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We report first results from an analysis based on a new multi-hadron correlation technique, exploring jet-medium interactions and di-jet surface emission bias at the BNL Relativistic Heavy Ion Collider (RHIC). Pairs of back-to-back high-transverse-momentum hadrons are used for triggers to study associated hadron distributions. In contrast with two- and three-particle correlations with a single trigger with similar kinematic selections, the associated hadron distribution of both trigger sides reveals no modification in either relative pseudorapidity $\Delta \eta$ or relative azimuthal angle $\Delta \varphi$ from $\text{d}+\text{Au}$ to central $\text{Au}+\text{Au}$ collisions. We determine associated hadron yields and spectra as well as production rates for such correlated back-to-back triggers to gain additional insights on medium properties.

We report STAR results on the azimuthal anisotropy parameter $v_2$ for strange particles $K_S^0$, $\Lambda$, and $\overline{\Lambda }$ at midrapidity in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ at the Relativistic Heavy Ion Collider. The value of $v_2$ as a function of transverse momentum, $p_t$, of the produced particle and collision centrality is presented for both particles up to $p_t\sim 3.0\mathrm{G}\mathrm{e}\mathrm{V}/c$. A strong $p_t$ dependence in $v_2$ is observed up to $2.0\mathrm{G}\mathrm{e}\mathrm{V}/c$. The $v_2$ measurement is compared with hydrodynamic model calculations. The physics implications of the $p_t$ integrated $v_2$ magnitude as a function of particle mass are also discussed.

Two-particle correlation measurements projected onto two-dimensional, transverse rapidity coordinates $(y_{T1},y_{T2})$ provide an independent, orthogonal view of the multidimensional correlation distribution that is most often studied via angular projections. As such, these independent transverse projections allow access to manifestations of dynamical fluctuations in relativistic heavy-ion collisions that angular-correlation measurements may not be sensitive to. We report nonidentified charged-particle correlations for $\text{Au}+\text{Au}$ minimum-bias collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV taken by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Correlations are presented as two-dimensional functions of transverse rapidity for like-sign, unlike-sign, and all charged-particle pairs, as well as for particle pairs whose relative azimuthal angles lie on the near-side, the away-side, or at all relative azimuth. The correlations are constructed using charged particles with transverse momentum $p_T\ge 0.15$ $\mathrm{GeV}/c$, pseudorapidity from $-1$ to 1, and azimuthal angles from $-\pi$ to $\pi$. The significant correlation structures that are observed evolve smoothly with collision centrality. The major correlation features include a saddle shape plus a broad peak with maximum near $y_T\approx 3$, corresponding to $p_T\approx 1.5$ $\mathrm{GeV}/c$. The broad peak is observed in both like- and unlike-sign charge combinations and in near- and away-side relative azimuthal angles. The all-charge, all-azimuth correlation measurements are compared with the predictions of hijing and epos to provide theoretical context for these new measurements. The results indicate that the correlations for peripheral to mid-central collisions can be approximately described as a superposition of nucleon $+$ nucleon collisions with minimal effects from the quantum chromodynamics medium. Strong medium effects are indicated in mid- to most-central collisions.

We report a new measurement of the midrapidity inclusive jet longitudinal double-spin asymmetry, $A_{LL}$, in polarized $pp$ collisions at center-of-mass energy $\sqrt{s}=200\mathrm{GeV}$. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep-inelastic scattering (DIS), semi-inclusive DIS, and RHIC $pp$ data. The measured asymmetries provide evidence at the $3\sigma$ level for positive gluon polarization in the Bjorken-$x$ region $x>0.05$.

We present measurements of the differential cross sections of inclusive $J/\psi$ meson production as a function of transverse momentum ($p_T^{J/\psi }$) using the ${\mu }^{+}{\mu }^{-}$ and $e^{+}{e}^{-}$ decay channels in $\mathrm{proton}+\mathrm{proton}$ collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the ${\mu }^{+}{\mu }^{-}$ channel is for $0<p_T^{J/\psi }<9\mathrm{GeV}/c$ and rapidity range $|y^{J/\psi }|<0.4$, and that from the $e^{+}{e}^{-}$ channel is for $4<p_T^{J/\psi }<20\mathrm{GeV}/c$ and $|y^{J/\psi }|<1.0$. The $\psi (2S)$ to $J/\psi$ ratio is also measured for $4<p_T^{\text{meson}}<12\mathrm{GeV}/c$ through the $e^{+}{e}^{-}$ decay channel. Model calculations, which incorporate different approaches toward the $J/\psi$ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties.

