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DOI: 10.1016/j.physletb.2016.05.027
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DIRETORIA DE TRATAMENTO DA INFORMAÇÃO
Cidade Universitária Zeferino Vaz Barão Geraldo
CEP 13083-970 – Campinas SP
Fone: (19) 3521-6493
http://www.repositorio.unicamp.br
Physics
Letters
B
www.elsevier.com/locate/physletb
Multi-strange
baryon
production
in
p–Pb
collisions
at
√
s
NN
=
5
.
02 TeV
.
ALICE
Collaboration
a
r
t
i
c
l
e
i
n
f
o
a
b
s
t
r
a
c
t
Article history:
Received16January2016
Receivedinrevisedform10May2016 Accepted10May2016
Availableonline12May2016 Editor:L.Rolandi
The multi-strange baryon yields in Pb–Pb collisions have been shown to exhibit an enhancement relative topp reactions.Inthiswork, and productionrateshave beenmeasured withtheALICE experimentasafunctionoftransversemomentum,pT,inp–Pbcollisionsatacentre-of-massenergyof
√s
NN=5.02 TeV.Theresultscoverthekinematicranges0.6 GeV/c<pT<7.2 GeV/c and 0.8 GeV/c<
pT<5 GeV/c,forandrespectively,inthecommonrapidityinterval−0.5<yCMS<0.Multi-strange
baryonshavebeenidentifiedbyreconstructingtheirweakdecaysintochargedparticles.The pTspectra
are analysedas afunctionofevent charged-particle multiplicity,whichinp–Pb collisionsranges over one order ofmagnitude and lies betweenthose observed in ppand Pb–Pb collisions. The measured
pT distributions are compared to the expectations from a Blast-Wave model. The parameters which
describetheproductionoflighterhadronspeciesalsodescribethehyperonspectrainhighmultiplicity p–Pb collisions. Theyield ofhyperonsrelativetocharged pionsis studiedand comparedwith results frompp andPb–Pb collisions.Acontinuousincreaseintheyieldratiosasafunctionofmultiplicityis observed inp–Pb data, thevalues of whichrangefrom those measuredin minimum biaspp tothe ones inPb–Pb collisions.Astatistical model qualitatively describesthismultiplicity dependenceusing acanonicalsuppression mechanism,inwhichthesmallvolumecauses arelativereductionofhadron productiondependentonthestrangenesscontentofthehyperon.
©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP3.
1. Introduction
Collisionsofheavynucleiatultra-relativisticenergiesallowthe studyofadeconfinedstateofmatter,theQuark–Gluon Plasma,in whichthedegreesoffreedom are partonic,ratherthan hadronic. The role ofstrange hadron yields in searchingfor thisstate was pointedout atan earlystage [1].It was subsequentlyfound that inhighenergynucleus–nucleus(A–A)collisionsattheSuper Pro-ton Synchrotron (SPS), the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) the abundances of strange andmulti-strange baryonsare compatiblewith those from ther-malstatisticalmodelcalculations[2–10].
Insmaller collision systems atthe same centre-of-mass ener-gies,inparticularproton–proton(pp)collisions,therelative abun-danceof multi-strangebaryonsis lowerwithrespect toA–A col-lisions, whether normalised to participant nucleons or produced particles (pions or charged hadrons). This led to the interpre-tation that strangeness enhancement is observed in A–A colli-sions.Attempts to explain thisphenomenon include the applica-tionofacanonicalformalisminthestatisticalmodel,replacingthe grandcanonical approach, in which the requirementto conserve
E-mail address:[email protected].
thestrangenessquantumnumberwhenproducing(multi-)strange baryonsinsmallsystemsisimposed[11].Thismeansthatstrange hadrons are produced with a lower relative abundance in small systems,aneffectknownascanonicalsuppression.Sucha theoret-icalframeworkhasbeenusedtomakepredictionsforLHCenergies [12]. Further complications in the interpretation arise when the producedsystem,althoughsmall,isformedinperipheralA–A col-lisions wheretheparticleproductionmaynot befroma contigu-ous volume dueto core-corona effects [13,14]. Evidence for this effectwas seenatRHICwhereacanonicalsuppressioncalculation basedontheestimatednumberofparticipantnucleons couldnot successfullyreproducethedata[15].A cleanerwaytoinvestigate canonicalsuppressioneffectsisprovidedbyproton–nucleus(p–A) collisions.
