Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Probe the QCD phase boundary with elliptic flow in relativistic heavy
  ion collisions at STAR   We present measurement of elliptic flow, $v_2$, for charged and identified
particles at midrapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7 - 39 GeV.
We compare the inclusive charged hadron $v_2$ to those from transport model
calculations, such as UrQMD model, AMPT default model and AMPT string-melting
model. We discuss the energy dependence of the difference in $v_2$ between
particles and anti-particles. The $v_2$ of $\phi$ meson is observed to be
systematically lower than other particles in Au+Au collisions at
$\sqrt{s_{NN}}$ = 11.5 GeV.

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The categories of this paper are: adap-org
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The categories of this paper are: stat
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The categories of this paper are: supr-con
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The categories of this paper are: ao-sci
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The categories of this paper are: solv-int
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: q-alg
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The categories of this paper are: quant-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-fin
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The categories of this paper are: bayes-an
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-th
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The categories of this paper are: q-bio