We present high precision measurements of elliptic flow near midrapidity ($|y|<1.0$) for multi-strange hadrons and $phi$ meson as a function of centrality and transverse momentum in Au+Au collisions at center of mass energy $sqrt{s_{NN}}=$ 200 GeV. W
e observe that the transverse momentum dependence of $phi$ and $Omega$ $v_{2}$ is similar to that of $pi$ and $p$, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0-30$%$ and 30-80$%$ collision centrality. There is an indication of the breakdown of previously observed mass ordering between $phi$ and proton $v_{2}$ at low transverse momentum in the 0-30$%$ centrality range, possibly indicating late hadronic interactions affecting the proton $v_{2}$.
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 (RH
IC). 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.
Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, $v_2{2}$ and $v_2{4}$, for charged hadr
ons from U+U collisions at $sqrt{s_{rm NN}}$ = 193 GeV and Au+Au collisions at $sqrt{s_{rm NN}}$ = 200 GeV. Nearly fully overlapping collisions are selected based on the amount of energy deposited by spectators in the STAR Zero Degree Calorimeters (ZDCs). Within this sample, the observed dependence of $v_2{2}$ on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. An initial-state model with gluon saturation describes the slope of $v_2{2}$ as a function of multiplicity in central collisions better than one based on Glauber with a two-component multiplicity model.
We present measurements of $pi^-$ and $pi^+$ elliptic flow, $v_2$, at midrapidity in Au+Au collisions at $sqrt{s_{_{rm NN}}} =$ 200, 62.4, 39, 27, 19.6, 11.5 and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{ch}$, based on data from
the STAR experiment at RHIC. We find that $pi^-$ ($pi^+$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $sqrt{s_{_{rm NN}}} = text{27 GeV}$ and higher. At $sqrt{s_{_{rm NN}}} = text{200 GeV}$, the slope of the difference of $v_2$ between $pi^-$ and $pi^+$ as a function of $A_{ch}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.
We report on measurements of dielectron ($e^+e^-$) production in Au$+$Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at RHIC. Systematic measurements of the dielectron yield as a function of trans
verse momentum ($p_{rm T}$) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region ($M_{ee}<$ 1 GeV/$c^2$). This enhancement cannot be reproduced by the $rho$-meson vacuum spectral function. In minimum-bias collisions, in the invariant-mass range of 0.30 $-$ 0.76 GeV/$c^2$, integrated over the full $p_{rm T}$ acceptance, the enhancement factor is 1.76 $pm$ 0.06 (stat.) $pm$ 0.26 (sys.) $pm$ 0.29 (cocktail). The enhancement factor exhibits weak centrality and $p_{rm T}$ dependence in STARs accessible kinematic regions, while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 $pm$ 0.10. Models that assume an in-medium broadening of the $rho$ meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of $omega$ and $phi$-meson production through their $e^+e^-$ decay channel. These measurements show good agreement with Tsallis Blast-Wave model predictions as well as, in the case of the $phi$-meson, results through its $K^+K^-$ decay channel. In the intermediate invariant-mass region (1.1$<M_{ee}<$ 3 GeV/$c^2$), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed.
We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in $p^uparrow+p$ collisions at RHIC. These correlations directly probe quark transversity distributions. We
measure signals in excess of five standard deviations at high transverse momenta, at high pseudorapidities eta>0.5, and for pair masses around the mass of the rho-meson. This is the first direct transversity measurement in p+p collisions. Comparing the results to data from lepton-nucleon scattering will test the universality of these spin-dependent quantities.
We report the measurement of the leptonic ($e^{+}e^{-}$) decay channel of the $phi$(1020) meson in Au+Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV by the STAR experiment. The transverse momentum ($p_{rm T}$) spectrum is measured for 0.1 $le p_{rm T}
le 2.5$ GeV/$c$ at mid-rapidity ($|y|le1$). We obtain the $p_{rm T}$-integrated $phi$(1020) mass $M_{phi}=1017.7pm0.8 (rm {stat.}) pm0.9 (rm {sys.})$ MeV/$c^{2}$ and width $Gamma_{phi} = 8.0pm 2.5(rm {stat.}) pm 2.3(rm {sys.}) textrm{MeV/}c^{2}$, which are within 1.5,$sigma$ and 1.1,$sigma$ of the vacuum values, respectively. No significant difference is observed in the measured $p_{rm T}$ spectrum, $dN/dy$, or $left<p_{rm T}right>$ of the $phi$(1020) meson between the $e^{+}e^{-}$ and hadronic ($K^{+}K^{-}$) decay channels as measured by the same experiment. The experimental results are compared to a theoretical model including medium-modified $phi$(1020).
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 a
t $sqrt{s_{NN}}$ = 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.
Dihadron correlations are analyzed in $sqrt{s_{_{rm NN}}} = 200$ GeV $d$+Au collisions classified by forward charged particle multiplicity and zero-degree neutral energy in the Au-beam direction. It is found that the jetlike correlated yield increase
s with the event multiplicity. After taking into account this dependence, the non-jet contribution on the away side is minimal, leaving little room for a back-to-back ridge in these collisions.
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 c
ollision 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 $Kpi$, $ppi$, 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 $rm u_{dyn}$ was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the $Kpi$, $ppi$, and $Kp$ pairs. The energy dependences of these fluctuations from central 0-5% Au+Au collisions all demonstrate a smooth evolution with collision energy.
