Dielectron and heavy-quark production in inelastic and high-multiplicity proton–proton collisions at s=13TeV

www.elsevier.com/locate/physletb

Dielectron

and

heavy-quark

production

in

inelastic

and

high-multiplicity

proton–proton

collisions

at

√

s

=

13 TeV

.

ALICE

Collaboration



a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received23May2018

Receivedinrevisedform8October2018 Accepted6November2018

Availableonline9November2018 Editor: M.Doser

Themeasurementofdielectronproductionispresentedasafunctionofinvariantmassandtransverse momentum(pT)atmidrapidity(|ye|<0.8)inproton–proton(pp)collisionsatacentre-of-massenergy of√s=13 TeV.The contributionsfromlight-hadrondecaysare calculatedfromtheirmeasured cross sections in pp collisions at √s=7 TeV or13 TeV. The remaining continuum stems from correlated semileptonicdecays ofheavy-flavour hadrons.Fitting the data withtemplates fromtwo differentMC eventgenerators,PYTHIAandPOWHEG,thecharmandbeautycrosssectionsatmidrapidityareextracted for the first timeat thiscollision energy:d

σ

cc¯/d y|y=0=974±138(stat.)±140(syst.)±214(BR) μb andd

σ

b¯b/d y|y=0=79±14(stat.)±11(syst.)±5(BR)μb usingPYTHIAsimulationsandd

σ

cc¯/d y|y=0= 1417±184(stat.)±204(syst.)±312(BR) μb andd

σ

_b¯b/d y|y=0=48±14(stat.)±7(syst.)±3(BR) μb for POWHEG. These values, whose uncertainties are fully correlated between the two generators, are consistent with extrapolations fromlower energies. The different results obtained withPOWHEG and PYTHIA implydifferent kinematic correlations ofthe heavy-quark pairs inthese two generators. Furthermore, comparisons of dielectron spectra in inelastic events and in events collected with a trigger on high charged-particle multiplicities are presented in various pT intervals. The differences are consistentwith thealreadymeasuredscaling oflight-hadronand open-charmproductionathigh charged-particle multiplicity as a function of pT. Upper limits for the contribution of virtual direct photonsareextractedat90%confidencelevelandfoundtobeinagreementwithpQCDcalculations.

©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3_.

1. Introduction

Heavy-flavour quarks (charm and beauty) are copiously pro-duced by inelastic partonic scatterings in high-energy proton– proton (pp) collisions at the CERN Large Hadron Collider (LHC). Theirlarge masses(mQ) make it possible to calculate their pro-ductioncrosssectionswithperturbative quantum chromodynam-ics (pQCD) [1–3]. Hence, experimental measurements of heavy-quarkproductionprovideanexcellent testofpQCDinthisenergy regime.Flavourconservationallowsheavy quarksto beonly pro-duced in pairs. Charm hadrons and their decay products reflect theinitialangularcorrelationoftheheavy-quarkpairs,whereasin thecaseofdecaysofbeautyhadronsthecorrelation isweakened dueto their large masses. The contribution from the simultane-oussemileptonicdecaysofthecorrespondingheavy-flavourhadron pairsdominatesthedileptonyieldintheintermediatemassregion (IMR)1

<

m

<

3 GeV

/

c2.Hence,dielectronmeasurementscanbe

usedtostudycharmandbeautyproduction.

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

The ALICE Collaboration has reported charm and beauty pro-duction cross section measurements at midrapidity (

|

y

|

<

0

.

5) in pp collisions at centre-of-mass energies of

√

s

=

2

.

76 and 7 TeV [4–10]. The charm measurement at

√

s

=

7 TeV is com-plementedby ATLASdataextendingtohighertransverse momen-tum(pT) and

|

y

|

<

2

.

1 [11].Furthermore, theCMS Collaboration has provided a variety of charm and bottom measurements at midrapidityat

√

s

=

2

.

76,5and7 TeV [12–20].Atforward rapid-ity(2

<

y

<

5), theLHCb Collaborationhas measuredcharm and beauty productioncross sectionsin pp collisions at

√

s

=

5,7, 8 and 13 TeV [21–24]. These results are generally in good agree-ment with pQCD calculations at next-to-leading order (NLO) in the strong coupling constant (

α

s) with all-order resummation of the logarithms of pT

/

mQ (FONLL) [1–3]. Though the measured charm production cross sections consistently lie on the upper edge of the systematic uncertainties of the theory calculations. Recently, the ALICE Collaboration has measured the charm and beautyproductioncrosssectionsinpp collisionsat

√

s

=

7 TeV us-ing electron–positron pairs(dielectrons) fromcorrelated semilep-tonicdecaysofheavy-flavourhadrons [25].Suchanapproachwas firstperformedby thePHENIXCollaborationinpp andd–Au col-lisions at

√

sNN

=

200 GeV at the Relativistic Heavy Ion Collider

https://doi.org/10.1016/j.physletb.2018.11.009

0370-2693/©2018TheAuthor.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3_.

(RHIC) [26–28].Thesemeasurementshavetheadvantagethatthey probethefull pT rangeofheavy-quarkpairsandcontain comple-mentaryinformationabouttheinitialcorrelationofcharmquarks, i.e.theunderlyingproductionmechanism,whichisnotaccessible inconventionalsingleheavy-flavourmeasurements.

