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DOI: 10.1016/j.physletb.2018.03.061
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DIRETORIA DE TRATAMENTO DA INFORMAÇÃO
Cidade Universitária Zeferino Vaz Barão Geraldo
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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
r
a
c
t
Articlehistory:
Received15December2017 Receivedinrevisedform5March2018 Accepted21March2018
Availableonline27March2018 Editor:L.Rolandi
The production of the charm-strange baryon 0c is measured for the first time at the LHC via its
semileptonicdecayintoe+−
ν
e inpp collisionsat√s=7 TeVwiththeALICEdetector.Thetransversemomentum(pT)differentialcrosssectionmultipliedbythebranchingratioispresentedintheinterval
1<pT<8 GeV/c at mid-rapidity,|y|<0.5.Thetransversemomentumdependenceofthe 0c baryon
production relativeto the D0 mesonproduction iscomparedto predictionsof event generators with
various tunesofthehadronisationmechanism,whichare foundtounderestimatethemeasured cross-sectionratio.
©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
Quantum Chromodynamics(QCD) as the theory of thestrong interactionhasbeenacornerstoneoftheStandardModelfor sev-eraldecades. It hasbeen testedthrough measurements ine+e−, pp,pp andep collisionsatmomentum-transferscales where per-turbative techniques are applicable [1]. In particular, measure-ments 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-scatteringcrosssectionatpartoniclevelandthe fragmen-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-leadingorderinα
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 pro-vide predictions for heavy-baryon production due to the lack of knowledgeaboutthefragmentationfunctionofcharmquarksinto baryonicstates.Measurementsoftheproductionofcharmbaryons, suchas
+c and
0c,areessentialtodevelopandtestmodelsofthe hadronisationprocess.
While a variety of new charm-baryon resonances, such as
0c [11],
++cc [12],haverecentlybeenfound,charm-hadron cross-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
0c, the existing measurements are currently limitedtoe+e− collisions [23–27].New measurementsof charm-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 of charm-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].Measurementsof charm-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 -dif-ferentialproductioncrosssectionof
0c multipliedbythe branch-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 pT, up to 8 GeV/c. The absolute branchingratioofthis0c decayiscurrentlyunknown [34].Using adatasampleofpp collisionsat
√
s=
7 TeVrecordedwiththe AL-ICEdetectorin2010, themeasurementisperformedbyanalysing e+− pairs formed by combining positrons and
− baryons re-constructedwiththedetectorsoftheALICEcentralbarrel,covering the pseudorapidity interval
|
η
|
<
0.
9. The missingmomentum of theneutrinoiscorrectedusingunfoldingtechniques.Charge conju-gatemodesareimpliedeverywhere,unlessotherwisestated.Only thesub-detectors relevantforthisdataanalysisaredescribed be-low.AmorecompleteanddetaileddescriptionoftheALICE detec-toranditsperformancecanbefoundinRefs. [35,36].https://doi.org/10.1016/j.physletb.2018.03.061
0370-2693/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
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.TheITSconsistsofsixcylindricallayersof sil-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 withη
=
0 [39].The TPCalso pro-videsparticleidentificationcapabilitiesviathemeasurementofthe specificionisationenergyloss,dE/
dx,witharesolutionof approx-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,anarrayofCherenkov coun-tersplacedat+
370 cmand−
70 cmfromthenominalinteraction 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 Lint=
5.
9±
0.
2 nb−1.The
0c candidatesaredefinedfrome+
− pairsby combining atrackoriginatingfromtheprimaryvertex(denoted by“electron track”inthe following) andareconstructed
− baryon. Electron trackssatisfying
|
η
|
<
0.
8 and pT>
0.
5 GeV/c arerequiredtohave atleast100associatedclustersintheTPC(outofwhichatleast80 areusedforthecalculationofthedE/
dx signal),aχ
2 normalised tothe numberofTPC clusterssmallerthan 4andat least4hits inthe ITS. It is also required that the electron trackhas associ-atedhitsinthetwoinnermostlayersoftheITS,inordertoreject electronsfromphotonconversionsoccurringinthedetector mate-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 nTPCσ andnTOFσ variables definedas the difference betweenthemeasureddE/
dx ortime-of-flightvaluesandtheone expectedforelectrons,dividedbythecorrespondingdetectorres-Fig. 1. Invariant-massdistributionof−→π−(andchargeconjugate)candidates integratedoverpT.Thearrowindicatestheworldaverage−massfromRef. [34]
andthedashedlinesindicatetheselectedintervalforthe−candidates.
olution.Thefollowingselectioncriteriaareapplied:
|
nTOFσ
|
<
3 and−
3.
