First measurement of Ξc0 production in pp collisions at √s = 7 TeV

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Physics Letters B

www.elsevier.com/locate/physletb

First measurement of ⁰ _c production in pp collisions at √

s = ^{7 TeV}

.ALICE Collaboration

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received15December2017 Receivedinrevisedform5March2018 Accepted21March2018

Availableonline27March2018 Editor:L.Rolandi

The production of the charm-strange baryon ⁰_c is measured for the ﬁrst time at the LHC via its semileptonicdecayintoe⁺⁻νe inpp collisionsat√

s=^{7 TeV}^with^the^ALICE^detector.^The^transverse momentum(pT)differentialcrosssectionmultipliedbythebranchingratioispresentedintheinterval 1<pT<8 GeV/c atmid-rapidity,|^y|<0.5.Thetransversemomentumdependenceofthe ⁰_c baryon production relativeto the D⁰ mesonproduction iscomparedto predictionsof event generators with various tunesofthehadronisationmechanism,whichare foundtounderestimatethemeasuredcross- sectionratio.

©²⁰¹⁸^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

Quantum Chromodynamics(QCD) as the theory of thestrong interactionhasbeenacornerstoneoftheStandardModelforsev- eraldecades. It hasbeen testedthrough measurements ine⁺e⁻, pp,pp andep collisionsatmomentum-transferscales whereper- turbative techniques are applicable [1]. In particular, measurements of charm hadrons have provided important tests of the theorybecauseperturbativetechniquesareapplicabledowntolow transversemomentum(pT)thankstothelargemassofthecharm quark compared to the QCD scale parameter (QCD∼^{200 MeV).}

Theproductioncrosssectionsofcharmhadronscanbecalculated using the factorisation approach as a convolution of three fac- tors [2]:thepartondistributionfunctionsoftheincomingprotons, thehard-scatteringcrosssectionatpartoniclevelandthefragmen- tation functions of charm quarks into charm hadrons. There are severalstate-of-the-artcalculationsadoptingdifferentfactorisation schemes. The collinear factorisation scheme is used by calcula- tionsatnext-to-leadingorderin

α

s,suchasthegeneral-massvari- ableﬂavour numberscheme(gm-vfns) [3–5] and theﬁxed order with next-to-leading-log resummation (fonll) [6,7] approaches, while the kT factorisation scheme is employed at leading order in Refs. [8–10]. However, some of thesecalculations do not provide predictions for heavy-baryon production due to the lack of knowledgeaboutthefragmentationfunctionofcharmquarksinto baryonicstates.Measurementsoftheproductionofcharmbaryons, suchas⁺_c and⁰_c,areessentialtodevelopandtestmodelsofthe hadronisationprocess.

While a variety of new charm-baryon resonances, such as ⁰_c [11],⁺⁺_cc [12],haverecentlybeenfound,charm-hadroncross- section measurements at the Large Hadron Collider (LHC) are mainly limited to mesons [13–21], apart from a few measure-

E-mailaddress:alice-publications@cern.ch.

ments of the ⁺_c cross section in pp and p–Pb collisions [16, 22]. In the case of ⁰_c, the existing measurements are currently limitedtoe⁺e⁻ collisions [23–27].New measurementsofcharm- baryonproductionarethereforeneededtoprovidefurtherinsights intothehadronisationprocessesinpp collisions.Forexample,in- teractions at the partonic level among the produced quarks and gluons,such ascolourreconnection,couldbestrongerinpp colli- sionsthanine⁺e⁻collisions,resultinginanenhancedproduction of baryonsrelative tomesons [28]. The measurements ofcharm- baryon production in pp collisions also serve as a reference for heavy-ioncollisions,whereamodiﬁcationofthebaryon-to-meson ratio isexpectedifa substantial fractionof charmquarks hadro- nises via recombination with other quarks from the deconﬁned mediumcreatedinthecollision [29–33].Measurementsofcharm- strange baryons, e.g. ⁰_c, could also provide additional input to betterunderstandthehadronisationmechanismofstrange quarks inpp collisionsbecauseoftheirvalencequarkcomposition.

