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Physics Letters B
www.elsevier.com/locate/physletb
3
H and 3
H lifetime measurement in Pb–Pb collisions at
√ s NN = 5 . 02 TeV via two-body decay
.ALICE Collaboration
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received26July2019
Receivedinrevisedform22August2019 Accepted29August2019
Availableonline3September2019 Editor: L.Rolandi
Animprovedvalueforthelifetimeofthe(anti-)hypertritonhasbeenobtainedusingthedatasampleof Pb–Pbcollisionsat√s
NN=5.02 TeVcollectedbytheALICEexperimentattheLHC.The(anti-)hypertriton has been reconstructedvia its charged two-body mesonic decay channel and the lifetime has been determinedfromanexponentialfittothedN/d(ct)spectrum.Themeasuredvalue,τ=242+−3438(stat.)± 17(syst.)ps,iscompatiblewithrepresentativetheoreticalpredictions,thuscontributingtothesolution ofthelongstandinghypertritonlifetimepuzzle.
©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Hypernuclei are bound states of nucleons and hyperons and they are mainly produced by means of (K−,
π
−), (π
+,K+) and (e,eK+)reactions onstablenucleartargets[1,2].Hypernuclei are particularly interesting because they can be used as experimen- talprobes forthestudyofthehyperon-nucleon(Y–N)interaction.The knowledge of this interaction has become more relevant in recentyears dueto its connectionto the modeling of astrophys- icalobjects like neutron stars [3,4]. Inthe inner core ofneutron stars,the creation of hyperons isenergetically favored compared toapurelynucleonicmattercomposition[5].The presenceofhy- peronsas additional degrees of freedom leads to a considerable softeningofthematterequationofstate(EOS).The resultingEOS inhibitstheformationoflargemassneutron stars.Thisisincom- patible with the observation of neutron stars as heavy as two solarmasses[3],constituting whatisreferred toasthe“hyperon puzzle”. Many attempts were made to solve this puzzle, e.g. by introducing three-body forces leading to an additional repulsion thatcancounterbalancethelargegravitationalpressureandallow forlarger star masses. To constrain the parameter space of such models, a detailed knowledge of the Y–N interaction and of the three-bodyY–N–Ninteractionismandatory,including,and states.The lifetimeofahypernucleus dependson thestrengthof theY–Ninteraction, andthereforeaprecise determinationofthe lifetimeof hypernuclei provides informationon the Y–N interac- tionstrength[6,7].
Therecentobservationofhypernucleiandthedeterminationof theirlifetimesinexperimentswithrelativisticheavyioncollisions
E-mailaddress:alice-publications@cern.ch.
hastriggereda particularinterest.Alltheresultspublishedso far arerelatedtothelightesthypernucleus,thehypertriton3H,which isa bound stateformed by aproton,a neutronanda ,andits charge conjugatethe anti-hypertriton 3
H. The results have been obtainedatthe RelativisticHeavy IonCollider (STAR experiment) [8],attheSIS18(HypHICollaboration)[9] andattheLargeHadron Collider(ALICECollaboration)[10].
The separation energy of the in this hypernucleus is only about130keV[11],whichresultsinan RMSradius(average dis- tance of the to the deuteron) of 10.6 fm [12,13]. A very low binding energy implies a small change of the wave function of the in a nucleus and hence one can expect the lifetime of the hypertriton to be very close to that of the free hyperon (
τ
=(263.2±2.0)ps [14]).Earlyhypertritonlifetimemeasurementsweredonewithimag- ing techniques(i.e. emulsions, bubble chambers) and the results are lower than or consistent with the value of the free life- time[15–20].However,mostofthemeasurementsperformedwith thesetechniquesarebasedonvery smallsamplesofevents,thus resulting in a large statistical uncertainty. The recent measure- ments of the lifetime of (anti-)3H produced in ultra-relativistic heavy-ioncollisions orinrelativisticionfragmentation[21], even though affected by statistical andsystematic uncertainties bigger than10%,areinagreementamongeach otherandarelowerthan thefreelifetime[9,10,22].
