"fig5_b $\pi$" of "Disappearance of partonic collectivity in $\sqrt{s_{NN}}$ = 3 GeV Au+Au collisions at RHIC"

We report on the measurements of directed flow $v_1$ and elliptic flow $v_2$ for hadrons $(\pi^{\pm}, K^{\pm}, K_S^0, p, \phi, \Lambda$ and $\Xi^-)$ from Au+Au collisions at $\sqrt{s_{NN}} $ = 3GeV and $v_2$ for $(\pi^{\pm}, K^{\pm}$, $p$ and $\bar{p})$ at 27 and 54.4 GeV with the STAR experiment. While at the two higher energy midcentral collisions the number-of-constituent-quark (NCQ) scaling holds, at 3 GeV the $v_{2}$ at midrapidity is negative for all hadrons and the NCQ scaling is absent. In addition, the $v_1$ slopes at midrapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative $v_2$ and positive $v_1$ slope at 3 GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that the medium in such collisions is likely characterized by baryonic interactions.

本工作报道了利用STAR实验（STAR experiment）开展的重离子碰撞物理测量结果：在核子-核子质心系能量平方根√s_NN=3GeV的金金（Au+Au）碰撞中，针对强子（hadrons）π^±、K^±、K_S^0、质子p、φ介子、Λ超子与Ξ^-的有向流（directed flow）v₁与椭圆流（elliptic flow）v₂进行了测量；同时在27GeV与54.4GeV能量下，针对π^±、K^±、质子p与反质子p̄测量了其椭圆流v₂。在两种更高能量的半中心碰撞中，组分夸克数标度（number-of-constituent-quark scaling, NCQ）成立；但在3GeV能量下，快度中心区的所有强子的v₂均为负值，且不存在NCQ标度。此外，几乎所有观测到的强子在快度中心区的v₁斜率均为正值，这表明碰撞体系中存在占主导地位的排斥性重子相互作用。在输运模型（transport model）计算中，通过引入重子平均场，可以复现3GeV能量下v₂为负、v₁斜率为正的实验特征。上述结果表明，此类碰撞所形成的核介质的相互作用特征很可能由重子相互作用主导。