The Effects of Varying the Correlation Volume on Strangeness Production in High Energy Collisions

0.08 MB·English

by  Helen Caines

submittedtoactaphysicaslovaca 1–4
TheEffectsofVaryingtheCorrelationVolumeonStrangenessProductioninHighEnergy
Collisions
H.Caines1fortheSTARCollaboration
WNSL,PhysicsDepartment,YaleUniversity,NewHaven,CT06520,U.S.A.
6
Preliminary results on strange particle production versus collision centrality are presented.
0
0 STARmeasurementsfrom√sNN=200GeVheavy-ionandp-pcollisionsarecomparedto
2 SPSmeasurements. Asystematicstudyofstrangeparticleproductionispresentedwiththe
n aimof establishing how thecorrelation volume of theproduced source affectsthescale of
a strange particle creation, including that of the multi-strange baryons. A linear increase of
J strangenessproductionwithvolumehasbeensuggestedbythermalmodelsasanindication

0 thatthecollisionregionhasreachedsufficientsizesuchthatsmallvolumeeffectscanbene-
1
glected. AnalysisofpreliminaryresultsfromSTARshowthat,usingtheassumptionthatthe

numberofparticipantsislinearlycorrelatedwiththevolume,nosuchregimewasobtained.
1
v Thissuggeststhatthecorrelationvolume”seen”bystrangequarksisnotmerelythatofthe
4 initialoverlap.
1
0
1
PACS:...
0
6
Bystudyingthespectraandyieldsofthestrangeparticlesproducedinheavy-ioncollisionsand
0
/ comparingthemtothoseresultingfromelementaryp-pcollisions,inwhichaQGPphaseisnot
x
expected,we gaininsightinto the propertiesof themedium. While we cannotmeasurea state
e
- offreequarksandgluonsdirectly,indirectmethodscanbeusedtoprovideevidencethatsucha
cl phasedidexistandwhetherthestatewasinchemicaland/orthermalequilibrium.Itispredicted
u thatstrangenessissuppressedinp-pdueto a lackofphase space[1]. As thecollisionvolume
n increases,thissuppressionreduces.Therelevantvolumeisbelievedtobelinearlyproportionalto
:
v thenumberofparticipantsinthecollision, Npart . Calculationsalsoshowthatthep-psuppres-
h i
i sionincreaseswiththestrangenesscontentoftheparticleanddecreaseswithincreasedcollision
X
energy. Thescale ofthe suppressionis verysensitiveto the temperaturesof the producedsys-
r
a tems. For large systems this phase space suppression is removedand strangeness is produced
freely, resulting in yields that are proportional to the volume. It is then appropriate to apply
the Grand Canonical approach, where quantum numbers need only be conserved on average,
tocalculatetheyields. Experimentallythescaleofthesuppressionismeasuredviaa so-called
enhancementfactor:
E(i)=(Yield(i) /N )/(Yield(i) /N ) (1)
AA part pp part
1E-mailaddress:[email protected]
InstituteofPhysics,SAS,Bratislava,Slovakia 1
2 H.Cainesetal.
Detailedpredictionsof the phasespace suppressioneffectsdonotyetexistfor 200GeV colli-
sions.However,forcentraleventsusingT=177MeVandµ =29MeVenhancementsofE(Ξ−)=3.05
b
andE(Λ)=1.44arecalculated.ForT=170MeVandµ =29MeV,E(Ξ−)=4.5andE(Λ)=1.77[2].
b
WepresentherepreliminarydatatakenwiththeSTARexperimentfromAu-Auandp-pcollisions
at √sNN= 200 GeV. The pT spectra and yields have been analyzed as a functionof centrality
andaGlaubermodelisusedtocalculate N and N foreachcentralitybin[3].
part bin
h i h i
e to pp10 √sNN = 200 GeV e to pp10 STAR Preliminary RAA3.45 STAR PRELIMINARY
v v Au-Au: 0-5%
ati Λ ati Λ
el el + 3 p+p (TOF+RICH)
rNPart Ξ- rNPart Ξp 2.5 ΞΛ+Ξ
d/ d/
Yiel Yiel 2
1.5
1
1 11 binary scaling
0.5
participants scaling
0 1 2 3 4 5
p (GeV/c)
1 10 102 1 10 102 T
N N
Part Part
Fig. 1. Preliminary measured enhancements for Fig. 2. Preliminary measurements of the nuclear
