11 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 the measurements of proton-deuteron (𝑝-𝑑) and deuteron-deuteron (𝑑-𝑑) correlation functions in Au + Au collisions at $$\sqrt{s_{NN}}$$ = 3 GeV using fixed-target mode with the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). For the first time, the source size (𝑅_𝐺), scattering length (𝑓₀), and effective range (𝑑₀) are extracted from the measured correlation functions with a simultaneous fit. The spin-averaged 𝑓₀ for 𝑝-𝑑 and 𝑑-𝑑 interactions are determined to be -5.28 ± 0.11(stat.) ± 0.82(syst.) fm and -2.62 ± 0.02(stat.) ±0.24(syst.) fm, respectively. The measured 𝑝-𝑑 interaction is consistent with theoretical calculations and low energy scattering experimentmore » results, demonstrating the feasibility of extracting interaction parameters using the femtoscopy technique. The reasonable agreement between the experimental data and the calculations from the transport model indicates that deuteron production in these collisions is primarily governed by nucleon coalescence.« 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

We report on the charged-particle multiplicity dependence of net-proton cumulant ratios up to sixth order from $$\sqrt{s}$$ = 200 GeV p+p collisions at the Relativistic Heavy Ion Collider (RHIC). The measured ratios C₄/C₂, C₅/C₁, and C₆/C₂ decrease with increased charged-particle multiplicity and rapidity acceptance. Neither the Skellam baselines nor PYTHIA8 calculations account for the observed multiplicity dependence. In addition, the ratios C₅/C₁ and C₆/C₂ approach negative values in the highest-multiplicity events, which implies that thermalized QCD matter may be formed in p+p collisions.

This White Paper presents an overview of the current status and future perspective of QCD research, based on the community inputs and scientific conclusions from the 2022 Hot and Cold QCD Town Meeting. We present the This White Paper presents an overview of the current status and future perspective of QCD research, based on the community inputs and scientific conclusions from the 2022 Hot and Cold QCD Town Meeting. We present the progress made in the last decade toward a deep understanding of both the fundamental structure of the sub-atomic matter of nucleon and nucleus in cold QCD, and themore » hot QCD matter in heavy ion collisions. We identify key questions of QCD research and plausible paths to obtaining answers to those questions in the near future, hence defining priorities of our research over the coming decades.progress made in the last decade toward a deep understanding of both the fundamental structure of the sub-atomic matter of nucleon and nucleus in cold QCD, and the hot QCD matter in heavy ion collisions. We identify key questions of QCD research and plausible paths to obtaining answers to those questions in the near future, hence defining priorities of our research over the coming decades.« less

We report results on an elastic cross section measurement in proton–proton collisions at a center-of-mass energy $$\sqrt{s}$$ = 510 GeV, obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section is measured in the four-momentum transfer squared range 0.23 ≤ −t ≤ 0.67 GeV². This is the only measurement of the proton-proton elastic cross section in this t range for collision energies above the Intersecting Storage Rings (ISR) and below the Large Hadron Collider (LHC) colliders. We find that a constant slope B does not fit the datamore » in the aforementioned t range, and we obtain a much better fit using a second-order polynomial for B(t). This is the first measurement below the LHC energies for which the non-constant behavior B(t) is observed. The t dependence of B is also determined using six subintervals of t in the STAR measured t range, and is in good agreement with the phenomenological models. The measured elastic differential cross section dσ/dt agrees well with the results obtained at $$\sqrt{s}$$ = 540 GeV for proton–antiproton collisions by the UA4 experiment. We also determine that the integrated elastic cross section within the STAR t-range is σ$$^{fid}_{el}$$ = 462.1 ± 0.9(stat.) ± 1.1(syst.) ± 11.6(scale) μb.« less

A decisive experimental test of the Chiral Magnetic Effect (CME) is considered one of the major scientific goals at the Relativistic Heavy-Ion Collider (RHIC) towards understanding the nontrivial topological fluctuations of the Quantum Chromodynamics vacuum. In heavy-ion collisions, the CME is expected to result in a charge separation phenomenon across the reaction plane, whose strength could be strongly energy dependent. The previous CME searches have been focused on top RHIC energy collisions. In this Letter, we present a low energy search for the CME in Au+Au collisions at $$\sqrt{s_{NN}}$$ = 27 GeV. We measure elliptic flow scaled charge-dependent correlators relativemore » to the event planes that are defined at both mid-rapidity |η| < 1.0 and at forward rapidity 2.1 < |η| < 5.1. We compare the results based on the directed flow plane ( $$Ψ$$₁ ) at forward rapidity and the elliptic flow plane ( $$Ψ$$₂ ) at both central and forward rapidity. The CME scenario is expected to result in a larger correlation relative to $$Ψ$$₁ than to $$Ψ$$₂, while a flow driven background scenario would lead to a consistent result for both event planes. In 10-50% centrality, results using three different event planes are found to be consistent within experimental uncertainties, suggesting a flow driven background scenario dominating the measurement. We obtain an upper limit on the deviation from a flow driven background scenario at the 95% confidence level. This work opens up a possible road map towards future CME search with the high statistics data from the RHIC Beam Energy Scan Phase-II.« less