Measurement of electrons from semileptonic heavy-flavour hadron decays at midrapidity in pp and Pb–Pb collisions at sNN=5.02 TeV

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Physics

Letters

B

www.elsevier.com/locate/physletb

Measurement

of

electrons

from

semileptonic

heavy-flavour

hadron

decays

at

midrapidity

in

pp

and

Pb–Pb

collisions

at

√

s

_NN

=

5

.

02 TeV

.

ALICE

Collaboration



a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received16November2019

Receivedinrevisedform8February2020 Accepted17March2020

Availableonline20March2020 Editor:L.Rolandi

Thedifferentialinvariantyieldasafunctionoftransversemomentum(pT)ofelectronsfromsemileptonic

heavy-flavourhadrondecayswasmeasuredatmidrapidityincentral(0–10%),semi-central(30–50%)and peripheral (60–80%)lead–lead(Pb–Pb)collisionsat√sNN=5.02 TeV inthe pTintervals0.5–26 GeV/c

(0–10%and 30–50%)and 0.5–10 GeV/c (60–80%). The productioncross sectioninproton–proton (pp) collisions at √s=5.02 TeV was measured as well in 0.5<pT<10 GeV/c and it lies close to

the upper band of perturbative QCD calculation uncertainties up to pT=5 GeV/c and close to the

mean value for larger pT. The modification of the electron yield with respect to what is expected

for an incoherent superposition ofnucleon–nucleon collisions is evaluatedby measuring the nuclear modificationfactor RAA.Themeasurementofthe RAAindifferentcentralityclassesallowsin-medium

energylossofcharmandbeautyquarkstobeinvestigated.TheRAAshowsasuppressionwithrespectto

unityatintermediate pT,whichincreaseswhilemovingtowardsmorecentralcollisions.Moreover,the

measured RAAissensitivetothemodificationofthepartondistributionfunctions (PDF)innuclei,like

nuclearshadowing, whichcauses asuppression oftheheavy-quarkproductionatlow pTinheavy-ion

collisionsatLHC.

©2020ConseilEuropéenpourlaRechercheNucléaire.PublishedbyElsevierB.V.Thisisanopenaccess articleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

ThemaingoalofALICEisthestudyoftheQuark-GluonPlasma (QGP),astate ofmatterwhichisexpectedtobecreatedin ultra-relativisticheavy-ioncollisionswherehightemperaturesandhigh energy densities are reached at the LHC [1]. Due to their large masses (mc

≈

1

.

5 GeV/c2, mb

≈

4

.

8 GeV/c2), charm and beauty

quarks (heavy-flavour) are mostly produced via partonic scatter-ing processes with highmomentum transfer, which have typical timescalessmallerthantheQGPthermalisationtime(1fm/c [2]). Furthermore, additional thermal production, as well as annihila-tionrates,ofcharmandbeautyquarksinthestrongly-interacting matter areexpected to be smallin Pb–Pb collisions even atLHC energies[3,4].Consequently,charmandbeautyquarksexperience thefullevolutionofthehotanddensemediumproducedin high-energyheavy-ion collisions,thereforetheyare idealprobesto in-vestigatethepropertiesoftheQGP.

Quarksandgluonsinteractstronglywiththemediumandthey are expected to lose energy through elastic collisions [5,6] and radiative processes [7,8]. Quarks have a smaller colour coupling factorwithrespect togluons, hencetheenergylossforquarksis expectedtobesmallerthanthatforgluons.Inaddition,the

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

cone effectis expectedto reduce small-angle gluon radiation for heavyquarkswithmoderateenergytomassratio[9],thusfurther attenuatingtheeffectofthemedium.Thecombinationofallthese effectsresultsintheobservedhierarchicalmassdependentenergy loss[8,10–22].

In order toquantify medium effects on heavy-flavour observ-ables measured in heavy-ion collisions, they are compared with measurements in proton–proton (pp) collisions, where these ef-fectsareexpectedtobeabsent.

In pp collisions, heavy-quark production can be described by perturbativeQuantumChromodynamics(pQCD)calculationsforall transversemomenta,whereaspQCDisnotapplicableforthe calcu-lationoflight quark andgluonproduction atlowtransverse mo-menta [3]. Moreover, measurements of heavy-flavour production crosssectionsinpp collisionsprovidethenecessaryexperimental referenceforheavy-ioncollisions.

Themediumeffectsonheavyquarksarequantifiedthroughthe measurementofthenuclearmodificationfactor,definedasthe ra-tio between the yield ofparticles produced in ion–ion collisions (d2NAA

/

dpTd y) andthe crosssection measured inproton-proton

collisions at the same energy (d2

_σ

pp

/

dpTd y), normalised by the

averagenuclearoverlapfunction



TAA



: RAA

(

pT

,

y

)

=

1



TAA



·

d2NAA

/

dpTd y d2

_σ

pp

/

dpTd y

.

(1) https://doi.org/10.1016/j.physletb.2020.135377

0370-2693/©2020ConseilEuropéenpourlaRechercheNucléaire.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

The



TAA



is definedas the average number ofnucleon–nucleon

collisions



Ncoll



, whichcan be estimatedvia Glauber model

cal-culations [23,24], divided by the inelastic nucleon-nucleon cross section. In-medium energy loss shifts the transverse momenta towards lower values, therefore at intermediate and high pT

(pT



2 GeV/c) a suppression of the production is expected

(RAA

<

1).Assumingthetotalcrosssectionevaluatedusing



Ncoll



scaling is not modified, the nuclear modification factor is ex-pectedto increasetowards lower pT,compensating thedepletion

at higher momenta. Such a rise was measured by the PHENIX andSTARexperiments atRHIC inAu–Au andCu–Cu collisions at

√

sNN

=

200 GeV for electrons from heavy-flavour hadron

de-cays [25–27]. The nuclear modification factor for electrons from semileptonic heavy-flavour hadron decays was also measured by the ALICE collaboration in Pb–Pb collisions at

√

sNN

=

2

.

76 TeV

[28,29], where the mentioned trend of RAA was also observed.

At low pT, the nuclear modification factor reaches a maximum

around1 GeV/c andtendstodecreaseatlower pT.Thistrendcan

be explained by initial and final state effects, like the collective expansionofthehotanddensesystem[30–32], theinterplay be-tweenhadronisationvia fragmentationandcoalescence[22,33,34] andthemodificationofthepartondistributionfunctions(PDF) in-sideboundnucleons[35].

