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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 ra 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√s
NN=2.76 TeV, byinvariant mass analysisinthe decaymodes n→dπ+ and H- dibaryon→pπ−.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–PbcollisionsattheLargeHadronCol- lider(LHC)hasbeenextensivelystudied[1–3].Theobservedpro- duction pattern is rather well described in equilibrium thermal models[4–7].Withinthisapproach,thechemical freeze-outtem- perature Tchem, the volume V and the baryo-chemical potential
μ
B are theonlythreefree parameters.Evenlooselyboundstates suchasthedeuteronandhypertritonandtheiranti-particleshave beenobserved[8–10]andtheirrapiditydensitiesareproperlyde- scribed[11–17].Consequentlyother looselyboundstates1 suchas theH-dibaryonandthen areexpectedtobeproducedwithcor- respondingyields.ThediscoveryoftheH-dibaryonorthen 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–interactionthroughthemeasure-
E-mailaddress:[email protected].
1 TheexpectedmassesofthesestatesaresomeMeVbelowthesumofthemass oftheirconstituents.
ment of correlations [24]; thisand a theoretical analysis of thesedata[25]didnotrevealasignal.Manytheoreticalinvestiga- 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 →pπ
− are pre- sented. Theanalysisfocuseson then boundstate becausepro- duction ofanti-particlesin the detectormaterial isstrongly sup- pressedandthussecondarycontaminationofthesignalisreduced.FortheH-dibaryonboththeandtheporiginatefromsecondary 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 techniqueusedtoidentifythedecaydaughtersandreconstructin- variantmassdistributions.Toassessthepossibleexistenceofthese stateswecomparetheexperimental distributionswiththemodel predictions.
2. Detectorsetupanddatasample
TheALICEdetector[32]isspecificallydesignedtostudyheavy- 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,twoscin- 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 pz (themomentumpoftheparticledividedbyitschargenumberz).
TheTPCisalsoused forparticle identificationviatheenergyde- positdE/dxmeasurement(seesection3).
A complete description ofthe performance of the ALICEsub- 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.Thisallowstheidentificationofallchargedstableparti- cles,fromthelightest(electron)totheheaviestones(anti-alpha).
TheenergydepositresolutionoftheTPCincentralPb–Pbcollisions (investigatedhere)isaround7%.Thecorresponding particlesepa- ration poweris demonstrated in Fig. 1. This technique was used inthefollowingtoidentifythedeuterons,protons andpions.The secondmethodmakesuseofspecifictopologiesfromweakdecays, whichresultintypicalV0decaypatterns.Thisisusedhereforthe detectionof then 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.TPCdE/dxspectrumfornegativeparticlesinasampleofthreedifferenttrig- gertypes(minimumbias,semi-centralandcentral).Thedashedlinesareparametri- 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 vertexreconstruc- tion canbe foundin [3,37,41], wherealsotheclearandeffective identificationofbaryonsisdisplayed usingtheaforementioned technique. The selection criteria, described below, are optimised usinga MonteCarlosetwherethe simulatedexoticboundstates are assumedtoliveaslongasafree baryon. Thisisareason- able assumption forall strange dibaryons, which areexpected to livearound2–4×10−10s[42–44] intheregionsofbindingener- giesinvestigatedhere.
4.1. nboundstate
In analogyto recenthypertritonmeasurements [8,9] we focus here on the expected two-body decay n→d
π
+. For the data analysisthefollowingstrategyisused:firstdisplaced verticesare identifiedusingITSandTPCinformation.Inasecondsteptheneg- ative trackof the V0 candidateis identified asan anti-deuteron viatheTPCdE/dxinformation.Iftheseconddaughterisidentified asapion,theinvariantmassofthepairisreconstructed.Bothpar- 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
V0and kinematic selection criteria
Pointing angle <0.045 rad
DCA between theV0daughters DCA<0.3 cm Momentumptotof the anti-deuteron ptot>0.2 GeV/c Energy deposit dE/dxanti-deuteron dE/dx>110 (fromFig. 1)
PID cut for daughters ±3σ(TPC)
Fig. 2.Invariantmass 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,reflectingthekinematicrangeofiden- tifieddaughtertracks,isdisplayedinFig. 2.
4.2.H-dibaryon
Thesearch fortheH-dibaryonisperformedinthedecaychan- 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-dibaryondecayver- 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 thefour- vectorsofthe proton andpionat thedecay vertex. A 3
σ
dE/dx cut in theTPC is used to identifythe protons and thepions for boththecandidateandthe V0 topologyattheH-dibaryonde- cayvertex.Fig. 3.Invariantmassdistributionfor pπ−forthePb–Pbdatacorrespondingto 19.3×106 centralevents.Theleftarrowindicatesthesumofthemassesofthe constituents()ofthepossibleboundstate.Asignalfortheboundstateisex- 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
V0selection criteria
DCAV0daughters DCA<1 cm
DCA positiveV0daughter – H decay vertex DCA>2 cm DCA negativeV0daughter – 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
Tocopewiththehugebackgroundcausedbyprimaryandsec- 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 rangeoftheidenti- 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 andin transverse momentum pT.Inordertodealwiththeunknownlifetime,differ- entdecaylengthsareinvestigated,rangingfrom4 cmup to3 m.
