Rapidity dependence of the proton-to-pion ratio in Au+Au and p+p collisions at sqrt(s_NN) = 62.4 and 200 GeV

0.16 MB·English

by  BRAHMS Collaboration

Rapidity dependence of the proton-to-pion ratio in Au+Au and p+p collisions at
I
√s = 62.4 and 200 GeV
NN
I.G.Arsenej,3,I.G.Beardend,D.Beavisa,S.Bekelei,4,C.Besliuh,B.Budickc,H.Bøggildd,C.Chasmana,
C.H.Christensend,P.Christiansend,5,H.H.Dalsgaardd,R.Debbea,J.J.Gaardhøjed,K.Hagelf,H.Itoa,A.Jipah,
E.B.Johnsoni,8,C.E.Jørgensend,6,R.Karabowicze,N.Katryn´skae,E.J.Kima,i,7,T.M.Larsenj,J.H.Leea,
G.Løvhøidenj,Z.Majkae,A.Marcineke,M.J.Murrayf,i,J.Natowitzf,B.S.Nielsend,C.Nygaardd,D.Pali,
A.Ovillerj,R.Płanetae,F.Ramib,C.Ristead,O.Risteah,D.Ro¨hrichg,S.J.Sandersi,P.Staszele,1,T.S.Tveterj,
F.Videbæka,2,R.Wadaf,H.Yangg,9,Z.Ying,10,andI.S.Zgurah
0 aBrookhavenNationalLaboratory,Upton,NewYork11973
1 bInstitutPluridisciplinaireHubertCurienCRNS-IN2P3etUniversite´LouisPasteur,Strasbourg,France
0 cNewYorkUniversity,NewYork10003
2 dNielsBohrInstitute,Blegdamsvej17,UniversityofCopenhagen,Copenhagen2100,Denmark
eSmoluchowskiInst.ofPhysics,JagiellonianUniversity,Krako´w,Poland
n fTexasA&MUniversity,CollegeStation,Texas,77843
a gUniversityofBergen,DepartmentofPhysicsandTechnology,Bergen,Norway
J hUniversityofBucharest,Romania
8 iUniversityofKansas,Lawrence,Kansas66045
jUniversityofOslo,Dep.ofPhysics,Blindern,0316Oslo,Norway

]
x
e
-
l
c
Abstract
u
n
[ Theproton-to-pionratiosmeasuredintheBRAHMSexperimentforAu+Auandp+pcollisionsat √sNN =62.4and
200GeVarepresentedasafunctionoftransversemomentumandcollisioncentralityatselectedpseudorapiditiesin

2 therangeof0to3.8.Astrongpseudorapiditydependenceoftheseratiosisobserved.Wealsocomparethemagnitude
v andp -dependenceofthe p/π+ ratiosmeasuredinAu+Aucollisionsat √s =200GeVandη 2.2withthesame
8 T NN ≈
ratiomeasuredat √s =62.4GeVandη=0. Thegreatsimilarityfoundbetweentheseratiosthroughoutthewhole
2 NN
3 pT range(upto2.2GeV/c)isconsistentwithparticleratiosinA+Acollisionsbeingdescribedwithgrand-canonical
3 distributionscharacterizedbythebaryo-chemicalpotentialµ . Atthecollisionenergyof62.4GeV,wehaveobserved
B
. auniquepointinpseudorapidity,η = 3.2,wherethe p/π+ ratioisindependentofthecollisionsystemsizeinawide
0
1 pT-rangeof0.3 pT 1.8GeV/c.
≤ ≤
9
0 Keywords: heavyioncollision,particleratios,forwardrapidity,hadronization
: PACS:25.75.q,25.75.Dw,25.40.-h,13.75.-n
v
i
X
r 1. Introduction
a ITheBRAHMSCollaboration
1Correspondingauthor.Email:[email protected] The ongoing dialogue between theory and experi-
2Spokesperson.Email:[email protected] ment is delivering an increasingly clear picture of the
3Current Address: ExtreMe Matter Institute EMMI, GSI
QCDphasediagraminitspartonicandhadronicphases.
