Azimuthal anisotropy of produced particles is one of the most important observables used to access the collective properties of the expanding medium created in relativistic heavy-ion collisions. In this paper, we present second ($v_{2}$) and third ($v_{3}$) order azimuthal anisotropies of $K_{S}^{0}$, $\phi$, $\Lambda$, $\Xi$ and $\Omega$ at mid-rapidity ($|y|<$1) in Au+Au collisions at $\sqrt{s_{\text{NN}}}$ = 54.4 GeV measured by the STAR detector. The $v_{2}$ and $v_{3}$ are measured as a function of transverse momentum and centrality. Their energy dependence is also studied. $v_{3}$ is found to be more sensitive to the change in the center-of-mass energy than $v_{2}$. Scaling by constituent quark number is found to hold for $v_{2}$ within 10%. This observation could be evidence for the development of partonic collectivity in 54.4 GeV Au+Au collisions. Differences in $v_{2}$ and $v_{3}$ between baryons and anti-baryons are presented, and ratios of $v_{3}$/$v_{2}^{3/2}$ are studied and motivated by hydrodynamical calculations. The ratio of $v_{2}$ of $\phi$ mesons to that of anti-protons ($v_{2}(\phi)/v_{2}(\bar{p})$) shows centrality dependence at low transverse momentum, presumably resulting from the larger effects from hadronic interactions on anti-proton $v_{2}$.
$v_{2}(p_{T})$ for $K_{S}^{0}$ (Centrality:0-10%)
$v_{2}(p_{T})$ for $K_{S}^{0}$ (Centrality:10-40%)
$v_{2}(p_{T})$ for $K_{S}^{0}$ (Centrality:40-80%)
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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We present measurements of $\Omega$ and $\phi$ production at mid-rapidity from Au+Au collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). Motivated by the coalescence formation mechanism for these strange hadrons, we study the ratios of $N(\Omega^{-}+\Omega^{+})/(2N(\phi))$. These ratios as a function of transverse momentum ($p_T$) fall on a consistent trend at high collision energies, but start to show deviations in peripheral collisions at $\sqrt{s_{NN}}$ = 19.6, 27 and 39 GeV, and in central collisions at 11.5 GeV in the intermediate $p_T$ region of 2.4-3.6 GeV/c. We further evaluate empirically the strange quark $p_T$ distributions at hadronization by studying the $\Omega/\phi$ ratios scaled by the number of constituent quarks. The NCQ-scaled $\Omega/\phi$ ratios show a suppression of strange quark production in central collisions at 11.5 GeV compared to $\sqrt{s_{NN}} >= 19.6$ GeV. The shapes of the presumably thermal strange quark distributions in 0-60% most central collisions at 7.7 GeV show significant deviations from those in 0-10% most central collisions at higher energies. These features suggest that there is likely a change of the underlying strange quark dynamics in the transition from quark-matter to hadronic matter at collision energies below 19.6 GeV.
Phi Meson Spectra.
Phi Meson Spectra.
Phi Meson Spectra.
A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy-ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical $K\pi$, $p\pi$, and $Kp$ fluctuations as measured by the STAR experiment in central 0-5\% Au+Au collisions from center-of-mass collision energies $\rm \sqrt{s_{NN}}$ = 7.7 to 200 GeV are presented. The observable $\rm \nu_{dyn}$ was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the $K\pi$, $p\pi$, and $Kp$ pairs. The energy dependences of these fluctuations from central 0-5\% Au+Au collisions all demonstrate a smooth evolution with collision energy.
$p\pi$, Kp, and $K\pi$ fluctuations as a function of collision energy, expressed as $v_{dyn,p\pi}$, $v_{dyn,Kp}$, and $v_{dyn,K\pi}$ respectively. Shown are data from central (0-5%) Au+Au collisions at energies from $\sqrt{s_{\rm NN}}$ = 7.7 to 200 GeV from the STAR experiment.
The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity $|y_{ee}|<1$ in minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened $\rho$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated dielectron excess yield at $\sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$ GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at $\sqrt{s_{NN}}$ = 17.3 GeV. For $\sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is higher than that at $\sqrt{s_{NN}}$ = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is longer than those in peripheral collisions and at lower energies.
Reconstructed dielectron unlike-sign pairs, like-sign pairs and signal distributions, together with the signal to background ratio (S/B). All columns are presented as a function of dielectron invariant mass in Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
Dielectron invariant mass spectrum in the STAR acceptance (|$y_{ee}$| < 1, 0.2 < $p_T^e$ < 3 GeV/c, |$\eta^e$ | < 1) after efficiency correction in Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
Hadronic cocktail consisting of the decays of light hadrons and correlated decays of charm in Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 GeV.