Initial-stateeffectsattheLHCareexploredwithproton–nucleus collisions, where an extended QGP phase is not expected to be formed. The nuclear modification factor ofelectrons fromcharm and beauty hadron decays [14,36] and of D mesons [37] in p–Pb collisions at

√

sNN

=

5

.

02 TeV was found to be consistent

withunitywithinuncertainties. From this,one canconcludethat the strong suppression observed in Pb–Pb collisions is due to substantial final-state interactions of heavy quarks with theQGP formed in thesecollisions. However, it is important to note that recently the measurement of the elliptic flow of electrons from semileptonic heavy-flavour hadron decays [38] and of D mesons [39] havebeenpublished,showingintriguingandnotyetfully un-derstood collective effects in high-multiplicity p–Pb collisions in theheavy-flavoursector.

This paper reports the measurement of the production cross section in pp collisions, the invariant yields and the nuclear modification factor, RAA, in Pb–Pb collisions as a function of

pT of electrons from semileptonic heavy-flavour hadron decays

at mid-rapidity at the centre-of-mass energy per nucleon pair

√

sNN

=

5

.

02 TeV.Inordertostudy howtheyieldandRAA change

withcentralityinPb–Pbcollisions,themeasurement wasdonein threerepresentativeclasses:the0-10%classforcentralPb–Pb col-lisions, the 30-50% for semi-central Pb–Pb collisions and60-80% forperipheralPb–Pbcollisions.

2. Experimentalapparatusanddatasample

The ALICE detector is described in detail in Refs. [1,40]. The experiment mainly consists of a central barrel at midrapidity (

|η|

<

0

.

9), embedded in a cylindrical solenoidwhich provides a magneticfieldof0.5Tparalleltothebeamdirection,andamuon spectrometeratforwardrapidity(

−

4

<

η

<

−

2

.

5).

Charged particles produced in the collisions and originating fromparticledecaysaretrackedbytheInnerTrackingSystem(ITS) [41] andtheTimeProjectionChamber(TPC)[42].TheITSdetector, composedoftheSiliconPixelDetector(SPD),SiliconDriftDetector (SDD), andSiliconStrip Detector (SSD),consistsof sixcylindrical silicon layers surrounding the beamvacuum pipe. Theseprovide measurementsofparticle momentaandenergyloss(dE/dx)used forcharged-particleidentification(PID),togetherwiththeTPC.The particleidentificationis complementedby aTime-Of-Flight (TOF) [43] detector,whichmeasures thetime-of-flightofcharged parti-cles.TheTOFdetectordistinguisheselectronsfromkaons,protons,

Table 1

NumberofeventsandTAA[46,47] usedintheanalysis,splitbycollisionssystem,

triggerconfiguration,andcentralityclass.

Centrality MB EMCal trigger TAA(mb−1)

pp – 881×106 _{– –} 0–10% 6×106 _1.2×106 ₂₃ .26±0.17 Pb–Pb 30–50% 12×106 _0.3×106 ₃ .917±0.065 60–80% 12×106 _{– 0.4188±0.0106} andpionsup to pT



2

.

5 GeV/c, pT



4 GeV/c and pT



1 GeV/c,

respectively. The ElectroMagneticCalorimeter (EMCal)[44] covers a pseudorapidity region of

|η|

<

0

.

7 and it is used to measure electrons, photons, and jets in an azimuthal region of

∼

107o. TheelectronidentificationintheEMCalisbasedonthe measure-mentofthe E

/

p ratio,whereE istheenergyoftheEMCalcluster matched to the prolongationof thetrack withmomentum p re-constructedwiththeTPCandITSdetectors.TheV0detectors[45] consist of twoarrays of32scintillator tiles coveringthe pseudo-rapidity ranges 2

.

8

<

η

<

5

.

1 (V0A) and

−

3

.

7

<

η

<

−

1

.

7 (V0C), respectively,andareusedforeventcharacterisation.

The resultspresentedinthispaperare basedondatasamples of Pb–Pb collisions recorded in 2015 andof pp collisions at the sameenergyrecordedin2017.Theanalysedeventswerecollected with a minimum bias (MB) trigger of a logic AND between the V0AandV0Cdetectors.Pb–Pbcollisionswerealsorecordedusing theEMCaltrigger,whichrequiresanEMCalclusterenergysummed overagroupof4

×

4calorimetercellslargerthananenergy thresh-oldof10GeV.TheEMCALtriggeredeventswereusedforelectron measurements for pT

>

12 GeV/c.The centralityclasseswere

de-fined intermsofpercentilesofthehadronicPb–Pbcross section, definedbyselectionsonthesumoftheV0signalamplitudes[46]. Forbothcollisionsystems,onlyeventswithatleasttwotracks andareconstructedprimaryvertexlocatedbetween

±

10cmwith respecttothenominalinteractionpointalongthe z-axisare con-sidered.Eventsaffectedbypile-upfromdifferentbunchcrossings, which constitute less than 1% of the recorded sample, were re-jected [28]. The number of events analysed in the two collision systemswiththedifferenttriggerconfigurationsissummarisedin Table 1, togetherwiththeaverage nuclearoverlapfunction



TAA



[46,47].

3. Dataanalysis

The pT-differentialyieldofelectronsfromsemileptonic

heavy-flavour hadron decays is computed by measuring the inclusive electron yield and subtracting the contribution of electrons that do not originate from semileptonic heavy-flavour hadron decays. In thefollowing,theinclusiveelectron identificationstrategyand the subtraction ofelectrons originatingfrom backgroundsources aredescribed.

3.1. Trackselectionandelectronidentification

The selection criteria are similar to the ones described in Refs. [28,29]. They are summarised together with the kinematic cutsappliedintheanalysesinTable2.

It isimportanttonote thatonly tracksthathavehitsonboth SPD layers are accepted so that electrons fromlate photon con-versions inthedetectormaterial are significantlyreduced. In the Pb–Pb analysis for pT

>

3 GeV/c, alsotracks witha single hit in

theSPDareconsidered,sincetheamountofphotonicbackground startstobecomenegligible.IntheanalysisinwhichtheEMCal de-tectorisused,specifictrack-clustermatchingcriteriaareadopted. As inthe procedure followed inRefs. [28,29], electron candi-datesareidentifiedaccordingtothecriterialistedinTable3.These requirements depend on the data sample and on the transverse momentumintervalinwhichtheanalysesareperformed.

Table 2

Trackselectioncriteriausedintheanalyses.“DCA”isanabbreviationfor“distanceofclosestapproach”ofatracktotheprimaryvertex.