9+
1.
2pT−
0.
094p2T<
nTPCσ(pT)
<
3.The pT-dependent lower limit on nTPCσ 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(
|
nTPCσ|
<
5 withoutTOF requirement)andareidentifiedasphotonicelectronsifthereisat leastonepairwithaninvariantmasssmallerthan50 MeV/c2. Set-tingsuchlooseelectronidentificationcriteriaismeanttoincrease theefficiencyoffindingthepartners.Thisimprovesthe signal-to-background ratiofor0c by about 50%, while thefraction of the signallostduetomisidentificationsislessthan2%.
The
−baryonsarereconstructedfromthedecaychain
−
→
π
−,followedby
→
pπ
−.Tracksusedtodefine− candidates are requiredto haveatleast80 clustersinthe TPCanda dE
/
dx signal intheTPCconsistent withthe expectedvaluesforprotons (pions)within4σ
.The−and
baryonshavelonglifetimes(c
τ
ofabout4.91 cmand7.89 cm,respectively [34]),andthustheycan beidentifiedusingtheircharacteristiccascade-likeorV-shaped de-caytopologies [43–45]. Pionsoriginatingdirectlyfrom
− decays are selected by requiringd0
>
0.
02 cm;protons and pions origi-natingfromdecaysarerequiredtohaved0
>
0.
07 cm.Thed0of thetrajectorytotheprimaryvertexisrequiredtobelargerthan 0.03 cm,whileitscosineofthepointingangle,whichistheangle betweenthereconstructed
momentumandthelineconnecting the
and
− decayvertices,is requiredto be larger than0.98. Thedistancesofthe
−and
decayverticesfromthebeamline are requiredtobelarger than0.4and2.7 cm,respectively. These selectioncriteriaaretunedtoreducethebackground,while keep-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 openinganglesmallerthan90 de-greesareusedfortheanalysis.Thebackgroundinthee+
− pair distribution is estimated by exploiting the fact that
0c 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 the0c raw yield. The procedure is verified with
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 the geant3transportcode [48],includingarealisticdescriptionof thedetectorresponseandalignmentduringthedatatakingperiod. AsimilarprocedurewasadoptedbytheARGUSandCLEO collabo-rationsstudyinge+e−collisions [24,25].
Fig.2(a)showsthe invariant-mass distributionsofRS andWS pairs, integrated over the whole pT interval. The invariant-mass distributionof
0c candidatesobtainedbysubtractingtheWSpair yieldfromtheRSoneisshowninFig.2(b)togetherwiththesignal distributionfromthesimulation,whichisnormalisedtothe mea-suredRS
−
WSyield.Theshapesofthetwodistributionsarefound tobe consistent witheach other.Dueto themissingmomentum oftheneutrino, theinvariant-mass distributionofthee+− pair doesnot peak at the
0c mass (2470
.
85−+00..2840 MeV/c2 [34]) indi-catedbyan arrowinFig.2(b).Theinvariant massofe+− pairs from
0c decays is bounded by the
0c mass due to the miss-ing momentum oftheneutrino. Thus onlye+
− pairs satisfying me
<
2.
5 GeV/c2 areselectedforfurtheranalysis.Inorder to obtain the pT-differential productioncross section of
0c 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
0c 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 backgroundcausedby
b
→
e−−
ν
eX decays,whichproduceWS pairs.Sincethe branchingratioofb intoe−
−
ν
eX and theb crosssectioninpp collisionsatLHCenergieshavenotbeen mea-suredyet,twoassumptionsaremadetoestimatethiscontribution. First,theshapeofthetransversemomentumdistributionofthe
b
baryonisassumedtobethesameasthatof0b,whichwas mea-suredforpT
>
10 GeV/c and|
y|
<
2 bytheCMScollaboration [49]. Thismeasurementisextrapolatedto pT=
0 usingtheTsallis func-tion,Fig. 3. Correlation betweenthe generated0
c-baryon pT and the reconstructed
e+− pair pT,obtainedfromthesimulationbasedon pythia 6describedinthe
text.(Forinterpretationofthecoloursinthefigure(s),thereaderisreferredtothe webversionofthisarticle.)
