H-3(Lambda) and (3)((Lambda)over-bar)(H)over-bar lifetime measurement in Pb-Pb collisions at root s(NN)=5.02 TeV via two-body decay

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DOI: 10.1016/j.physletb.2019.134905

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by Elsevier. All rights reserved.

DIRETORIA DE TRATAMENTO DA INFORMAÇÃO

Cidade Universitária Zeferino Vaz Barão Geraldo

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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

r

a

c

t

Articlehistory:

Received26July2019

Receivedinrevisedform22August2019 Accepted29August2019

Availableonline3September2019 Editor: L.Rolandi

Animprovedvalueforthelifetimeofthe(anti-)hypertritonhasbeenobtainedusingthedatasampleof Pb–Pbcollisionsat√sNN=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 presenceof hy-peronsas additional degrees of freedom leads to a considerable softeningofthematterequationofstate(EOS).The resultingEOS inhibitstheformationoflargemassneutron stars.Thisis incom-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,thehypertriton3

H,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]).

Earlyhypertritonlifetimemeasurementsweredonewith imag-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-)3

H produced in ultra-relativistic heavy-ioncollisions orinrelativisticionfragmentation[21], even though affected by statistical andsystematic uncertainties bigger than10%,areinagreementamongeach otherandarelowerthan thefree



lifetime[9,10,22].

However, the few existingtheoretical calculationspredict that the lifetime of the 3

H should be very close to the lifetime of free



. The most comprehensive 3_H 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 ofthe3_H lifetime.

Inthisletter,thelifetimeofthe (anti-)3_H 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 andsystematic un-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

AdetaileddescriptionoftheALICEapparatusanddata acquisi-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 the interac-tion region. They coverthe pseudorapidity ranges 2.8

<

η

<

5.1 and

−

3

.

7

<

η

<

−

1

.

7,respectively.TheV0detectorisusedto de-fine theMinimum Bias (MB) trigger, which ischaracterized by a coincidence signal in the V0Aand inthe V0C, andto determine thecentralityofthecollisions[28].TheITS[29] istheclosest de-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

≤

2

π

andcoversthepseudorapidityrange

|

η

|

<

0.9.TheTPC [30] isagaseousdetector,mainlyusedfortrackingandforparticle identification(PID)viathespecificenergyloss(dE/dx),witha to-talsensitivevolumeof90m3 filledwithamixtureof88%Arand 12%CO2.ThereconstructedclustersinTPCandITSarethestarting

point ofthetrackfinder algorithm,whichadopts theKalman fil-tertechnique[31].Thesetracksareusedtodeterminetheprimary collision vertexwith a precision better than 50 μmin the plane transversetothecollidingbeams[26].

3. Datasampleandanalysistechnique

Inthisletter,thelifetimeofthe(anti-)hypertritonisdetermined byexploitingthe2-body mesonicdecaychannelwithcharged pi-ons, namely3

H

→

3He

+

π

− and 3_H

→

3He

+

π

+.Both 3H and3

H candidatesareusedforthismeasurement.

TheanalysisisperformedusingthedatasampleofPb–Pb colli-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%.

The3_H and3

H identificationisbasedonthetopologyoftheir weakdecaysandonthereconstructionofthetracksoftheirdecay products, referred to as daughter particles. The weakly decaying hypernucleiarereconstructedusingthealgorithmwhichwas pre-viouslyusedfortheK0

S and



productionanalyses[32] andwhich

istypicallyadoptedfora2-bodyweakdecaytopology.Atfirst,the algorithmusestheTPCandITSclusterstoreconstructthedaughter tracksandthencombinestheminordertoobtainaV-shaped de-cayvertex.Moredetailsonthisalgorithmcanbefoundin[26,33].

Table 1

Selection criteriaappliedfor the identification ofthe daughter candidatetracksandforthereconstructionofthehypertriton can-didate. Selection criteria Track selections |η| <0.9 Number of TPC clusters >70 χ2_{per 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 3_{He p}

T(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,inordertoguaranteearesolution

σ

betterthan5% on track momentum andof about6% for the dE/dx [26]. Moreover, the

χ

2_{perTPCclusterisrequiredtobelessthan5andtrackswith}

kink topologies are rejected. The particle identification (PID) of the daughters(3_He, 3_He,