Parameter pp Pb–Pb Pb–Pb

(pT<3 GeV/c) (pT>3 GeV/c)

|y| <0.8 <0.8 <0.6

Number of clusters in TPC ≥100 ≥120 ≥80

TPC clusters in dE/dx calculation ≥80 ≥80 –

Number of clusters in ITS ≥3 ≥4 ≥3

Minimum number of clusters in SPD 2 2 1

|DCAxy| <1 cm <1 cm <2.4 cm

|DCAz| <2 cm <2 cm <3.2 cm

Found / findable clusters in TPC >0.6 >0.6 >0.6

χ2_{/clusters in TPC <4 <4 <4}

track-cluster matching in EMCal – – ϕ2_{+ η}2_<0.02 Table 3

Electron identification criteria. The following momentum-dependent function is used for the electron identification in pp collisions, based on the TPC dE/dx: f(p)=Min(0.12,0.02+0.07p).FortheelectronselectionbasedonclustersintheEMCAL,acriteriononthe“σ2

s”parameter[29],correspondingtotheshorter-axisof theshowershape,isused.Forbrevity,the“lowpT”labelisusedinplaceof“pT<3 GeV/c”,aswellas“highpT”inplaceof“pT>3 GeV/c”.

Centrality nTPC

σ,e nTOFσ,e nITSσ,e E/p Shower shape

pp (low pT) – [−0.5+f(p), 3] [−3,3] – – – pp (high pT) – [0.12, 3] – – – – 0–10% [−0.16,3] Pb–Pb (low pT) 30–50% [0,3] [−3,3] [−4,2] – – 60–80% [0.2,3] 0–10% Pb–Pb (high pT) 30–50% [−1,3] – – [0.8, 1.3] 0.01<σs2<0.35 60–80%

Theelectronidentificationinppcollisionsisperformedby eval-uatingthesignalfromtheTPCandTOFdetectors.Thediscriminant variable in the former detector is the deviation of dE/dx from theparameterisedelectronBethe-Bloch[48] expectationvalue, ex-pressedinunits ofthedE/dx resolution, nTPC_σ,e, whileinthelatter one the analogous variable nTOF_σ,e, referring to the particle time-of-flight,isconsidered. The criterion

|

nTOF_σ,e

|

<

3,used forelectron identificationuptopT

=

3 GeV/c,isrequiredtoreducebackground

fromkaonsandprotons.AmomentumdependentcriteriononnTPC_σ,e isadoptedtoguaranteeaconstantelectronidentificationefficiency of70%forpT

<

3 GeV/c andof50%forhighertransversemomenta

by reducing the selection window in nTPC_σ,e, in order to keep the hadron contamination sufficiently low. In the Pb–Pb analysis for pT

<

3 GeV/c, the electron identificationis performedby

apply-ingthesamerequirementonTOFandduetothelargedensitiesof tracks,aselectionbetween

−

4

<

nITS_σ,e

<

2 ontheenergydeposited inthe SDD and SSD detectorsis applied in all centrality classes. Finally,theselectiononnTPC_σ,e ensuresaconstantelectron identifica-tionefficiencyof50%forallcentralityclasses.Thehadron contam-inationfractionafterthePIDisestimatedbyfittingthenTPC_σ,e distri-butionforeachparticlespecieswithananalyticfunctionin differ-entmomentumintervals[28,29].Theinclusiveelectronsample is thenselectedbyapplyingafurthercriteriononnTPC_σ,e,whichis cho-seninordertohaveaconstantefficiencyasafunctionofthe mo-mentum,aswellastohavethehadroncontamination under con-trol.Thiscriterionisloosened for pT

>

3 GeV/c,duetothelower

amountofselectedhadronswhentheEMCaldetectorisemployed. InthePb–PbanalysisforpT

>

3 GeV/c,theelectroncandidates

arefirstselected bythe measurementofthe TPCdE/dx withthe criterion

−

1

<

nTPC_σ,e

<

3.Then,theselection0

.

8

<

E

/

p

<

1

.

3 onthe energyover momentum ratiois applied. Unlike for hadrons, the ratio E

/

p is close to 1 for electrons because they deposit most of their energy in the EMCal. Furthermore, the electromagnetic showersofelectronsaremorecircularthantheonesproducedby hadrons.Generally,theshowershapeproducedinthecalorimeter hasanellipticalshapewhichcanbecharacterisedbyitstwoaxes:

σ

2

l forthelong,and

σ

2

s fortheshortaxis.Aratherloseselection of 0

.

01

<

σ

2

s

<

0

.

35 is chosen, since it reducesthe hadron con-taminationwhileatthesametime itdoesnotaffectsignificantly the electron signal [29]. The residual hadron background in the electronsampleisevaluatedusingtheE

/

p distributionfor hadron-dominatedtracksselectedwithnTPC_σ,e

<

−

3

.

5.The E

/

p distribution ofthehadronsisthennormalisedtomatchthedistributionofthe electron candidatesin0

.

4

<

E

/

p

<

0

.

7 (awayfromthe true elec-tron peak), so that the fraction of contaminating hadrons under theelectronpeakcanbeestimated.

In pp events, the hadron contamination is below 1% at low pT, while it reaches about 40% at pT

=

10 GeV/c. In Pb–Pb, the

largesthadroncontaminationismeasuredinthemostcentral col-lisions,whereacontaminationofabout7% and10% mainlydueto kaonandprotoncrossingtheelectronbandatpT

=

0

.

5 GeV/c and

pT

=

1 GeV/c respectivelyispresent.Thetotalhadron

contamina-tioncontributionamountsto5% atpT

=

3 GeV/c incentralevents

and tends to decrease towards moreperipheral collisions. In the EMCalanalysisamaximumresidualcontaminationofabout10% is subtracted atthe highesttransverse momentain the 0–10% cen-tralityclass.Inbothcollisionsystems,thehadroncontaminationis subtractedstatisticallyfromtheinclusiveelectroncandidateyield. InPb–Pbcollisions,therapidityrangesusedintheITS-TPC-TOF (pT

<

3GeV/c) andTPC-EMCal (pT

>

3GeV/c) analyses are

re-strictedto

|

y

| <

0.8and

|

y

| <

0.6,respectively,toavoidtheedges ofthedetectors,wherethesystematicuncertaintiesrelatedto par-ticleidentificationincrease.