In this paper,we report the ﬁrst measurement ofthe pT-differentialproductioncrosssectionof⁰_c multipliedbythebranch- ing ratio(BR) intothesemileptonicdecaymode, ⁰_c →^e⁺⁻

ν

e, and its ratio to the measured production cross section of D⁰ mesons [21] as a function of p_T, up to 8 GeV/c. The absolute branchingratioofthis⁰_c decayiscurrentlyunknown [34].Using adatasampleofpp collisionsat√

s=^{7 TeV}^recorded^with^the^AL- ICEdetectorin2010, themeasurementisperformedbyanalysing e⁺⁻ pairs formed by combining positrons and⁻ baryons re- constructedwiththedetectorsoftheALICEcentralbarrel,covering the pseudorapidity interval |

η

_|<0.9. The missingmomentum of theneutrinoiscorrectedusingunfoldingtechniques.Chargeconju- gatemodesareimpliedeverywhere,unlessotherwisestated.Only thesub-detectors relevantforthisdataanalysisaredescribed be- low.AmorecompleteanddetaileddescriptionoftheALICEdetec- toranditsperformancecanbefoundinRefs. [35,36].

https://doi.org/10.1016/j.physletb.2018.03.061

0370-2693/©²⁰¹⁸^TheÂuthor(s).^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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Thedetectors used inthis analysisincludethe Inner Tracking System(ITS), the Time Projection Chamber (TPC) andthe Time- Of-Flight detector (TOF). These detectors are located in a large solenoid magnet producing a magnetic ﬁeld of 0.5 T parallel to theLHCbeamaxis.TheITSconsistsofsixcylindricallayersofsil- icon detectors,placed atradial distances ranging from3.9 cm to 43 cmfromthenominalbeamaxisandcoveringthefullazimuth.

Thetwo innermostlayersconsist ofSiliconPixelDetectors (SPD), the two intermediate layers of Silicon Drift Detectors (SDD) and thetwooutermostlayersofSiliconStripDetectors(SSD).Thetotal materialbudgetoftheITSisonaverage7.7%ofaradiationlength, for particles with

η

₌0 [37]. The ITS spatial resolution enables themeasurementofthedistanceofclosestapproach(d₀)oftracks totheprimary vertexwitharesolutionbetter than75 μminthe transverseplane for pT>1 GeV/c in pp collisions [38]. The TPC isa cylindricalgaseous detectorwith a volume of about90 m³. TheTPCprovidestrackreconstructionwithupto159spacepoints at radial distances from the beam axis ranging between 85 cm and 247 cm, within the full azimuth. The TPC cluster-position resolutionis about500 μm along thebeam directionandin the transversedirectionfortracks with

η

₌0 [39].The TPCalso pro- videsparticleidentificationcapabilitiesviathemeasurementofthe specificionisationenergyloss,dE/dx,witharesolutionofapprox- imately 5.2% in pp collisions [36]. The TOF detector consists of multi-gap resistive platechambers placed at a radial distance of 3.7 m fromthe beamaxis andalso covers thefull azimuth. The TOF detector, witha timing resolutionof about 80 ps, measures thetime-of-flightofparticles relativetothetime ofthecollision, which is determined by the arrival time of the particles at the TOFdetectorandbytheT0detector,anarrayofCherenkovcoun- tersplacedat+^{370 cm}ând−^{70 cm}^from^the^nominalinteraction pointalongthebeamaxis [40].

The analysed data sample consists of pp collisions at √ s= 7 TeV recorded during the 2010 LHC data taking period with a minimumbias triggerthat requires atleastone hit in eitherthe SPDortheV0detectors.ThetwolayersoftheSPDdetectorcover

|

η

_|<2.0. The two V0 detectors, each comprising 32 scintillator tiles,areinstalledonbothsidesoftheinteractionpointandcover

−³.7<

η

<−¹.7 and2.8<

η

<5.1.Thetriggerconditioncaptures 87% ofthe pp inelastic cross section [41]. The collision vertexis reconstructed with an efficiency of 88% and only events with a reconstructed vertexwithin 10 cm fromthe nominal interaction point along the beamdirectionare used in thisanalysis. Pile-up eventsareidentifiedby searchingforasecond interactionvertex, reconstructedwithatleastthreeSPDtracklets(thataretwo-point tracksegmentsconnectinghitsinthetwoSPDlayers)pointingto acommon vertex, which isseparatedfrom thefirst vertexby at least8mm.Aftertheselections, theanalysedsamplecorresponds toanintegratedluminosity L_int=⁵.9±⁰.2 nb⁻¹.