Thelowerlimit isdetermined bythesecondaryvertexfindingef- ficiency and the upper limit by the requirement that there is a significantprobability fordecaysinsidetheTPC2 (thefinal accep-
2 FortheH-dibaryonthereisalsoatheoreticalmaximaldecaylengthcalculated fortheinvestigateddecaychannel[45].
tance×efficiencydropsdownto1%forthen and10−3 forthe H-dibaryon).Theshapeoftransversemomentumspectrainheavy- ioncollisions is described well by the blast-waveapproach, with radialflowparameter βandkinetic freeze-outtemperatureTkin asin[46].The trueshapeofthe pT spectrum isalsonot known, thereforeitis estimatedfromthe extrapolationofblast-wavefits todeuterons and3He spectraatthe sameenergy[10].Toobtain finalefficiencies,theresultingblast-wavedistributionsconstructed fortheexoticboundstatesarenormalisedtounityandconvoluted withthecorrectionfactors(efficiency×acceptance).
Typical valuesofthe finalefficiency areofthe orderofa few percentassumingthelifetimeofthefree.Theuncertaintyinthe shapeofthe pT distributionsisthemainsource ofsystematicer- ror.Blast-wave fitsof deuteronand3He 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 alluncertain- tiesareaddedinquadrature.
Forthen boundstateanalysisthepossibleabsorptionofthe anti-deuteronsandthe bound state itself whencrossing material hastobetakenintoaccount.Forthis,thesameprocedureasused fortheanti-hypertritonanalysis[9]isutilised.Theabsorptioncor- rectionrangesfrom3to40%(dependingonthelifetimeofthen 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,reflectingthemomentumdistributionofthepar- 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/dy the method discussed in [47] is utilised.Inparticular,weapplythesoftwarepackageTRolkeasim- plemented in ROOT [48]. This method needs as input mass and experimentalwidth(3
σ
)ofthehypotheticalboundstates.Theob- 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 from3 NotethatahypotheticalH-dibaryonwithamassabovethe p thresholdwould notbeobservableinthepresentanalysis.
Fig. 4.Upperlimitoftherapiditydensityasfunctionofthedecaylengthshownfor 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 then boundstatethebranchingratiointhed+
π
+decaychan- nelisexpectedtobe54%[49].Thebranchingratiofora1 MeVor lessboundH-dibaryondecayingintopπ
−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. Forthen theab- sorptioncorrectionsarealsoconsideredinthefigure,whichcauses theupperlimitstobeshiftedupwards.
The obtained upper limits can now be compared to model predictions. The rapidity densities dN/dy from a thermal model prediction fora chemicalfreeze-out temperatureof,forexample, 156 MeV, are dN/dy=4.06×10−2 for the n bound state and dN/dy=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 afactorofmorethan5belowalltheorypredictionsforabranch- ingratioofatleast5%forthen boundstateandatleast20%for theH-dibaryon.
Fig. 5.Experimentallydeterminedupperlimit,undertheassumptionofthelifetime ofafree . Intheupperpanel shownfor then boundstateand for theH- 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]. (Forinterpretationoftherefer- 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 thereforeassumethattheyieldofthen,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-dibaryonandn boundstate,ingeneralsig- nificantlybelowthe thermalmodel predictions.The upper limits areobtainedfordifferentlifetimes.Thevaluesarewellbelowthe modelpredictions whenrealistic branching ratiosandreasonable lifetimesare assumed. Thus,our resultsdo notsupport the exis- tenceoftheH-dibaryonandthen boundstate.
Acknowledgements
WethankS. Beane, M.Petrá ˇn, J. Schaffner-BielichandJ. Stein- heimerforusefulcorrespondence.
The ALICE Collaboration would like to thank all its engineers andtechniciansfortheir invaluablecontributions totheconstruc- tionoftheexperimentandtheCERNacceleratorteamsfortheout- 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;TheEuropeanResearchCouncilundertheEuropeanCommu- nity’sSeventhFrameworkProgramme;HelsinkiInstituteofPhysics andtheAcademyofFinland;FrenchCNRS-IN2P3, the‘RegionPays deLoire’,‘Region Alsace’,‘RegionAuvergne’andCEA,France;Ger- manBundesministeriumfurBildung,Wissenschaft,Forschungund Technologie (BMBF)andthe Helmholtz Association; GeneralSec- 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),MinisteriodeEconomíayCom- 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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