Proton–nucleus collisions providean opportunity tostudy the
pT-dependence of the particle spectra created in a system with
a different, more compact, initial geometry than A–A collisions whereasimilarnumberofchargedparticles areproduced. Study-ing thisdependenceis importantindeterminingtheapplicability ofhydrodynamics[16]whichhasbeensuccessfulindescribingthe particlespectrainA–Acollisions[17–19].
At theLHC the combinationofthe rise inparticle production pernucleon–nucleoncollisionwithincreasing
√
s andadedicated p–Pb data-taking period have enabled the ALICE experiment tohttp://dx.doi.org/10.1016/j.physletb.2016.05.027
0370-2693/©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP3.
collect a large sample of
± and
±. In thisLetter, we set out the methods for these studies, present the results obtained and discusshowtheyfitintoatheoreticalpicture.
2. Sampleanddataanalysis
TheresultspresentedinthisLetterwere obtainedfroma sam-pleof thedatacollected withthe ALICEdetector[20] duringthe LHCp–Pb runat
√
sNN=
5.
02 TeV inthebeginning of2013. ThetwoscintillatorarraysV0A (directionofPbbeam),andV0C (direc-tionofpbeam),coveringpseudo-rapidityrangesof2
.
8<
η
<
5.
1 and−
3.
7<
η
<
−
1.
7,respectively, served both astriggering de-tectorsandfordetermining theeventmultiplicity class[21].The trackingofparticlesinthecentralbarrel, covering|
η
|
<
0.
9,takes placeintheInnerTrackingSystem(ITS),whichconsistsofthetwo innermostsiliconpixellayers,surroundedbytwosilicondriftand two siliconstrip layers, all placed within a radius of43 cm, and theTimeProjectionChamber(TPC),alargecylindricaldrift cham-ber filled witha Ne–CO2 gas mixture [20]. Measurementsoftheenergyloss by chargedparticles in the gas allow particles to be identifiedwiththisdetector.
Atrigger requiringa coincidence within lessthan1 ns inthe V0 detectorsselectedaround100millionevents,whicharemainly non-singlediffractive (NSD) events andcontain a negligible con-tribution from single diffractive (SD) and electromagnetic (EM) processes[22].A dedicatedradiator-quartz detector(T0)provided a measurement of the event time of the collisions. The V0 and T0 time resolutions allowed discrimination of beam–beam inter-actionsfrombackgroundeventsin theinteractionregion.Further backgroundsuppression was applied in theoffline analysisusing timeinformationfromtheneutronZeroDegreeCalorimeteronthe Pb-going side. Primaryvertices (PVs) were selected iftheir posi-tionalong the beamaxiswas reconstructed within10 cmofthe geometricalcentreofthedetector.InMonteCarlo(MC)studiesan efficiency of99.2% forthis trigger was obtained, while the joint trigger andprimary vertexreconstruction efficiencylies at97.8% [22].Theestimatedmeannumberofinteractionsperbunch cross-ingwasbelow1%inthesamplechosenforthisanalysis.
The analysed eventswere divided into seven multiplicity per-centile classes according to the total number of particles mea-suredintheforwardV0A detector.The efficiency-corrected mean numberofchargedprimaryparticlesperunitrapidity(
dNch/dη
) within−
0.
5<
η
<
0.
5 inthelaboratory referenceframeforeach ofthesemultiplicitybinswerepublishedin[23].Due to the asymmetric energies of the proton and lead ion beams, a consequence of the 2-in-1 magnet design of the LHC, the nucleon–nucleon centre-of-mass system is shifted by 0.465 unitsofrapidity inthedirectionoftheprotonbeamwithrespect to the laboratory frame. The measurements reported inthis Let-terwere performedinthecentralrapiditywindowdefinedinthe centre-of-massframe within
−
0.