The⁰_c candidatesaredeﬁnedfrome⁺⁻ pairsby combining atrackoriginatingfromtheprimaryvertex(denoted by“electron track”inthe following) andareconstructed ⁻ baryon. Electron trackssatisfying|

η

|<0.8 and pT>0.5 GeV/carerequiredtohave atleast100associatedclustersintheTPC(outofwhichatleast80 areusedforthecalculationofthedE/dxsignal),a

χ

² normalised tothe numberofTPC clusterssmallerthan 4andat least4hits inthe ITS. It is also required that the electron trackhas associ- atedhitsinthetwoinnermostlayersoftheITS,inordertoreject electronsfromphotonconversionsoccurringinthedetectormate- rialoutsidetheinnermostSPDlayer [13].Electronsare identified usingthe dE/dx measurement in the TPCand the time-of-flight measurement ofthe TOF detector. In both cases,the selection is applied on the n^TPCσ andn^TOFσ variables definedas the difference betweenthemeasureddE/dxortime-of-flightvaluesandtheone expectedforelectrons,dividedbythecorrespondingdetectorres-

Fig. 1.Invariant-massdistributionof⁻→π⁻(andchargeconjugate)candidates integratedoverpT.Thearrowindicatestheworldaverage⁻massfromRef. [34]

andthedashedlinesindicatetheselectedintervalforthe⁻candidates.

olution.Thefollowingselectioncriteriaareapplied:|ⁿ^TOF_σ |<3 and

−³.9+¹.2pT−⁰.094p²_T<n^TPCσ (pT)<3.The pT-dependent lower limit on n^TPCσ was optimised to reject hadrons. Thus, an electron purityof98%isachievedoverthewhole pTrange.

The background from “photonic” electrons (originating from Dalitz decays of neutral mesons and photon conversions in the detectormaterial)remaining intheelectronsampleareidentiﬁed usingatechniquebasedontheinvariantmassofe⁺e⁻ pairs [42].

The electrontracksare pairedwithopposite-signtracks fromthe sameeventpassinglooseselectioncriteria(|ⁿ^TPCσ |<5 withoutTOF requirement)andareidentifiedasphotonicelectronsifthereisat leastonepairwithaninvariantmasssmallerthan50 MeV/c².Set- tingsuchlooseelectronidentificationcriteriaismeanttoincrease theefficiencyoffindingthepartners.Thisimprovesthesignal-to- background ratiofor ⁰_c by about 50%, while thefraction of the signallostduetomisidentificationsislessthan2%.

The⁻baryonsarereconstructedfromthedecaychain⁻→

π

⁻,followedby→^p

π

⁻.Tracksusedtodeﬁne⁻ candidates are requiredto haveatleast80 clustersinthe TPCanda dE/dx signal intheTPCconsistent withthe expectedvaluesforprotons (pions)within4

σ

.The⁻andbaryonshavelonglifetimes(c

τ

ofabout4.91 cmand7.89 cm,respectively [34]),andthustheycan beidentiﬁedusingtheircharacteristiccascade-likeorV-shapedde- caytopologies [43–45]. Pionsoriginatingdirectlyfrom⁻ decays are selected by requiringd0>0.02 cm;protons and pionsorigi- natingfromdecaysarerequiredtohaved0>0.07 cm.Thed0of thetrajectorytotheprimaryvertexisrequiredtobelargerthan 0.03 cm,whileitscosineofthepointingangle,whichistheangle betweenthereconstructedmomentumandthelineconnecting the and⁻ decayvertices,is requiredto be larger than0.98.

Thedistancesofthe⁻anddecayverticesfromthebeamline are requiredtobelarger than0.4and2.7 cm,respectively. These selectioncriteriaaretunedtoreducethebackground,whilekeep- ingahigheﬃciencyforthesignal.Fig.1showsthe⁻peakinthe

π

⁻invariant-massdistributionintegratedoverpT.Only⁻can- didateswithinvariantmasseswithin8 MeV/c²fromthe⁻ mass (1321.71±⁰.07 MeV/c² [34])indicated byan arrowinFig.1are keptforfurtheranalysis. Inthisinterval,thesignal-to-background ratioisabout8.

The e⁺⁻ pairs are formed from selected positrons and ⁻ candidates.Onlypairswithan openinganglesmallerthan90de- greesareusedfortheanalysis.Thebackgroundinthee⁺⁻ pair distribution is estimated by exploiting the fact that ⁰_c baryons decayintoe⁺⁻

ν

e (right-sign,RS),butnotintoe⁻⁻

ν

e (wrong- sign,WS),whilemostofthebackgroundsourcescontributeequally to RS and WS pairs. The yield of WS pairs is therefore used to estimate the background and is subtracted from the yield of RS pairs to obtain the ⁰_c raw yield. The procedure is veriﬁed with

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Fig. 2.(a)Invariant-massdistributionsofright-signandwrong-sign(andchargeconjugate)pairsintegratedoverthewholepTinterval.(b)Invariant-massdistributionof⁰c candidatesobtainedbysubtractingthewrong-signpairyieldfromtheright-signonecomparedwiththesignaldistributionfromthesimulation,whichisnormalisedtothe measuredRS−WSyield.Thearrowindicatesthe⁰_cmass [34].

pythia6.4.21 [46] simulations using thePerugia-0 tune [47] and thegeant3transportcode [48],includingarealisticdescriptionof thedetectorresponseandalignmentduringthedatatakingperiod.

