Probe the QCD phase boundary with elliptic flow in relativistic heavy ion collisions at STAR

0.35 MB·English

by  Shusu Shi

Cent. Eur. J.Phys.• 1-6
Authorversion
Central European Journal of Physics
Probe the QCD phase boundary with elliptic flow in
relativistic heavy ion collisions at STAR
Editorial
2
Shusu Shi1,2 ∗ (for the STAR collaboration)
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0
2 1 InstituteofParticlePhysics,CentralChinaNormalUniversity,Wuhan,Hubei,430079,China

n 2 TheKeyLaboratoryofQuarkandLeptonPhysics(CentralChinaNormalUniversity),MinistryofEducation,Wuhan,
a Hubei,430079,China
J

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1 Abstract: cWoellipsiroensesnattm√esaNsuNre=me7n.7t-of3e9llGipetVic.flWowe,cvo2m,pfoarrechtharegiendclaunsdiveidcehnatirfigeeddphaardtrioclnesva2ttmoitdhroaspeidfriotymintrAanus+pAorut
] model calculations, such as UrQMD model, AMPT default model and AMPT string-melting model. We
x
e discuss the energy dependence of the difference in v2 between particles and anti-particles.√The v2 of φ
- meson is observed to be systematically lower than other particles in Au+Au collisions at sNN = 11.5
l GeV.
c
u PACS (2008): 25.75.Ld,25.75.Dw
n
Keywords: relativisticheavyioncollision• ellipticflow• beamenergyscan
[
© VersitaWarsawandSpringer-VerlagBerlinHeidelberg.
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v
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1. Introduction
3
.
1
0
Searching for the region of a possible phase transition between the Quark Gluon Plasma (QGP) and the hadron
2
1
gas phase in the QCD phase diagram is one of the main goals of the Beam Energy Scan (BES) at RHIC. Due to
:
v
the sensitivity of underlying dynamics in the early stage of the collisions, the elliptic flow, v , could be used as a
i 2
X √
powerful tool [1]. In the top energy ( s = 200 GeV) of RHIC Au+Au and Cu+Cu collisions, the number of
r NN
a
constituent quark (NCQ) scaling in v reflects that the collectivity has been built up at the partonic stage [2–4].
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Especially,theNCQscalingofmulti-strangehadrons,φandΩ,providestheclearevidenceofpartoniccollectivity
because they are less sensitive to the late hadronic interactions [5, 6]. Further, a study based on a multi-phase
transport model (AMPT) indicates the NCQ scaling is related to the degrees of freedom in the system [7]. The
holdingoftheNCQscalingreflectsthepartonicdegreeoffreedom,whereasthebreakingofthescalingreflectsthe
hadronic degree of freedom. In reference [8], the importance of φ meson has been emphasized. Without partonic
∗ E-mail: [email protected]
1
ProbetheQCDphaseboundarywithellipticflowinrelativisticheavyioncollisionsatSTAR
phase. the φ meson v could be small or zero. Thus, the measurements of elliptic flow with the Beam Energy
2
Scan data offer us the opportunity to investigate the phase boundary in the QCD phase diagram.
(a1) 7.7 GeV (b1) 11.5 GeV (c1) 39 GeV
0.2 Fit lines 0.2 Au+Au, 20-30%, v2{4} 0.2
STAR data
0.15 0.15 AMPT default 0.15
AMPT SM (3 mb)
2 UrQMD
v 0.1 0.1 0.1
0.05 0.05 0.05
0 0 0
0 0.5 1 1.5 20 0.5 1 1.5 2 0 0.5 1 1.5 2
(a2) Ratio to AMPT def (b2) Ratio to AMPT def (c2) Ratio to AMPT def
2.5 2.5 2.5
o
2 2 2
i
t
a
1.5 1.5 1.5
R
1 1 1
0 0.5 1 1.5 20 0.5 1 1.5 20 0.5 1 1.5 2
p (GeV/c)
T
Figure 1. (Color online) The v2{4} as a function of pT for 20−30% in Au+Au collisions at √sNN = 7.7, 11.5 and 39 GeV