However, the few existingtheoretical calculationspredict that the lifetime of the 3H should be very close to the lifetime of free . The most comprehensive 3H lifetime calculation is from RayetandDalitz[23];theyobtainedanestimateintherangefrom 239.3–255.5ps.MorerecentcalculationsfromCongleton[24] and Kamadaetal.[7] yieldavalueof232ps and256ps,respectively.
https://doi.org/10.1016/j.physletb.2019.134905
0370-2693/©2019TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
Thisscenariostimulated,inthelastyears,anewinterestfromboth experimentalistsandtheoreticiansformoreprecisemeasurements ofthe3H lifetime.
Inthisletter,thelifetimeofthe (anti-)3H measuredinPb–Pb collisions at √
sNN =5.02 TeV by the ALICE experiment is pre- sented. In Section 2,the ALICE detectoris briefly described. The details ofthedatasample, analysistechnique andsystematicun- certaintiesare presented inSection 3,wherealso a newanalysis approach to crosscheck the results is introduced in the subsec- tion 3.1. Finally the result is compared with previous measure- mentsandwiththeoreticalpredictionsinSection4.
2. TheALICEapparatus
AdetaileddescriptionoftheALICEapparatusanddataacquisi- tionframework canbefound in[25,26]. Themaindetectorsused in this analysis are the V0 detector, the Inner Tracking System (ITS) andthe Time Projection Chamber (TPC), which are located inside a solenoid creatinga magnetic field of 0.5 T.The V0 de- tector[27] consistsoftwoarraysofscintillatorcounters(V0Aand V0C), placedaround the beam-pipeonboth sidesof theinterac- tion region. They coverthe pseudorapidity ranges 2.8 <
η
< 5.1 and−3.7<η
<−1.7,respectively.TheV0detectorisusedtode- fine theMinimum Bias (MB) trigger, which ischaracterized by a coincidence signal in the V0Aand inthe V0C, andto determine thecentralityofthecollisions[28].TheITS[29] istheclosestde- tectortotheinteraction pointwithinALICE. Itiscomposedofsix layersofsilicondetectors,withradiibetween3.9and43cmfrom theinteraction point. Thesix layersuse threedifferent technolo- gies:silicon pixeldetector (SPD), silicondrift detector(SDD) and silicon strip detector (SSD). The ITS has full azimuthal coverage 0≤ϕ
≤2π
andcoversthepseudorapidityrange|η
|<0.9.TheTPC [30] isagaseousdetector,mainlyusedfortrackingandforparticle identification(PID)viathespecificenergyloss(dE/dx),withato- talsensitivevolumeof90m3 filledwithamixtureof88%Arand 12%CO2.ThereconstructedclustersinTPCandITSarethestarting point ofthetrackfinder algorithm,whichadopts theKalmanfil- tertechnique[31].Thesetracksareusedtodeterminetheprimary collision vertexwith a precision better than 50 μmin the plane transversetothecollidingbeams[26].3. Datasampleandanalysistechnique
Inthisletter,thelifetimeofthe(anti-)hypertritonisdetermined byexploitingthe2-body mesonicdecaychannelwithchargedpi- ons, namely3H →3He +
π
− and 3H → 3He +
π
+.Both 3H and3H candidatesareusedforthismeasurement.
TheanalysisisperformedusingthedatasampleofPb–Pbcolli- sionsat√
sNN=5.02 TeVcollectedbytheALICEexperimentatthe endof 2015. To ensure a uniformacceptance andreconstruction efficiencyinthepseudorapidityregion|
η
|<0.9,onlythoseevents areselectedwhosereconstructed primaryvertexwaswithin ±10 cmfrom the nominalposition ofthe interaction point along the beamaxis.Theanalyzed samplecontainsapproximately90million eventsinthecentralityinterval0-90%.The3H and3
H identificationisbasedonthetopologyoftheir weakdecaysandonthereconstructionofthetracksoftheirdecay products, referred to as daughter particles. The weakly decaying hypernucleiarereconstructedusingthealgorithmwhichwaspre- viouslyusedfortheK0S andproductionanalyses[32] andwhich istypicallyadoptedfora2-bodyweakdecaytopology.Atfirst,the algorithmusestheTPCandITSclusterstoreconstructthedaughter tracksandthencombinestheminordertoobtainaV-shapedde- cayvertex.Moredetailsonthisalgorithmcanbefoundin[26,33].
