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 ra 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√_s

NN=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 ofmatterwhichisexpectedtobecreatedinultra- relativisticheavy-ioncollisionswherehightemperaturesandhigh energy densities are reached at the LHC [1]. Due to their large masses (mc ≈¹.5 GeV/c², m_b ≈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 thefullevolutionofthehotanddensemediumproducedinhigh- energyheavy-ion collisions,thereforetheyare idealprobestoin- 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,thedead-

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,whereaspQCDisnotapplicableforthecalcu- lationoflight quark andgluonproduction atlowtransverse mo- menta [3]. Moreover, measurements of heavy-flavour production crosssectionsinpp collisionsprovidethenecessaryexperimental referenceforheavy-ioncollisions.

Themediumeffectsonheavyquarksarequantifiedthroughthe measurementofthenuclearmodificationfactor,definedasthera- tio between the yield ofparticles produced in ion–ion collisions (d²NAA/dpTdy) andthe crosssection measured inproton-proton collisions at the same energy (d²

σ

pp/dp_Tdy), normalised by the averagenuclearoverlapfunction^TAA^:

R_AA

(

p_T

,

y

) =

¹

^TAA

·

^d2NAA

/

dpTdy

d²

σ

pp

/

dp_Tdy

.

(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 of}nucleon–nucleon collisions ^Ncoll^{, whichcan be estimatedvia Glauber modelcal-} 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 (R_AA<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 R_AA 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], theinterplaybe- 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,showingintriguingandnotyetfullyun- 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–Pbcol- 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-flightofchargedparti- 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 pT².5 GeV/c, pT^{4 GeV/c andp}T^{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. TheelectronidentificationintheEMCalisbasedonthemeasure- mentofthe E/pratio,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 overagroupof4×⁴calorimetercellslargerthananenergythresh- oldof10GeV.TheEMCALtriggeredeventswereusedforelectron measurements for pT>12 GeV/c.The centralityclasseswere de- fined intermsofpercentilesofthehadronicPb–Pbcross section, definedbyselectionsonthesumoftheV0signalamplitudes[46].

Forbothcollisionsystems,onlyeventswithatleasttwotracks andareconstructedprimaryvertexlocatedbetween±^10cmwith respecttothenominalinteractionpointalongthe z-axisarecon- 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 p_T-differentialyieldofelectronsfromsemileptonicheavy- 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.IntheanalysisinwhichtheEMCalde- 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/dxcalculation ≥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+η²<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“σs²”parameter[29],correspondingtotheshorter-axisof theshowershape,isused.Forbrevity,the“lowpT”labelisusedinplaceof“pT<3 GeV/c”,aswellas“highpT”inplaceof“pT>3 GeV/c”.

Centrality n^TPC_σ,e n^TOF_σ,e n^ITS_σ,e E/p Shower shape

pp (lowpT) – [−⁰.5+^f(p), 3] [−³,3] ^{– – –}

pp (highpT) – [⁰.12, 3] ^{– – – –}

0–10% [−⁰.16,3]

Pb–Pb (lowpT) 30–50% [0,3] [−3,3] [−4,2] – –

60–80% [0.2,3] 0–10%

Pb–Pb (highpT) 30–50% [−1,3] – – [0.8, 1.3] 0.01<σs²<0.35

60–80%

Theelectronidentificationinppcollisionsisperformedbyeval- uatingthesignalfromtheTPCandTOFdetectors.Thediscriminant variable in the former detector is the deviation of dE/dx from theparameterisedelectronBethe-Bloch[48] expectationvalue,ex- pressedinunits ofthedE/dx resolution,n^TPCσ,e, whileinthelatter one the analogous variable n^TOFσ,e, referring to the particle time- of-flight,isconsidered. The criterion |^nTOFσ,e|<3,used forelectron identificationuptopT=^{3 GeV/c,isrequiredtoreducebackground} fromkaonsandprotons.Amomentumdependentcriteriononn^TPCσ,e isadoptedtoguaranteeaconstantelectronidentificationefficiency of70%forpT<3 GeV/c andof50%forhighertransversemomenta by reducing the selection window in n^TPCσ,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<n^ITSσ,e<2 ontheenergydeposited inthe SDD and SSD detectorsis applied in all centrality classes.