comparedtocorrespondingresultsfromUrQMD,AMPTdefaultversionandAMPTwithstringmeltingversion. The
bottompanelsshowtheratiosofSTARdataandtheresultsofAMPTString-MeltingtotheresultsofAMPTdefault.
DashlinesrepresentthefitlinesofafifthorderpolynomialfunctiontotheresultsofAMPTdefault.
In this paper, we present the v results of charged and identified hadrons from the STAR experiment in Au+Au
2
√
collisions at s = 7.7 - 39 GeV. STAR’s Time Projection Chamber (TPC) [9] is used as the main detector
NN
for event plane determination. The centrality was determined by the number of tracks from the pseudorapidity
region |η| ≤ 0.5. The particle identification for π±, K± and p (p) is achieved via the energy loss in the TPC
and the time of flight information from the multi-gap resistive plate chamber detector [10]. Strange hadrons are
reconstructed with the decay channels: K0 →π++π−, φ→K++K−, Λ →p+π− (Λ→p+π+), and Ξ− →
S
Λ + π− (Ξ+ → Λ+ π+)). The detailed description of the procedure can be found in Refs. [2, 3, 11]. The event
plane method [12] and cumulant method [13, 14] are used for the v measurement.
2
2. Results and Discussions
Figure1showstheresultsoftransversemomentum(p )dependenceofv {4}forchargedhadronsfrom20−30%
T 2
√
centralityclassinAu+Aucollisionsat s =7.7GeV(a1),11.5GeV(b1)and39GeV(c1). Toinvestigatethe
NN
partonicandhadroniccontributiontothefinalv resultsfromdifferentbeamenergy,transportmodelcalculations
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fromAMPT[15]andUrQMD[16]arecomparedwiththeSTARdatapresented. TheAMPTdefaultandUrQMD
modelsonlytakethehadronicinteractionsintoconsideration,whiletheAMPTString-meltingversionincorporates
2
ShusuShi, (fortheSTARcollaboration)
Λ-Λ
)
% p-p
( + -
K -K
)
X π+-π-
50
(
2
v
/
)
)
X
(
2
v
-
)
X
( 0
2
v
( STAR Preliminary
0 20 40 60
s (GeV)
NN
Figure2. (Coloronline)Thedifferenceofv2 forparticlesandanti-particles(v2(X)−v2(X))dividedbyparticlev2 (v2(X))asa
functionofbeamenergyinAu+Aucollisions(0-80%).
bothpartonicandhadronicinteractions. Largerthepartoncrosssectionindicateslaterthehadroncascadestarts.
The AMPT String-Melting model with a partonic cross section of 10 mb best describes 200 GeV data; while the
AMPT default version falls short by about 40% [15]. It suggests that the partonic interactions have to be
introduced for the v at 200 GeV. The comparison shows that UrQMD and AMPT model with default setting
2
√
underpredictthemeasurementsat s =39GeVformostofthep rangestudied,thedifferencesgetreduced
NN T
√
asthebeamenergydecreases. ThedatafromAu+Aucollisionsat s =7.7GeVisprettyclosetotheresults
NN
of AMPT default and UrQMD models when p is less than 1 GeV/c. For clarity we show the ratios of STAR
T
data and the results of AMPT String-Melting to the results of AMPT default. The STAR data is closer to the
AMPTdefaultandUrQMDmodelsinthelowerbeamenergy. Itindicatesthehadronicinteractionsbecomemore
dominant in the lower beam energy.
Figure 2 shows the excitation function for the relative difference of v between particles and anti-particles. In
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order to reduce the non-flow effect, the η-sub event plane method is used for the measurement. The η-sub event
plane method is similar to the event plane method, except one defines the flow vector for each particle based
on particles measured in the opposite hemisphere in pseudorapidity. An η gap of |η| < 0.05 is used between