5<
y<
0,wherenegative rapid-itycorrespondstothesideofthedetectorintowhichthePbbeam travels.The identification of multi-strange baryons was based on the topology of their weak decays through the reconstruction ofthe tracksleftbehindbythedecayproducts,referredtoasthe daugh-terparticles.Thedaughtersofthe
−
→
π
−(BR:99.9%),−
→
K− (BR:67.8%)andthesubsequent
→
pπ
−(BR:63.9%)weak decays [24], aswell asthe corresponding decays of the+ and
+,werereconstructed bycombiningtrackinformationfromthe TPC and the ITS [25]. Proton, anti-proton and charged
π
and K trackswereidentifiedintheTPCviatheirmeasuredenergy depo-sition,whichwascomparedwithamass-dependent parameterisa-tionofionisationlossintheTPCgasasafunctionofmomentum [26]. All daughter candidates were required to lie within 4σ
ofTable 1
Theparametersfor
V
0(and ¯)andcascades(±and±)selectioncriteria.Whereacriterionfor±and± findingdiffers,thevalueforthe±caseisinparentheses. DCArepresents“distanceofclosestapproach,”PVthe pri-maryvertex,θistheanglebetweenthemomentumvector ofthereconstructed
V
0orcascade,andthedisplacementvectorbetweenthedecayandprimaryvertices.The curva-tureofthecascadeparticle’strajectoryisneglected.
V0finding criteria
DCA: h±to PV >0.04(0.03)cm DCA: h−to h+ <1.5 standarddeviations mass (mV0) 1.108<mV0<1.124 GeV/c2
Fiducial volume (R2D) R2D>1.1(1.2)cm
V0pointing angle cosθ V0>0.97
Cascade finding criteria
Proper decay length <3×meandecaylength DCA:π±(K±) to PV >0.04 cm
DCA: V0to PV >0.06 cm
DCA:π±(K±) to V0 <1.3 cm
Fiducial volume (R2D) R2D>0.5(0.6)cm
Cascade pointing angle cosθcasc>0.97
their characteristic Bethe–Bloch energy loss curve. Multi-strange candidates were selected through the geometrical association of the V0 component(
or
¯
decay)toafurther secondary, ‘bach-elor’ track (identifiedasπ
± orK±). Inthis process,several geo-metricalvariables were measuredforeach candidate,andcriteria were setonthem inordertopurify theselectedsample: numer-ical valuesfor theselection cutsapplied are reportedinTable 1. Theseselectionsaresimilartothoseintheppmeasurements[25], a consequenceofthelow multiplicitiespresentinthedetectorin the p–Pb collisions. As a result the correction factors for the ef-ficiencyarealsosimilar. Inadditiontothesettingsontopological variables,acuthasbeenappliedontheV0invariantmasswindow of±
8 MeV/
c2 fromthenominalmass[24].Furtherrestrictions
were set on the proper lifetime of the
± and
±. By requir-ing thisvariable to be less than 3times the mean decaylength (4.91 cmand2.46 cm,respectively),wediscardedlow-momentum secondaryparticlesandfalsemulti-strangecandidates,the daugh-tertracksofwhichoriginatedfrominteractionswithdetector ma-terial.
The invariant mass of the
and
hyperons was calculated byassumingtheknownmasses[24]ofthe
andofthebachelor track. The masswas reconstructed twice for each cascade candi-date, once assuming the bachelor to be a
π
andonce a K. This allowedtheremovalofanimportantfractionofthebackground, which containeda largecontribution fromthe
candidates that passthe
selectioncriteria.Mostofthesefalse
wereremoved discarding allcandidatesthat couldbe reconstructedas
witha masswithin10 MeV/c2 oftheknownmass[24] ofthe
baryon. Fig. 1 showstheinvariantmass distributionsforthe
− and
−
hadronsinwellpopulatedpT binsforthelowestandhighest
mul-tiplicityclasses.