We report the first measurement of the inclusive jet and the dijet longitudinal double-spin asymmetries, $A_{LL}$, at midrapidity in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s}=510\mathrm{GeV}$. The inclusive jet $A_{LL}$ measurement is sensitive to the gluon helicity distribution down to a gluon momentum fraction of $x\approx 0.015$, while the dijet measurements, separated into four jet-pair topologies, provide constraints on the $x$ dependence of the gluon polarization. Both results are consistent with previous measurements made at $\sqrt{s}=200\mathrm{GeV}$ in the overlapping kinematic region, $x>0.05$, and show good agreement with predictions from recent next-to-leading order global analyses.

We present the first measurement of charge-dependent directed flow in $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics model, which suggests that most of the electric charges, i.e., quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, $1\mathrm{fm}/c$.

Rapidity-odd directed-flow measurements at midrapidity are presented for $\mathrm{\Lambda }$, $\overline{\mathrm{\Lambda }}$, $K^{\pm }$, $K_s^0$, and $\varphi$ at $\sqrt{s_{NN}}=7.7$, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV in $\mathrm{Au}+\mathrm{Au}$ collisions recorded by the Solenoidal Tracker detector at the Relativistic Heavy Ion Collider. These measurements greatly expand the scope of data available to constrain models with differing prescriptions for the equation of state of quantum chromodynamics. Results show good sensitivity for testing a picture where flow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. The pattern of departure from a coalescence-inspired sum rule can be a valuable new tool for probing the collision dynamics.

Measurements of the elliptic flow, $v_2$, of identified hadrons (${\pi }^{\pm }$, $K^{\pm }$, $K_s^0$, $p$, $\overline{p}$, $\varphi$, $\Lambda$, $\overline{\Lambda }$, ${\Xi }^{-}$, ${\overline{\Xi }}^{+}$, ${\Omega }^{-}$, ${\overline{\Omega }}^{+}$) in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=7.7$, 11.5, 19.6, 27, 39, and 62.4 GeV are presented. The measurements were done at midrapidity using the time-projection chamber and the time-of-flight detectors of the Solenoidal Tracker at RHIC experiment during the beam-energy scan program at Relativistic Heavy Ion Collider. A significant difference in the $v_2$ values for particles and the corresponding antiparticles was observed at all transverse momenta for the first time. The difference increases with decreasing center-of-mass energy, $\sqrt{s_{\mathit{NN}}}$ (or increasing baryon chemical potential, ${\mu }_B$), and is larger for the baryons as compared to the mesons. This implies that particles and antiparticles are no longer consistent with the universal number-of-constituent quark (NCQ) scaling of $v_2$ that was observed at $\sqrt{s_{\mathit{NN}}}=200$ GeV. However, for the selected group of particles (${\pi }^{+}$, $K^{+}$, $K_s^0$, $p$, $\Lambda$, ${\Xi }^{-}$, ${\Omega }^{-}$) NCQ scaling at $(m_T-m_0)/n_q>0.4$ GeV/$c^2$ is not violated within $\pm 10\%$. The $v_2$ values for $\varphi$ mesons at 7.7 and 11.5 GeV are approximately two standard deviations from the trend defined by the other hadrons at the highest measured $p_T$ values.

We report measurements of the third harmonic coefficient of the azimuthal anisotropy, $v_3$, known as triangular flow. The analysis is for charged particles in Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV, based on data from the STAR experiment at the BNL Relativistic Heavy Ion Collider. Two-particle correlations as a function of their pseudorapidity separation are fit with narrow and wide Gaussians. Measurements of triangular flow are extracted from the wide Gaussian, from two-particle cumulants with a pseudorapidity gap, and also from event plane analysis methods with a large pseudorapidity gap between the particles and the event plane. These results are reported as a function of transverse momentum and centrality. A large dependence on the pseudorapidity gap is found. Results are compared with other experiments and model calculations.

