

36

,

M. Krzewicki

42

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96

,

A.M. Kubera

20

,

V. Kuˇcera

82

,

Y. Kucheriaev

99

,

i

,

T. Kugathasan

36

,

C. Kuhn

54

,

P.G. Kuijer

80

,

I. Kulakov

42

,

J. Kumar

47

,

L. Kumar

78

,

86

,

P. Kurashvili

76

,

A. Kurepin

55

,

A.B. Kurepin

55

,

A. Kuryakin

98

,

S. Kushpil

82

,

M.J. Kweon

49

,

Y. Kwon

136

,

S.L. La Pointe

110

,

P. La Rocca

29

,

C. Lagana Fernandes

118

,

I. Lakomov

50

,

36

,

R. Langoy

41

,

C. Lara

51

,

A. Lardeux

15

,

A. Lattuca

27

,

E. Laudi

36

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R. Lea

26

,

L. Leardini

92

,

G.R. Lee

101

,

S. Lee

136

,

I. Legrand

36

,

J. Lehnert

52

,

R.C. Lemmon

81

,

V. Lenti

103

,

E. Leogrande

56

,

I. León Monzón

117

,

M. Leoncino

27

,

P. Lévai

134

,

S. Li

7

,

69

,

X. Li

14

,

J. Lien

41

,

R. Lietava

101

,

S. Lindal

22

,

V. Lindenstruth

42

,

C. Lippmann

96

,

M.A. Lisa

20

,

H.M. Ljunggren

34

,

D.F. Lodato

56

,

P.I. Loenne

18

,

V.R. Loggins

133

,

V. Loginov

75

,

C. Loizides

73

,

X. Lopez

69

,

E. López Torres

9

,

A. Lowe

134

,

X.-G. Lu

92

,

P. Luettig

52

,

M. Lunardon

30

,

G. Luparello

26

,

56

,

A. Maevskaya

55

,

M. Mager

36

,

S. Mahajan

89

,

S.M. Mahmood

22

,

A. Maire

54

,

R.D. Majka

135

,

M. Malaev

84

,

I. Maldonado Cervantes

62

,

L. Malinina

65

,

D. Mal’Kevich

57

,

P. Malzacher

96

,

A. Mamonov

98

,

L. Manceau

110

,

V. Manko

99

,

F. Manso

69

,

V. Manzari

36

,

103

,

M. Marchisone

27

,

J. Mareš

59

,

G.V. Margagliotti

26

,

A. Margotti

104

,

J. Margutti

56

,

A. Marín

96

,

C. Markert

116

,

M. Marquard

52

,

I. Martashvili

123

,

N.A. Martin

96

,

J. Martin Blanco

112

,

P. Martinengo

36

,

M.I. Martínez

2

,

G. Martínez García

112

,

M. Martinez Pedreira

36

,

Y. Martynov

3

,

A. Mas

118

,

S. Masciocchi

96

,

M. Masera

27

,

A. Masoni

105

,

L. Massacrier

112

,

A. Mastroserio

33

,

A. Matyja

115

,

C. Mayer

115

,

J. Mazer

123

,

M.A. Mazzoni

108

,

D. Mcdonald

120

,

F. Meddi

24

,

A. Menchaca-Rocha

63

,

E. Meninno

31

,

J. Mercado Pérez

92

,

M. Meres

38

,

Y. Miake

126

,

M.M. Mieskolainen

45

,

K. Mikhaylov

57

,

65

,

L. Milano

36

,

J. Milosevic

22

,

131

,

L.M. Minervini

103

,

23

,

A. Mischke

56

,

A.N. Mishra

48

,

D. Mi´skowiec

96

,

J. Mitra

130

,

C.M. Mitu

61

,

N. Mohammadi

56

,

B. Mohanty

130

,

78

,

L. Molnar

54

,

L. Montaño Zetina

11

,

E. Montes

10

,

M. Morando

30

,

S. Moretto

30

,

A. Morreale

112

,

A. Morsch

36

,

V. Muccifora

71

,

E. Mudnic

114

,

D. Mühlheim

53

,

S. Muhuri

130

,

M. Mukherjee

130

,

H. Müller

36

,

J.D. Mulligan

135

,

M.G. Munhoz

118

,

S. Murray

64

,

L. Musa

36

,

J. Musinsky

58

,

B.K. Nandi

47

,

R. Nania

104

,

E. Nappi

103

,

M.U. Naru

16

,

C. Nattrass

123

,

K. Nayak

78

,

T.K. Nayak

130

,

S. Nazarenko

98

,

A. Nedosekin

57

,

L. Nellen

62

,

F. Ng

120

,

M. Nicassio

96

,

M. Niculescu

36

,

61

,

J. Niedziela

36

,

B.S. Nielsen

79

,

S. Nikolaev

99

,

S. Nikulin

99

,

V. Nikulin

84

,

F. Noferini

104

,

12

,

P. Nomokonov

65

,

G. Nooren

56

,