Forthesignalextraction,apeakregionwasdefinedwithin4
σ
of the mean of a Gaussian invariant mass peak for every mea-sured pT interval. Adjacent background bands,covering an equal
combinedmassintervalasthepeakregion,weredefinedonboth sides of that central region. This isillustrated inFig. 1 with the shaded bandson eitherside ofthe peak. Thenumber ofbin en-tries inside the side-bands was subtracted from the number of candidateswithinthepeakregion,assumingthebackgroundtobe linearacrossthemassrangeconsidered.
The pTdistributionswerecorrectedfordetectoracceptanceand
reconstruction efficiencies. Thesewere estimatedwiththe useof DPMJet[27]simulatedMonteCarlo(MC)events,whichwere prop-agatedthroughthedetectorwithGEANT3[28].
Fig. 1. Invariantmassdistributionsofthe−and−inthe1.1–1.2 GeV/c and1.2–1.6 GeV/c pTbinsrespectively,fittedwithaGaussianpeakandlinearbackground(dashed
redcurves).Thedistributionsforhighest(left)andlowest(right)multiplicityclassesareshown.Thefitsonlyservetoillustratethepeakpositionwithrespecttowhichthe bandsweredefinedandthelinearbackgroundassumptionfortheappliedsignalextractionmethod.
2.1.Systematicuncertainties
Systematic uncertainties due to the choice of selection crite-riawereexaminedseparately ineach pTintervalofthemeasured
spectra.Individual settingswere loosenedandtightened,inorder tomeasurechangesinthesignallosscorrection.Forthe
hyper-ons,thesignalextractionaccountsforanuncertaintyofaround2% butreaches5%atlow-pTandinhighmultiplicityevents,whilefor
the
,uncertaintiesof3–5%were measured.Theuncertaintydue tothe topologicalselections isaround 2(3)% forthe main pT
re-gion,andupto3(5)%atlowmomentumfor
(
).Theconstraint onthe V0 masswindowcontributestothetotaluncertaintywith around 0.5(1)%andboth the TPCtracking andidentificationcuts with2(3)%.Theproperdecaylengthcutgivesanother3(5)% uncer-taintyatlow pT.A 4%errorwasaddedduetothematerialbudget,
andforthe
± only, anadditional 3% duetothe mass
hypothe-siscut.Alltheseindividualerrorcontributions,whicharelistedin Table 2,areadded inquadrature.Apartfromthelowmomentum region,no pT dependenceisobservedinthetotaluncertainty.The
totalsystematicerrorliesbetween5–6(8)%acrossthewhole spec-trum, reaching up to 8(14)% inthe lowest pT bins for the
(
)
baryons.
Thefractionofthesystematicerrorthatisuncorrelatedacross multiplicity was calculated byusing thesame methodapplied in [23], in which spectra deviations in specific multiplicity classes were compared to thoseobserved in theintegrated datasample. Thechoiceofthetopologicalparametervaluesandtheapplied sig-nalextractionmethodgeneratesthedominantcontributiontothe uncorrelateduncertainties across multiplicity. Theseuncertainties weremeasuredtobewithin2%inthecaseofthe
and3%inthe case ofthe
,which constitutes a fractionthat liesbetween 20 and40%ofthetotalsystematicuncertainties.
Table 2
Contributionstothetotalsystematicuncertaintiesforthe±and± spec-trameasurements.Thevaluesinbracketsindicatethemaximum uncertain-tiesmeasuredforlow-pTcascades(seetext).