The measurement of direct photons, i.e. those produced in hardscatteringsbetweenincomingpartonsinhadroniccollisions, provides another important test of pQCD. Furthermore, at pT

<

3 GeV

/

c, where the applicability of perturbation theory may be questionable,experimentaldataofdirect-photonproductioninpp collisions serve as a crucial reference to establish the presence ofthermal radiation fromthe hot and dense medium createdin heavy-ioncollisions [29–32]. The measurementof real(massless) directphotonsatlow pT ischallenging becauseofthelarge back-groundofhadron decay photons. Thisbackgroundcan be largely reducedby measuring thecontribution of virtual direct photons, i.e. direct e+e− pairs, to the dielectron invariant-mass spectrum abovethe

π

0 _{mass [29,30].}

Proton–proton collisions in which a large number of charged particles are producedhave recentlyattractedthe interest ofthe heavy-ion community [33,34]. These events exhibit features that are similar to those observed in heavy-ion collisions, e.g. collec-tive effects, such as long-range angular correlations [35–40] or enhanced strangeness production [41]. Charged-hadron pT spec-tra in pp collisions at

√

s

=

13 TeV show a hardening with in-creasingmultiplicity,an effectthat arisesnaturally fromjets [42]. Also, heavy-quark production is found to scale faster than lin-early with the charged-particle multiplicity in pp collisions at

√

s

=

7 TeV [43,44]. This motivates the study of dielectron pro-ductioninhigh-multiplicity pp collisions.In thelow massregion (mee

<

1 GeV

/

c2), dielectron measurements provide further in-sight into possible modifications of the light vector and pseudo-scalar meson production via their resonance and/or Dalitz de-cays, whereas in the IMR they allow for complementary studies oftheheavy-flavourproduction.AtLHCenergies,thecontribution fromopen charmalready dominatesthedielectron continuum at

mee

≈

0

.

5 GeV

/

c2.Moreover,ifathermalisedsystemwerecreated insuchhigh-multiplicitypp collisions,asignalofthermal(virtual) photonsshouldbepresent.

Inthisletter,firstresultsofcharmandbeautyproductioncross sections at midrapidity in inelastic (INEL) and high-multiplicity (HM)pp collisions at

√

s

=

13 TeV are reported.Thepaperis or-ganisedasfollows:theALICEapparatusandthedatasamplesused intheanalysisaredescribedinSection 2,thedataanalysisis dis-cussed in Section 3, Section 4 introduces the cocktail of known hadronicsources, andthe results are presentedand discussedin Section5.

2. TheALICEdetectoranddatasamples

A detailed description of the ALICE apparatus andits perfor-mancecanbefoundin [45–48].Thedetectorsusedinthisanalysis arebrieflydescribedbelow.

TrajectoriesofchargedparticlesarereconstructedintheALICE central barrelwiththeInner TrackingSystem(ITS) andthe Time Projection Chamber (TPC) that reside within a solenoid, which provides a homogeneous magneticfield of0.5 T along the beam direction. The ITS consists of sixcylindrical layers of silicon de-tectors,withradialdistancesfromthebeamaxisbetween3.9 cm and43 cm. The two innermost layers are equipped with Silicon PixelDetectors(SPD),thetwointermediatelayersarecomposedof SiliconDriftDetectors, andthetwo outermostlayersare madeof SiliconStripDetectors.TheTPC,maintrackingdeviceintheALICE central barrel, isa 5 m longcylindricalgaseous detector extend-ing from 85 cmto 247 cm in radial direction. It provides up to

159spacialpointspertrackforcharged-particlereconstructionand particle identification(PID)throughthemeasurement ofthe spe-cificionisationenergylossdE

/

dx inthegasvolume.

The PID is complementedby theTime-Of-Flight (TOF) system located at a radial distance of 3.7 m from the nominal interac-tionpoint.It measuresthearrivaltime ofparticles relativetothe eventcollisiontime providedby theTOFdetectoritselforby the T0detectors,twoarraysofCherenkovcounterslocatedatforward rapidities [49].

CollisioneventsaretriggeredbytheV0detectorthatcomprises two plasticscintillatorarraysplacedon bothsidesofthe interac-tionpointatpseudorapidities2

.

8

<

η

<

5

.

1 and

−

3

.

7

<

η

<

−

1

.

7. TheV0isalsousedtorejectbackgroundeventslikebeam-gas in-teractions, collisionswithde-bunchedprotonsorwithmechanical structuresofthebeamline.

ThedatasamplesusedinthisletterwererecordedwithALICE in2016duringtheLHCpp runat

√

s

=

13 TeV.Forthe minimum-bias eventtrigger that is used to define thedata sample for the analysis of inelastic pp collisions, coincident signals in both V0 scintillatorsarerequiredtobesynchronouswiththebeamcrossing time definedby theLHCclock. Eventswithhighcharged-particle multiplicities are triggered on by additionally requiring the total signal amplitudemeasured intheV0detectortoexceedacertain threshold. At the analysis level, the 0.036 percentile of inelastic events withthe highest V0 multiplicity (V0M) isselected to de-fine the high-multiplicity event class. This value is low enough to avoid inefficiencies due to trigger threshold variations during data taking. Track segmentsreconstructed inthe SPDare extrap-olated back to the beam line to define the interaction vertex. Events with multiple verticesidentified with the SPDare tagged as pile-up and removed from the analysis [48]. The vertex in-formation may be improved based on the information provided by tracks reconstructed in the ITSand TPC. To assure a uniform detector coverage within

|

η

|

<

0

.

8, the vertexposition along the beamdirectionisrestrictedto

±

10 cmaroundthenominal inter-action point.A totalof 455

×

106 _{minimum-bias(MB) pp events} and79

.

2

×

106 high-multiplicitypp eventsareconsideredfor fur-ther analysis, which corresponds to an integrated luminosity of

L

MB

int

=

7

.

