Systematics of Global Observables in Cu+Cu and Au+Au Collisions at RHIC Energies

0.08 MB·English

by  Rachid Nouicer

Systematics of Global Observables in Cu+Cu and
Au+Au Collisions at RHIC Energies
Rachid NOUICER for the PHOBOS Collaboration1
ChemistryDepartment,BrookhavenNationalLaboratory,Upton,NY11973-5000,USA
Abstract. Charged particles produced in Cu+Cu collisions at √s = 200 and 62.4 GeV have been
NN
measured in the PHOBOS experimentat RHIC. The comparison of the results for Cu+Cu and Au+Au
forthemostcentralcollisionsatthesameenergyrevealsthattheparticledensitypernucleonparticipant
pairandtheextendedlongitudinalscalingbehavioraresimilarinbothsystems.Thisimpliesthatforthe
mostcentraleventsin symmetricnucleus-nucleuscollisionsthe particle densityper nucleonparticipant
pair does not depend on the size of the two colliding nuclei but only on the collision energy. Also the
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extendedlongitudinalscalingseemsindependentofthecollidingenergyandspeciesforcentralcollisions.
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Inaddition,thereisanoverallfactorizationofdN /dh shapesasafunctionofcollisioncentralitybetween
0 ch
Au+AuandCu+Cucollisionsatthesameenergy.
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n Keywords: RHIC,QuarkGluonPlasma,ParticleDensity,SystemSize,Cu+Cu,Au+Au
a PACS: 25.75.-q,25.75.Dw,25.75.Gz
J
INTRODUCTION
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1
In relativistic heavy ion physics, it is well established that the global observables and their