Finally,theselectiononn^TPCσ,e ensuresaconstantelectronidentifica- tionefficiencyof50%forallcentralityclasses.Thehadroncontam- inationfractionafterthePIDisestimatedbyfittingthen^TPCσ,e distri- butionforeachparticlespecieswithananalyticfunctionindiffer- entmomentumintervals[28,29].Theinclusiveelectronsample is thenselectedbyapplyingafurthercriteriononn^TPCσ,e,whichischo- seninordertohaveaconstantefficiencyasafunctionofthemo- mentum,aswellastohavethehadroncontamination undercon- trol.Thiscriterionisloosened for pT>3 GeV/c,duetothelower amountofselectedhadronswhentheEMCaldetectorisemployed.

InthePb–PbanalysisforpT>3 GeV/c,theelectroncandidates arefirstselected bythe measurementofthe TPCdE/dxwiththe criterion−¹<n^TPCσ,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

_l² forthelong,and

σ

_s² fortheshortaxis.Aratherloseselection of 0.01<

σ

_s²<0.35 is chosen, since it reducesthe hadron con- taminationwhileatthesametime itdoesnotaffectsignificantly the electron signal [29]. The residual hadron background in the electronsampleisevaluatedusingtheE/pdistributionforhadron- dominatedtracksselectedwithn^TPCσ,e <−³.5.The E/p distribution ofthehadronsisthennormalisedtomatchthedistributionofthe electron candidatesin0.4<E/p<0.7 (awayfromthe trueelec- 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} largesthadroncontaminationismeasuredinthemostcentralcol- lisions,whereacontaminationofabout7% and10% mainlydueto kaonandprotoncrossingtheelectronbandatpT=⁰.5 GeV/cand pT=^{1 GeV/c} respectivelyispresent.Thetotalhadroncontamina- 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,wherethesystematicuncertaintiesrelatedtopar- ticleidentificationincrease.

3.2. Subtractionofelectronsfromnonheavy-flavoursources

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

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

π

⁰ and

η

, andfromphoton conversions inthe detectorma-

Table 4

Selectioncriteriafortaggingphotonicelectrons.

Associated electron pp Pb–Pb Pb–Pb

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

p^min_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/dxcalculation ≥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

n^TPC_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: K^0/± →^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 p_T=⁰.5 GeV/c [28]. Theircontributiondecreases with pT reaching 25% at about 3 GeV/c. The contribution from di- electrondecays oflightvector mesons(

ρ

,

ω

andφ) 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.Associatedelectronsareselectedwiththecri- 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.Primarypar- 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.Thesimulatedp_T distributions of

π

⁰ or

η

mesons are weighted in MC to matchthe measured spectra.Inboth ppandPb–Pb collisions,the weightingfactor for

π

⁰ isprovidedbyusingthemeasureddistributionsofchargedpi- 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 andrisesuntilp_T=^{3 GeV/c,whereitflattensat0}.7.InPb–Pbcol- 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.Atlowerandhighermo- 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 therel- ativecontribution from K_e3 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 p_T. These contributions are obtained from calculations using the POWHEG eventgenerator[57] forppcollisions andscalingitby ^Ncoll^,as- suming R_AA=^1.The contributionfrom W decaysincreasesfrom 1% atpT=^{10 GeV/c toabout20%atp}T=^{25 GeV/c inthe0–10%}

centralityclass,whilethe Z contributionreachesabout10% atthe sametransversemomentum.