CpT
⎡
⎢
⎣
1+
p2T+
m2−
m nT⎤
⎥
⎦
(1)whose parameters were also determined by the CMS collabora-tion by fitting the measured distribution. The fit parameters are consistent with those determined by the LHCb collaboration for the measurement of
0b down to pT
=
0 at forward rapidity (2<
y<
4.
5) [50]. The second hypothesis is made for the total yield ofb
→
e−−
ν
eX , whichisdeterminedby usingthe mea-surements of BR(
b→
b)·
BR(b
→
−l−ν
X) [51] and BR(
b→
0b
)
·
BR(
0b→
l−ν
X)
[52] ine+e− collisions andby assuming that thefractionofbeautyquarksthathadronise into0b and
b
baryons are the same asthose in e+e− collisions. This assump-tionissupportedbyB-mesonmeasurements,whichshowthatthe yield of B0s mesons relative to non-strange B mesons is consis-tent ine+e− andpp collisions [53].The
b
distributionobtained withtheseassumptions is furtherprocessed totake into account thedetectoracceptance,efficiencyandthemomentumcarriedby non-reconstructed decayparticles.Thisisdonewiththe pythia 6 simulation using geant3forparticletransport throughthe detec-tor.ThecorrectionincreaseswithpTandreaches2%atthehighest pT interval.The transverse momentum distribution of e+
− pairs is cor-rectedforthemissingmomentumoftheneutrinousingunfolding techniques.Theresponsematrixtocorrectforthemissingneutrino momentumisgeneratedbasedonthecorrelationbetweenthe pT ofthe
0c baryonandthatofthereconstructede+
−pair,which is obtainedfromthesimulation describedabove andisshownin Fig.3.Theresponsematrixincludesboththedecaykinematicsand
Table 1
SummaryofsystematicuncertaintiesonthepT-differentialcrosssectionof0c →e+−νefor5pTintervals.
TheuncertaintyonthemissingneutrinomomentumisdenotedaspνTinthetable.
Source Relative systematic uncertainty (%) in the measured pTintervals (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
pνT 29 8 6 7 10
Normalisation 3.5
theinstrumental effects, such asenergyloss andbremsstrahlung inthe detectormaterial. The response matrixneeds to be deter-mined using a realistic
0c-baryon pT 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
0c momentum distribution is used to produce the re-sponsematrixfortheseconditeration.Theunfoldingisperformed withtheRooUnfold [54] implementationoftheBayesianunfolding technique [55],whichisaniterativemethodbasedonBayes’ the-orem.ConvergenceoftheBayesianmethodisachievedafterthree iterations.
ThepT-differentialproductioncrosssectionof
0c baryons mul-tiplied by the branching ratio into the considered semileptonic decaychannel is calculated fromthe yields obtainedby the un-foldingapproachasfollows:
BR
·
d 2σ
0c dpTd y=
N0 c 2·
pTy
· (
A×
ε
)
·
Lint·
BR−,
(2) where N0c is the yield in a given pT interval with width
p
T.Theyieldisdividedbythe integratedluminosity Lintofthe anal-ysed sample and by the product of the branching ratios of the decays
−
→
π
−(99.887
±
0.035% [34])and→
pπ
−(63.9±
0.5% [34]),whichisindicatedasBR−.Thefactor1/2isneeded be-causethecrosssectioniscomputedfortheaverageof0c and
0c, whiletherawyieldincludesbothcontributions.Thefactor
(A
×
ε
)
isthe product of the geometrical acceptance ( A) andthe recon-struction and selection efficiency (ε
) for0c
→
e+−
ν
e decays determinedfor0c generatedin
|
y|
<
0.
8.Finally,theyieldis nor-malised toone unitof rapidityby dividingit byy
=
1.