87

±

0

.

40 nb−

1 _and

_L

HM

int

=

2

.

79

±

0

.

15 pb−

1_, respec-tively.The luminosity determinationisbased onthe visiblecross sectionfortheV0-basedminimum-biastrigger,measuredinavan derMeerscancarriedoutin2015 [50].Aconservativeuncertainty of5%isassignedtothismeasurement,toaccountforpossible vari-ationsofthecrosssectionbetweenthetwodata-takingperiods.

3. Dataanalysis

Electron1 candidates are selected fromcharged-particle tracks reconstructedintheITSandTPCinthekinematicrange

|

η

e

| <

0

.

8 and pT,e

>

0

.

2 GeV

/

c. Basic track quality criteria are applied, e.g. a sufficient number of space points measured in the TPC and ITS as well as a good track fit. The contribution from sec-ondary tracks is reduced by requiring a maximum distance of closest approach (DCA) to the primary vertex in the transverse plane (DCAxy

<

1

.

0 cm)andinthelongitudinaldirection(DCAz

<

3

.

0 cm).Tofurthersuppressthecontributionfromphoton conver-sions inthedetectormaterial,electroncandidates arerequiredto have a hitin the firstSPDlayer andno ITSclustersshared with otherreconstructedtracks.

The electronidentification isbasedon thecomplementary in-formationprovidedbytheTPCandTOF.ThedetectorPIDresponse,

1 _{Theterm‘electron’isusedforbothelectronsandpositronsifnotstated}

Fig. 1. Opposite-signspectrumN+−,thecombinatorialbackgroundB andthesignalS inminimum-bias(left)andhigh-multiplicity(right)events.Onlystatisticaluncertainties areshown.

Table 1

Sourcesofsystematicuncertainties.

Source Minimum bias High multiplicity Trackreconstruction 13% 13% Electronidentification 2% 2% Conversionrejection (mee<0.14 GeV/c2) 2% 2% Acceptancecorrection factor(R) 2% 2%

Vertexdistributionbias – 6% Multiplicitydependence

oftrackingandPID

– 6%

Total 14% 15%

n

(

σ

DET

i

)

,isexpressedintermsofthedeviationbetweenthe

mea-suredandexpectedvalue ofthespecificionisationenergylossin theTPCortime-of-flightintheTOFforagivenparticlehypothesis

i andmomentum,normalisedbythedetectorresolution

(

σ

DET

₎

_.In theTPC, electronsareselectedintherange



n



σ

TPC

e



<

3 and pi-onsarerejectedbyrequiringn



σ

TPC

π



>

4.Furthermore,kaonsand protonsarerejectedwith



n



σ

TPC

K



>

4 and



n



σ

TPC

p



>

4,unless thecandidateispositivelyidentifiedasanelectronintheTOF,i.e. fulfilling



n



σ

TOF

e



<

3.Forparticlesthatareoutside



n



σ

TPC K



<

4 and



n



σ

TPC

p



<

4 theTOFinformationisonlyusedtoselect elec-troncandidateswith



n



σ

TOF

e



<

3 ifthetrackhasan associated hitintheTOFdetector.

Sinceexperimentally theoriginofeach electronorpositronis unknown,allelectroncandidatesarepairedconsidering combina-tionswithopposite

(

N₊₋

)

butalsosame-signcharge

(

N_±±

)

.Most oftheelectronpairs arisefromthecombinationoftwo electrons originatingfromdifferentmotherparticles.Thesepairsgiveriseto thecombinatorialbackgroundB thatisestimatedviathe geomet-ric mean of same-sign pairs

√

N₊₊N₋₋ within the same event. Opposite- andsame-sign pairsincludecorrelatedbackground,e.g. originating from

π

0 _{decays with two e}+_e− _{pairs in the final} state (

π

0

_→

_γ

(∗)

_γ

(∗)

_→

_e+_e−_e+_e−_{), which includes decay}

chan-nelswithrealphotonsandtheirsubsequentconversionindetector material.Such processes lead to opposite andsame-sign pairsat equalrate.Thebackgroundestimateneedstobecorrectedforthe differentdetectoracceptanceofoppositeandsame-signpairs.This correctionfactoris determinedby dividingtheyields of uncorre-latedopposite

(

M₊₋

)

andsame-signpairs

(

M_±±

)

inmixedevents:

R

=

M₊₋

/(

2

√

M₊₊M₋₋

)

. The dielectron signal is then obtained as S

=

N₊₋

−

B

=

N₊₋

−

2R

√

N₊₊N₋₋. The signal S is shown togetherwiththeopposite-signspectrumN₊₋andthe combinato-rialbackgroundB inFig.1forminimum-biasandhigh-multiplicity

events.In themass interval 0

.

2

<

mee

<

3 GeV

/

c2, the signal-to-backgroundratiovariesinMBeventsbetween0.3and0.04witha minimumaroundmee

≈

0

.

5 GeV

/

c2andisroughlyconstantat0.2 intheIMR [51].In HMevents,theminimumreaches0.01 andis about0.08intheIMR.

Electron–positron pairs from photon conversion in the de-tector material, contributing to the low mass spectrum below 0

.

14 GeV

/

c2,are removed by using their distinct orientation rel-ativetothemagneticfield [25].