Source ± ±
Material budget 4% 4%
Competing mass hypothesis – 3%
Topological variables 2–3(5)% 3–5%
Signal extraction 2(5)% 3(5)%
Particle identification 2% 3%
Track selection 2% 3%
Proper decay length 1(3)% 2(5)%
V0mass window 0.5% 1%
Fig. 2. (Colouronline.)Invariant
p
T-differentialyieldsof(−+ +)/2 and(−+ +)/2 indifferentmultiplicityclasses.Datahavebeenscaledbysuccessivefactorsof 2forbettervisibility.Statistical(bars),fullsystematic(boxes)anduncorrelatedacrossmultiplicity(transparentboxes)uncertaintiesareplotted.Thedashedcurvesrepresent Blast-Wavefitstoeachindividualdistribution.
3. Results
3.1. Transversemomentumspectra
The pT distributionsof
−,
+,
− and
+ in
−
0.
5<
y<
0are showninFig. 2 fordifferentmultiplicity intervals, asdefined in[23].Sinceantiparticleandparticlespectraareidenticalwithin uncertainties,theaverageofthetwoisshown.Thespectraexhibit aprogressiveflatteningwithincreasingmultiplicity,whichis qual-itativelyreminiscentofwhatisobservedinPb–Pb collisions[10].
The calculation of pT-integrated yields can be performed by
usingdatainthemeasuredregionandaparametrisation-based ex-trapolationelsewhere.The Boltzmann–GibbsBlast-Wave(BG–BW) model[16]givesagooddescriptionofeach pT spectrumandhas
beenusedasatool forthisextrapolation.Otheralternatives,such astheLevy–Tsallis[29]andBoltzmanndistributions,wereusedfor estimatingthesystematicuncertaintyduetotheextrapolation.
The extrapolation in the unmeasured
± (
±) low-pT
re-giongrowsprogressivelywithdecreasingmultiplicity,fromaround 16%(19%)ofthetotalyieldinthe0–5%multiplicityclasstoaround 27%(40%) in the 80–100% class. The systematic uncertainty as-signedtotheyieldduetotheextrapolationtechniqueis2.8%(7.8%) forhighmultiplicities andrises to5.2%(14.5%) inthecasewhere thefractionoftheextrapolatedyieldishighest.
3.2. ComparisontoBlast-Wavemodel
In order to investigate whether the observed spectral shapes are consistent witha systemthat exhibits hydrodynamical radial
expansion, the measured distributions have been further stud-ied in the context of the BG–BW model [16]. This model as-sumesa locally thermalisedmediumthatexpandscollectivelywith a common velocity field and then undergoes an instantaneous freeze-out.Inthisframework,asimultaneousfittoidentified par-ticle spectra allows for the determination of common freeze-out parameters. These can be used to predict the pT distribution for
other particlespecies in a collective expansion picture. It should be noted that such asimultaneous fit differsfrom theindividual fits mentionedintheprevioussection andusedonlyfor extrapo-latingthespectra.
The
−,
+,
− and
+ pT spectrainthe0–5%and80–100%
multiplicity classesare comparedtopredictions fromtheBG–BW model with parameters acquired froma simultaneous fit to
π
±, K±,p(
p)
and( ¯
)
inFig. 3 [23].The modeldescribesthe mea-sured shapes within uncertainties up to a pT of approximately4 GeV/c for
and5 GeV/c for
inthehighestmultiplicityclass. This indicates that multi-strange hadrons also follow a common motionwiththelighterhadronsandissuggestiveofthepresence ofradial flowinp–Pbcollisions. However, itisworth notingthat some final state effects could alsomodify the spectra in a simi-lar manner to radial flow. For example,PYTHIA [30] implements the colourreconnectionmechanism, which fusesstrings originat-ing from independent parton interactions, leading to fewer but more energetic hadrons, which has been shown to mimic radial flow[31].
Applying the same technique to results from the lower mul-tiplicity classes reveals that the agreement of the data with the
Fig. 3. (Colouronline.)(−+ +)/2 and(−+ +)/2 pTspectrainthe0–5%(left)and80–100%(right)multiplicityclassescomparedtopredictionsfromtheBG–BWmodel
(upperpanels)withtheratiosonalinearscale(lowerpanels).Theparametersarebasedonsimultaneousfitstolighterhadrons[23].Seetextfordetails.