Table 1
Selection criteriaappliedfor the identification ofthe daughter candidatetracksandforthereconstructionofthehypertritoncan- didate.
Selection criteria Track selections
|η| <0.9
Number of TPC clusters >70
χ2per TPC cluster <5
Kink topology Rejected
|nσ|for TPC PID ≤3
Daughter candidate selections
πpT(GeV/c) 0.2-1.2
DCA betweenπ±and primary vertex (cm) >0.1
3HepT(GeV/c) ≥1.8
DCAtracks(cm) <0.7
Hypertriton candidate selections
cos(θpointing) ≥0.995
|y| ≤0.8
pT(GeV/c) 2–9
The daughter tracks are selectedin the pseudorapidity region
|
η
|< 0.9andarerequiredtohaveatleast70clustersout of159 intheTPC,inordertoguaranteearesolutionσ
betterthan5% on track momentum andof about6% for the dE/dx [26]. Moreover, theχ
2perTPCclusterisrequiredtobelessthan5andtrackswith kink topologies are rejected. The particle identification (PID) of the daughters(3He, 3He,π
±) isperformedfollowingthemethod described in [33], which is used in many analyses of the ALICE Collaboration.Itisbasedonthedifferencebetweenthemeasured andtheexpecteddE/dxforaselectedparticlespeciesnormalized totheenergylossresolutioninthedetector,σ
forshort,andisre- ferred toasthenσ
methodinthisletter.Inparticular,an |nσ
|≤3isrequired,inatrack-by-trackapproach,withrespecttotheex- pected
π
and3He specificenergy lossinthe TPC. Thepions can beidentifieduptoamomentumofabout1.2GeV/c,beyondwhich there isconsiderablecontamination fromkaons andprotons.The 3He, havinga charge of z=2e,can beidentified cleanly upto 7 GeV/c.The 3He is alsoproduced in the detectormaterial due to spallation.Theseare producedatlow transversemomenta,asre- ported by the ALICEexperiment [34]. As a consequence the 3He candidateisrequiredtohaveatransversemomentum(pT)greater than1.8GeV/c,wherethespallationprocessesarenegligible.The3H and3
H candidatesareselectedbyapplyingtopological and kinematic selection criteriaon the decay products. The dis- tanceofclosestapproach (DCA)betweenthetwodaughtertracks andtheDCAof
π
±tracksfromtheprimaryvertexarerequiredto belowerthan0.7cmandlargerthan0.1cmrespectively.Thecan- didates areselected whosecosineof theanglebetweenthe total momentumofthedaughtertracksatthesecondaryvertexandthe vectorconnectingtheprimaryandsecondaryvertex(pointingan- gle)islargerthan0.995.Twoadditionalselectionsonthe3H and 3H rapidity (|y|< 0.8) andtransverse momentum (2 < pT < 9 GeV/c) are applied. Allthe selection criteriapreviously described havebeenstudiedwithadedicatedMonteCarloproduction,inor- der to improvethebackground rejection,andare summarizedin Table1.
The sample of3H and 3
H candidatesis divided in fourct= M Lc/p intervals for the lifetime determination, where c is the speedoflight,tisthepropertimeofthecandidate,Misthemass ofthecandidate,Listhedecaydistanceandpisthereconstructed momentum. Themass M of thehypertriton isobtainedfromthe measured values of massof p, n and [14] andof the binding energy[11],andhasbeenfixedatM=2.99116±0.00005 GeV/c2. Thefourct intervalsare 4≤ct<7 cm,7≤ct<10 cm,10≤ct<
Fig. 1.Invariantmassdistributionof(3He,π−)and(3He,π+)forthefourctintervalsusedtodeterminethe3H and3H lifetime.Thesolidbluecurverepresentsthe functionusedtoperformthefitandthereddashedcurverepresentsthebackgroundcomponent.