The data are corrected for the reconstruction efficiencies us-ing detailed Monte Carlo (MC) simulations. For this, pp events are generated with the Monash 2013 tune of Pythia 8 [52] for light-hadrondecays andthe Perugia 2011tune of Pythia 6.4 for heavy-flavour decays [53] and the resulting particles are prop-agated through a detector simulation using Geant 3 [54]. The choiceofthedifferent Pythia versionsismotivatedbythefactthat the Perugia 2011 tune describes reasonably well the transverse momentum spectra of heavy-flavour hadrons while the Monash 2013 tune reproduces many of the relevant light-hadron multi-plicities [55,56].Thesignalreconstructionefficiencieswerestudied asa function of mee and pairtransverse momentum pT,ee sepa-ratelyforthedifferente+e− sources:resonanceandDalitzdecays of relevant mesons as well as correlated semileptonic decays of charm and beauty hadrons. The total signal reconstruction effi-ciency is obtainedby weighting the efficiencyof each dielectron sourceby itsexpectedcontributionandisfoundtobe about20% in0

.

7

<

mee

<

1

.

2 GeV

/

c2andapproaches30%atlowerandhigher masses.

Different aspects of the analysis are considered as possible sources ofsystematicuncertainties, whichare summarisedin Ta-ble1.Thesystematicuncertaintiesduetothetrackreconstruction areestimatedbycomparingtheefficiencyoftheITS–TPCmatching, therequirementofahitinthefirstSPDlayer,andtherequirement ofnosharedITSclustersinMCsimulationsanddata.Theresidual disagreementsbetweendataandMCaddtoa6.5%uncertaintyon thesingle tracklevel,which leads toa 13%uncertainty forpairs. TheMC simulations werealsocheckedtoreproduce alldetails of the PIDselection withina systematicuncertaintyof 2%for e+e− pairs.The purityofthe electronsample is estimatedtobe

>

93% over therelevant pT range,witha pT-integratedhadron contam-ination of about4%. The resulting hadron contamination on the dielectronsignalisfoundtobenegligible.Formee

<

0

.

14 GeV

/

c2, a 2%uncertainty onthe conversionrejectionwas estimatedfrom the yield change when tightening the selection to reject photon conversions.A2%uncertaintyonthesignalyieldduetothe correc-tionfactorR isobtainedbyrepeatingtheeventmixingindifferent eventclasses,definedbythepositionofthereconstructedprimary

vertex andby the charged-particle multiplicity. The efficiency of theminimum-biastriggertoselectinelasticeventswithan e+e− pairintheALICEacceptance(

|

η

e

|

<

0

.

8 and pT,e

>

0

.

2 GeV

/

c) is estimatedtobe

(

99

±

1

)

% fromtheMonash2013tuneof Pythia 8. Thisand the luminosity uncertaintyof 5% [50] are global uncer-tainties, which are not included in the point-to-point uncertain-ties. No significant variation of systematic uncertainties on mass orpT,eeisobservedintheanalysis,andthesametotaluncertainty of14%isassignedaspoint-to-pointcorrelateduncertaintiesonthe differentialdielectroncrosssectionininelasticpp collisions.

The analysis of the high-multiplicity data has additional sys-tematicuncertainties.First,nodedicatedhigh-multiplicityMC sim-ulation was performed. In such events the vertex distribution is biasedmore than inMB eventsby the asymmetric pseudorapid-itycoverage ofthe two V0detectors. Thechange ofthe detector acceptancewithvertexposition could lead toa difference inthe numberofreconstructedelectronsofupto3%,whichresultsinan uncertainty of6% for e+e− pairs. Second, a possible multiplicity dependenceofthereconstructionandPIDefficiencyiscoveredby anuncertaintyof6% [57].Addedinquadrature,thisamountstoa totaluncertaintyof15%.

4. Cocktailofknownhadronicsources

The dielectron spectrum measured in pp collisions at

√

s

=

13 TeV iscomparedwiththeexpectationsfromallknownhadron sources, i.e. the hadronic cocktail, contributing to the dielectron spectrum in the ALICE central barrel acceptance (

|

η

e

|

<

0

.

8 and

pT,e

>

0

.

2 GeV

/

c). A fast MC simulation is used to estimate the contributionfrom

π

0_,

_η

_,

_η

_,

_ρ

_,

_ω

_and

_φ

_{decaysinpp collisions,as} detailedin [25].

Followingtheapproachoutlinedin [58],thepionpT-spectrum at

√

s

=

13 TeV is approximated by scaling the pT-spectrum of charged hadrons [42] by the pion-to-hadron ratio measured at

√

s

=

7 TeV [59,60]. The difference with respect to the same procedure based on the pion-to-hadron ratio measured at

√

s

=

2

.

76 TeV [59,61] issmaller than1% atlow pT andreaches5% at high pT.The chargedhadron pT-spectraat

√

s

=

13 TeV are nor-malised toINEL

>

0 events, i.e.inelasticcollisions that produce at leastonechargedparticlein

|

η

|

<

1,ratherthanINELevents.This iscorrectedtakingthe21%differenceinthepTintegrateddNch

/

d

η

valuesforthesetwoeventclasses [42].Aconservativeuncertainty of10%isassignedonthisextrapolation.

Afit of theobtained charged-pion pT-spectrum witha modi-fiedHagedornfunctionisthentakenasproxyfortheneutral-pion

pT-distribution. The simulated cross section per unit rapidity of the

π

0 _{is d}

_σ

_/

_{d y}

_|y

=0

=

155

.

2 mb. For the

η

meson a fit ofthe measured

η

/

π

0 _{ratioinpp collisions at}

√

_s

₌

_{7 TeV isused [62].} TheMonash2013tuneof Pythia 8describesthe

ρ

/

π

0 _and

_ω

_/

_π

0 ratiosmeasured in pp collisionsat

√

s

=

2

.