AsimilarprocedurewasadoptedbytheARGUSandCLEOcollabo- rationsstudyinge⁺e⁻collisions [24,25].

Fig.2(a)showsthe invariant-mass distributionsofRS andWS pairs, integrated over the whole pT interval. The invariant-mass distributionof⁰_c candidatesobtainedbysubtractingtheWSpair yieldfromtheRSoneisshowninFig.2(b)togetherwiththesignal distributionfromthesimulation,whichisnormalisedtothemea- suredRS−^WS^yield.^The^shapes^of^the^twodistributionsarefound tobe consistent witheach other.Dueto themissingmomentum oftheneutrino, theinvariant-mass distributionofthee⁺⁻ pair doesnot peak at the ⁰_c mass (2470.85⁺₋⁰₀^._.²⁸₄₀ MeV/c² [34]) indi- catedbyan arrowinFig.2(b).Theinvariant massofe⁺⁻ pairs from ⁰_c decays is bounded by the ⁰_c mass due to the miss- ing momentum oftheneutrino. Thus onlye⁺⁻ pairs satisfying m_e<2.5 GeV/c² areselectedforfurtheranalysis.

Inorder to obtain the pT-differential productioncross section of ⁰_c baryons, the background-subtracted (WS-subtracted) yield needs to be corrected for: the signal loss due to misidentiﬁca- tion of photonic electrons, the b contribution in the WSpairs, themissingneutrinomomentum,thedetectoracceptanceandthe track-reconstruction and the candidate-selection eﬃciencies. No correction isappliedforpossible differencesintheacceptanceof RSandWSpairs,whicharefoundtobe negligibleforthecurrent analysisbasedonastudywiththemixed-eventtechnique(i.e.by pairingelectronsand⁻fromdifferentevents).

The firstcorrection accountsforthe signal losscausedby the misidentification of photonic electrons. The misidentification oc- curs when electrons from ⁰_c decaysaccidentally have opposite- signpartnersgivingrisetoaverysmallinvariantmassofthee⁺e⁻ pair.Themisidentificationprobabilityisestimatedtobe lessthan 2% by applying the tagging algorithm to e⁺e⁺ and e⁻e⁻ pairs.

The correction is applied as a function of the pT of the e⁺⁻ pair.

The second correction accounts for the overestimation of the backgroundcausedbyb→^e⁻⁻

ν

eX decays,whichproduceWS pairs.Sincethe branchingratioof_b intoe⁻⁻

ν

eX andthe_b crosssectioninpp collisionsatLHCenergieshavenotbeenmea- suredyet,twoassumptionsaremadetoestimatethiscontribution.

First,theshapeofthetransversemomentumdistributionoftheb baryonisassumedtobethesameasthatof⁰_b,whichwasmea- suredforp_T>10 GeV/cand|^y|<2 bytheCMScollaboration [49].

Thismeasurementisextrapolatedto pT=^{0 using}^the^Tsallis^function,

Fig. 3. Correlation betweenthe generated⁰_c-baryon pT and the reconstructed e⁺⁻ pair pT,obtainedfromthesimulationbasedonpythia6describedinthe text.(Forinterpretationofthecoloursintheﬁgure(s),thereaderisreferredtothe webversionofthisarticle.)

Cp_T

⎡

⎢ ⎣

¹

+

p²_T

+

^m²

−

^m nT

⎤

⎥ ⎦

⁽¹⁾

whose parameters were also determined by the CMS collaboration by fitting the measured distribution. The fit parameters are consistent with those determined by the LHCb collaboration for the measurement of ⁰_b down to p_T=^{0 at} ^forward ^rapidity (2<y<4.5) [50]. The second hypothesis is made for the total yield ofb→ê⁻⁻

ν

eX,whichisdeterminedby usingthemea- surements of BR(b→b)·^BR(b→⁻l⁻

ν

X) [51] and BR(b→ ⁰_b)·^BR(⁰_b→l⁻

ν

X)[52] ine⁺e⁻ collisions andby assuming that thefractionofbeautyquarksthathadronise into⁰_b andb baryons are the same asthose in e⁺e⁻ collisions. This assump- tionissupportedbyB-mesonmeasurements,whichshowthatthe yield of B⁰_s mesons relative to non-strange B mesons is consistent ine⁺e⁻ andpp collisions [53].The b distributionobtained withtheseassumptions is furtherprocessed totake into account thedetectoracceptance,eﬃciencyandthemomentumcarriedby non-reconstructed decayparticles.Thisisdonewiththepythia6 simulation usinggeant3forparticletransport throughthedetec- tor.ThecorrectionincreaseswithpTandreaches2%atthehighest p_T interval.