3.2. Subtractionofelectronsfromnonheavy-flavoursources

The selected inclusive electron sample does not only contain electrons fromopenheavy-flavourhadron decays,butalso differ-entsourcesofbackground:

1. electrons from Dalitz decays of light neutral mesons,mainly

π

0 _and

_η

_{, andfromphoton conversions inthe detector}

ma-Table 4

Selectioncriteriafortaggingphotonicelectrons.

Associated electron pp Pb–Pb Pb–Pb (pT<3 GeV/c) (pT>3 GeV/c) pmin T (GeV/c) 0.1 0.1 0.2 |y| <0.8 <0.8 <0.9 Number of clusters in TPC ≥60 ≥60 ≥70 TPC clusters in dE/dx calculation ≥60 ≥60 – Number of clusters in ITS ≥2 ≥2 ≥2

|DCAxy| <1 cm <1 cm <2.4 cm

|DCAz| <2 cm <2 cm <3.2 cm

Found / findable clusters in TPC >0.6 >0.6 –

χ2_{/d.o.f TPC}

<4 <4 <4 nTPC

σ,e [−3,3] [−3,3] [−3,3]

me+e−(MeV/c2) <140 <140 <100

terialaswell asfromthermalandhard scatteringprocesses, calledphotonicinthefollowing;

2. electrons from weak decays of kaons: K0/±

_→

_e±

_π

∓/0(

_ν

−) e (Ke3);

3. di-electrondecaysofquarkonia:J

/ψ

,

ϒ

→

e+e−;

4. di-electrondecaysoflightvectormesons:

ω,

φ, ρ

0

→

e+e−;

5. electronsfromW andZ

/

γ

∗.

The photonic tagging method [21,28,29,36,49] is the technique adoptedinthepresentanalysestoestimatethecontributionfrom photonic electrons. With a contribution of 80% to the inclusive electron sample, photonic electrons constitute the main back-groundat pT

=

0

.

5 GeV/c [28]. Theircontributiondecreases with

pT reaching 25% at about 3 GeV/c. The contribution from

di-electrondecays oflightvector mesons(

ρ

,

ω

and

φ

) isnegligible comparedtothecontributionsfromthephotonicsources[50].

Photonic electrons are reconstructed statistically by pairing electron(positron)trackswithoppositechargetracksidentifiedas positrons (electrons), calledassociatedelectrons in thefollowing, forming the so-called unlike-sign pairs. The combinatorial back-groundissubtractedusingthelike-signinvariantmassdistribution inthesameinterval.Associatedelectronsareselectedwiththe cri-terialistedinTable4,whichareintentionallylooserthantheones applied forthe inclusive electron selection, shownin Table 2, in ordertomaximisetheprobabilitytofindthephotonicpartners.

Duetothelimitedacceptanceofthedetectorandtherejection of some associated electrons by applying the mentioned criteria, a certain fraction of photonic pairs is not reconstructed. There-fore, the raw yield of tagged photonic electrons is corrected for efficiencyto findthe associatedelectron (positron),the so called taggingefficiency(

ε

tag).ThisisevaluatedusingMonteCarlo(MC)

simulations;ppandPb–PbcollisionsaresimulatedbythePYTHIA 6[51] andHIJING[52] eventgenerators,respectively.Primary par-ticlegenerationisfollowedbyparticletransportwithGEANT3[53] and a detailed detector response simulation and reconstruction. The tagging efficiency is defined as the ratio of the number of true reconstructed unlike-sign pair electrons and the number of thosegeneratedinthesimulations.ThesimulatedpT distributions

of

π

0 _or

_η

_{mesons are weighted in MC to matchthe measured}

spectra.Inboth ppandPb–Pb collisions,the weightingfactor for

π

0 _{isprovidedbyusingthemeasureddistributionsofcharged}

pi-ons [54]. The weighting factor for

η

mesons is computed using anmT–scalingapproach[55,56].Thetotaltaggingefficiencyhasa

monotonictrend.Inppcollisions,itstartsat0

.

4 forpT

=

0

.

5 GeV/c

andrisesuntilpT

=

3 GeV/c,whereitflattensat0

.

7.InPb–Pb

col-lisions,itfollowsthesametrend,increasingfrom0

.

3 to0

.

7 inthe samepT range.

It was observed in the previous analysis[28] thatthe contri-butionfrom J/

ψ

decaysreachesa maximumofaround 5% inthe region2

<

pT

<

3 GeV/c incentralPb–Pbcollisions,decreasingto

afewpercentinmoreperipheralevents.Atlowerandhigher mo-menta,thiscontributionquicklydecreasesandbecomesnegligible, henceit isnot subtractedin thepresentanalyses.The associated systematicuncertaintyistakenfromsimilarworks[28,29].Dueto the requirementofhitsinbothpixel layers,it was alsoobserved from similarstudies in previous measurements [28] that the rel-ativecontribution from Ke3 decaystothe electron backgroundis negligible,hencethiscontributionisnotsubtractedinthepresent analyses.Additionalsourcesofbackground,suchaselectronsfrom W and Z

/

γ

∗ decays,are subtracted fromthefullycorrectedand normalised electron yield in Pb–Pb collisions at high pT. These

contributions are obtained from calculations using the POWHEG eventgenerator[57] forppcollisions andscalingitby



Ncoll



,

as-suming RAA

=

1.The contributionfrom W decaysincreasesfrom

1% atpT

=

10 GeV/c toabout20%atpT

=

25 GeV/c inthe0–10%

centralityclass,whilethe Z contributionreachesabout10% atthe sametransversemomentum.

3.3. Efficiencycorrectionandnormalisation

After the statistical subtraction of the hadron contamination andthebackgroundfromphotonicelectrons,therawyieldof elec-trons and positrons in bins of pT is divided by the number of

analysedevents(N_evMB),bythetransversemomentumvalueatthe bin centre pcentre_T and the bin width



pT, by the width



y of

thecoveredrapidityinterval,bythegeometricalacceptance(

ε

geo₎

timesthereconstruction(

ε

reco_{)andPIDefficiencies(}

_ε

eID_),andby

afactoroftwotoobtainthechargeaveragedinvariantdifferential yield, since in the analyses the distinction between positive and negativechargesisnotdone:

1 2

π

pT d2Ne± dpTd y

=

1 2 1 2

π

pcentre T 1 NevMB 1



y



pT Ne± raw

(

pT

)

(

ε

geo

_×

_ε

reco

_×

_ε

eID

₎

.

