Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split the $\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance ($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} + N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$ ($\bar{\Lambda}$) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of $\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive conclusions concerning the CME or the magnetic field, and $\Delta n$ has not been measured previously. Correlations among these observables may reveal new insights. This paper reports measurements of correlation between $\Delta n$ and $\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
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The elliptic ($v_2$) and triangular ($v_3$) azimuthal anisotropy coefficients in central $^{3}$He+Au, $d$+Au, and $p$+Au collisions at $\mbox{$\sqrt{s_{\mathrm{NN}}}$}$ = 200 GeV are measured as a function of transverse momentum ($p_{\mathrm{T}}$) at mid-rapidity ($|\eta|<$0.9), via the azimuthal angular correlation between two particles both at $|\eta|<$0.9. While the $v_2(p_{\mathrm{T}})$ values depend on the colliding systems, the $v_3(p_{\mathrm{T}})$ values are system-independent within the uncertainties, suggesting an influence on eccentricity from sub-nucleonic fluctuations in these small-sized systems. These results also provide stringent constraints for the hydrodynamic modeling of these systems.
v2 and v3 in 0-10% He+Au collisions at 200 GeV
v2 and v3 in 0-10% d+Au collisions at 200 GeV
v2 and v3 in UC p+Au collisions at 200 GeV
The polarization of $\Lambda$ and $\bar{\Lambda}$ hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}}$ = 200 GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild $p_T$ dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagree with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and $p_T$ dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.
$sgn(\alpha_H)\langle\cos(\theta_p^{\ast})\rangle$ of $\Lambda$ and $\bar{\Lambda}$ as a function of hyperon azimuthal angle relative to the second-order event plane in isobar collisions at 200 GeV.
$sgn(\alpha_H)\langle\cos(\theta_p^{\ast})\rangle$ of $\Lambda$ and $\bar{\Lambda}$ as a function of hyperon azimuthal angle relative to the third-order event plane in isobar collisions at 200 GeV.
$P_z$ sine coefficients of $\Lambda+\bar{\Lambda}$ as a function of centrality in isobar collisions at 200 GeV.
We measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion Collider. A significant $v_3$ signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at mid-rapidity, $dv_3/dy|_{(y=0)}$, opposite in sign compared to the slope for directed flow. No significant $v_3$ signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.
Event plane resolutions for calculating $v_3\{\Psi_1\}$ as a function of centrality from $\sqrt{s_{\textrm{NN}}}=3$ GeV Au+Au collisions at STAR.
Event plane resolutions for calculating $v_3\{\Psi_1\}$ as a function of centrality from $\sqrt{s_{\textrm{NN}}}=3$ GeV Au+Au collisions at STAR.
$v_3\{\Psi_1\}$ vs. centrality for $\pi^+$, $\pi^-$, and protons using the event plane method in $\sqrt{s_{\textrm{NN}}}=3$ GeV Au+Au collisions at STAR.
The longitudinal and transverse spin transfers to $\Lambda$ ($\overline{\Lambda}$) hyperons in polarized proton-proton collisions are expected to be sensitive to the helicity and transversity distributions, respectively, of (anti-)strange quarks in the proton, and to the corresponding polarized fragmentation functions. We report improved measurements of the longitudinal spin transfer coefficient, $D_{LL}$, and the transverse spin transfer coefficient, $D_{TT}$, to $\Lambda$ and $\overline{\Lambda}$ in polarized proton-proton collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment at RHIC. The data set includes longitudinally polarized proton-proton collisions with an integrated luminosity of 52 pb$^{-1}$, and transversely polarized proton-proton collisions with a similar integrated luminosity. Both data sets have about twice the statistics of previous results and cover a kinematic range of $|\eta_{\Lambda(\overline{\Lambda})}|$$<$ 1.2 and transverse momentum $p_{T,{\Lambda(\overline{\Lambda})}}$ up to 8 GeV/$c$. We also report the first measurements of the hyperon spin transfer coefficients $D_{LL}$ and $D_{TT}$ as a function of the fractional jet momentum $z$ carried by the hyperon, which can provide more direct constraints on the polarized fragmentation functions.
'$D_{LL}$ as a function of $\cos\theta^{*}$ at $0 < \eta_{\Lambda(\overline{\Lambda})} < 1.2$ and $3 < p_{T} < 4 GeV/c$'
'$D_{TT}$ as a function of $\cos\theta^{*}$ at $0 < \eta_{jet} < 1.0$ and $0.5 < z < 0.7$'
'$\Lambda$ $D_{LL}$ as a function of $p_{T}$ at $0 < \eta_{\Lambda(\overline{\Lambda})} < 1.2$'
We report the triton ($t$) production in mid-rapidity ($|y| <$ 0.5) Au+Au collisions at $\sqrt{s_\mathrm{NN}}$= 7.7--200 GeV measured by the STAR experiment from the first phase of the beam energy scan at the Relativistic Heavy Ion Collider (RHIC). The nuclear compound yield ratio ($\mathrm{N}_t \times \mathrm{N}_p/\mathrm{N}_d^2$), which is predicted to be sensitive to the fluctuation of local neutron density, is observed to decrease monotonically with increasing charged-particle multiplicity ($dN_{ch}/d\eta$) and follows a scaling behavior. The $dN_{ch}/d\eta$ dependence of the yield ratio is compared to calculations from coalescence and thermal models. Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10% most central collisions at 19.6 and 27 GeV, with a significance of 2.3$\sigma$ and 3.4$\sigma$, respectively, giving a combined significance of 4.1$\sigma$. The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation, and decreases with a smaller $p_{T}$ acceptance. The physics implications of these results on the QCD phase structure and the production mechanism of light nuclei in heavy-ion collisions are discussed.
Particle yield ratios at 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, and 200 GeV, 0%-10% centrality
Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at sqrt(s_NN)=130 GeV using the STAR TPC at RHIC. The elliptic flow signal, v_2, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.
Elliptic flow as a function of centrality defined as nch/nmax. Also given is epsilon, the initial space eccentricity of the overlap region, as well as the cumulative fraction of events starting with the most central. From the results of the study of non-flow contributions by different subevent selections and the maximum magnitudes of the first and higher-order harmonics, we estimate a systematic error for v2 of about 0.007, with somewhat smaller uncertainty for the mid-centralities where the resolution of the event plane is high.
Elliptic flow as a function of transverse momen-tum for minimum bias events
We present the first measurements of transverse momentum spectra of $\pi^{\pm}$, $K^{\pm}$, $p(\bar{p})$ at midrapidity ($|y| < 0.1$) in U+U collisions at $\sqrt{s_{NN}}$ = 193 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The centrality dependence of particle yields, average transverse momenta, particle ratios and kinetic freeze-out parameters are discussed. The results are compared with the published results from Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV in STAR. The results are also compared to those from A Multi Phase Transport (AMPT) model.
'Identified transverse momentum spectra of $\pi^{+}$ at midrapidity (|y| < 0.1) in U+U collisions at $\sqrt{s_{\rm NN}}$ = 193 GeV'
'Identified transverse momentum spectra of $K^{+}$ at midrapidity (|y| < 0.1) in U+U collisions at $\sqrt{s_{\rm NN}}$ = 193 GeV'
'Identified transverse momentum spectra of p at midrapidity (|y| < 0.1) in U+U collisions at $\sqrt{s_{\rm NN}}$ = 193 GeV'
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