J. Norman

122

,

A. Nyanin

99

,

J. Nystrand

18

,

H. Oeschler

92

,

S. Oh

135

,

S.K. Oh

66

,

A. Ohlson

36

,

A. Okatan

68

,

T. Okubo

46

,

L. Olah

134

,

J. Oleniacz

132

,

A.C. Oliveira Da Silva

118

,

M.H. Oliver

135

,

J. Onderwaater

96

,

C. Oppedisano

110

,

A. Ortiz Velasquez

62

,

A. Oskarsson

34

,

J. Otwinowski

96

,

115

,

K. Oyama

92

,

M. Ozdemir

52

,

Y. Pachmayer

92

,

P. Pagano

31

,

G. Pai ´c

62

,

C. Pajares

17

,

S.K. Pal

130

,

J. Pan

133

,

A.K. Pandey

47

,

D. Pant

47

,

V. Papikyan

1

,

G.S. Pappalardo

106

,

P. Pareek

48

,

W.J. Park

96

,

S. Parmar

86

,

A. Passfeld

53

,

V. Paticchio

103

,

B. Paul

100

,

T. Pawlak

132

,

T. Peitzmann

56

,

H. Pereira Da Costa

15

,

E. Pereira De Oliveira Filho

118

,

D. Peresunko

75

,

99

,

C.E. Pérez Lara

80

,

V. Peskov

52

,

Y. Pestov

5

,

V. Petráˇcek

39

,

V. Petrov

111

,

M. Petrovici

77

,

C. Petta

29

,

S. Piano

109

,

M. Pikna

38

,

P. Pillot

112

,

O. Pinazza

104

,

36

,

L. Pinsky

120

,

D.B. Piyarathna

120

,

M. Płosko ´n

73

,

M. Planinic

127

,

J. Pluta

132

,

S. Pochybova

134

,

P.L.M. Podesta-Lerma

117

,

M.G. Poghosyan

85

,

B. Polichtchouk

111

,

N. Poljak

127

,

W. Poonsawat

113

,

A. Pop

77

,

S. Porteboeuf-Houssais

69

,

J. Porter

73

,

J. Pospisil

82

,

S.K. Prasad

4

,

R. Preghenella

104

,

36

,

F. Prino

110

,

C.A. Pruneau

133

,

I. Pshenichnov

55

,

M. Puccio

110

,

G. Puddu

25

,

P. Pujahari

133

,

V. Punin

98

,

J. Putschke

133

,

H. Qvigstad

22

,

A. Rachevski

109

,

S. Raha

4

,

S. Rajput

89

,

J. Rak

121

,

A. Rakotozafindrabe

15

,

L. Ramello

32

,

R. Raniwala

90

,

S. Raniwala

90

,

S.S. Räsänen

45

,

B.T. Rascanu

52

,

D. Rathee

86

,

V. Razazi

25

,

K.F. Read

123

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J.S. Real

70

,

K. Redlich

76

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R.J. Reed

133

,

A. Rehman

18

,

P. Reichelt

52

,

M. Reicher

56

,

F. Reidt

92

,

36

,

X. Ren

7

,

R. Renfordt

52

,

A.R. Reolon

71

,

A. Reshetin

55

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F. Rettig

42

,

J.-P. Revol

12

,

K. Reygers

92

,

V. Riabov

84

,

R.A. Ricci

72

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T. Richert

34

,

M. Richter

22

,

P. Riedler

36

,

W. Riegler

36

,

F. Riggi

29

,

C. Ristea

61

,

A. Rivetti

110

,

E. Rocco

56

,

M. Rodríguez Cahuantzi

11

,

2

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A. Rodriguez Manso

80

,

K. Røed

22

,

E. Rogochaya

65

,

D. Rohr

42

,

D. Röhrich

18

,

R. Romita

122

,

F. Ronchetti

71

,

L. Ronflette

112

,

P. Rosnet

69

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A. Rossi

36

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F. Roukoutakis

87

,

A. Roy

48

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C. Roy

54

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P. Roy

100

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A.J. Rubio Montero

10

,

R. Rui

26

,

R. Russo

27

,

E. Ryabinkin

99

,

Y. Ryabov

84

,

A. Rybicki

115

,

S. Sadovsky

111

,

K. Šafaˇrík

36

,

B. Sahlmuller

52

,

P. Sahoo

48

,

R. Sahoo

48

,

S. Sahoo

60

,