Helmholtzzentrum fu¨r Schwerionenforschung GmBH, Darmstadt,
Inparticular,recentmid-rapidityRHICresultsarecon-
Germany
4Current Address: Dept of Physics, Tennessee Tech Univer- sideredasevidenceforasmoothcrossoverfromadense
sity,Cookeville,Tennessee38505 and opaque partonic phase into a hadronic gas at tem-
5Current Address: Div. of Experimental High-Energy Physics,
peraturesaround 170 MeV and small values of baryo-
LundUniversity,Lund,Sweden
6RisøNationalLaboratory,Denmark chemical potential µB [1]. Measurements of the rela-
7CurrentAddress: DivisionofScienceEducation, ChonbukNa-
tionalUniversity,Jeonju,561-756,Korea
8Current Address: Radiation Monitoring Devices, Cambridge, Heidelberg,Germany
Massachusetts 10CurrentAddress:InstituteofParticlePhysics,HuazhongNormal
9Current Address: Physics Institute, University of Heidelberg, University,Wuhan,China
PreprintsubmittedtoElsevier January8,2010
tive abundances of hadronic species constrain statisti- denceof the ratiosisinvestigatedand comparedto the
cal modelsused to describe the chemicalfreeze-outin smallest system; p+p collisions. The comparison of
nucleus-nucleusinteractionsatdifferentenergies.Some the p/π+ measured at two differentenergiesand pseu-
of these models show a remarkable behavior at low dorapidities is used to establish a connection between
baryo-chemical potential where the curves of temper- particleabundancesinthesesystemsandtheirpossible
ature vs. baryo-chemicalpotential representing chem- descriptionby a grand-canonicaldistribution. We also
ical freeze-outtend to merge with the phase boundary presentan interestingfeaturein the pseudorapidityde-
betweenpartonicandhadronicmedia[2]. Insuchapic- pendenceofthe p/π+ ratioat √sNN =62.4GeV.There
ture,hadronsareproducedveryclosetofreeze-out,and the ratio has the same magnitude and p dependence
T
one could safely state that the features of the partonic atη = 3.2whenmeasuredatallAu+Aucentralitiesas
medium are transmitted to the final bulk hadrons via wellasp+pcollisions. Wecomparetheratiosextracted
hadronizationprocesses. Indeed,suchappearstobethe fromthemostcentralAu+Aueventstoahydrodynami-
case for the constituent quark scaling found in elliptic calmodelandtoapartoniccascademodelthatincludes
flow v measurements [3] as well as the enhancement hadronicre-scatteringinthefinalstate.
2
of baryon-to-mesonratios that scale with the collision
sizearoundmid-rapidity(lowµ )[4,5]. Thisincrease
B
2. Experimentalsetup
inthebaryon-to-mesonratios( 1fortheso-calledin-
≈
termediate p valuesrangingfrom2 to 6 GeV/c), first
T The BRAHMS detector setup [14] consists of two
reported by the PHENIX Collaboration [6], deviates
movable, small acceptance spectrometers: the Mid-
remarkablyfrom calculations that include parton frag-
Rapidity Spectrometer (MRS), which operates in the
mentation in the vacuum. The p -dependence of the
T polar angle interval from 90 θ 30 (correspond-
baryon-to-mesonratiointheintermediate pT rangeap- ing to a pseudorapidity inter◦va≥l of≥0 ◦η 1.3) and
pearstoberelatedtoamodificationofhadronizationin ≤ ≤
the Forward Spectrometer (FS), which operates in the
the partonic mediumcreated by the collision. This ef-
range from 15 θ 2.3 (2 η 4). The
fect could be caused by the different quark content of ◦ ≥ ≥ ◦ ≤ ≤
BRAHMS setup also includes detectors used to deter-
thebaryonsandmesons[7]orbecauseoftheirdifferent
mine global features of the collision such as the over-
massesthroughtheeffectofradialflow[8].