76 and7 TeV, respec-tively [55,56]. Therefore,MC simulations obtained withthistune at

√

s

=

13 TeV are used to obtain the

ρ

/

π

0 _and

_ω

_/

_π

0 _ratios. Based on the

η

/

π

0_,

_ρ

_/

_π

0 _and

_ω

_/

_π

0 _{data,the ratios athigh p}

T are 0

.

5

±

0

.

1, 1

.

0

±

0

.

2 and 0

.

85

±

0

.

17, respectively.The

η

 and

φ

mesons are generated assuming mT scaling, replacing pT with



m2

₋

_m2

π

+ (

pT

/

c

)

2 [63]. For themT scaling, particle yields are normalisedathighpT relative tothe

π

0 yield:0

.

40

±

0

.

08 for

η

 (from Pythia 6calculations) and0

.

13

±

0

.

04 for

φ

[64]. The de-tector response,includingmomentumandangularresolutions, as wellasBremsstrahlungeffectsobtainedfromfullMCsimulations, isappliedtothedecayelectrons asafunctionofpT,e,

η

eandthe azimuth

ϕ

e.Thisresultsinamassresolutionofapproximately1%. Thefollowingsources ofsystematicuncertaintieswere evaluated: the input parameterisations of the measured spectra as a func-tion of pT (

π

±,

η

/

π

0 and

ω

/

π

0), the branching fractions of all

includeddecaymodes,themT scalingparametersandthe resolu-tionsmearing.Forthehigh-multiplicitycocktail,theinputhadron

pT-distributions are adjusted according to the measured modifi-cations of the charged-hadron pT spectra [42]. The uncertainties of the cocktail from light-hadron decaysare about

±

15%, reach-ing up to

+

50% intheregion dominatedby the

η

mesondueto uncertainties in theextrapolation tolow pT. The multiplicity de-pendencehasanuncertaintythatvariesbetweenabout12%atlow

pT and40%athighpT.

The Perugia 2011 tune of Pythia 6.4, which includes NLO parton showering processes, is used to estimate the contri-butions of correlated semileptonic decays of open charm and beauty hadrons [53,65]. As an alternative, the NLO event gener-ator Powheg is also considered [66–69]. The resultingsame-sign spectrum is subtracted fromthe opposite-sign distribution as in the data analysis. Detector effects are implemented as for the light-hadron cocktail. The spectra are normalised to cross sec-tions atmidrapiditythatarebasedonFONLL [1–3] extrapolations of the ALICE measurements at 7 TeV [8–10]. Following the de-scription in [70], this leads to cross sections per unit rapidity of d

σ

cc

/

d y

|y=

0

=

1296+₋172162 μb and d

σ

bb

/

d y

|y=

0

=

68+₋1516 μb at

√

s

=

13 TeV.Thequoteduncertainties takeintoaccountboththe measured uncertaintyand the FONLL extrapolationuncertainties. Thelatter(dominatedbyscaleuncertaintiesbutalsoincludingPDF andmass uncertainties)are considered to be fullycorrelated be-tweenthetwoenergies [71].Forthehigh-multiplicitycocktail,the opencharmcontributionisweightedasafunctionofpTaccording tothemeasuredenhancementofD mesonswith pT

>

1 GeV

/

c at

√

s

=

7 TeV [43].Thesameweightsareappliedtotheopenbeauty contribution as no significant difference between the production ofD mesonsandJ

/ψ

frombeauty-hadrondecaysisobserved [43]. For electrons originating from charm or beauty hadrons with

pT

<

1 GeV

/

c,thesameweightasfor1

<

pT

<

2 GeV

/

c isassumed intheabsenceofa measurement.Thisleads toanuncertaintyon themultiplicity dependenceofabout40%atlow pT decreasingto 20%athighpT.

The J

/ψ

contribution is simulated with Pythia 6.4 and nor-malised to the cross section at

√

s

=

13 TeV, extrapolated with FONLL [9] fromthemeasurementat

√

s

=

7 TeV bytheALICE Col-laboration [72].Inthehigh-multiplicitycocktail, theJ

/ψ

isscaled according to a dedicated, pT-integrated measurement [44]. The

ψ(

2S

)

contributionisnormalisedtotheJ

/ψ

basedonacross sec-tionratioof

σ

ψ (2S)→e+e−

/

σ

J/ψ→e+e−

= (

1

.

59

±

0

.

17

)

% [73].

5. Results

The dielectron cross sections are reported within the ALICE central barrel acceptance

|

η

e

|

<

0

.

8 and pT,e

>

0

.

2 GeV

/

c, i.e. withoutcorrection tofullphase space.The result,integratedover

pT,ee

<

6 GeV

/

c, isshown asa functionof mee in theleft panel of Fig. 2. The data are compared withthe expectation from the hadronic decaycocktail, using Pythia for the heavy-flavour com-ponents,andfoundtobeinagreementwithinuncertainties.Good agreementbetweendataandcocktail calculationsisalsofoundas a functionof pT,ee,whichisshownforthreemeeintervalsinthe rightpanelofFig.2.

Figs. 3 and 4 show the ratios of the dielectron spectra in high-multiplicity over inelastic events as a function of mee for different pT,ee intervals. To account for the trivial scaling with charged-particle multiplicity, the ratio is scaled by the factor dNch

/

d

η

(

HM

)/



dNch

/

d

η

(

INEL

)



=

6

.

27

±

0

.

22, where dNch

/

d

η

(

HM

)

=

33

.

29

±

0

.

39 and



dNch

/

d

η

(

INEL

)



=

5

.

31

±

0

.

18 are thecharged-particlemultiplicitiesin

|

η

ch

|

<

0

.

