HΛ3 and H‾Λ‾3 lifetime measurement in Pb–Pb collisions at sNN=5.02 TeV via two-body decay

Contents lists available atScienceDirect

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

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+₋1.02_1.13 (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+₋1.20_0.93(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

τ

visual

=

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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31

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10

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48

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53

, Z. Ahammed

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C. Grigoras

34

,

V. Grigoriev

91

,

A. Grigoryan

1

,

S. Grigoryan

74

,

O.S. Groettvik

22

,

J.M. Gronefeld

105

,

F. Grosa

31

,

J.F. Grosse-Oetringhaus

34

,

R. Grosso

105

,

R. Guernane

77

, B. Guerzoni

27

,

M. Guittiere

114

,

K. Gulbrandsen

87

,

T. Gunji

132

, A. Gupta

99

,

R. Gupta

99

,

I.B. Guzman

44

,

R. Haake

146

,

M.K. Habib

105

, C. Hadjidakis

61

,

H. Hamagaki

80

,

G. Hamar

145

,

M. Hamid

6

,

R. Hannigan

119

,

M.R. Haque

63

,

A. Harlenderova

105

,

J.W. Harris

146

,

A. Harton

11

,

J.A. Hasenbichler

34

, H. Hassan

77

,

D. Hatzifotiadou

10

,

53

, P. Hauer

42

,

S. Hayashi

132

,

A.D.L.B. Hechavarria

144

,

S.T. Heckel

68

,

E. Hellbär

68

,

H. Helstrup

36

, A. Herghelegiu

47

,

E.G. Hernandez

44

,

G. Herrera Corral

9

, F. Herrmann

144

, K.F. Hetland

36

,

T.E. Hilden

43

,

H. Hillemanns

34

,

C. Hills

127

,

B. Hippolyte

136

,

B. Hohlweger

103

,

D. Horak

37

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S. Hornung

105

, R. Hosokawa

16

,

133

,

P. Hristov

34

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C. Huang

61

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C. Hughes

130

,

P. Huhn

68

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T.J. Humanic

95

,

H. Hushnud

108

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L.A. Husova

144

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N. Hussain

41

, S.A. Hussain

15

,

T. Hussain

17

, D. Hutter

39

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D.S. Hwang

19

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J.P. Iddon

34

,

127

,

R. Ilkaev

107

,

M. Inaba

133

, M. Ippolitov

86

,

M.S. Islam

108

,

M. Ivanov

105

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V. Ivanov

96

,

V. Izucheev

89

, B. Jacak

78

, N. Jacazio

27

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P.M. Jacobs

78

,

M.B. Jadhav

48

,

S. Jadlovska

116

,

J. Jadlovsky

116

,

S. Jaelani

63

,

C. Jahnke

121

,

M.J. Jakubowska

142

,

M.A. Janik

142

,

M. Jercic

97

,

O. Jevons

109

,

R.T. Jimenez Bustamante

105

,

M. Jin

125

, F. Jonas

94

,

144

,

P.G. Jones

109

,

A. Jusko

109

,

P. Kalinak

64

,

P. Karczmarczyk

34

,

E. Karpechev

62

,

U. Kebschull

73

,

R. Keidel

46

,

M. Keil

34

,

B. Ketzer

42

, Z. Khabanova

88

,

A.M. Khan

6

,

S. Khan

17

,

S.A. Khan

141

,

A. Khanzadeev

96

, Y. Kharlov

89

,

A. Khatun

17

, A. Khuntia

49

,

118

,

B. Kileng

36

,

B. Kim

60

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B. Kim

133

,

D. Kim

147

,

D.J. Kim

126

, E.J. Kim

13

, H. Kim

147

, J. Kim

147

,

J.S. Kim

40

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J. Kim

102

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J. Kim

147

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J. Kim

13

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M. Kim

102

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S. Kim

19

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T. Kim

147

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T. Kim

147

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S. Kirsch

39

, I. Kisel

39

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S. Kiselev

90

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142

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J.L. Klay

5

, C. Klein

68

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J. Klein

58

, S. Klein

78

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C. Klein-Bösing

144

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S. Klewin

102

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A. Kluge

34

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M.L. Knichel

34

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A.G. Knospe

125

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C. Kobdaj

115

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M.K. Köhler

102

, T. Kollegger

105

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A. Kondratyev

74

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N. Kondratyeva

91

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E. Kondratyuk

89

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P.J. Konopka

34

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L. Koska

116

,

O. Kovalenko

83

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V. Kovalenko

112

,

M. Kowalski

118

, I. Králik

64

, A. Kravˇcáková

38

,

L. Kreis

105

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M. Krivda

64

,

109

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F. Krizek

93

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K. Krizkova Gajdosova

37

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M. Krüger

68

,

E. Kryshen

96

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M. Krzewicki

39

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A.M. Kubera

95

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V. Kuˇcera

60

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C. Kuhn

136

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P.G. Kuijer

88

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L. Kumar

98

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48

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S. Kundu

84

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P. Kurashvili

83

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A. Kurepin

62

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A.B. Kurepin

62

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A. Kuryakin

107

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S. Kushpil

93

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J. Kvapil

109

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M.J. Kweon

60

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60

, Y. Kwon

147

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S.L. La Pointe

39

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P. La Rocca

28

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Y.S. Lai

78

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R. Langoy

129

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K. Lapidus

34

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146

, A. Lardeux

21

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P. Larionov

51

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E. Laudi

34

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R. Lavicka

37

, T. Lazareva

112

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R. Lea

25

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L. Leardini

102

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S. Lee

147

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F. Lehas

88

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S. Lehner