negative/positiveη sub-eventtoguaranteethatnon-floweffectsarereducedbyenlargingtheseparationbetween
√
the correlated particles. The difference of v for baryons is within 10% at s = 39 and 62.4 GeV, while the
2 NN
√
difference increases as decreasing beam energy below 39 GeV. At s = 7.7 GeV, the difference of protons
NN
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ProbetheQCDphaseboundarywithellipticflowinrelativisticheavyioncollisionsatSTAR
0.1 11.5 GeV h -sub EP0.1 39 GeV
p + p
K0 L
s
f X -
q q
c 0.05 0.05
n c
n
⁄ /
2 2
v v
0 0
STAR preliminary
0 0.5 1 1.50 0.5 1 1.5
(m - m) ⁄ n (GeV/c2)
cq
T
Figure 3. (Color online) The number of constituent quark (ncq) scaled v2 as a function o√f transverse kinetic energy over ncq
((mT −m0)/ncq)forvariousidentifiedparticlesinAu+Au(0-80%)collisionsat sNN =11.5and39GeV.
versus anti-protons is around 60%. There is no obvious difference for π+ versus π− (within 3%) and K+ versus
√
K− (within2%)at s =39GeV.Asdecreasingbeamenergy,π+ versusπ− andK+ versusK− starttoshow
NN
thedifference. Thev ofπ− islargerthanthatofπ+ andthev ofK+ islargerthanthatofK−. Thisdifference
2 2
betweenparticlesandanti-particlesmightbeduetothebaryontransporteffecttomidrapidity[17]orabsorption
effect in the hadronic stage. The results could indicate the hadronic interaction become more dominant in lower
beamenergy. Theimmediateconsequenceofthesignificantdifferencebetweenbaryonandanti-baryonv isthat
2
√
the NCQ scaling is broken between particles and anti-particles when s < 39 GeV. Figure 3 shows the p
NN T
(cid:112)
differentialv fortheselectedidentifiedparticles. Thev andm −m (m = p2 +m2)aredividedbynumber
2 2 T 0 T T 0
√
of constituent quark in each hadron. The similar scaling behavior at s =200 GeV is observed in Au+Au
NN
√
collisions at s = 39 GeV. Especially, the φ mesons which are not sensitive to the later hadronic interactions
NN
followsthesametrendofotherparticles. Itsuggeststhatthepartonicdegreeoffreedomandcollectivityhasbeen
√ √
built up at s = 39 GeV. Whereas, at s = 11.5 GeV, the v for φ mesons falls off from other particles.
NN NN 2
√
Themeandeviationtothev ofpionsis2.6σ. Itindicatesthatthehadronicinteractionsaredominantat s
2 NN
= 11.5 GeV.
4
ShusuShi, (fortheSTARcollaboration)
3. Summary
In summary, we present the v measurements for charged hadrons and identified hadrons in Au+Au collisions
2
√
at s = 7.7 - 39 GeV. Comparison of charged hadron v is made with transport model calculations show
NN 2
agreement between data, UrQMD and AMPT models decreases as beam energy increases. The data at lower
beamenergyisclosertoAMPTdefaultandUrQMDmodels. Thecomparisonsuggeststhehadronicinteractions
are more dominant in the lower beam energy. The difference between the v of particles and anti-particles is
2
√
observed. The baryon and anti-baryon v show significant difference in s < 39 GeV. The difference of v
2 NN 2
between difference particles and anti-particles (pions, kaons, protons and Λs) increases with decreasing of the
√
beamenergy. Thev ofφmesonfallsofffromotherparticlesat s =11.5GeV.Experimentaldataindicates
2 NN
√
the hadronic interactions are dominant when s ≤ 11.5 GeV.
NN
4. Acknowledgments
ThisworkwassupportedinpartbytheNationalNaturalScienceFoundationofChinaundergrantNo. 11105060,
10775060 and 11135011.
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ProbetheQCDphaseboundarywithellipticflowinrelativisticheavyioncollisionsatSTAR
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