(2) Theproductioncrosssectioninppcollisionsiscalculatedby mul-tiplyingtheinvariantyieldofEq. (2) bytheminimumbiastrigger cross section at

√

s

=

5

.

02 TeV, that is 50.9

±

0.9 mb[58]. The per-eventyieldofelectronsfromtheEMCaltriggeredsamplewas scaled tothe minimumbiasyield bynormalisation factors deter-minedwithadata-drivenmethodbasedontheratiooftheenergy distributionsofEMCalclustersforthetwotriggers,asdescribedin Ref. [29].Thenormalisationis64.5

±

0.5in0–10%and246

±

2.6 in30–50%centralityintervals,respectively.

The efficiencies are determined usingspecific MC simulations, where every collisioneventis produced withatleasteithera cc orbb pairandheavy-flavourhadronsareforcedtodecay semilep-tonically to electrons [28,29]. The underlyingPb–Pb events were simulated usingthe HIJINGgenerator [52] and heavy-flavour sig-nalswereaddedusingthePYTHIA6generator[51].Theefficiency ofreconstructingelectronsfromsemileptonicheavy-flavourhadron decays isabout20% at pT

=

0

.

5 GeV/c, thenit increaseswith pT

upto58% inppcollisions.InPb–Pbcollisions,itfollowsthesame trend,increasingfrom5% to10% inthesamepT range.

3.4. Systematicuncertainties

The overallsystematicuncertaintiesonthe pT spectraare

cal-culated summinginquadraturethedifferentcontributions,which are assumedtobe uncorrelated. Theyare summarisedinTable 5

anddiscussedinthefollowing.

The systematic uncertainties on the total reconstruction effi-ciencyarisingfromthecomparisonbetweenMCanddataare esti-matedbyvaryingthetrackselectionandPIDrequirementsaround thedefaultvalueschosenintheanalyses.Theanalysisisrepeated

Table 5

Contributionstothesystematicuncertaintiesonthecrosssection(yield)ofelectronsfromheavy-flavourhadrondecaysinpp(Pb–Pb)collisions,quotedforthetransverse momentumintervals0.5<pT<0.7 GeV/c and8<pT<10 GeV/c.ThesepTintervalsarelistedbecausethedetectorsusedforparticleidentificationinthetwocasesare

different.Inaddition,theyalsorepresentthefirstandthelastpTintervalsincommonforthecentralityclassesinPb–Pbcollisions,aswellasfortheppcrosssection.At

higherpTtheuncertaintiesaregenerallylower,apartfromtheonerelatedtotheelectroweakbackground,whichstaysbelow4%.Theuncertaintiesquotedwith*arenot

summedinquadraturetogetherwiththeothers,becausetheyaretheRAAnormalizationuncertainties.

pp Pb–Pb (0–10%) Pb–Pb (30–50%) Pb–Pb (60–80%) pT(GeV/c) 0.5–0.7 8–10 0.5–0.7 8–10 0.5–0.7 8–10 0.5–0.7 8–10 Track selections 1% 1% 4% 2% 1% 2% 2% 2% Photonic tagging 4% – 13% 4% 7% 4% 7% 4% SPD hit requirement 3% 3% 10% – – – – – J/ψ→e – – 2% – 2% – 2% – W→e – – – <4% – <1% – <1% Z/γ∗→e – – – <1% – <1% – <1% nTPC σ,eselection – 5% – 5% – 5% – 2% E/p selection – – – 6% – 6% – 6% Hadron contamination – 5% 6% – 2% – – – ITS–TPC matching 2% 2% 2% 2% 2% 2% 2% 2% TPC–TOF matching 2% – 3% – – – – – η 5% 4% 10% – 5% – 5% – ϕ – – 10% – – – – – Interaction rate – – 5% – – – – – Centrality limit* – – <1% <1% 2% 2% 3% 3% Luminosity* 2.1% 2.1% – – – – – – Total uncertainty 9% 9% 24% 9% 9% 9% 9% 8%

withtighter andlooserconditionswithrespectto thedefault se-lectioncriteriaandthesystematicuncertaintyiscalculatedasthe rootmean square (RMS) of thedistribution of the resulting cor-rectedyields (or cross sections in pp) in each centrality and pT

interval. The systematic uncertainty estimatedin pp collisions is lessthan2%,whileinPb–Pbcollisionsitreachesamaximumvalue of4%in0–10%centralityclassforpT

<

0

.

9 GeV/c.

Similarly,thesystematicuncertaintyarisingfromthe photonic-electronsubtractiontechniqueisestimatedastheRMSofthe dis-tributionofyields obtainedbyvaryingtheselectioncriterialisted in Table 4. In pp collisions this contribution has a maximum of 4%for0

.

5

<

pT

<

0

.

7 GeV/c and thenitgradually decreaseswith

increasing pT,whilein the0–10% Pb–Pbcentrality classit isthe

dominantsourceofsystematicuncertainty, being13%in thefirst pTinterval.Thissystematicuncertaintymainlyariseswhenthe

in-variantmasscriteriononthephotonicpairsisvariedanditreflects thelargecontributionofphotonicelectronsinthelow-pT region.

Inordertofurthertesttherobustnessofthephotonicelectron tagging, the requirement on the number of clusters for electron candidatesintheSPDisrelaxedinordertoincreasethefractionof electronscomingfromphotonconversionsinthedetectormaterial. Avariationof3%isobservedforthemeasuredppcrosssectionin thefull pT range,while in central Pb–Pb collisions the observed

deviation amounts to 10% for 0

.

5

<

pT

<

0

.

7 GeV/c, decreasing

withincreasing pT.Thissystematicuncertaintyislessrelevantin

semi-centralcollisions,anditiscompatiblewiththevariation de-terminedinppmeasurementsfor1

.