Two-particle azimuthal ($\Delta\phi$) and pseudorapidity ($\Delta\eta$) correlations using a trigger particle with large transverse momentum ($p_T$) in $d$+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV and 200~GeV from the STAR experiment at RHIC are presented. The \ns correlation is separated into a jet-like component, narrow in both $\Delta\phi$ and $\Delta\eta$, and the ridge, narrow in $\Delta\phi$ but broad in $\Delta\eta$. Both components are studied as a function of collision centrality, and the jet-like correlation is studied as a function of the trigger and associated $p_T$. The behavior of the jet-like component is remarkably consistent for different collision systems, suggesting it is produced by fragmentation. The width of the jet-like correlation is found to increase with the system size. The ridge, previously observed in Au+Au collisions at $\sqrt{s_{{NN}}}$ = 200 GeV, is also found in Cu+Cu collisions and in collisions at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV, but is found to be substantially smaller at $\sqrt{s_{{NN}}}$ =\xspace 62.4 GeV than at $\sqrt{s_{{NN}}}$ = 200 GeV for the same average number of participants ($ \langle N_{\mathrm{part}}\rangle$). Measurements of the ridge are compared to models.
Parameterizations of the transverse momentum dependence of the reconstruction efficiency of charged particles in the TPC in various collision systems, energies and centrality bins for the track selection cuts used in this analysis.
The raw correlation in $\Delta\eta$ for di-hadron correlations for 3 $<$ $p_T^{trigger}$ $<$ 6 GeV/$c$ and 1.5 GeV/$c$ $<$ $p_T^{associated}$ $<$ $p_T^{trigger}$ for 0-12% central \Au collisions for $|\Delta\phi|<$ 0.78 before and after the track merging correction is applied. The data have been reflected about $\Delta\eta$=0.
Sample correlations in $\Delta\eta$ ($|\Delta\phi|<$ 0.78) for 3 $<$ $p_T^{trigger}$ $<$ 6 GeV/$c$ and 1.5 GeV/$c$ $<$ $p_T^{associated}$ $<$ $p_T^{trigger}$ for 0-60% Cu+Cu at $\sqrt{s_{NN}}$ = 62.4 GeV, 0-80% Au+Au at $\sqrt{s_{NN}}$ = 62.4 GeV, 0-95% $d$+Au at $\sqrt{s_{NN}}$ = 200 GeV, 0-60% Cu+Cu at $\sqrt{s_{NN}}$ = 200 GeV, 40-80% Au+Au at $\sqrt{s_{NN}}$ = 200 GeV, and 0-12% central Au+Au at $\sqrt{s_{NN}}$ = 200 GeV. The data are averaged between positive and negative $\Delta\eta$. 5% systematic uncertainty due to track reconstruction efficiency not listed below.
We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, and 39 GeV using identified hadrons ($\pi^\pm$, $K^\pm$, $p$ and $\bar{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity ($|y|<$0.1) results for multiplicity densities $dN/dy$, average transverse momenta $\langle p_T \rangle$ and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.
The average number of participating nucleons (⟨Npart⟩) for various collision centralities in Au+Au collisions at √sNN = 7.7–39 GeV.
Midrapidity (|y| < 0.1) transverse momentum spectra for (b) π- in Au+Au collisions at √sNN = 7.7 GeV for different centralities. The spectra for centralities other than 0–5% are scaled for clarity as shown in the figure. The curves represent the Bose-Einstein, mT -exponential, and double-exponential function fits to 0–5% central data for pions, kaons, and (anti)protons, respectively. The uncertainties are statistical and systematic added in quadrature.
Midrapidity (|y| < 0.1) transverse momentum spectra for (a) π+ in Au+Au collisions at √sNN = 7.7 GeV for different centralities. The spectra for centralities other than 0–5% are scaled for clarity as shown in the figure. The curves represent the Bose-Einstein, mT -exponential, and double-exponential function fits to 0–5% central data for pions, kaons, and (anti)protons, respectively. The uncertainties are statistical and systematic added in quadrature.