The transverse momentum distribution of e⁺⁻ pairs is cor- rectedforthemissingmomentumoftheneutrinousingunfolding techniques.Theresponsematrixtocorrectforthemissingneutrino momentumisgeneratedbasedonthecorrelationbetweenthe p_T ofthe⁰_c baryonandthatofthereconstructede⁺⁻pair,which is obtainedfromthesimulation describedabove andisshownin Fig.3.Theresponsematrixincludesboththedecaykinematicsand

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Table 1

SummaryofsystematicuncertaintiesonthepT-differentialcrosssectionof⁰_c →^e⁺⁻νefor5pTintervals.

Theuncertaintyonthemissingneutrinomomentumisdenotedasp^ν_Tinthetable.

Source Relative systematic uncertainty (%) in the measuredpTintervals (GeV/c)

1–2 2–3.2 3.2–4.4 4.4–6 6–8

Raw yield 5 5 5 5 5

(A×ε) 30 22 16 13 14

p^ν_T 29 8 6 7 10

Normalisation 3.5

theinstrumental effects, such asenergyloss andbremsstrahlung inthe detectormaterial. The response matrixneeds to be determined using a realistic ⁰_c-baryon p_T distribution. However, the distributionis notknown apriori. Therefore,the responsematrix ispreparedintwo steps. Intheﬁrst step,theresponse matrixis obtained with the pT distribution generated with pythia 6. The resulting ⁰_c momentum distribution is used to produce the re- sponsematrixfortheseconditeration.Theunfoldingisperformed withtheRooUnfold [54] implementationoftheBayesianunfolding technique [55],whichisaniterativemethodbasedonBayes’the- orem.ConvergenceoftheBayesianmethodisachievedafterthree iterations.

ThepT-differentialproductioncrosssectionof⁰_c baryonsmul- tiplied by the branching ratio into the considered semileptonic decaychannel is calculated fromthe yields obtainedby the un- foldingapproachasfollows:

BR

·

^d²

σ

⁰^c

dp_Tdy

=

^N⁰^c

2

·

p_T

y

· (

A

× ε ) ·

^Lint

·

^BR⁻

,

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where N0

c is the yield in a given p_T interval with width p_T. Theyieldisdividedbythe integratedluminosity Lintoftheanal- ysed sample and by the product of the branching ratios of the decays⁻→

π

⁻(99.887±0.035% [34])and→^p

π

⁻(63.9± 0.5% [34]),whichisindicatedasBR−.Thefactor1/2isneededbe- causethecrosssectioniscomputedfortheaverageof⁰_c and⁰_c, whiletherawyieldincludesbothcontributions.Thefactor(A×

ε

) isthe product of the geometrical acceptance (A) andthe reconstruction and selection eﬃciency (

ε

) for ⁰_c →^e⁺⁻

ν

e decays determinedfor⁰_c generatedin|^y|<0.8.Finally,theyieldisnor- malised toone unitof rapidityby dividingit by y=¹.6 under theassumptionthat the rapiditydistribution of⁰_c isuniformin therange |^y|<0.8.Thisassumption isveriﬁed withan accuracy of1% usingpythia6.Notethat theﬂatnessoftherapiditydistri- butionin |^y|<0.8 isalso relevantforthe comparisonto the D⁰ mesoncrosssection,whichwasdeterminedin|^y|<0.5 [21].

Theacceptanceandtheeﬃciencyarecalculatedfromthesim- ulationswithanadditionalcorrectiontotakeintoaccountthefact that the elastic cross section of anti-protons is not accurate in geant3 [56]. The correctionis calculatedusingthe geant4trans- portcode [57],whichhasamoreaccuratedescriptionofthecross section,andfoundtobelessthan2%.Sincetheacceptanceandthe eﬃciencydependon the⁰_c-baryon p_T,the ⁰_c shouldbe gener- atedwitharealisticmomentumdistribution.Thiswasobtainedvia atwo-stepproceduresimilartothatusedfortheresponsematrix.