6 under theassumptionthat the rapiditydistribution of0
c isuniformin therange
|
y|
<
0.
8.Thisassumption isverified withan accuracy of1% using pythia 6.Notethat theflatnessoftherapidity distri-butionin|
y|
<
0.
8 isalso relevantforthe comparisonto the D0 mesoncrosssection,whichwasdeterminedin|
y|
<
0.
5 [21].Theacceptanceandtheefficiencyarecalculatedfromthe sim-ulationswithanadditionalcorrectiontotakeintoaccountthefact that the elastic cross section of anti-protons is not accurate in geant3 [56]. The correctionis calculatedusingthe geant4 trans-portcode [57],whichhasamoreaccuratedescriptionofthecross section,andfoundtobelessthan2%.Sincetheacceptanceandthe efficiencydependon the
0c-baryon pT,the
0c shouldbe gener-atedwitharealisticmomentumdistribution.Thiswasobtainedvia atwo-stepproceduresimilartothatusedfortheresponsematrix. Fig.4showstheproductofthegeometricalacceptanceandthe re-constructionand selectionefficiency
(A
×
ε
)
of0c asa function of pT.
Thesystematicuncertaintyonthe
0c crosssection has differ-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(relatedtothe un-foldingprocedure)andon thenormalisation.Table 1summarisesFig. 4. Productofacceptanceand efficiency( A×ε)of0
c baryonsgeneratedin
|y|<0.8 decayingintoe+−νeasafunctionof pT,determinedfromsimulations
pythia6(seetext).
theestimatedsystematicuncertainties,reportingtheirvaluesinall the pTintervals.Thetotalsystematicuncertaintyisdeterminedby addingtheindividualcontributionsinquadratureineachpT inter-val.
Thesystematicuncertaintyontherawyieldincludesthe uncer-taintiesduetotheWSsubtractionprocedureandtotheestimation ofthe
b
contribution.IntheWSsubtractionproceduredescribed above,itwas assumedthatall thebackgroundsourcescontribute equallytoRSandWSpairs.Thisistrueaslongasthebackground comprises uncorrelated pairs of electrons and−. A systematic uncertainty of 4% on the
0c signal yield due to possible differ-encesbetweenRSandWSisestimatedfromsimulationswiththe pythia6eventgeneratorbycheckingtheremainingcontamination ofbackgroundpairsintheRSyieldafterthesubtractionoftheWS pairs. The WS subtraction could also be affected by the amount ofhadroncontaminationintheelectronsampleandthe signal-to-backgroundratioofthe
0
c signal.Thiseffectisstudiedby repeat-ing theanalysiswithdifferentelectron identificationcriteria.The resultsobtainedwiththesemodifiedcriteriaarefoundtobe con-sistentwiththeonesfromthedefaultselectionsandthereforeno systematicuncertaintyisassigned.Thesystematicuncertaintydue to the
b contribution to the WS pairs is estimated by varying the
b
momentumdistribution within thequoted uncertainty of about50%onthecrosssectionof0b inpp collisions [49] andthe quoteduncertaintyofabout50%ontheratioofthefragmentation fractionsofbeautyquarksinto
0b and
b
ine+e−collisions [51, 52].Theeffectonthefinalresultsisfoundtobeabout1%because thecontributionfromb
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 thestatistical lim-itationsofthe
0
c sample,theelectronefficienciesarestudiedvia variationsofthetrack-qualitycriteriaandofthenσ valuesforthe electronidentificationwithTPCandTOF inthe
+c
→
e+ν
e de-cays,whichareanalysedwiththesameprocedureandhavehigher statisticalsignificance.TheRMSofthedeviationsofthecorrectedFig. 5. Inclusive0
c-baryonpT-differentialproductioncross sectionmultipliedby
thebranchingratiointoe+−νe,asafunctionof pTfor|y|<0.5,inpp collisions
at√s=7 TeV.Theerrorbarsandboxesrepresentthestatisticalandsystematic 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-structionandselectionefficiencyisestimatedfromtheRMS devi-ationoftheinclusive
− correctedyieldagainstvariations ofthe criteriaappliedtoselectthe
− decaytracksanditscascade de-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,asystematic uncer-tainty mayalsoarise fromanimperfect descriptionofthe accep-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 thepriordistributiontotheBayesianunfoldingandbyusing differ-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 ofthe
0
c-baryon pT distributionsusedasinputfortheefficiency calculationandtheunfoldingprocedurefromthesimulation.Itis estimatedfromthedifferenceinduced intheresultby addingan additionalstepintheiterativeproceduredescribedabovetoobtain theinput pT distributions. Thesesystematicuncertainties add up toanuncertaintyrangingbetween6%and29%dependingon pT.