15 cmand15≤ct<28 cm.Thecorrespondinginvariantmassdis- tributions are shownin Fig.1 andare fitted, ineach ct interval, withafunctionwhichisthesumofaGaussian,usedtointerpolate the signal, and a second order polynomial, used to describe the background. The fit is performed using the maximum-likelihood estimateandthefitfunctionisrepresentedasasolidblueline.
From the fit, the mean values
μ
and the widthsσ
of each distributionareextracted. Inparticular,thesignal widthisinthe range1.7–2.1 MeV/c2,depending onthe ct interval,andisdriven bythedetectorresolution.Therawyieldofthesignalisdefinedas theintegraloftheGaussianfunctionina±3σ
regionaroundthe meanvalueabovethebackground.Thesignificanceofthesignalin thefourct intervalsvariesintherange3.1–4.9.The yield is corrected in each ct bin for the detector accep- tance,thereconstruction efficiencyandtheabsorption ofthe3H (3
H)inthedetectormaterial.Theefficiency×acceptanceisdeter- minedwithadedicatedMonteCarlosimulation,wherethe3H and 3
H are injectedontopofaHIJING event[35] andare allowedto decayintochargedtwo-bodyandthree-bodyfinalstates.Thesim- ulatedparticles arepropagatedthrough theALICEdetectorsusing theGEANT3transportcode[36] andthenreconstructedfollowing thesameprocedureasadoptedforthedata.
Theaforementionedtransport codedoesnot properlydescribe the interactions of the (anti-)(hyper-)nuclei with the material of
the apparatus. Thus, acorrection factor forthe absorptionof 3H (3
H) and3He (3He) isestimated, based onthe p (p) absorption probabilitymeasuredintheALICEdetector[37].Theusage ofthis experimental measurement offers the advantage of taking auto- maticallyintoaccount thecrosssection andtheeffectivematerial of the detector crossed by a charged particle. The same absorp- tion probability forprotons and neutrons hasbeen assumedand the3He(3He)hasbeenconsideredasastateofthreeindependent p (p)asverified in[10]. Theabsorption probability,computedas the third powerof that of one proton,goesfrom 11% atlow pT to 6% athigh pT for3He while it isconstant at6% for 3He.The evaluationof the3H (3
H) absorption probability isdone follow- ing the same approach. However, to take into account the small separationenergy(B=0.13±0.05 MeV[11]),the3H absorp- tion cross-sectionis increasedby 50% withrespectto theone of the 3He [38,39], as described in the ALICE measurement in Pb–
Pbcollisionsat√
sNN=2.76 TeV[10].Thisleadstoanabsorption probabilitybetween16% and9% for3
H asafunctionof pT while itisconstantat9% for3H.Thecorrectionfactortobeappliedis:
k
=
kabs,3H
+ (
1−
kabs,3H
)
kabs,3He (1)where kabs,3
H is the probability that the 3H is absorbed be- tween the primary and the secondary vertex while kabs,3He is
Fig. 2.Efficiency×acceptance asa functionofct for 3H (redsquare),3H (blue square)and3H+3H (blackopencircle)inthesamectintervalsselectedforthe rawyieldsextraction.
the probability that the daughter 3He is absorbed between the secondary vertex and the TPC inner wall. For each ct interval, the efficiency x acceptance has been calculated using the ab- sorption corrected numbers of reconstructed 3H and 3
H. Fig. 2 shows the efficiency×acceptance (black marker) which is used forthe lifetime determination andis obtained by combining 3H and 3
H after the absorption correction is applied. Thisdistribu- tion is alsoshown separately for 3H and 3
H and the difference is dueto the absorption correction which is bigger forthe anti- matter.