113

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J. Lehrbach

39

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R.C. Lemmon

92

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I. León Monzón

120

, E.D. Lesser

20

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M. Lettrich

34

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P. Lévai

145

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X. Li

12

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X.L. Li

6

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J. Lien

129

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R. Lietava

109

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18

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S. Lindal

21

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39

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127

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C. Lippmann

105

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95

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V. Litichevskyi

43

, A. Liu

78

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S. Liu

95

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W.J. Llope

143

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I.M. Lofnes

22

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V. Loginov

91

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C. Loizides

94

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P. Loncar

35

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X. Lopez

134

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E. López Torres

8

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68

, J.R. Luhder

144

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M. Lunardon

29

, G. Luparello

59

, M. Lupi

73

, A. Maevskaya

62

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M. Mager

34

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S.M. Mahmood

21

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T. Mahmoud

42

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A. Maire

136

,

R.D. Majka

146

,

M. Malaev

96

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Q.W. Malik

21

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L. Malinina

74

,

iii

,

D. Mal’Kevich

90

, P. Malzacher

105

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A. Mamonov

107

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G. Mandaglio

55

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V. Manko

86

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F. Manso

134

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V. Manzari

52

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Y. Mao

6

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M. Marchisone

135

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J. Mareš

66

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G.V. Margagliotti

25

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A. Margotti

53

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J. Margutti

63

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A. Marín

105

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C. Markert

119

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68

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N.A. Martin

102

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P. Martinengo

34

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J.L. Martinez

125

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M.I. Martínez

44

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G. Martínez García

114

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M. Martinez Pedreira

34

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S. Masciocchi

105

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26

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A. Masoni

54

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61

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E. Masson

114

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A. Mastroserio

138

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A.M. Mathis

103

,

117

, O. Matonoha

79

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P.F.T. Matuoka

121

, A. Matyja

118

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C. Mayer

118

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M. Mazzilli

33

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M.A. Mazzoni

57

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68

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F. Meddi

23

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Y. Melikyan

91

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71

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C. Mengke

6

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E. Meninno

30

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M. Meres

14

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124

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Y. Miake

133

, L. Micheletti

26

, M.M. Mieskolainen

43

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D.L. Mihaylov

103

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K. Mikhaylov

74

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90

,

A. Mischke

63

,

i

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A.N. Mishra

69

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D. Mi´skowiec

105

,

C.M. Mitu

67

,

A. Modak

3

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N. Mohammadi

34

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A.P. Mohanty

63

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B. Mohanty

84

, M. Mohisin Khan

17

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iv

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M. Mondal

141

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M.M. Mondal

65

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C. Mordasini

103

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D.A. Moreira De Godoy

144

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L.A.P. Moreno

44

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S. Moretto

29

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A. Morreale

114

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A. Morsch

34

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T. Mrnjavac

34

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V. Muccifora

51

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E. Mudnic

35

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D. Mühlheim

144

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S. Muhuri

141

, J.D. Mulligan

78

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M.G. Munhoz

121

,

K. Münning

42

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R.H. Munzer

68

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H. Murakami

132

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S. Murray

72

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L. Musa

34

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J. Musinsky

64

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C.J. Myers

125

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J.W. Myrcha

142

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48

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R. Nair

83

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B.K. Nandi

48

, R. Nania

10

,

53

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E. Nappi

52

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M.U. Naru

15