We present the first measurement of midrapidity vector meson $\phi$ production in Au+Au collisions at RHIC $(\sqrt{s_{\mathrm{NN}}}=130\mathrm{}\hspace{0.5em}\mathrm{GeV})$ from the STAR detector. For the 11% highest multiplicity collisions, the slope parameter from an exponential fit to the transverse mass distribution is $T=379\mathrm{}\pm 50\left(\mathrm{stat}\right)\pm 45\left(\mathrm{syst}\right)\hspace{0.5em}\mathrm{MeV},$ the yield $dN/dy=5.73\mathrm{}\pm 0.37\left(\mathrm{stat}\right)\pm 0.69\left(\mathrm{syst}\right)$ per event, and the ratio $N_{\phi }{/N}_{h^{-}}$ is found to be $0.021\pm 0.001\left(\mathrm{stat}\right)\pm 0.004\left(\mathrm{syst}\right).\mathrm{}$ The measured ratio $N_{\phi }{/N}_{h^{-}}$ and T for the $\phi$ meson at midrapidity do not change for the selected multiplicity bins.

Yields, correlation shapes, and mean transverse momenta $p_T$ of charged particles associated with intermediate- to high-$p_T$ trigger particles ($2.5<p_T<10$ GeV/$c$) in $d+$ Au and Au $+$ Au collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV are presented. For associated particles at higher $p_T\gtrsim 2.5$ GeV/$c$, narrow correlation peaks are seen in $d+$ Au and Au $+$ Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $p_T<2$ GeV/$c$, a large enhancement of the near- ($\Delta \varphi ~0$) and away-side ($\Delta \varphi ~\pi$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au $+$ Au collisions compared to $d+$ Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $\Delta \varphi ~\pi$ in central Au $+$ Au collisions.

We report on new measurements of inclusive $J/\psi$ polarization at midrapidity in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$ by the STAR experiment at the Relativistic Heavy Ion Collider. The polarization parameters, ${\lambda }_{\theta }$, ${\lambda }_{\varphi }$, and ${\lambda }_{\theta \varphi }$, are measured as a function of transverse momentum ($p_{\mathrm{T}}$) in both the helicity and Collins-Soper (CS) reference frames within $p_{\mathrm{T}}<10\mathrm{GeV}/c$. Except for ${\lambda }_{\theta }$ in the CS frame at the highest measured $p_{\mathrm{T}}$, all three polarization parameters are consistent with 0 in both reference frames without any strong $p_{\mathrm{T}}$ dependence. Several model calculations are compared with data, and the one using the Color Glass Condensate effective field theory coupled with nonrelativistic QCD gives the best overall description of the experimental results, even though other models cannot be ruled out due to experimental uncertainties.

The balance function is a new observable based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for charged particle pairs and identified charged pion pairs in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ at the Relativistic Heavy Ion Collider using STAR. Balance functions for peripheral collisions have widths consistent with model predictions based on a superposition of nucleon-nucleon scattering. Widths in central collisions are smaller, consistent with trends predicted by models incorporating late hadronization.

The STAR Collaboration reports measurements of the transverse single-spin asymmetry (TSSA) of inclusive ${\pi }^0$ at center-of-mass energies ($\sqrt{s}$) of 200 GeV and 500 GeV in transversely polarized proton-proton collisions in the pseudo-rapidity region 2.7 to 4.0. The results at the two different energies show a continuous increase of the TSSA with Feynman-$x$, and, when compared to previous measurements, no dependence on $\sqrt{s}$ from 19.4 GeV to 500 GeV is found. To investigate the underlying physics leading to this large TSSA, different topologies have been studied. ${\pi }^0$ with no nearby particles tend to have a higher TSSA than inclusive ${\pi }^0$. The TSSA for inclusive electromagnetic jets, sensitive to the Sivers effect in the initial state, is substantially smaller, but shows the same behavior as the inclusive ${\pi }^0$ asymmetry as a function of Feynman-$x$. To investigate final-state effects, the Collins asymmetry of ${\pi }^0$ inside electromagnetic jets has been measured. The Collins asymmetry is analyzed for its dependence on the ${\pi }^0$ momentum transverse to the jet thrust axis and its dependence on the fraction of jet energy carried by the ${\pi }^0$. The asymmetry was found to be small in each case for both center-of-mass energies. All the measurements are compared to QCD-based theoretical calculations for transverse-momentum-dependent parton distribution functions and fragmentation functions. Some discrepancies are found, which indicates new mechanisms might be involved.