allchargedparticlemultiplicity,thecollisionvertexand
Both radial flow and in-medium quark coalescence
the centrality of the collision. The MRS is composed
areexpectedtoenhanceprotonsoverpionsatinterme-
of a single dipole magnet placed between two Time
diate p . Inparticular,thePHENIX p¯/π dataatmid-
T − Projection Chambers (TPC), which provide a momen-
rapidityis well describedby the Greco, Ko, and Levai
tummeasurement.Particleidentification(PID)isbased
quarkcoalescence model[9] where the introducedco-
on Time-of-Flight (TOF) measurements [14]. The FS
alescence involvespartonsfrom the medium (thermal)
has two TPCs, which are capable of track recognition
and partonsfrom mini-jets. The Hwa and Yang quark
in a high multiplicity environment close to the inter-
recombinationmodel[10]describeswelltheBRAHMS
action region, and, at the far end of the spectrometer,
and PHENIX p/π+ ratios around mid-rapidity. These
threeDriftChambers. Intheaggregate,theFScande-
mid-rapidityresultsareconsistentwithhadronizationin
tect particle track segments with high momentum res-
theintermediatep rangebeingdominatedbypartonre-
T olution (δp/p = 0.0008p at the highest field setting)
combinationwith negligiblefinal state interactionsbe-
usingthreedipolemagnets.Particleidentificationinthe
tweentheproducedhadrons. Incontrast,atforwardra-
FS is provided by TOF measurements in two separate
pidity(largeµ ),asignificantgapbetweenthetemper-
B hodoscopesforlow andmediumparticlemomenta,re-
atureofthetransitionfromthepartonictothehadronic
spectively. Highmomentumparticlesareidentifiedus-
phase,T ,andthetemperatureofchemicalfreeze-outis
c ingaRingImagingCherenkovdetector(RICH)[15].
predictedbyQCDlatticecalculation[11]. Inthatenvi-
ronment,hadronicre-scatteringmayplayan important
role,makingstatisticalmodelsmoresuitabletodescribe 3. Theanalysis
particleabundancesathighrapidity[12,13].
We present in this Letter the p/π ratios for both The analysis reported in this Letter consists of the
charges extracted from Au+Au collisions at different comparison of proton and pion yields as a function of
centralitiesaswellasp+pat √s =62.4and200GeV. p forseveralpseudorapidityintervals. Theanalysisis
NN T
The ratios are presented as function of pseudorapidity carriedoutinηversusp space,sinceforanygivenan-
T
andtransversemomentum p . Thesystemsize depen- gle and field configuration the acceptance is the same
T
2
for pions and protons. For a given η-p bin the p/π studying yields of tracks identified in the TOF detec-
T
ratiosarecalculatedonasettingbysettingbasis. Inor- toraspionsbuthavingnoassociatedringintheRICH.
dertoavoidmixingdifferentPIDtechniques,whichcan Thisstudywasdoneforalowmomentumrangewitha
leadto differentsystematic uncertainties,the ratiosare good kaon/pionseparation in TOF. It is found that the
calculatedseparatelyforPIDusingTOFandtheRICH pion efficiency grows rapidly from the pion threshold
detectors. In this way, all factors such as acceptance ( 2.3 GeV/c) and reaches a constant value of about
≈
corrections,trackingefficiencies, trigger normalization 97%above4GeV/c. TheRICH inefficiencyfoundfor
and bias related to the centrality cut cancel out in the pions is then applied to kaons (and the other species)
ratio.Theremainingspeciesdependentcorrectionsare: assumingthatCherenkovradiationdependsonlyonthe
γ factor of the particle. A more detailed description
(i) decays in flight and interactions with the beam
of the RICH inefficiency analysis is given in [15] and
pipeandothermaterial,and
a forthcoming publication. There are two sources of
(ii) thePIDefficiencycorrection.
systematicuncertaintiesassociatedwiththeRICHPID,
The corrections for (i) are determined from the sin- namely, the uncertaintyonthe RICH inefficiency,esti-
gleparticleresponseofpionsandprotonsinarealistic mated to be at the levelof 10%, andthe overlapin m2
GEANT [16] model description of the BRAHMS ex- betweenpionsandkaonshavingmomentaaboveabout
perimentalsetup. The magnitudeof this correction on 30GeV/c.