5

<

pT

<

3 GeV/c.

Inaddition, thesystematic uncertaintyrelatedto the subtrac-tionofthebackgroundelectronsfromWandZ

/

γ

∗isestimatedby propagating 15% ofuncertainty, whichcovers the possible differ-encebetweenthe measurements and thetheoretical calculations [59–61]. The uncertaintyfromthesubtractionon thefinal result, whichisrelevantonlyathighpT,islessthan4%forelectronsfrom

semileptonicheavy-flavourhadrondecaysincentral(0–10%)Pb–Pb collisionsfor24

<

pT

<

26 GeV/c,andlessthan1% inother

cen-tralityclassesforthesamepTinterval.Intheppanalysis,a5%

sys-tematicuncertaintyisfound whilevarying theselection criterion intheTPCforpT

>

8 GeV/c duetotheincreasingrelativeamount

ofhadrons.An additionalsystematicuncertaintyof5%,relatedto theprecision of the estimatedhadron contamination, isassigned forpT

>

8 GeV/c.InPb–Pbcollisions,a10%systematicuncertainty

isassignedforpT

>

12 GeV/c duetothevariationofelectron

iden-tificationintheTPC,whilethiscontributioniswithin5%atlower pT.Moreover, a6% uncertaintyis assignedduetothe E

/

p

selec-tioncriterion. Finally,for pT

<

3 GeV/c,differentfunctional forms

are used for the parametrisation of the pion contribution in the fittingprocedureadoptedtoevaluatethehadroncontamination.A systematic uncertaintyof about 6% is assigned for pT

<

3 GeV/c

inthe0–10%centralityclass,whilethiscontributiondecreasesfor moreperipheralcollisions.

Inthepp(Pb–Pb)analysis,asystematicuncertaintyofabout2% (3%)isassignedduetotheincompleteknowledgeoftheefficiency inmatching tracksreconstructed intheITS andTPC andanother 2%(5%)forthetrackmatchingbetweenTPCandTOF.

The effectsdue tothe presence ofnon-uniformity in the cor-rection for the space-charge distortion in the TPC drift volume or irregularities in the detector coverage are then evaluated by repeatingtheanalysisindifferentgeometricalregions.Inpp colli-sions,amaximumsystematicuncertaintyof5%isestimatedfrom varying thepseudorapidityrange usedforthecrosssection mea-surement. Thesamevalue isassignedinthe30–50% and60–80% Pb–Pb centrality intervals, while a 10% systematic uncertainty is assignedfor0

.

5

<

pT

<

0

.

7 GeV/c inthe0–10%centralityinterval,

duetothelargersensitivitytotheelectrons fromphoton conver-sions.Anadditionaluncertaintyof10%forpT

<

1 GeV/c andof5%

uptopT

=

3 GeV/c isestimatedfromvaryingtheazimuthalregion

incentralPb–Pbcollisions.Furthermore,theanalysisofPb–Pb col-lisions is repeatedusing different interaction rateregimes. A 5% deviationis observedatlow pT incentral Pb–Pb collisionswhen

selecting only high(

>

5kHz) or low (

<

5kHz) interaction rate events.

TheuncertaintyfromtheEMCaltriggernormalisationinPb–Pb collisionsatpT

>

12 GeV/c isestimatedastheRMSoftherejection

factorvaluescomputedatdifferenttransversemomenta[29].The RMSis4%andassignedasthesystematicuncertainty.

The uncertainties on the RAA normalisationare the quadratic

sumoftheuncertaintiesontheaveragenuclearoverlapfunctions inTable1,thenormalisationuncertaintyduetotheluminosityand theuncertaintyrelatedtothedeterminationofthecentrality inter-vals,whichreflectstheuncertaintyonthefractionofthehadronic cross section used in the Glauber fit to determine the centrality [16,62].

Fig. 1. pT-differentialinvariantproductioncrosssectionofelectronsfrom semilep-tonicheavy-flavourhadrondecaysinppcollisionsat√s=5.02 TeV.The measure-mentiscomparedwiththeFONLLcalculation[63].Inthebottompanel,theratios withrespecttothecentralvaluesoftheFONLLcalculationareshown.Anadditional 2.1%normalisationuncertainty,duetothemeasurementoftheminimumbias trig-geredcrosssection[46],isnotshownintheresults.

4. Results

4.1. pT-differentialcrosssectioninppcollisionsandinvariantyieldin

Pb–Pbcollisions

The pT-differential production cross section of electrons from

semileptonic heavy-flavour hadron decays in pp collisions at

√

s

=

5

.

02 TeV is shown in Fig. 1. The data in the region 0

.

5

<

pT

<

10 GeV/c is compared with the

Fixed-Order-Next-to-Leading-Log (FONLL) [63] pQCD calculation. The uncertainties of the FONLL calculations(dashed area) reflect differentchoices for thecharm andbeautyquark masses, thefactorisationand renor-malisationscales aswell asthe uncertaintyon theset of parton distributionfunctions(PDF)usedinthepQCDcalculation(CTEQ6.6 [64]). The measured cross section is close to the upper edge of thetheoretical prediction up to pT



5 GeV/c, asobserved inpp

collisions at

√

s

=

2

.

76 and 7 TeV[28,49,50],while athigher pT,

whereelectrons from semileptonicbeautyhadron decays are ex-pectedtodominate, themeasurement isclosetothe meanvalue oftheFONLLprediction.

The pT-differential invariant yield of electrons from

semilep-tonic heavy-flavour hadron decays measured in central (0–10%), semi-central (30–50%), and peripheral (60–80%) Pb–Pb collisions at

√

sNN

=

5

.

02 TeV is shown in Fig. 2. The measurements are

performedin the pT interval 0.5–26 GeV/c in the 0–10% and in

the30–50% centralityintervals, andonlyup to pT =10 GeV/c in

the60–80%centralityclassduetolimitedstatisticsinPb-Pbdata recordedin2015.

4.2. Nuclearmodificationfactor

The nuclear modification factor of electrons from semilep-tonic heavy-flavour hadron decays measured in central (0–10%), semi-central (30–50%), and peripheral (60–80%) Pb–Pb collisions at

√

sNN

=

5

.

02 TeV isshown inFig. 3.The measured cross

sec-Fig. 2. pT-differentialinvariantyieldincentral(0–10%),semi-central(30–50%),and peripheral(60–80%)Pb–Pbcollisionsat√sNN=5.02 TeV.

tion in pp collisions at

√

s

=

5

.

02 TeV (Fig. 1) is used as a ref-erence up to pT

=

10 GeV/c. For pT

>

10 GeV/c, the reference

is obtained by a pT-dependent scaling of the measurement at

√

s

=

7 TeV by the ATLAS collaboration [65] with the ratio of the cross section at the two collision energies computed with the FONLL calculation [66]. This ratio is performed by consid-ering the different rapidity coverage of the ATLAS measurement (

|

y

|

<

2 excluding 1

.

37

<

|

y

|

<

1

.

52). The systematic uncertain-tiesofthecrosssection at

√

s

=

5

.

02 TeV rangefrom13%to 18% dependingonthepT interval,andtheyarecomputedasthe

prop-agationoftheuncertaintiesassociatedwithFONLLcalculationsat

√

s

=

5

.