The main sources of systematic uncertainties on each ct bin usedforthe lifetimeevaluationaretheabsorptioncorrection,the singletrackefficiencyandtheuncertaintyonthedetectormaterial budget.Thesystematicuncertaintyontheabsorptioncorrectionis mainlyduetotheassumptionusedforthe3H (3
H)cross-section.
Thisuncertaintyisevaluatedby varyingthisassumptionbetween alowerandanupperlimit.Thefirstoneisobtainedbysettingthe 3
H (3
H) cross-section equal to the 3He (3He) absorption cross- section and the second one as twice the 3He (3He) absorption cross-section.Thisleads to an uncertaintyof5.2% for each ct in- terval,asreportedinTable2.
The second source ofsystematic uncertainty is relatedto the material budget description inthe simulation. An uncertaintyon theknowledgeoftheALICEdetectormaterialbudgetof4.5% was determinedinapreviousstudy[26].Thesystematicuncertaintyis estimated usingtwo dedicated Monte Carloproductions, varying thematerialbudgetaccordingly,andamountsto1% fortheyields inallctintervals.
The systematic uncertainty due to the single-track efficiency andthedifferentchoices ofthetrackquality selectionshasbeen investigated[40] andamountsto4%. Fortheanalysisofthetwo- bodydecayof3H anuncertaintyof8% isassumedinallct inter- vals. The summary of thesystematic uncertainties is reportedin Table2,wherethetotaluncertaintyisobtainedassuminquadra- tureofeachcontributionoftheindividualsources.
ThecorrecteddN/d(ct) spectrumisshowninFig. 3wherethe blue markers are the corrected yield withtheir statistical uncer- tainty,whiletheboxrepresentsthesystematicuncertainty.
Thelifetimeisdeterminedwithanexponentialfit(redline)and thesloperesultsinaproperdecaylengthofc
τ
=7.25+−11..0213 (stat.)± 0.51 (syst.) cm,corresponding to a lifetime
τ
=242+−3438 (stat.)± 17(syst.) ps. Thesystematic uncertaintyforthe lifetimevalue
Table 2
Summary ofthe systematic uncertainties usedin the lifetimeanalysis.Thetotaluncertaintyassignedineach ctintervalisthesuminquadratureofthesinglesources.
Systematic uncertainties
Source Value
Absorption 5.2%
Material budget 1%
Single track efficiency 8%
Total 9.5%
Fig. 3.CorrecteddN/d(ct)spectrumfittedwithanexponentialfunction(redline) usedtoextractthe(3H+3H)lifetime.Thebarsandboxesrepresentthestatistical andsystematicuncertainties,respectively.
isdeterminedbyassumingthesystematicuncertaintiesineachct intervalasuncorrelated.
3.1. Unbinnedfitmethodforlifetimeextraction
In order to enforce the result described in Sec. 3, an addi- tionalanalysisonthesamedatasamplehasbeencarriedoutthat reliesonatwo-dimensional(invariantmassvs.ct)unbinnedfitap- proach.Themethodcanbesummarizedinthreesteps:i)fittothe ct-integratedinvariantmassdistribution;ii)tunethefunctionused todescribethecombinatorialbackground;iii)fittothectdistribu- tion withafunction whichisthe sumof threeexponentials, one todescribethesignalandtwotodescribethebackground.
The firststep isperformedwithafunction that isthesumof a Gaussian,forthesignal, andasecond orderpolynomial,forthe background. The mean value
μ
and theσ
are 2.9913 ± 0.0004 GeV/c2 and 0.0020 ± 0.0005 GeV/c2 respectively and are used to define theboundaries ofthe signal region andthe sidebands, which correspond to the intervalsμ
± 3σ
and ± 3σ
to ± 9σ
withrespecttothemeanvalue,respectively.
The second step consists in fitting the ct distribution of the background inthe sidebands usinga function that isthe sumof two exponentials. Thefitis performedsimultaneouslyinthetwo sidebandregions withtheROOFITpackage [41].The resultisthen used asbackground parameterizationforthe fit inthe signal re- gion.
The(3H+3H)lifetimemeasurementisobtainedbyperform- ing the unbinned fit to the ct distribution in the signal region.