5 measuredin high-multiplicityandinelasticpp collisions,respectively [42].Inthis ra-tio,themultiplicity-independentuncertaintiescancelandthetotal

Fig. 2. Thedielectroncrosssectionininelasticpp collisionsat√s=13 TeV asafunctionofinvariantmass(left)andofpairtransversemomentumindifferentmassintervals (right).Theglobalscaleuncertaintyonthepp luminosity(5%)isnotshown.Thestatisticalandsystematicuncertaintiesofthedataareshownasverticalbarsandboxes. Theexpectationfromthehadronicdecaycocktailisshownasaband,andthebottomleftplotshowstheratiodatatococktailtogetherwiththecocktailuncertainty.

Fig. 3. RatioofdielectronspectrainHMandINELeventsscaledbythe charged-particle multiplicity.The statistical andsystematic uncertaintiesofthe data are shownasverticalbarsandboxes.Theexpectationfromthehadronicdecay cock-tailcalculationisshownasagreyband.

systematicuncertaintyreducesto 9%. The ratioisin good agree-mentwiththehadronicdecaycocktailcalculationsoverthewhole measuredmeeandpT,eerange.Thisisthefirstmeasurement sen-sitive to the production of

π

0_,

_η

_,

_ω

_and

_φ

_{in high-multiplicity} pp collisions.The result confirms the hypothesis that these light mesons have the same multiplicity dependence as a function of mT,whichwasusedintheconstructionofthehigh-multiplicity hadron cocktail. From the agreement between data and cocktail inthe high-pT range(3

<

pT,ee

<

6 GeV

/

c), which isdominated by open beauty,it can be also concluded for the first time that the open beauty production has a multiplicity dependence sim-ilar to that of open charm. This puts additional constraints on mechanisms used to describe heavy-flavour production in high-multiplicity pp collisions, such as multiple parton interactions, percolationorhydrodynamicmodels.

In the intermediate mass region (1

.

03

<

mee

<

2

.

86 GeV

/

c2), whichisdominatedbyopenheavy-flavourdecays,thedataare fit-ted simultaneously in mee and pT,ee (for pT,ee

<

6 GeV

/

c) with Pythiaand Powheg templatesofopen charmandbeauty produc-tion,keepingthelight-flavourandJ

/ψ

contributions fixed,which introducesnegligibleuncertaintiesontheheavy-flavourcross sec-tion.The Pythia and Powheg least-squarefitsofdielectronspectra ininelasticeventsprojectedover pT,eeandmee areshowninthe

left and right panels of Fig. 5, respectively. The resulting cross sections are summarised in Table 2. The first uncertainty is the statistical uncertainty resulting from the fits and the second is the systematic uncertainty, which is determined by moving the data points coherently upward and downward by their system-aticuncertaintiesandrepeatingthefits.Thebranchingfractionof charm-hadrondecaystoelectronsistakenas

(

9

.

6

±

0

.

4

)

% [74].An additional uncertaintyof 9.3% is added in quadratureto account fordifferencesinthe



c

/

D0 ratiomeasured byALICEinpp

colli-sions at

√

s

=

7 TeV,whichis 0

.

543

±

0

.

061

(

stat.

)

±

0

.

160

(

syst.

)

for pT

>

1 GeV

/

c [75], and the LEP average of 0

.

113

±

0

.

013

±

0

.

006 [76].Thistranslatesintoa22%uncertaintyatthepairlevel. Thebranching fractionofbeautyhadronsdecayingintoelectrons, includingviaintermediatecharmhadrons,is

(

21

.

53

±

0

.

63

)

% [74], whichleadstoa6%uncertaintyonthedielectron-basedcross sec-tionmeasurement.Likethestatisticalandsystematicuncertainties, thesebranchingfractionuncertaintiesarefullycorrelatedbetween the Pythia and Powheg basedresults.

Theresultsareconsistentwithextrapolationsfromlower ener-giesbasedonpQCDcalculationsdiscussedintheprevioussection. There is a strong anti-correlation between the fitted charm and beautycrosssections.Thesizeabledifferenceinthecrosssections betweenthetwo MCeventgenerators arecomparabletowhat is observedat

√

s

=

7 TeV [25].Thedifferentcrosssectionsobtained fromfits with Pythia and Powheg simulationsare causedby ac-ceptancedifferencesofe+e− pairsfromheavy-flavourhadron de-cays inthesetwoeventgeneratorsbecauseofdifferentkinematic correlationsoftheheavyquarkpairs,inparticularinrapidity.The fractionofe+e− pairsthatfall intotheALICEacceptanceandthe intermediate mass region originating from cc pairs at midrapid-ityis14% in Pythia and10%in Powheg.Thispointstoimportant differences in the heavy quark production mechanisms between thetwogenerators.Itshouldbestressedthatsingleheavy-flavour measurementsappearinsensitivetothesedifferencesasthecross sectionsobtainedfromsuchmeasurements agreebetween Pythia and Powheg based extrapolations [7,11,22]. Therefore,dielectrons provide complementary information on heavy-flavour production that,ifproperlymodelled,shouldleadtoconsistentcrosssections with Pythia and Powheg.

Table 2 also summarises the corresponding cross sections for thehigh-multiplicitydata.Incaseof Pythia,themeasured charm crosssectiontranslatesintoanenhancementof1

.

86

±

0

.

40

(

stat.

)

±

0

.

40

(

syst.