Table 3
Themid-rapiditydNch/dηvaluesforeachofthe7multiplicityclassesandthe−+ +and−+ +
inte-gratedyieldsperunitrapiditynormalisedtothevisiblecrosssection.Thestatisticaluncertaintyontheyieldsis followedbythesystematic uncertainty.
Event class dNch/dη |ηlab|<0.5 dN/dy(−+ +) dN/dy(−+ +) 0–5% 45±1 0.2354±0.0020±0.0161 0.0260±0.0011±0.0034 5–10% 36.2±0.8 0.1861±0.0016±0.0138 0.0215±0.0008±0.0029 10–20% 30.5±0.7 0.1500±0.0010±0.0112 0.0167±0.0006±0.0022 20–40% 23.2±0.5 0.1100±0.0006±0.0085 0.0120±0.0005±0.0016 40–60% 16.1±0.4 0.0726±0.0006±0.0065 0.0072±0.0003±0.0010 60–80% 9.8±0.24 0.0398±0.0004±0.0031 0.0042±0.0002±0.0006 80–100% 4.3±0.1 0.0143±0.0003±0.0015 0.0013±0.0003±0.0003
Blast-Wavepredictionsbecome progressivelyworse. The compari-sonbetweenlowestandhighestmultiplicitycasescanbe seenin Fig. 3,where their respective ratiosto the modelpredictions are showninthelowerpanels.Theseobservationsindicatethat com-monkinetic freeze-out conditionsare able tobetter describe the spectrainhighmultiplicityp–Pb collisions.
The multi-strange baryon spectra in central Pb–Pb collisions [10]havealsobeeninvestigatedinacommonfreeze-outscenario [17,18] and similar studies were performed for Au–Au collisions [19].Incontrasttohighmultiplicityp–Pb collisions,whereall sta-ble and long-lived hadron spectra are compatible with a single setof kinetic freeze-out conditions(the temperatureTfo andthe
meantransverseflowvelocity
β
T),multi-strangeparticlesincen-tral heavy-ion collisions seem to experience less transverse flow andmay freezeout earlierin the evolution of the systemwhen comparedtomostoftheotherhadrons.
3.3.Hyperontopionratios
Themeasuredintegratedyieldsinthesevenmultiplicityclasses aregiveninTable 3.Tostudytherelativeproductionofstrangeness andcompare itwithresultsinpp andPb–Pb collisions,theyield ratios to pions were calculated as a function of charged par-ticle multiplicity. Both the
(
−+
+)/(
π
++
π
−)
and(
−+
+
)/(
π
++
π
−)
ratios are observedto increase asa function of multiplicity, asseen in Fig. 4. The relative increase is more pro-nouncedforthe− and
+ thanfor
−and
+,being approx-imately 100%fortheformerand60%forthelatter.Theserelative increasesare larger than the30% increase observed forthe
/
π
ratio[23],indicatingthatstrangenesscontentmaycontroltherate ofincreasewithmultiplicity.
Theseratiosarefurthercomparedtomeasurementsperformed in the pp [25,34] and Pb–Pb [10] collision systems. The
(
−+
+
)/(
π
++
π
−)
ratioforthehighestp–Pb multiplicityis compat-ible withthePb–Pb measurementsinthePb–Pb 0–60%centrality rangeandthe(
−+
+)/(
π
++
π
−)
reachesavalueslightly be-lowitsPb–Pb equivalentinthiscentralityrange,althoughtheerror barsstilloverlap.Itisalsonoteworthythatthevaluesobtainedfor thep–Pb 80–100%multiplicityeventclassaresimilar totheones measuredinminimumbiasppcollisions.Finally,the hyperontopionratios canalso becompared with thevaluesintheGrandCanonical(GC)limitobtainedfromglobal fits toPb–Pb data. Twodifferentimplementations ofthe thermal modelare showninFig. 4,wherethedashed lines representthe valuesfromtheTHERMUS 2.3model[36]andthesolidlines repre-sentpredictionsfromtheGSI-Heidelbergmodel[35].Bothmodels provide values that are consistent with the most central Pb–Pb measurements.