1 spatial distributions are indicators for the global reaction dynamics and kinematics. They
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are very useful tools for event characterization. But one can also study their systematics
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2 as a function of beam energy and system size which may shed light on the onset of the
0 formation of a new state of deconfinement. Recently, the multiplicity of charged particles
1
0 produced in Cu+Cu collisions at √sNN = 200 and 62.4 GeV has been measured with the
6 PHOBOS detector at RHIC. These measurements provide an excellent opportunity to study
0 the systematics of charged particle multiplicityby comparing the results obtained in Cu+Cu
/
x and Au+Aucollisionsofthesameenergies.
e
The Cu+Cu data were collected using a silicon multiplicity array, which consists of an
-
l octagonal multiplicity detector and six forward silicon counters, three on each side of the
c
u interaction point. Two analysis methods: a “hit-counting” method and an “analog” method
n were used [1]. The measured dN /dh was corrected for particles which were absorbed
: ch
v or produced in the surrounding material and for feed-down products from weak decays of
i
X neutralstrangeparticles.Thecentralitydeterminationwasobtainedbyusingchargedparticles
r detectedintwosetsof16scintillatorscounterslocatedatz= 3.21metersfromthenominal
a ±
interactionpointalong thebeam axis.Themultiplicityarray,theanalysisprocedures andthe
centralitydeterminationused forCu+Cu andAu+Au collisionsarethesame.
1 Collaboration: B.Alver4, B.B.Back1, M.D.Baker2, M.Ballintijn4, D.S.Barton2, R.R.Betts6, R.Bindel7,
W.Busza4, Z.Chai2, V.Chetluru6, E.García6, T.Gburek3, K.Gulbrandsen4, J.Hamblen8, I.Harnarine6,
C.Henderson4, D.J.Hofman6, R.S.Hollis6, R.Hołyn´ski3, B.Holzman2, A.Iordanova6, J.L.Kane4, P.Kulinich4,
C.M.Kuo5, W.Li4, W.T.Lin5, C.Loizides4, S.Manly8, A.C.Mignerey7, R.Nouicer2, A.Olszewski3, R.Pak2,
C.Reed4, E.Richardson7, C.Roland4, G.Roland4, J.Sagerer6, I.Sedykh2, C.E.Smith6, M.A.Stankiewicz2,
P.Steinberg2,G.S.F.Stephans4,A.Sukhanov2,A.Szostak2,M.B.Tonjes7,A.Trzupek3,G.J.vanNieuwenhuizen4,
S.S.Vaurynovich4, R.Verdier4, G.I.Veres4, P.Walters8, E.Wenger4, D.Willhelm7, F.L.H.Wolfs8, B.Wosiek3,
K.Woz´niak3, S.Wyngaardt2, B.Wysłouch4. 1 Argonne National Laboratory, Argonne, IL 60439-4843, USA.
2BrookhavenNationalLaboratory,Upton,NY11973-5000,USA.3InstituteofNuclearPhysicsPAN,Kraków,
Poland.4 MassachusettsInstituteofTechnology,Cambridge,MA 02139-4307,USA. 5 NationalCentralUni-
versity,Chung-Li,Taiwan.6 UniversityofIllinoisatChicago,IL60607-7059,USA.7 UniversityofMaryland,
MD20742,USA.8 UniversityofRochester,Rochester,NY14627,USA.
300 300
0-6% (a) 25-35% 0-6% (b)
25-35%
250 Cu+Cu at 200 GeV 250 Cu+Cu at 62.4 GeV
6-15% 35-45% 6-15%
Preliminary Preliminary
200 15-25% 45-55% 200 15-25% 35-45%
hd hd
/h150 /h150
Nc Nc
d d
100 100
50 50
0 0
-4 -2 0 2 4 -4 -2 0 2 4
h h
5 5
4.5 AGS: Au+Au (c) 4.5 (d) AuAu 200
æ2 4 SPS: Pb+Pb æ2 4
/Npart3.5 PHOBOS: Au+Au /Npart3.5 CAuuACuu 123000 (Prelim.)
Æ/ 3 PHOBOS: Cu+Cu (Prel.) Æ/ 3
hd hd CuCu 62.4 (Prelim.)
/dNch2.52 -0.4+0.39·ln(s) fit /dNch2.52 AuAu 62.4
AuAu 19.6
1.5 1.5
1 1
0.5 0.5
0
0
1 10 102 103 -5 -4 -3 -2 -1 0 1 2
s |h | - y
NN beam
FIGURE 1. Panels a) and b): Measured dN /dh distributions of chargedparticles from Cu+Cu collisions
ch
at 200 and 62.4 GeV as a function of collision centrality (preliminary results). Shaded bands represent the
systematic errors. Panel c): Particle density per nucleon participant pair produced in central (6%) nucleus-
nucleus collisions as a function of energy from AGS, SPS and RHIC data [2, 3]. Panel d): Pseudorapidity
distributionsforAu+AuandCu+Cucentralcollisions(6%)atseveralRHICenergies[2, 4].Thedistributions
havebeenshiftedto h y inordertoeffectivelystudythefragmentationregionsintherestframeofone
beam
| |−
nucleus.
RESULTS AND CONCLUSIONS
ThedN /dh distributionsmeasuredforCu+Cucollisionsat200and62.4GeVarepresented
ch
in Figures 1.a) and b), respectively. We observe that there is a smooth transition between
the mid-rapidity plateau and the fragmentation region [2]. Figure 1.c) shows the charged
particle density per nucleon participant pair in the mid-rapidity region ( h 1) for A+A
| |≤
collisions from AGS to RHIC energies. The comparison of the results for Au+Au and
Cu+Cumostcentralcollisionsindicate,thatforthemostcentraleventsinsymmetricnucleus-
nucleus collisions the density per nucleon participant pair does not depend on the size of
the two colliding nuclei but only on the collision energy [2]. In the fragmentation region,
the comparison of dN /dh distributions per nucleon participants pair produced in Cu+Cu
ch
and Au+Au collisions at several energies are presented in Figure 1.d). We observe that the
Cu+Cu and Au+Au collisions exhibit the same extended longitudinal scaling for central
collisions[4].
Cu+Cu centrality 6-15% Cu+Cu centrality 25-35% 40 Cu+Cu centrality 45-55%
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150 RACuu(0-6%) * Au+Au(6-15%) RACuu (0-6%) * Au+Au(25-35%) RACuu(0-6%) * Au+Au(45-55%)
30
hd 60
/
h
c100
N 20
40
d
50
20 10
Preliminary Preliminary Preliminary
0 0 0
-4 -2 h0 2 4 -4 -2 h0 2 4 -4 -2 h0 2 4
FIGURE2. ComparisonofdN /dh distributionsforCu+CuandAu+Aucollisionsatthesamecentralityand
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energy(200GeV),presentedfordifferentcentralitybins[2]. ThedN /dh distributionsofAu+Aucollisions
ch
havebeenmultipliedby the ratioof the measureddN /dh distributionsof Cu+Cu central(6%) collisionsto
ch
themeasureddN /dh ofAu+Aucentral(6%)collisions.Forclarity,thesystematicerrorsarenotshown.
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For 0-6% central collisions we find that the multiplicity shapes are essentially identical
for Au+Au and Cu+Cu collisions, differing only by a factor which is approximately h -
independent[2].TheratioofdN /dh distributionsmeasuredforthetwosystems(Cu+Cuto
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Au+Au)for6%mostcentralcollisionswasusedtoscalethedN /dh distributionsmeasured
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in Au+Au collisions with different centralities. Figure 2 shows that dN /dh for Cu+Cu
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collisionsand scaleddN /dh forAu+Au systemare verysimilar,indicatingthat theshapes
ch
of pseudorapidity distributionsare the same for both systems and all centrality bins [2]. The
small difference at mid-rapidity can be related to the difference of the mean P of charged
T
particle in Cu+Cu and Au+Au collisionsbut it falls well within the systematicerrors. It thus
appears that the dN /dh shapes are independent of the overall size of the colliding nuclei
ch
at leastbetween theCu+Cu and Au+Au systemsstudiedhere.
In this talk, we have shown that the particle density and extended longitudinal scaling of
charged particles are similar in Cu+Cu and Au+Au for central collisionsat the same energy.
InRef.[4]wehaveshownthatthedN /dh distributionsareidenticalfor h 4forCu+Cu
ch
| |≤
andAu+Aucollisionscharacterized bythesame N .Inthepresentworkweobservethat
part
at200GeVtheshapesofdN /dh distributionsahresimiilarforthetwosystemsinthefull h
ch
| |
rangeforcollisionsinthesamecentralityclassasdefinedbythefractionofthetotalinelastic
cross section.
This work was partially supportedby U.S. DOE grants DE-AC02-98CH10886,DE-FG02-93ER40802,DE-
FC02-94ER40818,DE-FG02-94ER40865,DE-FG02-99ER41099,andW-31-109-ENG-38,byU.S.NSFgrants
960!3486,0072204,and0245011,byPolishKBN grant1-P03B-062-27(2004-2007),byNSCofTaiwanCon-
tractNSC89-2112-M-008-024andbyHungarianOTKAgrant(F049823).
REFERENCES
1. B.B.Backetal.,Phys.Rev.Lett, 87,102303(2001).
2. R.Nouicer,AmericanInstituteofPhysicsinpress(2005),arXiv:nucl-ex/0512044.
3. B.B.Backetal.,Phys.Rev.C(RapidComm.),submitted(2005),arXiv:nucl-ex/0512044.
4. G.Roland(forPHOBOSCollaboration)ProceedingsQM,(2005),arXiv:nucl-ex/0510042.