theparticleratiosdependsontheparticlemomentaand Nocorrectionswereappliedforweakdecays. How-
thespectrometerpositions,butdoesnotexceed6%. We everwe applycutson trackand eventvertexmatching
estimate that the overall systematic uncertainty related tolimittheeffectsoffeeddownonparticleyields. The
tothiscorrectionisatthelevelof2%. ranges of the vertex cuts are determined by the mea-
The TOF PID is done separately for small momen- suredspatialresolutionoftheparticletrackprojectionto
tumbinsbyfittingamulti-Gaussianfunctiontotheex- theeventvertex,suchthat97%ofprimarytracksarein-
perimentalsquaredmassm2distributionandapplyinga cluded.UsingAMPT[17]modelcalculationswefound
3σ cut to select a given particle type. For measure- thatthesevertexcutsleadtoremnantcontaminationof
±
mentsdonewiththeFSspectrometerinthemomentum proton and pion yields mainly due to Λ hyperons and
range where pions overlap with kaons (usually above K0 meson decays. At mid-rapidity we estimated that
s
3.5GeV/c),theRICHdetectorcanbeusedinvetomode this contamination leads to a 10%(7%) enhancement
to select kaons with momentum smaller than the kaon of p/π+(p¯/π ) at p = 3.0 GeV/c. This enhancement
− T
Cherenkovthreshold,whichisabout9GeV/c.Thispro- increases toward low transverse momenta and reaches
cedureleadsto a relativelyclean sample of pionswith 20%(14%) for p/π+(p¯/π ) at p = 1.5 GeV/c. The
− T
somecontaminationbykaonshavingspuriousringsas- levelofcontaminationdecreasesgraduallytowardsfor-
sociated in the RICH counter. Together with the kaon ward rapidities and at η 3 the enhancementreaches
-protonoverlapatlargermomenta,thiscontamination values of 6%(5%) for p/≈π+(p¯/π ) at p = 3.0 GeV/c
− T
effectisa sourceofsystematicerrorswhichhavebeen and 10%(7%) for p/π+(p¯/π ) at p = 1.5 GeV/c. We
− T
estimatedtobeinthe orderofa fewpercentforall p also found from the model calculations that the ex-
T
values,exceptforlow p inthep¯/π ratios,wherethese tracted contaminations are proportionalto the primary
T −
uncertaintiescan reach high values ( 15%). At mid- Λ/pratios(Λ¯/p¯ratiosfornegativespecies). Thusifthe
≈
rapidity the systematic uncertainty reaches a value of AMPT modelunderpredictsthe data by 20%then the
5%at p > 2.5GeV/cdueto thelimited kaonto pion respectivecorrectionsduetocontaminationsshouldbe
T
separationatlargemomenta. increasedbyapproximately20%.
TheRICHPIDisalsobasedontheparticleseparation
inthem2 versusmomentumspace. TheRICHprovides
4. Results
direct proton identification above the proton threshold
momentumwhichisabout15GeV/c. However,anad- Figure1shows p/π+ ratiosmeasuredinAu+Aucol-
ditionalprotonidentification scheme is possible below lisions at √s = 200 GeV in two centrality classes
NN
thisvaluebutabovethekaonthreshold.Inthismomen- of events, namely, 0 10%(solid dots) and 40 80%
− −
tumrangeaprotonisassociatedwithtrackshavingmo- (open squares). The centrality selection is based on
menta above the kaon threshold, but no RICH signal thecharged-particlemultiplicitymeasuredin therange
(veto mode). The veto proton yields are corrected for 2.2<η<2.2asdescribedin[18]. Theshadedboxes,
−
pion and kaon contaminationdue to a small RICH in- plotted for the most central events, represent the sys-
efficiency. The RICH inefficiency was determined by tematicuncertaintiesdiscussedin theprevioussection.