The total probability densityfunction used forthefit is thesum ofthetwoexponentials(background)andtheexponentialadopted toreproduce thesignal.Sincethect distributionisunbinned,the efficiency×acceptancecorrection,evaluatedasdescribed inSec.3, is parametrized with a polynomial plus an exponential and it is
Fig. 4.Lifetimevalueτdeterminedfromtheminimizationofthelog-likelihoodratio –log(λ(τ)).Thestatisticaluncertaintyisevaluatedataconfidencelevelof68% (red dashedlines)withthelog-likelihoodratio(blueline).
usedtoscalethesignalfunction.Theobservedsignal distribution is described as the product of the function used for the signal andthe efficiencyparametrization. Thus, the lifetime isobtained withthe unbinnedmaximum-likelihoodestimate(MLE)fit tothe ctdistribution,performedinthesignalregion,leadingtoavalueof
τ
=240+−4031(stat.)±18(syst.)ps,asreportedinFig.4.Thestatis- ticaluncertaintyofthemeasurementisassessedbyprovidingthe intervaloftheestimatedτ
[42],ataconfidencelevelof68%,which isrepresentedbythereddashedlines,basedonthelog-likelihood ratio (logλ(τ
)), shown as a blue line. The result corresponds to aproperdecaylength cτ
=7.20+−10..2093(stat.)±0.54 (syst.) cm.The sources of systematicuncertainties are the sameas described in Sec.3(Table2)andcontributetoatotalsystematicuncertaintyof 9.5% ontheestimatedlifetime.The value obtained with this approach is in good agreement within1
σ
withthelifetimeestimationobtainedwiththemethod describedinSec.3,whichweconsiderasthefinalvalueforthe3H lifetime.Additionalfiguresanddetailsfortheunbinnedfitmethod arepresentedin[43].4. Discussionandconclusions
Thanks to the large data sample of heavy-ion collisions at
√sNN=5.02 TeVprovidedbytheLHCandtotheexcellenttracking andparticleidentificationperformance oftheALICEapparatuswe havedeterminedaprecisevalueforthe3H lifetime.Themeasured
τ
=242+−3438(stat.)± 17(syst.)ps isshownasafullred diamond inFig.5together withother experimental resultsandtheoretical estimates.Early experiments [15–20] were performed with visualizing techniques,namely photographic emulsion and3He filled bubble chambers,wherethetracksformedduetopassageofchargedpar- ticles were recorded visually. Most ofthe results obtainedusing thesetechniques hadlarge uncertainties due to the limited size ofthedatasample atdisposal.Furthermore,thesemeasurements preventedadefinite conclusion onthe agreementwiththetheo- retical predictions, whichforesee a lifetimecloseto the value of thefreehyperon. Itisworthwhiletonote thatthe smallbind- ingenergyofthehypertritonmakesthespendmostofthetime farfromthedeuteron corethereby not affectingthelifetimedue toY-Ninteraction.
The recent determination of the lifetime
τ
of (anti-)3H of 182+−8945 (stat.)± 27(syst.) ps,measured forthefirst timeinAu–Aucollisionsviatwo-bodydecaybytheSTARexperimentatRHIC [8], revived the interest for a more precise determination of the lifetime. The HypHI Collaborationat GSI reporteda value of
τ
= 183+−4232(stat.)± 37(syst.)ps[9],whichwas obtainedbystudying the projectilefragmentationof 6Liat2AGeV on acarbontarget.Very recently,the ALICEexperiment atthe LHCmeasured alife- time value
τ
=181+−5438(stat.)±33 (syst.) ps [10] using the data fromPb–Pbcollisions at√sNN=2.76 TeV and theinvariant mass analysisof the two-bodydecaychannel. The average value ofall resultsavailable upto2016was
τ
=215+−1816 ps [10],muchlower thanthetheoreticalestimates,motivatingtheneedforameasure- mentwithimprovedprecision.TheSTARCollaborationperformed a newanalysis[22] combiningthetwo-body andthethree-body decay channels usingthe datasample of the RHIC beam energy scan, resultinginaneven lowervalue ofτ
=142+−2421 (stat.)± 29 (syst.) ps.The ALICECollaborationexploitedthe datacollected in Pb–Pb collisions at√sNN=5.02 TeV to carry out a newanalysis ofthetwo-bodydecaychannel,reportedinthisletter.Thesetwo mostrecentvalues arereported inFig.5. Thenewmeasurement bySTARyieldsaverylowvalueascomparedtothelifetimeofthe free , while theresult presented inthis paperis in agreement withthetheoretical predictionsandit ischaracterized byan im- provedprecision withrespecttoprevious experiments.Thisvalue isalsoinagreementwiththepreviousALICEresult[10] obtained by analyzingthe datasample ofPb–Pb collisions at√
sNN=2.76 TeV.
