| Abstract: | Measurements of the elliptic flow, $v_{2}$, of identified hadrons
($pi^{pm}$, $K^{pm}$, $K_{s}^{0}$, $p$, $ar{p}$, $phi$, $Lambda$,
$ar{Lambda}$, $Xi^{-}$, $ar{Xi}^{+}$, $Omega^{-}$, $ar{Omega}^{+}$)
in Au+Au collisions at $sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV
are presented. The measurements were done at mid-rapidity using the Time
Projection Chamber and the Time-of-Flight detectors of the STAR experiment
during the Beam Energy Scan program at RHIC. A significant difference in the
$v_{2}$ values for particles and the corresponding anti-particles was observed
at all transverse momenta for the first time. The difference increases with
decreasing center-of-mass energy, $sqrt{s_{NN}}$ (or increasing baryon
chemical potential, $mu_{B}$) and is larger for the baryons as compared to the
mesons. This implies that particles and anti-particles are no longer consistent
with the universal number-of-constituent quark (NCQ) scaling of $v_{2}$ that
was observed at $sqrt{s_{NN}}=$ 200 GeV. However, for the group of particles
NCQ scaling at $(m_{T}-m_{0})/n_{q}>$ 0.4 GeV/$c^{2}$ is not violated within
$pm$10%. The $v_{2}$ values for $phi$ mesons at 7.7 and 11.5 GeV are
approximately two standard deviations from the trend defined by the other
hadrons at the highest measured $p_{T}$ values. |
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