aa r X i v : . [ nu c l - e x ] S e p Event anisotropy v at STAR Shusu Shi a , b , c for the STAR Collaboration a Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA b Institute of Particle Physics, Huazhong Normal University, Wuhan, Hubei, 430079, China c The Key Laboratory of Quark and Lepton Physics (Huazhong Normal University), Ministry of Education, Wuhan,Hubei, 430079, China
Abstract
Collective flow reflects the dynamical evolution in high-energy heavy ion collisions. In partic-ular, the elliptic flow reflects early collision dynamics [1]. We present a systematic analysis ofelliptic flow ( v ) for identified particles measured in Au + Au and Cu + Cu collisions at √ s NN =
200 GeV. Number of quark scaling is tested in the intermediate p T region and in the smallersystem (Cu + Cu ). The Cu + Cu collisions results are compared with those from ideal hydrody-namic model calculations.
1. Introduction
When two nuclei collide in non-central heavy-ion collisions, their overlap area in the trans-verse plane has a short axis, which is parallel to the impact parameter, and a long axis, which isperpendicular to it. This initial spatial anisotropy of the overlap region of the colliding nuclei istransformed into an anisotropy in momentum space through interactions between the particles.The magnitude of this e ff ect is characterized by elliptic flow, defined as v = h cos 2( φ − Ψ R ) i (1)where φ is azimuthal angle of an outgoing particle, Ψ R is the azimuthal angle of the impactparameter, and angular brackets denote an average over many particles and events.The characterization of the elliptic flow of produced particles by their azimuthal anisotropyhas proven to be one of the more fruitful probes of the dynamics in Au + Au collisions at theRelativistic Heavy Ion Collider(RHIC) [2, 3, 4], see recent review in [5, 6, 7]. It can providemuch information about the degree of thermalization of the hot and dense medium. A systematicstudy of the p T dependence of v for di ff erent particle species enables investigation of underlyingphenomena and the properties of the produced matter.
2. Methods and Analysis
In this proceeding, we report v measurements by the STAR experiment from √ s NN = + Au and Cu + Cu collisions. Data were taken from Run 5 (2005) and Run 7 (2007).STAR’s Time Projection Chamber (TPC) [8] is used as the main detector for particle identifica-tion and event plane determination. The centrality was determined by the number of tracks fromthe pseudorapidity region | η | ≤ .
5. Two Forward Time Projection Chambers (FTPCs) were alsoused for event plane determinations. The FTPCs cover 2.5 ≤ | η | ≤
4. The pseudorapidity gap
Preprint submitted to Nuclear Physics A July 19, 2018 etween FTPC and TPC allows us to reduce some of the non-flow e ff ects. In Au + Au colli-sions, the di ff erence between v (TPC) (event plane determined by TPC tracks) and v (FTPC)(event plane determined by FTPC tracks) has been used to estimate the systematic errors, wherein Cu + Cu collisions, we used v (FTPC) for the measurement, v AA − pp (FTPC) [9] (subtractingthe residual non-flow e ff ects based on the azimuthal correlations in p + p collisions) for thesystematic study.The PID is achieved via dE / dx in TPC and topologically reconstructed hadrons: K S → π + + π − , φ → K + + K − , Λ → p + π − ( Λ → p + π + ), Ξ − → Λ + π − ( Ξ + → Λ+ π + ) and Ω − → Λ + K − ( Ω + → Λ+ K + ). The detailed description of the procedure can be found in Refs. [10, 11, 12].
