5 Search Results

Rapidity-odd directed flow v₁ measurements are presented for $K^±$ and $$K^0_S$$ in Au + Au collisions for $$\sqrt{s_{NN}}$$ from 3.0 to 3.9 GeV with the STAR experiment. For comparison, v₁ of π^±, protons, and Λ from the same collisions are also discussed. The mid-rapidity v₁ slope dv₁/dy|_y=0 for protons and Λ is positive in these collisions. On the other hand, v₁ slope of kaons exhibits a strong dependence: negative at p_T < 0.6 GeV/c and positive at higher p_T. A similar p_T dependence is also evident for the v₁ slope of charged pions. Compared to the spectator-removed calculations in Au+Aumore » collisions at $$\sqrt{s_{NN}}$$ = 3.0–3.9 GeV, the JAM model demonstrates a pronounced shift of the v₁ slopes of mesons towards the negative direction. It suggests that the shadowing effect of the spectators plays an important role in the observed kaon anti-flow at low p_T in the high baryon density region of non-central collisions.« less

The correlation between the mean transverse momentum, [p_T], and the squared anisotropic flow, $$v^2_n$$, on an event-by-event basis has been suggested to be influenced by the initial conditions in heavy-ion collisions. We present measurements of the variances and covariance of [p_T] and $$v^2_n$$, along with their dimensionless ratio, for Au+Au collisions at various beam energies: $$\sqrt{s_{NN}}$$ = 14.6, 19.6, 27, 54.4, and 200 GeV. Our measurements reveal a distinct energy-dependent behavior in the variances and covariances. In addition, the dimensionless ratio displays a similar behavior across different beam energies. We compare our measurements with hydrodynamic models and similar measurements frommore » Pb+Pb collisions at the Large Hadron Collider (LHC). These findings provide valuable insights into the beam energy dependence of the specific shear viscosity (η/s) and initial-state effects, allowing for differentiating between different initial-state models.« less

The polarization of Λ, $$\overline{Λ}$$, $$Ξ$$^–, and $$\overline{Ξ}$$⁺ hyperons along the angular momentum of the system has been measured in isobar collisions of Ru+Ru and Zr+Zr at $$\sqrt{s_{NN}}$$ = 200 GeV with the STAR detector at RHIC. The polarization dependence on collision centrality exhibits an increasing trend in more peripheral collisions. Λ and $$\overline{Λ}$$ polarization dependence on the transverse momentum and pseudorapidity have been investigated, but no significant dependence was observed. The polarizations of Λ and $$\overline{Λ}$$ are found to be consistent with each other, indicating little contribution of the spin-magnetic coupling to the measured polarization. Comparison to previously measuredmore » polarization in Au+Au collisions show no obvious system size dependence. The results are qualitatively consistent with hydrodynamic calculations including contributions from shear-induced polarization and thermal vorticity. For the first time in heavy-ion collisions, the dependence of the global polarization on the hyperon’s emission azimuthal angle relative to the second-order event plane has been measured, indicating stronger polarization for the in-plane emitted hyperons at the level of 2.4σ significance in 20–50 % centrality. The $$Ξ$$ hyperon polarization measurements via polarization transfer analysis yield finite positive values with 2.9σ significance in 20–50 % centrality, slightly larger compared to the inclusive Λ polarization.« less

We report directed flow (v₁) of multistrange baryons ($$Ξ$$ and Ω) and improved v₁ data for K^–,$$\overline{p}$$, $$\overline{Λ}$$, and ϕ in Au+Au collisions at $$\sqrt{s_{NN}}$$ = 27 and 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). We focus on particles whose constituent quarks are not transported from the incoming nuclei but instead are produced in the collisions. At intermediate impact parameters, we examine quark coalescence behavior for particle combinations with identical quark content, and search for any departure from this behavior (“splitting”) for combinations having non-identical quark content. Under the assumption of quark coalescence formore » produced quarks, the splitting strength appears to increase with the electric charge difference of the constituent quarks in the combinations, consistent with electromagnetic effect expectations.« less