3.3. Efficiencycorrectionandnormalisation

After the statistical subtraction of the hadron contamination andthebackgroundfromphotonicelectrons,therawyieldofelec- trons and positrons in bins of p_T is divided by the number of analysedevents(N_ev^MB),bythetransversemomentumvalueatthe bin centre p^centre_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

p_T

d²N^e± dp_Tdy

=

¹

2 1 2

π

p^centre_T

1 N_ev^MB

1

y

p_T

N^e_raw^±

(

p_T

) ( ε

^geo

_× ε

^reco

_× ε

^eID

) .

(2) Theproductioncrosssectioninppcollisionsiscalculatedbymul- 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 factorsdeter- 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 orbbpairandheavy-flavourhadronsareforcedtodecaysemilep- 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- ciencyarisingfromthecomparisonbetweenMCanddataareesti- matedbyvaryingthetrackselectionandPIDrequirementsaround thedefaultvalueschosenintheanalyses.Theanalysisisrepeated

Table 5

Contributionstothesystematicuncertaintiesonthecrosssection(yield)ofelectronsfromheavy-flavourhadrondecaysinpp(Pb–Pb)collisions,quotedforthetransverse momentumintervals0.5<pT<0.7 GeV/cand8<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%

n^TPC_σ,eselection – 5% – 5% – 5% – 2%

E/pselection – – – 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 thedefaultse- 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,thesystematicuncertaintyarisingfromthephotonic- electronsubtractiontechniqueisestimatedastheRMSofthedis- tributionofyields obtainedbyvaryingtheselectioncriterialisted in Table 4. In pp collisions this contribution has a maximum of 4%for0.5<p_T<0.7 GeV/c andthenitgradually decreaseswith increasing pT,whilein the0–10% Pb–Pbcentrality classit isthe dominantsourceofsystematicuncertainty, being13%in thefirst p_Tinterval.Thissystematicuncertaintymainlyariseswhenthein- 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< p_T<0.7 GeV/c, decreasing withincreasing pT.Thissystematicuncertaintyislessrelevantin semi-centralcollisions,anditiscompatiblewiththevariationde- terminedinppmeasurementsfor1.5<pT<3 GeV/c.

Inaddition, thesystematic uncertaintyrelatedto the subtrac- tionofthebackgroundelectronsfromWandZ/

γ

^∗isestimatedby propagating 15% ofuncertainty, whichcovers the possiblediffer- 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/cduetothevariationofelectroniden- 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 thecor- rection for the space-charge distortion in the TPC drift volume or irregularities in the detector coverage are then evaluated by repeatingtheanalysisindifferentgeometricalregions.Inppcolli- 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/cinthe0–10%centralityinterval, duetothelargersensitivitytotheelectrons fromphoton conver- sions.Anadditionaluncertaintyof10%forp_T<1 GeV/candof5%

uptopT=^{3 GeV/cisestimatedfromvaryingtheazimuthalregion} incentralPb–Pbcollisions.Furthermore,theanalysisofPb–Pbcol- lisions is repeatedusing different interaction rateregimes. A 5%

deviationis observedatlow p_T incentral Pb–Pb collisionswhen selecting only high(> 5kHz) or low (< 5kHz) interaction rate events.

TheuncertaintyfromtheEMCaltriggernormalisationinPb–Pb collisionsatpT>12 GeV/cisestimatedastheRMSoftherejection factorvaluescomputedatdifferenttransversemomenta[29].The RMSis4%andassignedasthesystematicuncertainty.

The uncertainties on the RAA normalisationare the quadratic sumoftheuncertaintiesontheaveragenuclearoverlapfunctions inTable1,thenormalisationuncertaintyduetotheluminosityand theuncertaintyrelatedtothedeterminationofthecentralityinter- vals,whichreflectstheuncertaintyonthefractionofthehadronic cross section used in the Glauber fit to determine the centrality [16,62].