02 TeVand

√

s

=

7 TeVandthesystematicuncertaintiesof theATLASmeasurement.Thestatisticaluncertainties arefromthe ATLASmeasurement.

Statistical and systematic uncertainties of the pT-differential

yields andcrosssectionsin Pb–Pbandpp collisions, respectively, arepropagatedasuncorrelateduncertainties.Theuncertaintieson the RAAnormalisationarereportedinFig.3asboxesatunity.The

measured RAAshowsacleardependenceon thecollision

central-ity, since in most central events it reaches a minimum ofabout 0

.

3 around pT

=

7 GeV/c,whilemovingtomoreperipheralPb–Pb

collisions the RAA gets closer to unity at pT

>

3 GeV/c. Such a

suppression isnot observedinproton-lead collisionsatthe same energywherethe QGPisnotexpectedto beformed andthe nu-clearmodification factorisconsistent withunity[14,36,37].Thus thesuppressionofelectronproductionisduetofinal-stateeffects, such aspartonicenergylossinthemedium.Since electronsfrom semileptonic beauty decays are expected to dominate the spec-trumathighpT whilecharmproductiondominatesatlowpT[14],

themeasurementsshowthatcharmandbeautyquarksloseenergy inthe medium.The centralitydependenceofthe RAA is

compat-ible withthe hypothesisofapartonicenergylossdependenceon medium density,beinglargerinahotteranddenserQGP,likethe one created in the mostcentral collisions. In addition, it reflects a path-length dependence of energy loss. Moreover, it has been shown in Refs. [67,68] thata centrality selection bias is present inperipheralPb–Pbcollisionswhichreducesthe RAAbelowunity

evenintheabsenceofanynuclearmodificationeffects.Thiseffect mayberesponsibleforasignificant partoftheapparent suppres-sionseeninthe RAA ofelectronsfromsemileptonicheavy-flavour

hadrondecaysinthe60-80%centralityclass.

For pT

<

7 GeV/c, the RAA of electrons from semileptonic

heavy-flavour hadron decays increases with decreasing pT as a

Fig. 3. Nuclear modification factor of electrons from semileptonic heavy-flavour hadron decays measured in the three centrality intervals in Pb–Pb collisions at

√

sNN=5.02 TeV.

the number of binary collisions among nucleons in Pb–Pb colli-sions.Ontheotherhand,thenuclearmodificationfactoratlow pT

doesnotriseaboveunity.Thiskinematicregionissensitivetothe effectsofnuclearshadowing:thedepletion ofpartondensitiesin nucleiatlowBjorkenx valuescanreducetheheavy-quark produc-tioncrosssectionperbinarycollisioninPb–Pbwithrespecttothe ppcase[28]. Thisinitial-stateeffectisstudiedin p–Pbcollisions, however,the present uncertainties on the RpPb measurement do

notallowquantitativeconclusions onthemodificationofthePDF innucleiin thelow pT regiontobe made[36].Furthermore,the

amountof electrons fromsemileptonic heavy-flavour hadron de-caysisreducedduetothepresenceofhadrochemistryeffects.For example,



+c baryons decay into electrons with a branching

ra-tioof5%, whilefortheDmesonsthebranchingratioislessthan 10%.SinceinPb–Pbcollisionsmorecharmquarksmighthadronize intobaryons[69],thiseffectreducesthetotalamountofelectrons fromsemileptonicheavy-flavourhadrondecays.Additionaleffects, suchascollectivemotioninducedbythemedium,alsohavean in-fluenceonthemeasured RAA.Also,ithasbeenobservedthatthe

radialflowcanprovokean additionalyieldenhancementat inter-mediate pT [70–73].In thiscase, the radial flowpushes upslow

particlesto highermomenta, causinga smallincrease inthe nu-clearmodificationfactoraround pT

=

1 GeV/c.

ItshouldbenotedthattheRAAmeasurementsinthemost

cen-tralcollisionsat

√

sNN

=

2

.

76 TeV[28] and5.02 TeVarecompatible

withinuncertainties,asshowninFig.3.Thiseffectwas predicted by the Djordjevic model [74], and it results from the combina-tion of a higher medium temperature at 5.02TeV, which would decreasethe RAA by about10%, witha harder pT distributionof

heavyquarksat5.02TeV,whichwouldincreasethe RAA byabout

5%ifthemedium temperaturewere thesameasat2.76TeV.An analogousbehaviourbetweenthemeasured RAA atthetwo

ener-giesisalsoobservedfortheDmesons[16]. 4.3.Comparisonwithmodelpredictions

In Fig. 4 the measured RAA in the 0–10% (left panel) and

30–50%(rightpanel)centralityintervalsarecomparedwithmodel calculations[74–81].Themodelcalculationstakeintoaccount dif-ferenthypothesesaboutmassdependenceofenergylossprocesses, transportdynamics,charmandbeautyquarkinteractionswiththe QGP constituents, hadronisation mechanisms of heavy quarks in theplasma,andheavy-quarkproductioncrosssectioninnucleus– nucleuscollisions.

Mostofthemodelsprovideafairdescriptionofthedatainthe region pT

<

5GeV/c inboth centralityclasses,exceptforBAMPS

[76]. The predictions from the MC@sHQ+EPOS2 [81], PHSD [77],

TAMU[78],andPOWLANG[80] modelsalsoincludenuclear mod-ification of the partondistribution functions, which is necessary topredicttheobservedsuppressionofthe RAAatlow pT.The

fol-lowingobservationsaboutthecomparisonwithmodelcalculations arefullyinagreementwithwhatisobservedintheRAA

measure-mentsofDmesons[16].

The nuclear modification factor forcentral Pb–Pb collisions is well described by the TAMU [78] prediction at pT

<

3 GeV/c

withintheuncertaintiesrelatedtotheshadowingeffectoncharm quarks. However, this model tends to overestimate the RAA for

pT

>

3 GeV/c,probablyduetothemissingimplementationofthe

radiativeenergylossinthemodel,whichmaybecomethe domi-nantenergylossmechanismathighpT.

Theagreement withTAMU[78] atlow pT,ontheother hand,

confirms thedominance ofelastic collisionsatlow momenta, to-getherwiththeimportanceoftheinclusionofshadowingeffectsin themodelcalculations[35],whichreduce thetotal heavy-flavour productioninPb–Pbcollisionswithrespecttoanexpectationfrom thebinaryscaling.