Fig.4showstheproductofthegeometricalacceptanceandthere- constructionand selectioneﬃciency (A×

ε

) of⁰_c asa function ofpT.

Thesystematicuncertaintyonthe ⁰_c crosssection hasdiffer- ent contributions, which are the uncertainties on the raw yield (owingtotheprocedureofbackgroundestimation),onthe(A×

ε

) factor (due to imperfections in the simulated samples), on the correctionofthemissingneutrinomomentum(relatedtotheun- foldingprocedure)andon thenormalisation.Table 1summarises

Fig. 4.Productofacceptanceand eﬃciency(A×ε)of⁰c baryonsgeneratedin

|^y|<0.8 decayingintoe⁺⁻νeasafunctionofpT,determinedfromsimulations pythia6(seetext).

theestimatedsystematicuncertainties,reportingtheirvaluesinall the p_Tintervals.Thetotalsystematicuncertaintyisdeterminedby addingtheindividualcontributionsinquadratureineachpTinter- val.

Thesystematicuncertaintyontherawyieldincludestheuncer- taintiesduetotheWSsubtractionprocedureandtotheestimation oftheb contribution.IntheWSsubtractionproceduredescribed above,itwas assumedthatall thebackgroundsourcescontribute equallytoRSandWSpairs.Thisistrueaslongasthebackground comprises uncorrelated pairs of electrons and ⁻. A systematic uncertainty of 4% on the ⁰_c signal yield due to possible differ- encesbetweenRSandWSisestimatedfromsimulationswiththe pythia6eventgeneratorbycheckingtheremainingcontamination ofbackgroundpairsintheRSyieldafterthesubtractionoftheWS pairs. The WS subtraction could also be affected by the amount ofhadroncontaminationintheelectronsampleandthesignal-to- backgroundratioofthe⁰_c signal.Thiseffectisstudiedbyrepeat- ing theanalysiswithdifferentelectron identificationcriteria.The resultsobtainedwiththesemodifiedcriteriaarefoundtobecon- sistentwiththeonesfromthedefaultselectionsandthereforeno systematicuncertaintyisassigned.Thesystematicuncertaintydue to the _b contribution to the WS pairs is estimated by varying the b momentumdistribution within thequoted uncertainty of about50%onthecrosssectionof⁰_b inpp collisions [49] andthe quoteduncertaintyofabout50%ontheratioofthefragmentation fractionsofbeautyquarksinto⁰_b andb ine⁺e⁻collisions [51, 52].Theeffectonthefinalresultsisfoundtobeabout1%because thecontributionfrom b issmall.Thesesystematicuncertainties adduptoatotaluncertaintyof5%fortherawyieldextraction.

The systematic uncertainties arising from the reconstruction andselection efficiencies are estimated by repeating the analysis withdifferentselection criteriaforelectrons,⁻ ande⁺⁻pairs andbycomparingthe correctedyields.Dueto thestatisticallim- itationsofthe⁰_c sample,theelectronefficienciesarestudiedvia variationsofthetrack-qualitycriteriaandofthenσ valuesforthe electronidentificationwithTPCandTOF inthe⁺_c →ê⁺

ν

ede- cays,whichareanalysedwiththesameprocedureandhavehigher statisticalsigniﬁcance.TheRMSofthedeviationsofthecorrected

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Fig. 5.Inclusive⁰c-baryonpT-differentialproductioncross sectionmultipliedby thebranchingratiointoe⁺⁻νe,asafunctionofpTfor|y|<0.5,inpp collisions at√

s=^{7 TeV.}^The^error^bars^and^boxes^represent^thestatisticalandsystematic uncertainties,respectively.Thecontributionfrombdecaysisnotsubtracted.

yields relative to the value obtained with the standard selection criteria, whichamounts to 4% and3%, isthen assignedasa sys- tematicuncertaintyonthereconstructionandselectioneﬃciency.