Besidestheexperimentalresults,thetheoreticalpredictionsfor the 3H lifetime are reported in Fig. 5 for comparison with the data. The calculation performed by Dalitz and Rayet [23], repre- sented with a dot-long dashed cyan line, took into account the phasespacefactorsandthePauliprinciple,includingalsocorrec- tionstoaccountforfinalstatepionscatteringandthenon-mesonic weak decaychannel. More recently,a prediction forthe 3H life- time quitecloseto theoneofthe freehyperonwas published by Congleton [24] (dashed green line in Fig. 5), obtained using updatedvaluesforN–NandY–Npotentials.ThepredictionbyKa- mada et al.[7] (dotted-dashed blue line) was performed witha rigorousdeterminationofthe hypernucleuswave function andof thethreenucleonsscatteringstates,thusfindingavalueof256ps, whichistheclosesttothefreelifetimevalue.Recently,Garcilazo andGalperformedacalculation[44] usingthewavefunctiongen- erated by solving three-body Faddeev equations and adding the final-stateinteractions of thepions. Theirpredictionof213 ps is shownasadottedpurpleline.
Astatisticalcombinationofalltheexperimentalresults,includ- ing the most recent values determined by the STAR and ALICE experiment, leads to a world average of
τ
=206+−1513 ps for the 3H lifetimeandisrepresentedwithanorangeband inFig.5.The methodusedforthisevaluationisthesameasdescribedin[10].
Furthermore world averages were calculated grouping the mea- surementsonthe basisofthe experimentaltechniques,obtaining
τ
visual=224+−2320ps andτ
HI=189+−2220ps for the visualizing tech- niques andthe heavy-ion experiments,respectively. These values areconsistentandinagreement,alsowiththeworldaverage,and this suggests that the results are not affected by the technique usedforthemeasurement.Despite the addition of two recent high precision measure- ments of the 3H lifetime, one well below and the other closer to the theoretical predictions, the situation has hardly changed with the current world average, now more than 3
σ
below the lifetime of the free hyperon. In the future a very large data samplewillbecollectedwithheavy-ioncollisionsduringLHCRun 3(2021-2023) andRun4 (2027-2029)[45].At theendofRun4, ALICEexpectsto reducethestatisticaluncertaintyonthelifetime down to1% andsignificantly improvethe systematicuncertainty,Fig. 5.Collectionofthe3H lifetimemeasurementsobtainedwithdifferentexperimentaltechniques.Theverticallinesandboxesarethestatisticalandsystematicuncertain- tiesrespectively.Theorangebandrepresentstheaverageofthelifetimevaluesandthelinesattheedgecorrespondto1σ uncertainty.Thedashed-dottedlinesarefour theoreticalpredictions.
which at presentis 9.5%. Furthermore, it would be beneficial in viewofamoresolid comparisonwiththetheoreticalpredictions, tohave newmeasurements performedatlower energies atRHIC and SIS and by using different experimental techniques at the J-PARC and MAMI facilities. A measurement of the lifetime to a precisionofafewpercentwillguideandconstrainthetheoretical inputleading toamoreprecisedeterminationoftheY-Ninterac- tion,eventuallycontributingtosolvingthehyperonpuzzle.