_π

±_{) isperformedfollowingthemethod}

described in [33], which is used in many analyses of the ALICE Collaboration.Itisbasedonthedifferencebetweenthemeasured andtheexpecteddE/dx foraselectedparticlespeciesnormalized totheenergylossresolutioninthedetector,

σ

forshort,andis re-ferred toasthen

σ

methodinthisletter.Inparticular,an

|

n

σ

|

≤

3isrequired,inatrack-by-trackapproach,withrespecttothe ex-pected

π

and3_{He specificenergy lossinthe TPC. Thepions can}

beidentifieduptoamomentumofabout1.2GeV/c,beyondwhich there isconsiderablecontamination fromkaons andprotons.The

3_{He, 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,as re-ported by the ALICEexperiment [34]. As a consequence the 3_He

candidateisrequiredtohaveatransversemomentum(pT)greater

than1.8GeV/c,wherethespallationprocessesarenegligible. The3

H and3H candidatesareselectedbyapplyingtopological and kinematic selection criteriaon the decay products. The dis-tanceofclosestapproach (DCA)betweenthetwodaughtertracks andtheDCAof

π

±tracksfromtheprimaryvertexarerequiredto belowerthan0.7cmandlargerthan0.1cmrespectively.The can-didates areselected whosecosineof theanglebetweenthe total momentumofthedaughtertracksatthesecondaryvertexandthe vectorconnectingtheprimaryandsecondaryvertex(pointing an-gle)islargerthan0.995.Twoadditionalselectionsonthe3_H and

3

H rapidity (

|

y

|

<

0.8) andtransverse momentum (2

<

pT

<

9 GeV/c) are applied. Allthe selection criteriapreviously described havebeenstudiedwithadedicatedMonteCarloproduction,in or-der to improvethebackground rejection,andare summarizedin Table1.

The sample of3

H and 3H candidatesis divided in fourct

=

M Lc/p intervals for the lifetime determination, where c is the speedoflight,t isthepropertimeofthecandidate,M isthemass ofthecandidate,L isthedecaydistanceandp isthereconstructed 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(3_He,π−_)and(3_He,π+_{)forthefourct intervalsusedtodeterminethe}3

H and

3

H lifetime.Thesolidbluecurverepresentsthe

functionusedtoperformthefitandthereddashedcurverepresentsthebackgroundcomponent.

15 cmand15

≤

ct

<

28 cm.Thecorrespondinginvariantmass dis-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 widths

σ

of each distributionareextracted. Inparticular,thesignal widthisinthe range1.7–2.1 MeV/c2,depending onthe ct interval,andisdriven bythedetectorresolution.Therawyieldofthesignalisdefinedas theintegraloftheGaussianfunctionina

±

3

σ

regionaroundthe meanvalueabovethebackground.Thesignificanceofthesignalin thefourct intervalsvariesintherange3.1–4.9.

The yield is corrected in each ct bin for the detector accep-tance,thereconstruction efficiencyandtheabsorption ofthe3_H (3

H)inthedetectormaterial.Theefficiency

×

acceptanceis deter-minedwithadedicatedMonteCarlosimulation,wherethe3

H and

3

H are injectedontopofaHIJING event[35] andare allowedto decayintochargedtwo-bodyandthree-bodyfinalstates.The sim-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 3

H (3

H) and

3_{He (}3_{He) 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 the3_H (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 3_{He [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% for3

H.Thecorrectionfactortobeappliedis: k

=

k_abs,3

H

+ (1

−

kabs,3H

)

kabs,3He (1)

where k_abs,3

H is the probability that the

3

H is absorbed be-tween the primary and the secondary vertex while k_abs,3_He is

Fig. 2. Efficiency×acceptance asa functionofct for 3

H (redsquare),3H (blue

square)and3

H+3H (blackopencircle)inthesamect intervalsselectedforthe

rawyieldsextraction.

the probability that the daughter 3_{He 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 3_H and 3

H. Fig. 2 shows the efficiency

×

acceptance (black marker) which is used forthe lifetime determination andis obtained by combining 3_H and 3

H after the absorption correction is applied. This distribu-tion is alsoshown separately for 3

H and 3H 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 mainlyduetotheassumptionusedforthe3

H (3H)cross-section. Thisuncertaintyisevaluatedby varyingthisassumptionbetween alowerandanupperlimit.Thefirstoneisobtainedbysettingthe

3

H (

3

H) cross-section equal to the

3_{He (}3_{He) 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 inallct intervals.