Fig. 1.pT-differentialinvariantproductioncrosssectionofelectronsfromsemilep- tonicheavy-flavourhadrondecaysinppcollisionsat√

s=5.02 TeV.Themeasure- mentiscomparedwiththeFONLLcalculation[63].Inthebottompanel,theratios withrespecttothecentralvaluesoftheFONLLcalculationareshown.Anadditional 2.1%normalisationuncertainty,duetothemeasurementoftheminimumbiastrig- geredcrosssection[46],isnotshownintheresults.

4. Results

4.1. p_T-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, thefactorisationandrenor- 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 p_T-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 p_T 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 p}T>10 GeV/c, the reference is obtained by a p_T-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%

dependingonthep_T interval,andtheyarecomputedastheprop- agationoftheuncertaintiesassociatedwithFONLLcalculationsat

√s=⁵.02 TeVand√

s=^{7 TeVandthesystematic}uncertaintiesof 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 R_AA ofelectronsfromsemileptonicheavy-flavour hadrondecaysinthe60-80%centralityclass.

For pT <7 GeV/c, the RAA of electrons from semileptonic heavy-flavour hadron decays increases with decreasing p_T as a consequence of the scaling of the total heavy-flavour yield with

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 nucleiatlowBjorkenxvaluescanreducetheheavy-quarkproduc- tioncrosssectionperbinarycollisioninPb–Pbwithrespecttothe ppcase[28]. Thisinitial-stateeffectisstudiedin p–Pbcollisions, however,the present uncertainties on the R_pPb 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,alsohaveanin- fluenceonthemeasured RAA.Also,ithasbeenobservedthatthe radialflowcanprovokean additionalyieldenhancementatinter- mediate pT [70–73].In thiscase, the radial flowpushes upslow particlesto highermomenta, causinga smallincrease inthe nu- clearmodificationfactoraround p_T=^{1 GeV/c.}

ItshouldbenotedthattheR_AAmeasurementsinthemostcen- 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 R_AA by about10%, witha harder p_T 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].Themodelcalculationstakeintoaccountdif- ferenthypothesesaboutmassdependenceofenergylossprocesses, transportdynamics,charmandbeautyquarkinteractionswiththe QGP constituents, hadronisation mechanisms of heavy quarks in theplasma,andheavy-quarkproductioncrosssectioninnucleus–

nucleuscollisions.

Mostofthemodelsprovideafairdescriptionofthedatainthe region p_T < 5GeV/c inboth centralityclasses,exceptforBAMPS [76]. The predictions from the MC@sHQ+EPOS2 [81], PHSD [77],

TAMU[78],andPOWLANG[80] modelsalsoincludenuclearmod- ification of the partondistribution functions, which is necessary topredicttheobservedsuppressionofthe RAAatlow pT.Thefol- 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,whichmaybecomethedomi- nantenergylossmechanismathighpT.

Theagreement withTAMU[78] atlow p_T,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 R_AA for p_T<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 p_T > 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 p_T<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 √

s_NN=².76 TeV [28,29], the main sourceofbackgroundelectrons,constituted byphotonicelectrons,

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 p_T range is extended, andan additional centrality classisadded.

Themeasured RAA confirmsthe evidenceofastrongsuppres- sionwithrespecttowhatisexpectedfromasimplebinaryscaling forlarge p_T.Thisisa clearsignatureofthe mediuminduced en- ergylossonheavy quarkstraversingtheQGPproducedinheavy- 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.Thisindicatesthatthecentralitydependenceofradia- 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.Theimplemen- tationofthenuclear modificationofthePDF intheoreticalcalcu- lationsisanecessaryingredientinorderforthemodelpredictions tocorrectlydescribethemeasuredRAA [28].

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompetingfinan- cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

Acknowledgements

The ALICE Collaboration would like to thank all its engineers andtechnicians fortheir invaluablecontributionstotheconstruc- tion of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICECollab- oration gratefully acknowledges the resources and support pro- 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. AlikhanyanNationalScienceLab-

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