Similarly, a systematic uncertaintyof 1% on both the ⁻ recon- structionandselectionefficiencyisestimatedfromtheRMSdevi- ationoftheinclusive⁻ correctedyieldagainstvariations ofthe criteriaappliedtoselectthe⁻ decaytracksanditscascadede- cay topology. In addition, a systematicuncertainty of4% on the ⁻ efficiency dueto possibleimperfections inthe description of the detector material in the simulations [44] is considered and summed inquadraturewiththat estimatedfromthe variation of the selection criteria. The uncertainties on the electron and ⁻ track-quality criteria are considered as correlated and combined linearly.The uncertaintyon the e⁺⁻ pairselection efficiency is estimatedby varying the selection criteria on the opening angle andthe invariant mass of the pair and a systematicuncertainty of3–27%isassigneddepending onpT.Finally,asystematicuncer- tainty mayalsoarise fromanimperfect descriptionoftheaccep- tanceofe⁺⁻pairsinthesimulation.Itisestimatedtobe11%by comparing the azimuthal distributions of inclusive electrons and ⁻baryonsinthedataandinthesimulation.Theuncertaintyon thee⁺⁻ pairacceptanceissummedinquadraturewiththaton theelectron and ⁻ selection efficiencies, resultingin a system- aticuncertaintyonthe(A×

ε

)correctionfactorrangingfrom13%

to30%dependingon pT.

Thesystematicuncertaintyonthemissingneutrinomomentum correction with the unfolding procedure is evaluated by varying thepriordistributiontotheBayesianunfoldingandbyusingdiffer- entunfoldingtechniques,suchasthe

χ

²minimisationmethod [58, 59] andtheSingularValueDecomposition(SVD)method [60].The RMSdeviationoftheresults,rangingbetween4%and29%depend- ing on pT, is assigned as a systematic uncertainty. A systematic uncertaintyof3%isalsoassignedduetotheimperfectknowledge ofthe⁰_c-baryon pT distributionsusedasinputfortheeﬃciency calculationandtheunfoldingprocedurefromthesimulation.Itis estimatedfromthedifferenceinduced intheresultby addingan additionalstepintheiterativeproceduredescribedabovetoobtain theinput p_T distributions. Thesesystematicuncertainties add up toanuncertaintyrangingbetween6%and29%dependingonpT.

Finally, the results have a 3.5% normalisation systematic uncertainty arising from the uncertainty in the determination of the minimum-bias trigger cross section in pp collisions at √

s= 7 TeV [41].

The pT-differential cross section of ⁰_c baryons multiplied by the branching ratio into e⁺⁻

ν

e is shown in Fig. 5 for the pT interval 1<pT<8 GeV/c at mid-rapidity, |^y|<0.5. The error bars and boxes represent the statistical and systematic uncertainties, respectively. The feed down contribution from b, e.g.

Fig. 6.Ratioofthe pT-differentialcrosssectionsof⁰c baryons(multipliedbythe branchingratiointoe⁺⁻νe)andD⁰mesons [21] asafunctionofpTfor|y|<0.5, inpp collisionsat √

s=^{7 TeV.}^The^error^bars^and ^boxes^represent^the^statisti- calandsystematicuncertainties,respectively.Predictionsfromtheoreticalmodels, (a)pythia 8with differenttunes[28,62].(b)dipsy [63] andherwig7 [64],are shownasshadedbandsrepresentingtherangeofthecurrentlyavailabletheoretical predictionsforthebranchingratiooftheconsidered⁰c decaymode.

⁻_b →⁰_c

π

⁻ [61],isnotsubtractedduetothelackofknowledge oftheabsolutebranchingratiosofb→⁰_c+^X.

The ratioofthe pT-differentialcross section of⁰_c baryonsto that of D⁰ mesons [21] is shown in Fig. 6. The pT intervals of the cross-section measurements are combined to have the same p_T bin boundariesfor ⁰_c and D⁰. The systematicuncertainty in a merged pT interval is deﬁnedby propagating theyield extrac- tion uncertaintiesof theD⁰ measurementasuncorrelatedamong pT intervalsandalltheotheruncertaintiesoftheD⁰ and⁰_c mea- surementsascorrelated.Thesystematicuncertaintyonthe⁰_c/D⁰ ratioiscalculatedtreatingall theuncertaintiesonthe⁰_c andD⁰ crosssectionsasuncorrelated,exceptforthenormalisationuncer- tainty that cancels out in the ratio. The ratio integrated in the transverse momentum interval 1<p_T<8 GeV/c is found to be (7.0±¹.5(stat)±².6(syst))×¹⁰⁻³^.