The systematic uncertainty due to the single-track efficiency andthedifferentchoices ofthetrackquality selectionshasbeen investigated[40] andamountsto4%. Fortheanalysisofthe two-bodydecayof3_H anuncertaintyof8% isassumedinallct

inter-vals. The summary of thesystematic uncertainties is reportedin Table2,wherethetotaluncertaintyisobtainedassumin quadra-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

ct intervalisthesuminquadratureofthesinglesources. 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(3

H+

3

H)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)unbinnedfit ap-proach.Themethodcanbesummarizedinthreesteps:i)fittothe

ct-integratedinvariantmassdistribution;ii)tunethefunctionused todescribethecombinatorialbackground;iii)fittothect

distribu-tion withafunction whichisthe sumof threeexponentials, one todescribethesignalandtwotodescribethebackground.

The firststep isperformedwithafunction that isthesumof a Gaussian,forthesignal, andasecond orderpolynomial,forthe background. The mean value

μ

and the

σ

are 2.9913

±

0.0004 GeV/c2 _{and 0.0020}

_±

_{0.0005 GeV/c}2 _{respectively and are used}

to define theboundaries ofthe signal region andthe sidebands, which correspond to the intervals

μ

±

3

σ

and

±

3

σ

to

±

9

σ

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(3_H

+

3

H)lifetimemeasurementisobtainedby perform-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

ct distribution,performedinthesignalregion,leadingtoavalueof

τ

=

240+₋40₃₁(stat.)

±

18(syst.)ps,asreportedinFig.4.The statis-ticaluncertaintyofthemeasurementisassessedbyprovidingthe intervaloftheestimated

τ

[42],ataconfidencelevelof68%,which isrepresentedbythereddashedlines,basedonthelog-likelihood ratio (log

λ(

τ

)

), shown as a blue line. The result corresponds to aproperdecaylength c

τ

=

7

.

20₋+₀1._.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,whichweconsiderasthefinalvalueforthe3

H lifetime.Additionalfiguresanddetailsfortheunbinnedfitmethod arepresentedin[43].

4. Discussionandconclusions

Thanks to the large data sample of heavy-ion collisions at

√

sNN

=

5

.

02 TeVprovidedbytheLHCandtotheexcellenttracking

andparticleidentificationperformance oftheALICEapparatuswe havedeterminedaprecisevalueforthe3_H 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,wherethetracksformedduetopassageofcharged par-ticles were recorded visually. Most ofthe results obtainedusing thesetechniques hadlarge uncertainties due to the limited size ofthedatasample atdisposal.Furthermore,thesemeasurements preventedadefinite conclusion onthe agreementwiththe theo-retical predictions, whichforesee a lifetimecloseto the value of thefree



hyperon. Itisworthwhiletonote thatthe small bind-ingenergyofthehypertritonmakesthe



spendmostofthetime farfromthedeuteron corethereby not affectingthelifetimedue toY-Ninteraction.

The recent determination of the lifetime

τ

of (anti-)3_H 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 6_{Liat2AGeV on acarbontarget.}

Very recently,the ALICEexperiment atthe LHCmeasured a life-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,motivatingtheneedfora measure-mentwithimprovedprecision.TheSTARCollaborationperformed a newanalysis[22] combiningthetwo-body andthethree-body decay channels usingthe datasample of the RHIC beam energy scan, resultinginaneven lowervalue of

τ

=

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 3_H 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,includingalso correc-tionstoaccountforfinalstatepionscatteringandthenon-mesonic weak decaychannel. More recently,a prediction forthe 3_H life-time quitecloseto theoneofthe free



hyperonwas published by Congleton [24] (dashed green line in Fig. 5), obtained using updatedvaluesforN–NandY–Npotentials.Thepredictionby Ka-mada et al.[7] (dotted-dashed blue line) was performed witha rigorousdeterminationofthe hypernucleuswave function andof thethreenucleonsscatteringstates,thusfindingavalueof256ps, whichistheclosesttothefree



lifetimevalue.Recently,Garcilazo andGalperformedacalculation[44] usingthewavefunction gen-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

3

H lifetimeandisrepresentedwithanorangeband inFig.5.The methodusedforthisevaluationisthesameasdescribedin[10]. Furthermore world averages were calculated grouping the mea-surementsonthe basisofthe experimentaltechniques,obtaining

τvisual

=

224+₋23₂₀ps and

τ

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 3_H 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. Collectionofthe3

H lifetimemeasurementsobtainedwithdifferentexperimentaltechniques.Theverticallinesandboxesarethestatisticalandsystematic

uncertain-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-N interac-tion,eventuallycontributingtosolvingthehyperonpuzzle.