Fig. 4. (Colouronline.)(−+ +)/(π++π−)(left)and(−+ +)/(π++π−)(right)ratiosasafunctionofdNch/dηforallthreecollidingsystems.Theratiosforthe
sevenmultiplicityclassesinp–Pb dataliebetweentheMinimumBiaspp(√s=900 GeV[32,33]and√s=7 TeV[25,34])andperipheralPb–Pb results.ThePb–Pb points [10]represent,fromlefttoright,the60–80%,40–60%,20–40%and10–20%and0–10%centralityclasses.ThechemicalequilibriumpredictionsbytheGSI-Heidelberg[35] andtheTHERMUS 2.3[36]modelsarerepresentedbythehorizontallines.
In smallmultiplicity environments such asthose produced in p–Pb collisions, a grand canonical statisticaldescription may not beappropriate.Instead,localconservationlawsmightplayan im-portant role.The evolution ofhyperon topion ratiosin terms of theeventmultiplicitycanbecalculatedwithaStrangeness Canon-ical (SC) model implemented in THERMUS [36]. This model ap-plies a local conservationlaw to the strangeness quantum num-ber within a correlation volume Vc while treating the baryon and charge quantum numbers grand-canonically within the fire-ball volume V .This impliesa decrease of thestrangeness yields withrespect to the pion yields with a shrinking system size. To modelthis canonicalsuppression effect asa functionof pion ra-pidity density, yield calculations were repeated for varying sys-temsizes. Strangeness conservationwas imposed within the size of the fireball (Vc
=
V ), and the strangeness saturation parame-terγ
S was fixed to 1,thus changes in the hadron to pionratioswere due to the variations of the restraints on the system size only. The chemical potentials (
μ
) of the conserved strangeness, baryon and electric charge quantum numbers were set to zero. The obtained suppression curves for,
and
are shown in Fig. 5fora temperature of155 MeV, thevalue extractedfroma GCglobalfit to highmultiplicity Pb–Pb data, witha variation of
±
10 MeV (solidlines).Both thedataandmodelpoints were nor-malised to the highmultiplicity limit. For the data, this limit is themeanhyperontopionratiointhe0–60%mostcentralPb–Pb events, whereas for the model it corresponds to the GC limit. Thetheoreticalcurvesforstrangenesssuppressioncomputedwith THERMUS areinqualitativeagreementwiththeeffectobservedin thedata.4. Conclusions
In summary, a measurement of the pT spectra of
−,
+,
− and
+ for seven multiplicity classes in p–Pb collisions at
√
sNN
=
5.
02 TeV attheLHChasbeenpresented.Thesemeasure-ments represent an important contribution to the understanding of strangeness production, as hyperon production rates are now measured atLHC energies over a large rangein charged–particle multiplicity,frompptocentralPb–Pb collisions.
Themulti-strange baryonspectraexhibit a progressive flatten-ing withincreasing multiplicity suggestingthe presence ofradial
Fig. 5. (Colouronline.)Hyperontopionratiosasafunctionofpionyieldsforpp, p–Pb andPb–Pb collidingsystemscomparedtotheTHERMUS[36]strangeness sup-pressionmodelprediction,inwhichonlythesystemsizeisvaried.Theh/π are theratiosoftheparticleandantiparticlesums,exceptforthe2/(π−+π+)data pointsinpp[33],p–Pb[23]andPb–Pb[37].Allvaluesarenormalisedtothehigh multiplicity limit,whichisgivenbythe meanofthe0–60%highest multiplicity Pb–Pb measurementsforthedataandbytheGClimitforthemodel.
flow. A comparison with theBoltzmann–Gibbs Blast-Wavemodel indicatesacommonkineticfreeze-outwithlighterhadronsinthe highest multiplicity p–Pb collisions. This is in contrast to higher multiplicity heavy-ion collisions where there is an indication for anearlierfreeze-outoftheseparticles.