3
33..4455 η = 2.25 33..5544 η = 2.6 0000....7878 η = 2.25 0000....7878 η = 2.6
33 33 00..66 00..66
22..55 22..55 00..55 00..55
22 22 00..44 00..44
11..55 11..55 00..33 00..33
11 11 00..22 00..22
00..55 00..55 00..11 00..11
33..5544 00.. 55η =11 311..55.122 22..55 33 33..55 44 33..5544 00.. 55η =11 311..55.322 22..55 33 33..55 44 0000....7878 00.. 55η =11 311..55.122 22..55 33 33..55 44 0000....7878 00.. 55η =11 311..55.322 22..55 33 33..55 44
++ 33 33 -- 00..66 00..66
ππ 22..55 22..55 ππ 00..55 00..55
p /p /11..5522 11..5522 /p /p 0000....3434 0000....3434
11 11 00..22 00..22
00..55 00..55 00..11 00..11
33..4455 00.. 55η =11 311..55.522 22..55 33 33..55 44 33..5544 00.. 55η =11 311..55.822 22..55 330-331..550%44 0000....7878 00.. 55η =11 311..55.522 22..55 33 33..55 44 0000....7878 00.. 55η =11 311..55.822 22..55 330-331..550%44
33 33 40-80% 00..66 00..66 40-80%
22..55 22..55 p+p 00..55 00..55 p+p
22 22 00..44 00..44
11..55 11..55 00..33 00..33
11 11 00..22 00..22
00..55 00..55 00..11 00..11
00..55 11 11..55 22 22..55 33 33..55 44 00..55 11 11..55 22 22..55 33 33..55 44 00..55 11 11..55 22 22..55 33 33..55 44 00..55 11 11..55 22 22..55 33 33..55 44
pp [[GGeeVV//cc]] pp [[GGeeVV//cc]] pp [[GGeeVV//cc]] pp [[GGeeVV//cc]]
TT TT TT TT
Figure1:Centralitydependentp/π+ratiosforAu+Ausystemcollid- Figure2: Theratiosofp¯/π measuredinAu+Auandp+pcollisions
−
ingat √sNN = 200GeVforcentral(0 10%)andsemi-peripheral at √sNN =200GeV,areplottedforthesamecentralitysamplesused
−
(40 80%) reactions in comparison with p+p collision data at the inFig.1.
−
sameenergy. Theverticalbarsrepresentthestatisticalerrorsandthe
shadedboxes(plottedonlyforcentralAu+Au)showthesystematic
uncertainties.
the p/πratiovs. p shownosignificantdependenceon
T
theisospinoftheinitialNNsystem. Thusthedifference
betweenp+pandAu+Auisnotanisospineffect.
However, they do not include the uncertainties related
to the weak decay contamination. The ratios extracted The values of the p¯/π− ratios plotted in Fig. 2 are
fromp+pdata atthesame energyareplottedforcom- significantlylowerthanthe p/π+ ratios(notethediffer-
parison (solid triangles). The p coveragedependson ence in the vertical scale), however, the centrality de-
T
the pseudorapidity bins and for central Au+Au colli- pendence shows the same features as those observed
sionsextendsupto p = 4GeV/cforη = 2.6. Atlow in the p/π+ ratios, namely, that the ratios for different
T
p (< 1.5GeV/c)the p/π+ ratiosexhibitarisingtrend centrality classes are consistent with each other up to
T
withaweakdependenceoncentrality.Asignificantde- pT 1.2GeV/candastrongdependenceoncentrality
≈
pendence on centrality begins above 1.5 GeV/c. The appears at larger transverse momenta reaching a max-
ratios appear to reach a maximum value at pT around imum at similar pT as the positive particles. Looking
2.5GeV/cwhereverthe p coverageissufficienttode- atthep+pdataalone,onenotesthedifferenceinshape
T
terminethislimit. Themaximaoftheratiosaregreater between the p/π+ and p¯/π− ratios: a clear shift of the
for more central events and, at η = 3.1, are equal to p¯/π−peakstowardslowerpT,aswellasamuchbroader
about2.5 and 1.5 for the 0 10% and 40 80% cen- p/π+ peaks. The large difference between the Au+Au
tralitybins, respectively. Th−ep+pratiosar−e consistent and p+p both in shape and overall magnitude may re-
with Au+Au data at low p and begin to deviate sig- flectsignificantmediumeffectsinAu+Aucollisionsat
T
nificantly above pT = 1 GeV/c. At η = 3.1 a maxi- √sNN = 200 GeV in the pseudorapidityintervals cov-
mum value of the ratio of 0.55 is reached in p+p col- ered.
lisions which is a factor of 4.5 smaller than that ob- InFig. 3weexplorethepossibleconnectionbetween
servedforcentralAu+Aureactions.Wehaveperformed the measured p/π ratios in extended systems and the
PYTHIA[19]calculationsfornucleon-nucleoninterac- baryo-chemicalpotential µ used to characterize them
B
tionsineachpossibleisospinstate. Thecalculationsof as statistical systems. Such a connection is done by
4
η ≈ 2.67
10
1 1
+
π

/

p
10-1
centrality 0-10 %
10-1 Au+Au @ 62.4 GeV, η ≈ 0 0 η ≈ 03.5.2 1 1.5 2
Au+Au @ 200 GeV, η ≈ 2.2
10
p+p at 62.4 GeV, η ≈ 0
0.5 1 1.5 2 2.5 3 +
π
p [GeV/c]
T / 1 Au+cAeun tarat l itys := 0 6-120.4 % GeV
p centrality: 10-20 %
Figure3: p/π+ ratio inAu+Aucollisions forη = 0.0at √sNN = centrality: 20-40 %
62.4GeVmarkedwithopentriangles andtheAu+Aureactions for 10-1 centrality: 40-80 %
η=2.2at √sNN =200GeVmarkedwiththeblacktriangles. Stars p + p
shownreferencemid-rapiditydataforp+pat √sNN =62.4GeV.