In Fig. 6(a), the measured transverse momentum dependence ofthe ⁰_c/D⁰ ratioiscomparedwithpredictionsfromthepythia 8.211 eventgenerator [46,65].pythia8uses2→²^processes^followedby aleading-logarithmic pT-ordered partonshowerforthe charmquarkpairproductionandthehadronisationistreatedwith theLund stringmodel [66].Theﬁgureshowstheresultsobtained withdifferenttunes ofhadronisation:theMonash2013tune [62]

andtheMode 0tunefrom [28].Thelatterisbasedonamodelfor the hadronisation ofmulti-parton systems,which includes string formationbeyondtheleading-colourapproximation andisimple- mentedinpythia8withspeciﬁctuningofthecolourreconnection parameters. As compared to the Monash 2013 tune, this model provides a better description of the measured baryon-to-meson ratios in the light-ﬂavour sector. Two other tunes (Mode 2 and Mode 3)providedinRef. [28] givesimilar⁰_c/D⁰ratiosasMode 0.

In Fig. 6(b), the measured ratio is also compared to other models implementing differenthadronisation mechanisms: dipsy [63]

(6)

withtherope hadronisation [67] and herwig7.0.4 [64] with the clusterhadronisation [68].To comparethedata withthesemod- els, theoretical calculations of the branching ratio, which range between0.83%and4.2% [69–71],areused.Thisrangedeﬁnesthe widthofthebandsshownforthemodelcalculationsrepresented inFig.6.AlthoughthepredictionsoftheMode 0tuneofpythia8 are the closest to the data compared to the other models, all calculationsunderestimatethe measured ratio signiﬁcantly. Thus, thisnewmeasurementcanprovideanimportantconstrainttothe modelsofcharmquarkhadronisationinpp collisions,onceamea- surementof theabsolute branching ratioof the ⁰_c will become available.

Insummary,wereportedontheﬁrstLHCmeasurementofthe inclusive pT-differential production cross section of the charm- strangebaryon⁰_c multipliedbythebranchingratiointoe⁺⁻

ν

e inpp collisionsat√

s=7 TeV.The ratioofthismeasurementin- tegrated over 1<pT<8 GeV/c to the production cross section oftheD⁰ mesonintegratedoverthe same pT intervalwas found tobe(7.0±¹.5(stat)±².6(syst))×¹⁰⁻³^.^Several^event^generators withvarious models andtunes for thehadronisation mechanism underestimatethemeasuredratio.

Acknowledgements

The ALICECollaboration wouldlike to thank all its engineers andtechniciansfortheirinvaluablecontributionstotheconstruc- tionoftheexperimentandtheCERNacceleratorteamsfortheout- standingperformanceoftheLHCcomplex.TheALICECollaboration gratefully acknowledges the resources and support provided by allGrid centresandthe Worldwide LHCComputing Grid(WLCG) collaboration. The ALICE Collaboration acknowledges the follow- ingfundingagenciesfortheirsupportinbuildingandrunningthe ALICEdetector:A.I. AlikhanyanNationalScienceLaboratory (Yere- vanPhysicsInstitute)Foundation (ANSL),State CommitteeofSci- enceandWorldFederationofScientists (WFS),Armenia; Austrian AcademyofSciencesandNationalstiftungfürForschung,Technolo- gie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan;

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Fi- nanciadoradeEstudoseProjetos(Finep)andFundaçãodeAmparo à Pesquisa do Estado de São Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science, Education and Sports and CroatianScienceFoundation,Croatia; MinistryofEducation,Youth and Sports of the Czech Republic, Czech Republic; The Danish CouncilforIndependentResearch—NaturalSciences,theCarlsberg FoundationandDanishNationalResearchFoundation(DNRF),Den- mark;HelsinkiInstitute ofPhysics (HIP),Finland; Commissariatà l’Énergie Atomique (CEA) and Institut National de Physique Nu- cléaireet dePhysique desParticules(IN2P3) andCentreNational de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) andGSIHelmholtzzentrumfürSchwerionenforschungGmbH,Ger- many;GeneralSecretariatforResearchandTechnology,Ministryof Education,ResearchandReligions, Greece;NationalResearch,De- velopmentandInnovationOffice,Hungary;DepartmentofAtomic Energy, Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Com- mission,GovernmentofIndia(UGC) andCouncil ofScientificand Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi – Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Instituto Nazionale di Fisica Nucleare (INFN),Italy; Institute forInnovative Science and Tech-

nology, Nagasaki Institute of Applied Science (IIST), Japan Soci- ety for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tec- nología, through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie 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 EducationandScienceoftheRussianFederationandNationalRe- search Centre Kurchatov Institute, Russia; Ministry of Education, Science, ResearchandSportofthe 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 Development Agency(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;ScienceandTechnologyFacilitiesCoun- cil (STFC), United Kingdom; National Science Foundation of the UnitedStates (NSF)andUnitedStatesDepartmentofEnergy,Office ofNuclearPhysics(DOENP),UnitedStates.

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