8 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

High-energy, heavy-ion collisions can create local domains of chirality-imbalanced quarks, reflecting the topological features of quantum chromodynamics. The chiral magnetic effect (CME) predicts an electric charge separation of quarks in such topological domains along the magnetic field ($$\vec{B}$$) generated by the passing of two high-Z nuclei. Here, we use a correlation observable Δ⁢𝛾$$^{112}$$ between charged meson pairs to detect the CME-induced charge separation and a novel event shape selection (ESS) method to mitigate the background effects related to elliptic flow (𝑣₂). The ESS method classifies events based on the emission pattern of final-state particles and determines Δ⁢𝛾$$^{112}_{ESS}$$ from the zero-flowmore » limit. We reconstruct the $$\vec{B}$$ field direction from the spectator nucleons, which minimizes backgrounds unrelated to the collective motion of the system. In this work, we report the measurements of Δ⁢𝛾$$^{112}$$ and a background indicator Δ⁢𝛾$$^{132}$$ in Au+Au collisions from the Brookhaven National Laboratory Relativistic Heavy Ion Collider (RHIC) Beam Energy Scan phase II and at the top RHIC energy. After background suppression, Δ⁢𝛾$$^{132}_{ESS}$$ aligns with zero, and Δ⁢𝛾$$^{112}_{ESS}$$ is reduced to no more than 20% of Δ⁢𝛾$$^{112}$$. We observe a finite residual charge separation with 2.5⁢𝜎, 3⁢𝜎, and 3.2⁢𝜎 significance in the 20–50% centrality range of Au + Au collisions at 11.5, 14.6, and 19.6 GeV. The results at 17.3 and 27 GeV also show positive values but with a lower significance of 1.3⁢𝜎 and 1.1⁢𝜎, respectively. The corresponding Δ⁢𝛾$$^{112}_{ESS}$$ values at 7.7, 9.2, and 200 GeV are consistent with zero within uncertainties.« 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 welcome the comments provided by Dargent et al. (2018) and appreciate the effort they have made to evaluate our recently reported data on the stability of uranyl(VI) chloride complexes as function of temperature (Migdisov et al., 2018). We also appreciate the opportunity provided by the editor to clarify issues in our paper that were not clearly articulated or in error.

The speciation of U in NaCl-bearing solutions at temperatures up to 250 °C and concentrations of NaCl up to 1.5 m has been investigated using an in situ spectroscopic technique. The recorded spectra permit us to identify the species present in the solutions as UO₂²⁺, UO₂Cl⁺, and UO₂Cl₂°. UO₂Cl₃^- is also likely present at high temperatures and NaCl concentrations, but concentrations of this species are insufficient for derivation of the formation constants. No evidence was found for species of higher ligand (Cl-) number. Thermodynamic stability constants derived for these species show fair agreement with published data for 25 °C, butmore » differ significantly from those predicted by an earlier high-temperature study (Dargent et al., 2013), which suggested that UO₂Cl₄^2- and UO₂Cl₅^3- contribute significantly to the mass balance of uranyl chloride complexes, especially at high temperature. In contrast, our data suggest that the main uranyl-chloride complex present in aqueous solutions at T >150 °C and concentrations of NaCl relevant to natural hydrothermal systems is UO₂Cl₂°. The values of the logarithms of thermodynamic formation constants (β) for the reaction UO₂²⁺ + Cl^- = UO₂Cl⁺ are 0.02, 0.25, 0.55, 1.09, 1.59, and 2.28 derived at 25, 50, 100, 150, 200, and 250 °C, respectively. For the reaction UO₂²⁺ + 2Cl^- = UO₂Cl₂° the values of log β derived at these temperatures are 0.4, 0.58, 0.74, 1.44, 2.18, and 3.42. Values of the formation constant estimated for uranyl-chloride species predict the high concentrations of U observed by Richard et al. (2011) in fluid inclusions of the giant McArthur River unconformity-type uranium deposit.« less