Search

We present results of a search for strange quark matter (strangelets) in $11.5A\mathrm{GeV}/c\mathrm{Au}+\mathrm{Pb}$ collisions from the 1994 and 1995 runs of experiment E864 at Brookhaven's Alternating Gradient Synchrotron. We observe no strangelet candidates and set a 90% confidence level upper limit of approximately $3\times {10}^{-8\mathrm{}}$ per 10% central interaction for the production of $|Z|=1\mathrm{}$ and $|Z|=2\mathrm{}$ strangelets over a large mass range and with metastable lifetimes of about 50 ns or more. These results place constraints primarily on quark-gluon plasma based production models for strangelets.

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 discuss the problem of observing spontaneous parity and $\mathrm{CP}$ violation in collision systems. We discuss and propose observables which may be used in heavy-ion collisions to observe such violations, as well as event-by-event methods to analyze the data. Finally, we discuss simple Monte Carlo models of these $\mathrm{CP}$-violating effects which we have used to develop our techniques and from which we derive rough estimates of sensitivities to signals that may be seen at the relativistic heavy-ion collider.

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.

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 $x>0.05$.

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 measurements from experiment E864 at the BNL-AGS of the yields of particle unstable light nuclei in central collisions of ${}^{197}\mathrm{Au}$ with beam momentum of $11.5A$ $\mathrm{GeV}/c$ on ${}^{197}\mathrm{Pt}.$ Yields are reported as a function of rapidity for the nuclei ${}^4\mathrm{H},$ ${}^4\mathrm{Li},$ ${}^5\mathrm{He},$ and ${}^5\mathrm{Li}$ in the rapidity range from $y_{\mathrm{c}.\mathrm{m}.\mathrm{}}$ to $y_{\mathrm{c}.\mathrm{m}.\mathrm{}}+0.8\mathrm{}$ and in the transverse momentum range of approximately $0.1<~p_T/A<~0.4\mathrm{GeV}/c.\mathrm{}$ The yields are compared to previously reported yields and trends for the production of stable light nuclei. The nonobservation of two excited states ${}^5{\mathrm{He}}_{16.75\mathrm{MeV}}^{*}$ and ${}^5{\mathrm{Li}}_{16.66\mathrm{MeV}}^{*}$ is used to set an upper limit on the yields of these states.

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.

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 $13\%\pm 5\%$.

We report the measurements of $\Sigma (1385)$ and $\Lambda (1520)$ production in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ from the STAR Collaboration. The yields and the $p_T$ spectra are presented and discussed in terms of chemical and thermal freeze-out conditions and compared to model predictions. Thermal and microscopic models do not adequately describe the yields of all the resonances produced in central $\mathrm{Au}+\mathrm{Au}$ collisions. Our results indicate that there may be a time span between chemical and thermal freeze-out during which elastic hadronic interactions occur.

Transverse momentum spectra of ${\pi }^{\pm }$, $p$, and $\overline{p}$ up to $12\mathrm{GeV}/c$ at midrapidity in centrality selected $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ are presented. In central $\mathrm{Au}+\mathrm{Au}$ collisions, both ${\pi }^{\pm }$ and $p(\overline{p})$ show significant suppression with respect to binary scaling at $p_T\gtrsim 4\mathrm{GeV}/c$. Protons and antiprotons are less suppressed than ${\pi }^{\pm }$, in the range $1.5\lesssim p_T\lesssim 6\mathrm{GeV}/c$. The ${\pi }^{-}/{\pi }^{+}$ and $\overline{p}/p$ ratios show at most a weak $p_T$ dependence and no significant centrality dependence. The $p/\pi$ ratios in central $\mathrm{Au}+\mathrm{Au}$ collisions approach the values in $p+p$ and $d+\mathrm{Au}$ collisions at $p_T\gtrsim 5\mathrm{GeV}/c$. The results at high $p_T$ indicate that the partonic sources of ${\pi }^{\pm }$, $p$, and $\overline{p}$ have similar energy loss when traversing the nuclear medium.

We present results for the measurement of $\varphi$ meson production via its charged kaon decay channel $\varphi \to K^{+}{K}^{-}$ in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=62.4,130$, and 200 GeV, and in $p+p$ and $d+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV from the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC). The midrapidity ($\left|y\right|<0.5$) $\varphi$ meson transverse momentum ($p_T$) spectra in central $\mathrm{Au}+\mathrm{Au}$ collisions are found to be well described by a single exponential distribution. On the other hand, the $p_T$ spectra from $p+p$, $d+\mathrm{Au}$, and peripheral $\mathrm{Au}+\mathrm{Au}$ collisions show power-law tails at intermediate and high $p_T$ and are described better by Levy distributions. The constant $\varphi /K^{-}$ yield ratio vs beam species, collision centrality, and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for $\varphi$ production at RHIC. The $\Omega /\varphi$ yield ratio as a function of $p_T$ is consistent with a model based on the recombination of thermal $s$ quarks up to $p_T~4$ GeV/$c$, but disagrees at higher transverse momenta. The measured nuclear modification factor, $R_{d\mathrm{Au}}$, for the $\varphi$ meson increases above unity at intermediate $p_T$, similar to that for pions and protons, while $R_{\mathit{AA}}$ is suppressed due to the energy loss effect in central $\mathrm{Au}+\mathrm{Au}$ collisions. Number of constituent quark scaling of both $R_{\mathrm{cp}}$ and $v_2$ for the $\varphi$ meson with respect to other hadrons in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV at intermediate $p_T$ is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate $p_T$ region at RHIC.

Parity ($\mathcal{P}$)-odd domains, corresponding to nontrivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in noncentral collisions. To study this effect, we investigate a three-particle mixed-harmonics azimuthal correlator which is a $\mathcal{P}$-even observable, but directly sensitive to the charge-separation effect. We report measurements of this observable using the STAR detector in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ and 62 GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators and discuss in detail possible contributions from other effects that are not related to $\mathcal{P}$ violation.

Transverse mass and rapidity distributions for charged pions, charged kaons, protons, and antiprotons are reported for $\sqrt{s_{NN}}=200\mathrm{GeV}$ $pp$ and $\mathrm{Au}+\mathrm{Au}$ collisions at Relativistic Heary Ion Collider (RHIC). Chemical and kinetic equilibrium model fits to our data reveal strong radial flow and long duration from chemical to kinetic freeze-out in central $\mathrm{Au}+\mathrm{Au}$ collisions. The chemical freeze-out temperature appears to be independent of initial conditions at RHIC energies.

The transverse mass spectra and midrapidity yields for $\Xi$s and $\Omega$s are presented. For the 10% most central collisions, the ${\overline{\Xi }}^{+}/h^{-}$ ratio increases from the Super Proton Synchrotron to the Relativistic Heavy Ion Collider energies while the ${\Xi }^{-}/h^{-}$ stays approximately constant. A hydrodynamically inspired model fit to the $\Xi$ spectra, which assumes a thermalized source, seems to indicate that these multistrange particles experience a significant transverse flow effect, but are emitted when the system is hotter and the flow is smaller than values obtained from a combined fit to $\pi$, $K$, $p$, and $\Lambda$s.

Measurements of the production of forward high-energy ${\pi }^0$ mesons from transversely polarized proton collisions at $\sqrt{s}=200\mathrm{GeV}$ are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at $x_F$ below about 0.3, and becomes positive and large at higher $x_F$, similar to the trend in data at $\sqrt{s}\le 20\mathrm{GeV}$. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with $p_T>1\mathrm{GeV}/c$ at a polarized proton collider.