0 η ≈ 03.5.5 1 1.5 2
10
comparingtwo rapidityrangesthathave the same p¯/p
ratio but different √s . These are η = 0 at 62 GeV
NN 1
and η = 2.2 at 200 GeV. The pseudorapidity intervals
selected for this comparison correspond to similar ob-
served p¯/pratiosofapproximately0.45. Theverysim- 10-1
ilar p¯/pratiosinthesetwosystemshasbeenattributed
to their havinga commonvalue of the baryo-chemical 0 0.5 1 1.5 2
potentialµB of 62 MeV [20, 21]. The similarity of p [GeV/c]
∼
proton-to-pionratiosfortheseselectedheavyioncolli- T
sionssuggeststhatthebaryonandmesonproductionat
the p intervalstudied(upto2GeV/c)isdominatedby Figure 4: p/π+ ratio from p+p and Au+Au collisions at √sNN =
T 62.4GeVforη=2.67(toppanel), η=3.2(middlepanel)andη=
mediumeffectsand is determinedbythe bulk medium
3.5(bottompanel).Thecurvedrawninthemiddlepanelisapolyno-
properties. The considerablylower valuesof the p/π+ mialfittothemostcentralevents,itspurposeistoguidetheeyeand
ratio measured in the p+p system at √s = 200 GeV, bridgethegapinpT .
shown with stars in Fig. 3, can also be construed as
strongindicationthatmediumeffectsare the sourceof
theobservedenhancementofthe p/πasfunctionof pT a universal shape for the p/π+ ratios implies, that for
inthenucleus-nucleuscollisions. eachcentralityatthispseudorapidity,thenuclearmodi-
ThethreepanelsofFig.4displaythe p/π+ ratioex- ficationfactorsforprotonsandpionsareconsistentwith
tracted from p+p and Au+Au collisions at √s = eachotheratallmeasuredvaluesof p . The observed
NN T
62.4GeV measured at 3 differentpseudorapiditybins; crossingof p/π+ ratiosfordifferentsizesofthecollid-
η = 2.67 (top panel), η = 3.2 (middle panel) and η ingsystemsisconsistentwithrecentBRAHMSresults
= 3.5 (bottom panel). At η = 2.67, p/π+ is greater onthecentralitydependenceofnet-protonrapiditydis-
in Au+Au than in p+p collisions while at η = 3.5 tributioninAu+Aureactionsat √sNN =200GeV[22].
the situation is reversed. The middle panel of Fig. 4 The data show an increased baryon transport towards
showsthe p/π+ ratiosfromAu+Auandp+pcollisions mid-rapidityincentralcollisions. Suchincreasedstop-
at √sNN =62.4GeVatη=3.2.This”crossingpoint”in pingpowerdissipatesthebeamenergytoformadenser
pseudorapidityshowsaremarkablycompleteoverlapof system where recombinationmechanismsfavor proton
theratiosasfunctionofp notonlyforp+pand0 10% production at intermediate values of p . Both effects
T T
centralAu+Aureactions,butalsoforotherAu+A−ucen- will produce higher values of the p/π+ ratio in larger
tralities,namely10 20%,20 40%and40 80%.Such systems. On the other hand, at very forwardrapidities
− − −
5
η ≈ 0 η ≈ 2.25 η ≈ 2.65 η ≈ 3.05 η ≈ 3.5
3 data 3 3 3 3
2.5 THERMINATOR2.5 2.5 2.5 2.5
π+ 2 AMPT 2 2 2 2

/1.5 1.5 1.5 1.5 1.5
p
1 1 1 1 1
0.5 0.5 0.5 0.5 0.5
1 1 1 1 1
0.8 0.8 0.8 0.8 0.8
π- 0.6 0.6 0.6 0.6 0.6
/
p
0.4 0.4 0.4 0.4 0.4
0.2 0.2 0.2 0.2 0.2
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
p [GeV/c]
T
Figure5:Rapidityevolutionofp/π+andp¯/π−for0−10%centralAu+Aureactionat √sNN =200GeV(solidpoints),andcalculationsfromthe
THERMINATOR(dashedline)andtheAMPTmodels(solidline).