Acknowledgements
The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstotheconstruc- tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICECollab- oration gratefully acknowledges the resources and support pro- videdbyall GridcentresandtheWorldwide LHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfundingagencies fortheirsupport inbuildingandrun- ningtheALICEdetector: A.I. AlikhanyanNationalScience Labora- tory(YerevanPhysicsInstitute)Foundation(ANSL),StateCommit- teeofScienceandWorldFederationofScientists(WFS),Armenia;
Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung,Austria; MinistryofCommunicationsandHighTech- nologies, National Nuclear Research Center, Azerbaijan; Conselho NacionaldeDesenvolvimentoCientíficoeTecnológico(CNPq),Uni- versidadeFederal doRioGrande doSul(UFRGS), Financiadorade Estudos e Projetos (Finep) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science &
Technology of China (MSTC), National Natural Science Founda- tionof China(NSFC) andMinistryof EducationofChina (MOEC), China; Croatian Science Foundation and Ministry of Science and Education, Croatia; Centro de Aplicaciones Tecnológicas y Desar- rollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Educa- tion, Youth and Sports of the Czech Republic, Czech Republic;
The Danish Council forIndependent Research| Natural Sciences, the Carlsberg Foundation and Danish National Research Founda- tion (DNRF), Denmark; Helsinki Institute of Physics (HIP), Fin- land; Commissariat à l’Energie Atomique (CEA), Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and
Centre National de la Recherche Scientifique (CNRS) and Région des Pays de laLoire, France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionen- forschung GmbH, Germany; General Secretariat forResearch and Technology,MinistryofEducation,ResearchandReligions,Greece;
National Research, Development and Innovation Office, Hungary;
Department of Atomic Energy, Government of India (DAE), De- partment of Science andTechnology, Governmentof India (DST), University Grants Commission, Government of India (UGC) and Council ofScientific andIndustrialResearch(CSIR), India;Indone- sianInstituteofSciences,Indonesia;CentroFermi- MuseoStorico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innova- tive ScienceandTechnology,NagasakiInstituteofAppliedScience (IIST),Japan SocietyforthePromotion ofScience(JSPS)KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia y Tec- nología(CONACYT) throughFondodeCooperaciónInternacionalen CienciayTecnología(FONCICYT)andDirecciónGeneraldeAsuntos delPersonalAcademico(DGAPA),Mexico;NederlandseOrganisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Re- search Council of Norway, Norway; Commission on Science and Technology forSustainableDevelopmentintheSouth(COMSATS), Pakistan; Ministryof Science andHigher Education andNational Science Centre,Poland;Korea InstituteofScience andTechnology InformationandNationalResearchFoundationofKorea(NRF),Re- publicofKorea;MinistryofEducationandScientificResearch,In- stituteofAtomic PhysicsandMinistryofResearchandInnovation andInstituteofAtomicPhysics,Romania;JointInstituteforNuclear Research(JINR), MinistryofEducationandScience oftheRussian Federation, National ResearchCentre KurchatovInstitute, Russian Science Foundation and Russian Foundation for Basic Research, Russia; Ministryof Education,Science, ResearchandSport ofthe Slovak Republic, Slovakia; NationalResearch Foundation of South Africa,SouthAfrica;SwedishResearchCouncil(VR)andKnut&Al- iceWallenbergFoundation(KAW),Sweden;EuropeanOrganization for Nuclear Research, Switzerland;National Science andTechnol- ogy Development Agency (NSDTA), Suranaree University of Tech- nology(SUT)andOfficeoftheHigherEducationCommissionunder NRU projectofThailand,Thailand;TurkishAtomicEnergy Agency (TAEK),Turkey;NationalAcademyofSciencesofUkraine,Ukraine;
ScienceandTechnologyFacilitiesCouncil(STFC),UnitedKingdom;
NationalScienceFoundationoftheUnitedStatesofAmerica(NSF) andUnitedStatesDepartmentofEnergy,OfficeofNuclearPhysics (DOENP),UnitedStatesofAmerica.
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ALICECollaboration