We present the results of an elliptic flow analysis of Cu+Cu collisions recorded with the STAR detector at 62.4 and 200GeV. Elliptic flow as a function of transverse momentum is reported for different collision centralities for charged hadrons and strangeness containing hadrons $K_{S}^{0}$, $\Lambda$, $\Xi$, $\phi$ in the midrapidity region $|eta|<1.0$. Significant reduction in systematic uncertainty of the measurement due to non-flow effects has been achieved by correlating particles at midrapidity, $|\eta|<1.0$, with those at forward rapidity, $2.5<|\eta|<4.0$. We also present azimuthal correlations in p+p collisions at 200 GeV to help estimating non-flow effects. To study the system-size dependence of elliptic flow, we present a detailed comparison with previously published results from Au+Au collisions at 200 GeV. We observe that $v_{2}$($p_{T}$) of strange hadrons has similar scaling properties as were first observed in Au+Au collisions, i.e.: (i) at low transverse momenta, $p_T<2GeV/c$, $v_{2}$ scales with transverse kinetic energy, $m_{T}-m$, and (ii) at intermediate $p_T$, $2<p_T<4GeV/c$, it scales with the number of constituent quarks, $n_q$. We have found that ideal hydrodynamic calculations fail to reproduce the centrality dependence of $v_{2}$($p_{T}$) for $K_{S}^{0}$ and $\Lambda$. Eccentricity scaled $v_2$ values, $v_{2}/\epsilon$, are larger in more central collisions, suggesting stronger collective flow develops in more central collisions. The comparison with Au+Au collisions which go further in density shows $v_{2}/\epsilon$ depend on the system size, number of participants $N_{part}$. This indicates that the ideal hydrodynamic limit is not reached in Cu+Cu collisions, presumably because the assumption of thermalization is not attained.
Charged hadron azimuthal correlations as a function of pT in 0-60% Cu+Cu and p+p collisions at 200 GeV using TPC and FTPC flow vectors.
Charged hadron azimuthal anisotropy v2 as a function of pT in 0-60% Cu+Cu collisions at 200 GeV using TPC flow vectors, and those with subtracting the azimuthal correlations in p+p collisions.
Charged hadron azimuthal anisotropy v2 as a function of pT in 0-60% Cu+Cu collisions at 200 GeV using FTPC flow vectors, and those with subtracting the azimuthal correlations in p+p collisions.
We report results for $K/\pi$ fluctuations from Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. Our results for $K/\pi$ fluctuations in central collisions show little dependence on the incident energies studied and are on the same order as results observed by NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at $\sqrt{s_{NN}}$ = 12.3 and 17.3 GeV. We also report results for the collision centrality dependence of $K/\pi$ fluctuations as well as results for $K^{+}/\pi^{+}$, $K^{-}/\pi^{-}$, $K^{+}/\pi^{-}$, and $K^{-}/\pi^{+}$ fluctuations. We observe that the $K/\pi$ fluctuations scale with the multiplicity density, $dN/d\eta$, rather than the number of participating nucleons.
(Color online) The event-by-event $K/\pi$ ratio for 200 GeV Au+Au central collisions (0-5%) compared with the same quantity calculated from mixed events. The inset shows the ratio of the distribution from real events to that from mixed events. The errors shown are statistical.
(Color online) The event-by-event $K/\pi$ ratio for 200 GeV Au+Au central collisions (0-5%) compared with the same quantity calculated from mixed events. The inset shows the ratio of the distribution from real events to that from mixed events. The errors shown are statistical.
(Color online) Measured dynamical $K/\pi$ fluctuations in terms of σdyn for central collisions (0 - 5%) of 19.6, 62.4, 130, and 200 GeV Au+Au compared with the central collisions (0 - 3.5%) of Pb+Pb from NA49 [7] and the statistical hadronization (SH) model of Ref. [14]. The solid line represents the relationship of the incident energy dependence of $\sigma_{dyn}$ in central collisions to the collision centrality dependence of $\nu_{dyn,K\pi}$ at higher energies. Both statistical (vertical line with horizontal bar) and systematic (no vertical line) error bars are shown for the experimental data.
We present measurements of net charge fluctuations in $Au + Au$ collisions at $\sqrt{s_{NN}} = $ 19.6, 62.4, 130, and 200 GeV, $Cu + Cu$ collisions at $\sqrt{s_{NN}} = $ 62.4, 200 GeV, and $p + p$ collisions at $\sqrt{s} = $ 200 GeV using the dynamical net charge fluctuations measure $\nu_{+-{\rm,dyn}}$. We observe that the dynamical fluctuations are non-zero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate $1/N_{ch}$ scaling, but display approximate $1/N_{part}$ scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation.
(Color online) Dynamical net charge fluctuations, $\nu_{+−,dyn}$, of particles produced within pseudorapidity $|\eta|$ < 0.5, as function of the number of participating nucleons.
(Color online) Corrected values of dynamical net charge fluctuations ($\nu^{corr}_{+−,dyn}$) as a function of $\sqrt{s_{NN}}$. See text for details.
(Color online) Dynamical net charge fluctuations, $\nu_{+−,dyn}$, of particles produced with pseudorapidity $|\eta|$ < 0.5 scaled by (a) the multiplicity, $dN_{ch}/d\eta$. The dashed line corresponds to charge conservation effect and the solid line to the prediction for a resonance gas, (b) the number of participants, and (c) the number of binary collisions.
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