√sNN=200GeVAu-Aucollisions.Theuncertainties modification factor for baryons. Thedotted curve
shownareacombinationofstatisticalandsystematic. representsthechargehadronmeasurement.
Fig. 1 shows the preliminary measured enhancementfactors from STAR. The p show a linear
scaling with N and almost no enhancement relative to p-p. Meanwhile, the Λ and the
part
h i
Ξ exhibit significant enhancements, even for the most peripheraldata. The expected ordering
with strangeness content is also observed, i.e. the Ξ are more enhanced than the Λ. The par-
ticles and their anti-particles show similar enhancements - reflecting the near-zero net baryon
number. However, neitherthe Λ norΞ show a linear scalingwith N , suggestingthatthe
part
h i
GrandCanonicalregimeisnotreachedevenincentralevents.Incontrast,calculationsofγ ,the
s
strangenesssaturationfactor,usingathermalmodelshowthatitapproachesunityforthemore
centralbinsfromfitstoSTARdata[4].ThesefitsthereforesupporttheuseoftheGrandCanon-
icalapproachandsuggestthatthestrangenessyieldsshouldscalewiththecollisionvolume. As
theseresultsarecontradictory,weconcludethattherelevantvolumeforstrangenessproduction
isnotlinearlyproportionalto N andhencenotpurelyrelatedtothatoftheinitialcollision
part
h i
overlap.ThisideaallowsfortheGrandCanonicalregimetohavebeenreached,assuggestedby
thermalmodelfitstogetherwithalackofproportionalityoftheyieldswith N .
part
h i
In order to explore how strangeness production in the central Au-Au data compares to p-p as
a function of p , we calculate the nuclear modification factor, R , for baryons. The pre-
T AA
liminary results are shown in Fig. 2. Previously reported measurementsof R (the ratio of
CP
central/peripheral),showthatallbaryonsfollowthesamecurveandaresuppressedathighp ,
T
theresultof the modificationofjets bythe medium[5]. OurmeasuredR , however,reveals
AA
EffectsofVolumeonStrangenessProduction. 3
N*N)qbin1.28 Kφs0 ΛΛ ΞΞ-+ pΩ-+Ω+ e to pp/Be103 NSTAA57R ((ofuplel)n) e to pp/Be103 PAbu--PAbu √√ssNNNN==1270 0 GGeeVV
eld/1.6 ativ ativ
Yi el el
+(N*s1.4 eld r102 eld r102
N)part1.2 Yi Yi
d)/(N*q 1 10 10
el
N*Yiu/d0.8
(0.6 STAR Preliminary 1 1
Au-Au √sNN=200 GeV STAR Preliminary
0.4 50 100 150 200 250 300 350 1 10 102 1 10 102
Npart dNch/dη dNch/dη
Fig.3. PreliminaryAu-Audatascaledasdefinedby Fig. 4. Preliminary RHIC and SPSstrange baryon
Eqn.2,excepttheφwhichisscaledby N .All yieldsrelativetothemeasuredp-pyieldsasafunc-
part
h i
dataarenormalizedtothemostcentralbin. tionofdNch/dη.OpensymbolsarefortheSPSdata
andclosedsymbolsfromSTAR.
a striking dependency on the strangeness content. This can be explained by considering that
R usesperipheraleventstodeterminethebaselinespectraandR usesp-pdata. Fig.1has
CP AA
already shown that the peripheral Au-Au yields are enhanced compared to scaled p-p. Thus,
R reveals a combined effect of suppressed p-p production, due to phase space effects, and
AA
suppressed Au-Au production, due to ”jet quenching”. The R measure only reveals the jet
CP
quenching.Whatisunexpectedisthatthesoftphysicsphasespacesuppressioninp-pappearsto
dominatetheR ratioouttop >3GeV/c.
AA T
Since the Au-Au strangeness data appear to show that N , or the initial nuclear overlap,
part
h i
is not the appropriate scaling volume for strangeness production we take a closer look at the
centrality dependence. Our goal is to try to determine if there is a measurable quantity that
isproportionalto the strangenessproductionvolume in whichcase the yieldsasa functionof