(aroundoneunitbelowthebeamrapidity)abiggerfrac- ing a value of 2.5 at η 3.5 and p 3 GeV/c. In
T
≈ ≈
tionofthemeasuredparticlesareprotonsandtheirnum- contrast,the p¯/π ratio(bottomrow)decreaseswithin-
−
bersareevenhigherforlightersystemsduetoreduced creasingrapidity(from 1atη 0to0.4atη 3.5).
≈ ≈ ≈
stoppingpower. Thisobservationisconsistentwiththe Notethedifferentverticalscalesforpositiveandnega-
reversed p/π+ dependence on the system size seen in tivechargedparticles. Figure5comparesourresultsfor
the bottom panel. A crossing of p/π+ yields ratio at 0 10%centralAu+Aureactionsat √sNN = 200GeV
−
the energy of 62.4 GeV is predicted by UrQMD [23], to two modelcalculationsbased on: the THERMINA-
HIJING[24]andAMPTmodelcalculations. However, TOR model [25] (black dashed lines) and the AMPT
these models predictthe location of the crossing point model (solid lines). THERMINATOR is a 1+1-D hy-
in the interval 2 < y < 2.5, which is almost one unit drodynamic model that incorporates statistical particle
ofrapiditylowerthantheobservedvalue. Theseexper- production(includingexcitedstates),whichinturn,has
imental results then provide a strong constraint on the chemicalpotentialswithparameterizedrapiditydepen-
theoreticaldescriptionof baryonnumbertransportand dence. Alternatively,theAMPT(AMulti-PhaseParton
associated energy dissipation in relativistic nuclear re- Transport) model includes mini-jet parton production,
actions. Itisworthnotingthatat200GeV(seeFig. 1), partondynamics,hadronizationaccordingtotheLUND
evenforη=3.8,thep/π+ratiosmeasuredinAu+Aure- stringfragmentationmodel,andfinalstatehadronicin-
actionsarelargerthanthoseobservedinp+pcollisions, teractions in determining the final particle production.
sothepossiblecrossingpointathighenergyislocated In these simulations we duplicate experimental condi-
atlargerrapidity,beyondtheexperimentalacceptance. tionsregardingthe particle contaminationdue to weak
decays, the tracks were required to point to the event
vertexunderthesameconditionsasthoseappliedtothe
5. Modelcomparisons
experimentaldata.
To interpret these results, theoretical models of The THERMINATOR model tracks well the trends
nucleus-nucleuscollisionsareconfrontedwiththedata. of the data up to p 2.5 GeV/c for both positive
T
≈
The p/π+ ratio measured in central Au+Au collisions and negative particles. At η = 0 this model describes
at √sNN = 200 GeV, shownon the top row of Fig. 5, well the experimental p/π+ and p¯/π− ratios in the in-
hasastronggrowthwithrapidity;startingfromavalue termediate p range covered by the data (1 < p < 3
T T
of about 1.0 at η 0 and p 3 GeV/c and reach- GeV/c), however, it predicts the p¯/π ratios with val-
T −
≈ ≈
6
ues well above those measured at forward rapidities. the Polish Ministry of Science and Higher Education
Atlarge p (> 3 GeV/c)THERMINATORfails to de- (Contract no. 1248/B/H03/2009/36), and the Roma-
T
scribe the data. The mismatch is clearly seen in the nian Ministry of Education and Research (5003/1999,
η = 3.05 plot of Fig. 5, where the data have the best 6077/2000), and a sponsored research grant from Re-
p coverage. This mismatch is attributed to the fact naissanceTechnologiesCorp. Wethankthestaffofthe
T
that the modeldoes not include the productionof jets. Collider-AcceleratorDivisionandtheRHICcomputing
The AMPT model can qualitatively describe the pseu- facilityatBNLfortheirsupporttotheexperiment.
dorapiditytrends,butfailstoquantitativelydescribethe
data, namely, the model under predicts p/π+ and over
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7