For the first time, the lifting ofstrangeness suppression with systemsizehasbeenobservedwithmeasurementsinasingle col-lision system. Hyperontopion ratiosare showntoincrease with multiplicity in p–Pb collisions from the values measured in pp to those observed in Pb–Pb. The rate of increase is more pro-nounced for particles with higher strangeness content. Compar-ing these results to the trends observed in statistical hadroni-sation models that conserve strangeness across the created sys-tem indicates that the behaviour is qualitatively consistent with the lifting of canonical suppression with increasing multiplic-ity.
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 theALICEdetector:State CommitteeofScience,World Federation ofScientists (WFS) andSwiss Fonds Kidagan, Armenia; Conselho Nacionalde Desenvolvimento Científico e Tecnológico(CNPq), Fi-nanciadorade Estudose Projetos(FINEP),Fundação de Amparoà Pesquisa do Estado de São Paulo (FAPESP); NationalNatural Sci-enceFoundation ofChina (NSFC),the Chinese Ministryof Educa-tion(CMOE)andtheMinistryofScienceandTechnology ofChina (MSTC); Ministry ofEducation and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Founda-tionandtheDanish NationalResearchFoundation;The European ResearchCouncilundertheEuropeanCommunity’sSeventh Frame-work Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Re-gion Alsace’, ‘Region Auvergne’ and CEA, France; German Bun-desministerium fur Bildung, Wissenschaft, Forschung und Tech-nologie(BMBF)andtheHelmholtzAssociation;GeneralSecretariat forResearchandTechnology,MinistryofDevelopment,Greece; Na-tionalResearch,DevelopmentandInnovationOffice(NKFIH), Hun-gary;DepartmentofAtomicEnergyandDepartmentofScienceand TechnologyoftheGovernmentofIndia;IstitutoNazionalediFisica Nucleare(INFN) andCentro Fermi– MuseoStorico della Fisica e CentroStudieRicerche“EnricoFermi”,Italy;JapanSocietyforthe PromotionofScience(JSPS)KAKENHIandMEXT,Japan;Joint Insti-tuteforNuclearResearch,Dubna;NationalResearchFoundationof Korea(NRF);ConsejoNacionaldeCienca yTecnologia(CONACYT), Direccion General de Asuntos del Personal Academico (DGAPA), México,AmeriqueLatineFormationacademique–European Com-mission (ALFA-EC) and the EPLANET Program (European Particle PhysicsLatinAmericanNetwork);StichtingvoorFundamenteel On-derzoekderMaterie(FOM)andtheNederlandse Organisatievoor WetenschappelijkOnderzoek(NWO),Netherlands;Research Coun-cilofNorway(NFR); NationalScience Centre,Poland;Ministryof NationalEducation/InstituteforAtomicPhysicsandNational Coun-cil of Scientific Research in Higher Education (CNCSI–UEFISCDI), Romania;MinistryofEducationandScienceofRussianFederation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovation and The Russian Foundation for Basic Research; Ministry of Educa-tionofSlovakia; DepartmentofScience andTechnology, Republic ofSouth Africa, South Africa; Centrode Investigaciones Energet-icas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA), Ministerio de Economía yCompetitividad (MINECO) of Spain, Xunta de Galicia (Conselleríade Educación), Centro de AplicacionesTecnológicasy DesarrolloNuclear(CEADEN),Cubaenergía,Cuba, andIAEA (Inter-national Atomic Energy Agency); Swedish Research Council (VR) andKnut&AliceWallenbergFoundation(KAW);UkraineMinistry ofEducationandScience;UnitedKingdomScienceandTechnology FacilitiesCouncil(STFC);TheUnited StatesDepartmentofEnergy, theUnitedStatesNationalScienceFoundation,the StateofTexas, andtheStateofOhio;MinistryofScience,EducationandSportsof CroatiaandUnitythroughKnowledgeFund,Croatia;Councilof Sci-entificandIndustrial Research(CSIR),New Delhi,India;Pontificia UniversidadCatólicadelPerú.
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