Insemi-centralPb–Pbcollisions theTAMU[78] andPOWLANG [80] predictions are close to the lower edge of the uncertain-tiesofthemeasured RAA for pT

<

3 GeV/c.Thelattercalculation

describes the data better up to pT



8 GeV/c, while the former

provides a good description even at higher transverse momenta. The CUJET3.0 [75] andDjordjevic [74,79] models provide a good description of the RAA within the uncertainties in both

central-ity intervals for pT

>

5 GeV/c, suggesting that the dependence

ofradiative energyloss on thepath lengthin thehot anddense mediumiswellunderstood.

5. Conclusions

The invariant yield of electrons from semileptonic heavy-flavour hadron decays was measured in central (0–10%), semi-central (30–50%), and peripheral (60–80%) Pb–Pb collisions at

√

sNN

=

5

.

02 TeV. The measurement of the nuclear modification

factor in all the centrality classesfor pT

<

10 GeV/c isprovided

using asreferencethe crosssection measured in pp collisions at the same centre-of-mass energy. The systematic uncertainties of this measurement are reduced by a factor of about2 compared tothe published referencein ppcollisions at

√

s

=

2

.

76 TeV[28] andthemeasured crosssectionis closetotheupperedge ofthe FONLLuncertaintyband.Athigher pTthereferenceisobtainedby

a pT-dependentscalingofthemeasurementat

√

s

=

7 TeVbythe

ATLAScollaboration [65] withtheratioofthecrosssectionatthe two collisionenergies computedwiththeFONLL calculation[66]. As in the Pb–Pb analysis at

√

sNN

=

2

.

76 TeV [28,29], the main

Fig. 4. Nuclear modification factor of electrons semileptonic from heavy-flavour hadron decays measured in 0–10% and 30–50% centrality in Pb–Pb collisions at

√

sNN=5.02 TeVcomparedwithmodelpredictions[74–81].

isremovedviathephotonictaggingmethod.Inaddition,compared withthe measurements performedinpp and Pb–Pb collisions at 2

.

76 TeV, the pT range is extended, andan additional centrality

classisadded.

Themeasured RAA confirmsthe evidenceofastrong

suppres-sionwithrespecttowhatisexpectedfromasimplebinaryscaling forlarge pT.Thisisa clearsignatureofthe mediuminduced

en-ergylossonheavy quarkstraversingtheQGPproducedin heavy-ioncollisions.

Themeasurementofelectronsfromsemileptonicheavy-flavour hadron decays in different centrality classes exhibits the depen-dence of energy loss on the path length and energy density in thehot anddense medium. The RAA athigh pT (above 5GeV/c)

is fairly described in the 0–10% and 30–50% centrality intervals by model calculations that include both radiative andcollisional energyloss.Thisindicatesthatthecentralitydependenceof radia-tiveenergylossis theoreticallyunderstood.Furtherinvestigations andmeasurementofelectronsfromsemileptonicdecaysofbeauty hadronswillgivemoreinformationaboutthemassdependenceof theenergylossintheheavy-flavoursector.

With the good precision of the results presented here, the Pb–Pb dataexhibit theirsensitivitytothemodificationofthePDF in nuclei, like nuclearshadowing, which causes a suppression of theheavy-quarkproductionin heavy-ioncollisions.The implemen-tationofthenuclear modificationofthePDF intheoretical calcu-lationsisanecessaryingredientinorderforthemodelpredictions tocorrectlydescribethemeasuredRAA [28].

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

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-vided by all Grid centres and the Worldwide LHC Computing Grid(WLCG)collaboration. TheALICECollaborationacknowledges the following funding agencies fortheir support in building and runningtheALICEdetector:A.I. AlikhanyanNationalScience

Lab-oratory (YerevanPhysicsInstitute) Foundation(ANSL),State Com-mitteeofScienceandWorldFederationofScientists(WFS), Arme-nia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-N36] andÖsterreichischeNationalstiftung fürForschung, Technologie und Entwicklung, Austria; Ministry of Communica-tions and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (Finep), Fundação deAmparoà PesquisadoEstado deSão Paulo(FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foun-dation ofChina (NSFC), China;MinistryofScience andEducation and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research | NaturalSciences,theVillumFonden andDanishNationalResearch Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland;Commissariatàl’ÉnergieAtomique (CEA),InstitutNational de Physique Nucléaire etde 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-sian Institute ofScience,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),JapaneseMinistryofEducation,Culture,Sports,Scienceand Technology(MEXT)andJapanSocietyforthePromotionofScience (JSPS) KAKENHI,Japan;Consejo Nacionalde Ciencia(CONACYT)y Tecnología, through Fondo de Cooperación Internacionalen Cien-cia y Tecnología (FONCICYT) and Dirección General de Asuntos delPersonalAcademico(DGAPA),Mexico;NederlandseOrganisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Re-search Council of Norway, Norway; Commission on Science and Technology forSustainableDevelopmentintheSouth(COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry

of Science and Higher Education and National Science Centre, Poland;KoreaInstituteofScienceandTechnologyInformationand NationalResearch Foundation of Korea (NRF), Republic ofKorea; MinistryofEducation andScientific Research, Institute ofAtomic Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education andScience ofthe Russian Federa-tion,NationalResearchCentreKurchatovInstitute,RussianScience Foundation and Russian Foundation for Basic Research, Russia; Ministryof Education, Science, Research andSport of the Slovak Republic, Slovakia; National ResearchFoundation ofSouth Africa, South Africa; Swedish Research Council (VR) and Knut & Alice WallenbergFoundation(KAW),Sweden;EuropeanOrganizationfor Nuclear Research, Switzerland; Suranaree University of Technol-ogy(SUT), NationalScience andTechnology DevelopmentAgency (NSDTA) and Office of the Higher Education Commission under NRUprojectofThailand,Thailand;TurkishAtomic EnergyAgency (TAEK),Turkey;NationalAcademyofSciencesofUkraine,Ukraine; ScienceandTechnologyFacilitiesCouncil(STFC),UnitedKingdom; NationalScienceFoundationoftheUnitedStatesofAmerica(NSF) andUnitedStatesDepartmentofEnergy,OfficeofNuclearPhysics (DOENP),UnitedStatesofAmerica.

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34

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D. Bauri

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I.G. Bearden

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C. Bedda

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N.K. Behera

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I. Belikov

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34

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125

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G. Bencedi

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,

S. Beole

26

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A. Bercuci

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D. Berenyi

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R.A. Bertens

130

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D. Berzano

58

,

M.G. Besoiu

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,