We report a new measurement of transverse single-spin asymmetries for dijet production in collisions of polarized protons at $\sqrt{s}=200\mathrm{GeV}$. Correlations between the proton spin and the transverse momenta of its partons, each perpendicular to the proton momentum direction, are probed at high $Q^2\approx 160{\mathrm{GeV}}^2$. Evidence for nonzero Sivers effects is measured for the first time in dijets from proton-proton collisions, but only when the jets are sorted by their net charge, which enhances the otherwise canceling opposite-sign $u$- or $d$-quark contributions to separate data samples. The resulting asymmetries are compared to recent theoretical calculations. Separately, the associated Sivers observable $⟨k_T⟩$, the average parton transverse momentum, is extracted using a simple kinematics approach which further enables a determination of the individual partonic contributions to the observed asymmetries.

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for $\mathrm{Au}+\mathrm{Au}$ and $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Strong short- and long-range correlations (LRC) are seen in central $\mathrm{Au}+\mathrm{Au}$ collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral $\mathrm{Au}+\mathrm{Au}$ collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.

The STAR Collaboration at the Relativistic Heavy Ion Collider presents measurements of $J/\psi \to e^{+}{e}^{-}$ at midrapidity and high transverse momentum ($p_T>5\hspace{0.5em}\mathrm{GeV}/c$) in $p+p$ and central $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV. The inclusive $J/\psi$ production cross section for $\mathrm{Cu}+\mathrm{Cu}$ collisions is found to be consistent at high $p_T$ with the binary collision-scaled cross section for $p+p$ collisions. At a confidence level of $97\%$, this is in contrast to a suppression of $J/\psi$ production observed at lower $p_T$. Azimuthal correlations of $J/\psi$ with charged hadrons in $p+p$ collisions provide an estimate of the contribution of $B$-hadron decays to $J/\psi$ production of $13\%\pm 5\%$.

In heavy-ion collision experiments, the global collectivity of final-state particles can be quantified by anisotropic flow coefficients $\left(v_n\right)$. The first-order flow coefficient, also referred to as the directed flow $\left(v_1\right)$, describes the collective sideward motion of produced particles and nuclear fragments in heavy-ion collisions. It carries information on the very early stage of the collision, especially at large pseudorapidity $\left(\eta \right)$, where it is believed to be generated during the nuclear passage time. Directed flow therefore probes the onset of bulk collective dynamics during thermalization, providing valuable experimental guidance to models of the pre-equilibrium stage. In 2018, the Event Plane Detector (EPD) was installed in STAR and used for the Beam Energy Scan phase-II (BES-II) data taking. The combination of EPD $(2.1<|\eta |<5.1)$ and high-statistics BES-II data enables us to extend the $v_1$ measurement to the forward and backward $\eta$ regions. In this paper, we present the measurement of $v_1$ over a wide $\eta$ range in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=$ 19.6 and 27 GeV using the STAR EPD. The results of the analysis at $\sqrt{s_{NN}}=$ 19.6 GeV exhibit excellent consistency with the previous PHOBOS measurement, while elevating the precision of the overall measurement. The increased precision of the measurement also revealed finer structures in heavy-ion collisions, including a potential observation of the first-order event-plane decorrelation. Multiple physics models were compared to the experimental results. Only a transport model and a three-fluid hybrid model can reproduce a sizable $v_1$ at large $\eta$ as was observed experimentally. The model comparison also indicates $v_1$ at large $\eta$ might be sensitive to the QGP phase transition.