Acknowledgements

The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstothe construc-tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collab-oration gratefully acknowledges the resources and support pro-videdbyall GridcentresandtheWorldwide LHCComputingGrid (WLCG) collaboration. The ALICE Collaboration acknowledges the followingfundingagencies fortheirsupport inbuildingand run-ningtheALICEdetector: A.I. AlikhanyanNationalScience Labora-tory(YerevanPhysicsInstitute)Foundation(ANSL),State Commit-teeofScienceandWorldFederationofScientists(WFS),Armenia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung,Austria; MinistryofCommunicationsandHigh Tech-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 and Technol-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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98

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31

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48

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P. Ghosh

141

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S.K. Ghosh

3

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P. Gianotti

51

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P. Giubellino

58

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105

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P. Giubilato

29

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P. Glässel

102

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D.M. Goméz Coral

71

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A. Gomez Ramirez

73

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V. Gonzalez

105

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P. González-Zamora

44

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39

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L. Görlich

118

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S. Gotovac

35

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71

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142

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109

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78

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A. Grelli

63

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C. Grigoras

34

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V. Grigoriev

91

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A. Grigoryan

1

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S. Grigoryan

74

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22

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J.M. Gronefeld

105

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F. Grosa

31

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34

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105

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77

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27

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M. Guittiere

114

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K. Gulbrandsen

87

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T. Gunji

132

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A. Gupta

99

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99

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44

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146

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105

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61

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80

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145

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6

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119

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63

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146

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A. Harton

11

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J.A. Hasenbichler

34

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H. Hassan

77

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10

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53

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P. Hauer

42

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S. Hayashi

132

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A.D.L.B. Hechavarria

144

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S.T. Heckel

68

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68

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H. Helstrup

36

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44

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G. Herrera Corral

9

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144

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36

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T.E. Hilden

43

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H. Hillemanns

34

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C. Hills

127

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B. Hippolyte

136

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B. Hohlweger

103

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D. Horak

37

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S. Hornung

105

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R. Hosokawa

16

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133

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P. Hristov

34

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C. Huang

61

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130

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68

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T.J. Humanic

95

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H. Hushnud

108

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L.A. Husova

144

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N. Hussain

41

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15

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T. Hussain

17

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19

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34

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127

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R. Ilkaev

107

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M. Inaba

133

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M. Ippolitov

86

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M.S. Islam

108

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M. Ivanov

105

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V. Ivanov

96

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V. Izucheev

89

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B. Jacak

78

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N. Jacazio

27

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P.M. Jacobs

78

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M.B. Jadhav

48

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S. Jadlovska

116

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J. Jadlovsky

116

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63

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121

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142

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142

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97

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O. Jevons

109

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R.T. Jimenez Bustamante

105

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M. Jin

125

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F. Jonas

94

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144

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P.G. Jones

109

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A. Jusko

109

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P. Kalinak

64

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P. Karczmarczyk

34

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E. Karpechev

62

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U. Kebschull

73

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R. Keidel

46

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M. Keil

34

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B. Ketzer

42

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Z. Khabanova

88

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A.M. Khan

6

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S. Khan

17

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S.A. Khan

141

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A. Khanzadeev

96

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Y. Kharlov

89

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A. Khatun

17

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A. Khuntia

49

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118

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B. Kileng

36

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B. Kim

60

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133

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D. Kim

147

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D.J. Kim

126

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13

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147

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J.S. Kim

40

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J. Kim

102

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147

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13

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M. Kim

102

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19

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T. Kim

147

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147

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S. Kirsch

39

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I. Kisel

39

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90

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A. Kisiel

142

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5

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C. Klein

68

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J. Klein

58

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78

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144

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S. Klewin

102

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34

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A.G. Knospe

125

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115

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102

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T. Kollegger

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74

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L. Koska

116

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O. Kovalenko

83

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112

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M. Kowalski

118

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64

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A. Kravˇcáková

38

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L. Kreis

105

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M. Krivda

64

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109

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93

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37

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M. Krüger

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96

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M. Krzewicki

39

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95

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60

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136

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88

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98

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48

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34

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34

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6

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L. Malinina

74

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90

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69

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67

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3

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84

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17

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iv

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M. Mondal

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121

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10

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