Finally, the results have a 3.5% normalisation systematic un-certainty 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
0c 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 uncer-tainties, respectively. The feed down contribution fromb
, e.g.Fig. 6. Ratioofthe pT-differentialcrosssectionsof0c baryons(multipliedbythe
branchingratiointoe+−νe)andD0mesons [21] asafunctionof pTfor|y|<0.5,
inpp collisionsat √s=7 TeV.Theerrorbarsand boxesrepresentthe statisti-calandsystematicuncertainties,respectively.Predictionsfromtheoreticalmodels, (a) pythia 8with differenttunes[28,62].(b) dipsy [63] and herwig 7 [64],are shownasshadedbandsrepresentingtherangeofthecurrentlyavailabletheoretical predictionsforthebranchingratiooftheconsidered0
c decaymode.
−b
→
0cπ
− [61],isnotsubtractedduetothelackofknowledge oftheabsolutebranchingratiosofb
→
0c+
X .The ratioofthe pT-differentialcross section of
0c 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 pT bin boundariesfor
0c and D0. The systematicuncertainty in a merged pT interval is definedby propagating theyield extrac-tion uncertaintiesof theD0 measurementasuncorrelatedamong pT intervalsandalltheotheruncertaintiesoftheD0 and
0c mea-surementsascorrelated.Thesystematicuncertaintyonthe
0c
/
D0 ratioiscalculatedtreatingall theuncertaintiesonthe0
c andD0 crosssectionsasuncorrelated,exceptforthenormalisation uncer-tainty that cancels out in the ratio. The ratio integrated in the transverse momentum interval 1
<
pT<
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
0c
/
D0 ratioiscomparedwithpredictionsfromthe pythia 8.211 eventgenerator [46,65]. pythia 8uses2→
2processes fol-lowedby 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 andis imple-mentedin pythia 8withspecifictuningofthecolourreconnection 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] givesimilar0c/D0ratiosasMode 0. In Fig. 6(b), the measured ratio is also compared to other mod-els implementing differenthadronisation mechanisms: dipsy [63]
withtherope hadronisation [67] and herwig 7.0.4 [64] with the clusterhadronisation [68].To comparethedata withthese mod-els, theoretical calculations of the branching ratio, which range between0.83%and4.2% [69–71],areused.Thisrangedefinesthe widthofthebandsshownforthemodelcalculationsrepresented inFig.6.AlthoughthepredictionsoftheMode 0tuneof pythia 8 are the closest to the data compared to the other models, all calculationsunderestimatethe measured ratio significantly. Thus, thisnewmeasurementcanprovideanimportantconstrainttothe modelsofcharmquarkhadronisationinpp collisions,oncea mea-surementof theabsolute branching ratioof the
0
c will become available.
Insummary,wereportedonthefirstLHCmeasurementofthe inclusive pT-differential production cross section of the charm-strangebaryon
0c multipliedbythebranchingratiointoe+
−
ν
e inpp collisionsat√
s=
7 TeV.The ratioofthismeasurement in-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.Severaleventgenerators withvarious models andtunes for thehadronisation mechanism underestimatethemeasuredratio.Acknowledgements
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-ingfundingagenciesfortheirsupportinbuildingandrunningthe ALICEdetector:A.I. AlikhanyanNationalScienceLaboratory (Yere-vanPhysicsInstitute)Foundation (ANSL),State Committeeof Sci-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 EducationandScienceoftheRussianFederationandNational Re-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;ScienceandTechnologyFacilities Coun-cil (STFC), United Kingdom; National Science Foundation of the UnitedStates (NSF)andUnitedStatesDepartmentofEnergy,Office ofNuclearPhysics(DOENP),UnitedStates.
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