3. Results and Discussions (GeV/c) T p v TopBottom
Hydro modelK ΛΞ v S0 K Λ Ξ Cu + Cu at 200 GeV
STAR Preliminary
Figure 1: The v as a function of p T for K S , Λ and Ξ in 0 - 60 % (top), 0 - 20 % and 20 - 60 % (bottom) Cu + Cucollisions at √ s NN =
200 GeV. Dashed lines represent ideal hydrodynamical calculation [14].
Results using ideal hydrodynamical calculations [13] have been able to reproduce mass or-dering of v in the low p T region in Au + Au collisions. Figure 1 shows the v for K S , Λ and Ξ as a function of p T in di ff erent centrality selections for Cu + Cu collisions along with resultsof hydrodynamical calculations [14]. We observe that v for Λ is smaller than v for K S for p T < / c. For p T > / c, v for Λ becomes larger than that of K S . We have also found Ξ has sizable v in minimum bias 0 - 60 % centrality. The ideal hydrodynamical model doesnot describe the centrality dependence of our data. For 0 - 20 %, the model under-predicts thedata and for 20 - 60 %, it over-predicts the v . E ff ects not included in the model which maybe relevant are geometrical fluctuations in the initial conditions (particularly important in centralcollisions), finite viscosity e ff ects and incomplete thermalization. It remains to be seen if thesee ff ects can account for the di ff erence between the models and data.2 q / n v ΛΞ Ω ΛΞ Ω π K φ π K φ STAR Preliminary K ΛΞ PHENIX π Kp ) (GeV/c q - m) / n T (m Figure 2: Number of quark scaling of v as a function of m T − mass in 0 - 80 % Au + Au (left) and 0 - 50 % Cu + Cu(right) collisions at √ s NN =
200 GeV. Green and gray bands show non-flow systematic errors for strange hadrons (( m T − mass ) / n q > . GeV / c ) and π , p (( m T − mass ) / n q > . GeV / c ) respectively. PHENIX results were taken from [19]. Quark coalescence [15] or recombination [16] mechanisms in particle production predictthat at intermediate p T (2 GeV / c < p T < / c ) number of quark (NQ) scaled v will followa universal curve. Thus, the NQ scaling is considered evidence for partonic degrees of freedomin Au + Au collisions at √ s NN =
200 GeV [12]. With the large statistics from Run 7, we can testthe scaling in the large p T region. Figure 2 shows the number of quark scaled v for identifiedparticles as a function of ( m T − mass ) / n q in Au + Au and Cu + Cu collisions at √ s NN =
200 GeV.Proton and Λ begin to deviate from the NQ scaling when ( m T − mass ) / n q > GeV / c in theAu + Au case. Scaling behavior can be seen in the smaller system (Cu + Cu ) at the same energy. A n i s o t r op y P a r a m e t e r v π Ωφ
200 GeV Au+Au M.B. collisions
STAR Preliminary (GeV/c) T Transverse Momentum p
Figure 3: v as function of p T for π , p (left) and φ , Ω (right) in Au + Au minimum-bias collisions at √ s NN =
200 GeV.Open symbols represent results from PHENIX [17]. Lines represent NQ-inspired fit [18].
Figure 3 shows the v for φ and Ω together with v for π and p as a function of p T in minimum3ias Au + Au collisions at √ s NN =
200 GeV. The p T dependence of v for π and p is observedto be similar as the corresponding results for Ω baryons and φ mesons. This indicates that theheavier s quarks flow as strongly as the lighter u and d quarks, providing evidence for partoniccollectivity.
4. Summary
In summary, we present the results from a systematic analysis of the identified particleselliptic flow ( v ) measurement from Au + Au and Cu + Cu collisions at √ s NN =
200 GeV. Idealhydrodynamic model calculations fail to reproduce the data in Cu + Cu collisions. Proton and Λ begin to deviate from the NQ scaling when ( m T − mass ) / n q > GeV / c in Au + Au collisions;scaling behavior can be seen in the smaller system (Cu + Cu ). The fact that the φ and Ω v ( p T )follows a similar trend as that of π and p indicates that the heavier s quarks flow as strongly asthe lighter u and d quarks suggesting partonic collectivity has been established at RHIC .
5. Acknowledgments
The author was supported in part by the National Natural Science Foundation of China undergrant no. 10775058, MOE of China under project IRT0624 and MOST of China under Grant No2008CB817707.
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