centralitywillscalelinearly.Fig.1showsthattheanti-protonalonescaleswith N .Acloser
part
h i
lookatthestrangebaryonsrevealsthatasstrangequarksareaddedtotheparticle,thescaledyield
deviatesmorestronglyfromalineardependenceon N andinfactcomescloserto N
part bin
h i h i
scaling. Thissuggeststhatthestrangequarkshavealarge N contributionwhereasthelight
bin
h i
u/d quarks scale with N . This idea is supported by R (Fig. 2) which shows that the
part AA
h i
strangebaryonsscale significantlyabovethatofsimple N scalingatintermediatep . We
bin T
h i
thereforeconsideranormalizationthatisdependentonthequarkcontentoftheparticle:
C =N N /N +N N /N (2)
scaling light part q s bin q
∗ ∗
whereN isthenumberofquarksintheparticle,N isthenumberoflight(uandd)quarks,
q light
and N is the numberof strange quarks. While this scaling is mostly successful, the φ, which
s
accordingtoEqn.2shouldscalewith N ,appearstoscalewith N . This N scaling
bin part part
h i h i h i
4 H.Cainesetal.
isusedinFig.3,whileallotherparticlesarescaledbyEqn.2. Itcanbeseenthatthescalingis
goodto 20%level.
∼
ThePHOBOScollaborationhasreportedastrongcorrelationbetweenN /dη and N [8].
ch part
h i
Atwocomponentfit
N /dη =n ((1 x) N /2 +x N /2 (3)
ch pp part bin
− h i h i
was used to fit the data. Where npp = 2.29 (1.27) at √sNN= 200 (19.7) GeV and is the mid-
rapidity charged particle yield in p(p)-p collisions. x represents the contribution to the yield
from hard processes. Independent fits to the data using Eqn. 3, leaving x as a free parame-
ter, determined that the 200 GeV and 19.6 GeV Au-Au data resulted in the same value of x.
A simultaneous fit gave x = 0.13 0.01 0.05. This suggests that the contributions from
± ±
hardprocessesareindependentofcollisionenergy,aresultnotpredictedbypQCDcalculations.
Building on the idea from PHOBOS, M. Lisa et al. [9] have recently reported that when the
measuredHBT radiiare plottedversusdN /dη1/3, a linearscaling isobserved. Theirreason-
ch
ingisthatHBTcorrelationmeasurementsarerelatedtothesourcesizeandthereforetheymust
havesomedependenceontheinitialgeometricaloverlap,i.e. N 1/3. This, however,does
part
not give a perfect scaling, which could be due to the fact thath N i 1/3 relates to the origi-
part
h i
nal overlap volume and HBT measurements resolve the sources final state. Since dN /dη is
ch
closelyrelatedtofinalstatedensity,thisvariable,orratherdN /dη1/3 whichgivesanapprox-
ch
imatelengthscale, shouldresultina strongercorrelation,andindeeditdoes. Fig. 4showsthe
strangenessyieldsscaled bythe p-p(Be) data fromSTAR andNA57 atthe SPS [7]asa func-
tionofdN /dη, estimatedusingEqn.3andthecalculatedvaluesof N and N . This
ch part bin
h i h i
resultsin an apparentenergyindependentscaling forthe strange anti-baryonswhich are linear
indN /dη. Eqn.3showsthatdN /dηcombinescontributionsfrom N and N which
ch ch part bin
h i h i
seemstobenecessarytodescribestrangenessproductioninA-Acollisions.
In summary, strange baryonproductionis not trivially related to the number of participantsin
the collision and the numberof binarycollisions seems to havea significant impacton the p
T
integrated yields. The measured R of these baryons suggests that the strong effect of the
AA
phase space suppression extends out to high p . Meanwhile, three independent observations
T
appeartoprovideevidencethatdN /dη,whichiscloselyrelatedtotheentropyofthesystem,is
ch
theunderlyingdriveformanyoftheglobalobservablesmeasuredinheavy-ioncollisions.
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