)

relative to the charged-particle multiplicity increase. Thisisconsistentwiththemodelledmultiplicitydependenceused

Fig. 4. RatioofdielectronspectrainHMandINELeventsscaledbythecharged-particlemultiplicityindifferentpT,eeintervals.Thestatisticalandsystematicuncertaintiesof

thedataareshownasverticalbarsandboxes.Theexpectationfromthehadronicdecaycocktailcalculationisshownasagreyband.

Fig. 5. Projectionoftheheavy-flavourdielectronfit(greyline)ininelasticpp collisionsat√s=13 TeV ontothedielectronmass(left)andpT,ee(right)usingthe Pythia and

Powhegeventgenerators.Thelinesshowthecharm(red)andbeauty(magenta)contributionsafterthefit.Theglobalscaleuncertaintyonthepp luminosity(5%)isnot shown.Thestatisticalandsystematicuncertaintiesofthedataareshownasverticalbarsandboxes.Thefitswith Pythia and Powheg resultinaχ2_{/ndf of57.8/66 and}

52.6/66,respectively.

asinputforthecocktailinFigs.3and4.Forthebeautycross sec-tiontheobservedenhancementis1

.

63

±

0

.

50

(

stat.

)

±

0

.

35

(

syst.

)

. This is consistent withthe multiplicity dependence observed for open charm,buta scalingwithcharged-particle multiplicity can-notbeexcluded.

Thefractionofrealdirectphotonstoinclusivephotonscanbe extractedfromthedielectronspectrum atsmallinvariant masses assuming the equivalence between this fraction and the ratio of virtual direct photons to inclusive photons. The data are fit-ted minimising the

χ

2_{, in bins of p}

T,ee, with the sum of the light-flavour cocktail

(

fLF

(

mee

))

, open heavy-flavour contribution

(

fHF

(

mee

))

and a virtual direct photon component

(

fdirect

(

mee

))

, whose shape is described by the Kroll–Wada equation [77,78] in the quasi-real virtual photon regime

(

pT,ee

mee

)

. The nor-malisation of the open heavy-flavour component is fixed to the measured opencharmandbeautycrosssectionspresentedabove, using the Pythia simulations for the nominal fit. As systematic uncertaintyestimate, the Powheg simulationisused instead.The light-flavourcocktail andvirtual directphotontemplates are nor-malised independently to the data in mee

<

0

.

04 GeV

/

c2, i.e. in a mass window in which both Dalitz decays and direct pho-tons have the same 1

/

mee dependence. The direct-photon

frac-Pythia Powheg

dσcc/d y|y=0 974±138(stat.)±140(syst.)μb 1417±184(stat.)±204(syst.)μb

dσbb/d y|y=0 79±14(stat.)±11(syst.)μb 48±14(stat.)±7(syst.)μb

dσcc/d y|HMy=0 4.14±0.67(stat.)±0.66(syst.)μb 5.95±0.91(stat.)±0.95(syst.)μb

dσbb/d y| HM

y=0 0.29±0.07(stat.)±0.05(syst.)μb 0.17±0.07(stat.)±0.03(syst.)μb

Table 3

Upperlimitsat 90% C.L.on thedirect-photonfractions incomparisonwith the expectationininelasticpp collisionsbasedonaNLOpQCDcalculationfora fac-torisationandrenormalisationscalechoiceofμ=pT[80].

Data sample 1<pT,ee<2 2<pT,ee<3 3<pT,ee<6

GeV/c GeV/c GeV/c

Minimum bias 0.057 0.072 0.023 High multiplicity 0.060 0.083 0.055

pQCD 0.003 0.007 0.013

tion r is then extracted by fitting the data in the mass interval 0

.

14

<

mee

<

0

.

32 GeV

/

c2,i.e.abovethe

π

0 masstosuppressthe mostdominanthadronbackground,withthefollowingexpression: d

σ

/

dmee

=

r fdir

(

mee

)

+ (

1

−

r

)

fLF

(

mee

)

+

fHF

(

mee

)

.

Nosignificantdirectphotoncontributionisobservedinneither theinelasticnorthehigh-multiplicityevents [51].Upperlimitsat 90%confidencelevel(C.L.)areextractedwiththeFeldman–Cousins method [79] andsummarisedinTable3togetherwithpredictions fromperturbative QCD calculations for inelastic events [80]. The current uncertainties prevent any conclusions on the scaling of direct-photonproductionwithcharged-particlemultiplicity.

6. Summaryandconclusion

We have presented the first measurement of dielectron pro-ductionat midrapidity (

|

ye

|

<

0

.

8) in proton–protoncollisions at

√

s

=

13 TeV.The dielectroncontinuumcan bewell described by theexpectedcontributionsfromdecaysoflight- andheavy-flavour hadrons. The charm and beauty cross sections are extracted for the first time atmidrapidity at

√

s

=

13 TeV and are consistent withextrapolations fromlower energies based on pQCD calcula-tions.Thedifferencesobservedbetween Powheg and Pythia imply differentkinematiccorrelationsofthe heavy-quarkpairsinthese twoeventgenerators.Thereforedielectronsare uniquelysensitive to the heavy quark production mechanisms. The comparison of thedielectronspectraininelasticevents andin eventswithhigh charged-particlemultiplicitiesdoesnotrevealmodificationsofthe spectrum beyondthe already established onesof light and open charmhadrons.Theupperlimitsonthedirect-photonfractionsare consistentwithpredictionsfromperturbativequantum chromody-namicscalculations.

Acknowledgements

TheALICECollaborationwouldliketothankWernerVogelsang forprovidingtheNLOpQCDcalculationsfordirectphoton produc-tion.