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A.F. Nassirpour

79

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H. Natal da Luz

121

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130

, R. Nayak

48

, T.K. Nayak

84

,

141

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S. Nazarenko

107

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A. Neagu

21

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R.A. Negrao De Oliveira

68

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69

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S.V. Nesbo

36

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G. Neskovic

39

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D. Nesterov

112

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B.S. Nielsen

87

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86

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S. Nikulin

86

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V. Nikulin

96

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F. Noferini

10

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53

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74

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G. Nooren

63

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J. Norman

77

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N. Novitzky

133

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P. Nowakowski

142

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86

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J. Nystrand

22

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M. Ogino

80

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102

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J. Oleniacz

142

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A.C. Oliveira Da Silva

121

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M.H. Oliver

146

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C. Oppedisano

58

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R. Orava

43

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A. Ortiz Velasquez

69

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A. Oskarsson

79

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J. Otwinowski

118

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K. Oyama

80

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Y. Pachmayer

102

, V. Pacik

87

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D. Pagano

140

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G. Pai ´c

69

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P. Palni

6

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J. Pan

143

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A.K. Pandey

48

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S. Panebianco

137

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V. Papikyan

1

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P. Pareek

49

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J. Park

60

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J.E. Parkkila

126

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S. Parmar

98

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A. Passfeld

144

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S.P. Pathak

125

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R.N. Patra

141

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B. Paul

24

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58

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H. Pei

6

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T. Peitzmann

63

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X. Peng

6

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L.G. Pereira

70

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137

, D. Peresunko

86

,

G.M. Perez

8

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E. Perez Lezama

68

,

V. Peskov

68

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Y. Pestov

4

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V. Petráˇcek

37

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M. Petrovici

47

, R.P. Pezzi

70

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S. Piano

59

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M. Pikna

14

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P. Pillot

114

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L.O.D.L. Pimentel

87

, O. Pinazza

34

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53

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L. Pinsky

125

, C. Pinto

28

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51

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D. Pistone

55

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D.B. Piyarathna

125

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M. Płosko ´n

78

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M. Planinic

97

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F. Pliquett

68

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J. Pluta

142

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S. Pochybova

145

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M.G. Poghosyan

94

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B. Polichtchouk

89

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N. Poljak

97

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W. Poonsawat

115

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A. Pop

47

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144

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S. Porteboeuf-Houssais

134

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V. Pozdniakov

74

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S.K. Prasad

3

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R. Preghenella

53

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F. Prino

58

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143

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I. Pshenichnov

62

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26

,

34

, V. Punin

107

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K. Puranapanda

141

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J. Putschke

143

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125

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S. Ragoni

109

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S. Raha

3

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S. Rajput

99

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J. Rak

126

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137

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32

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136

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R. Raniwala

100

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100

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43

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68

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49

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V. Ratza

42

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I. Ravasenga

31

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K.F. Read

94

,

130

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K. Redlich

83

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v

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A. Rehman

22

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68

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34

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X. Ren

6

, R. Renfordt

68

,