The ALICECollaboration wouldlike to thank all its engineers andtechniciansfortheirinvaluablecontributionstothe construc-tionoftheexperimentandtheCERNacceleratorteamsforthe out-standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by allGrid centresandthe Worldwide LHCComputing Grid(WLCG)

collaboration. The ALICE Collaboration acknowledges the follow-ing funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (YerevanPhysicsInstitute)Foundation (ANSL),State Committeeof Science andWorld Federation ofScientists (WFS), Armenia; Aus-trian Academy of Sciences and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry of Communica-tions and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS),Financiadorade EstudoseProjetos(Finep)andFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; MinistryofScience &TechnologyofChina(MSTC),National Natu-ral ScienceFoundationofChina (NSFC)andMinistryofEducation ofChina (MOEC),China;MinistryofScienceandEducation, Croa-tia;MinistryofEducation,YouthandSportsoftheCzechRepublic, CzechRepublic;TheDanishCouncilforIndependentResearch Nat-ural Sciences, the CarlsbergFoundation and Danish National Re-search Foundation (DNRF), Denmark; HelsinkiInstitute of Physics (HIP),Finland;Commissariatàl’ÉnergieAtomique (CEA)and Insti-tut National de Physique Nucléaire et de Physique des Particules (IN2P3) andCentre Nationalde la Recherche Scientifique(CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum für Schwer-ionenforschungGmbH, Germany;GeneralSecretariat forResearch and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy, Government of India (DAE),DepartmentofScienceandTechnology,GovernmentofIndia (DST), University Grants Commission,Government ofIndia (UGC) andCouncilofScientific andIndustrialResearch(CSIR),India; In-donesian Institute of Sciences, Indonesia; Centro Fermi - Museo StoricodellaFisicaeCentroStudieRicercheEnricoFermiand Isti-tutoNazionalediFisicaNucleare(INFN),Italy;Institutefor Innova-tiveScience andTechnology,NagasakiInstituteofAppliedScience (IIST),Japan SocietyforthePromotionofScience (JSPS)KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONA-CYT)yTecnología,throughFondodeCooperaciónInternacionalen CienciayTecnología(FONCICYT)andDirecciónGeneraldeAsuntos delPersonalAcademico(DGAPA),Mexico;NederlandseOrganisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Re-search Council of Norway, Norway; Commission on Science and Technology forSustainable Developmentinthe South(COMSATS), Pakistan;PontificiaUniversidadCatólicadelPerú,Peru;Ministryof ScienceandHigherEducationandNationalScienceCentre,Poland; KoreaInstituteofScienceandTechnologyInformationandNational ResearchFoundationofKorea(NRF),RepublicofKorea;Ministryof EducationandScientific Research,InstituteofAtomic Physicsand RomanianNationalAgencyforScience,TechnologyandInnovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of EducationandScienceoftheRussianFederationandNational Re-search Centre Kurchatov Institute, Russia; Ministry of Education,

Science,Research andSportofthe Slovak Republic, Slovakia; Na-tionalResearchFoundationofSouthAfrica,SouthAfrica;Centrode AplicacionesTecnológicasyDesarrolloNuclear(CEADEN), Cubaen-ergía,CubaandCentrodeInvestigacionesEnergéticas, Medioambi-entalesyTecnológicas(CIEMAT),Spain;SwedishResearchCouncil (VR)andKnut&AliceWallenbergFoundation(KAW),Sweden; Eu-ropean Organization for Nuclear Research, Switzerland; National Science andTechnology DevelopmentAgency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Educa-tionCommissionunderNRUprojectofThailand,Thailand;Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sci-encesofUkraine,Ukraine;ScienceandTechnologyFacilities Coun-cil (STFC), United Kingdom; National Science Foundation of the UnitedStatesofAmerica(NSF)andU.S.DepartmentofEnergy, Of-ficeofNuclearPhysics(DOENP),UnitedStatesofAmerica.

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F. Torales-Acosta

20

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D. Adamová

93

,

J. Adolfsson

80

,

M.M. Aggarwal

98

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G. Aglieri Rinella

34

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M. Agnello

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N. Agrawal

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Z. Ahammed

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S.U. Ahn

76

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S. Aiola

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A. Akindinov

64

,

M. Al-Turany

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S.N. Alam

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,

D.S.D. Albuquerque

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,

D. Aleksandrov

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,

B. Alessandro

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,

R. Alfaro Molina

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,

Y. Ali

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,

A. Alici

10

,

27

,

53

,

A. Alkin

2

,

J. Alme

22

,

T. Alt

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,

L. Altenkamper

22

,

I. Altsybeev

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M.N. Anaam

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C. Andrei

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D. Andreou

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H.A. Andrews

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A. Andronic

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104

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M. Angeletti

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V. Anguelov

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F. Antinori

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N. Apadula

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A. Badalà

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40

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R. Bala

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A. Baldisseri

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M. Ball

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R.C. Baral

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F. Barile

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G.G. Barnaföldi

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L.S. Barnby

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V. Barret

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K. Barth

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N. Bastid

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S. Basu

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G. Batigne

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J.L. Bazo Alba

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I.G. Bearden

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H. Beck

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C. Bedda

63

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N.K. Behera

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I. Belikov

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F. Bellini

34

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H. Bello Martinez

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R. Bellwied

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L.G.E. Beltran

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V. Belyaev

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G. Bencedi

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A. Bercuci

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D. Berenyi

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R.A. Bertens

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D. Berzano

34

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58

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L. Betev

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P.P. Bhaduri

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J. Bhom

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A. Bilandzic

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103

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G. Biro

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J.T. Blair

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C. Blume

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A. Borissov

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M. Borri

127

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26

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C. Bourjau

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L. Bratrud

69

,