We report new results on identified (anti)proton and charged pion spectra at large transverse momenta (3<$p_{T}$<10 GeV/c) from Cu+Cu collisions at $\sqrt{s_{NN}}$=200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). This study explores the system size dependence of two novel features observed at RHIC with heavy ions: the hadron suppression at high-$p_{T}$ and the anomalous baryon to meson enhancement at intermediate transverse momenta. Both phenomena could be attributed to the creation of a new form of QCD matter. The results presented here bridge the system size gap between the available pp and Au+Au data, and allow the detailed exploration for the on-set of the novel features. Comparative analysis of all available 200 GeV data indicates that the system size is a major factor determining both the magnitude of the hadron spectra suppression at large transverse momenta and the relative baryon to meson enhancement.
Anti-particle to particle ratios, as a function of transverse momentum for pions (a) and protons (b). Data for the four centrality classes show little centrality dependence. Errors are statistical only.
Anti-particle to particle ratios, as a function of transverse momentum for pions (a) and protons (b). Data for the four centrality classes show little centrality dependence. Errors are statistical only.
(Color online) (a) Nuclear modification factor, RAA, for charged pions ($\pi^{+}+\pi^{−}$) in Cu+Cu (filled symbols) and Au+Au (open symbols) collisions at $\sqrt{s_{NN}}$=200 GeV. Error bands are shown for most peripheral and most central Cu+Cu data to represent evolution of the systematic uncertainties for this dataset. Error boxes at $R_{AA}$=1 represent Cu+Cu scale uncertainties due to the number of collisions and from $pp$ spectra normalization.
We present the multiplicity and pseudorapidity distributions of photons produced in Au+Au and Cu+Cu collisions at \sqrt{s_NN} = 62.4 and 200 GeV. The photons are measured in the region -3.7 < \eta < -2.3 using the photon multiplicity detector in the STAR experiment at RHIC. The number of photons produced per average number of participating nucleon pairs increases with the beam energy and is independent of the collision centrality. For collisions with similar average numbers of participating nucleons the photon multiplicities are observed to be similar for Au+Au and Cu+Cu collisions at a given beam energy. The ratios of the number of charged particles to photons in the measured pseudorapidity range are found to be 1.4 +/- 0.1 and 1.2 +/- 0.1 for \sqrt{s_NN} = 62.4 GeV and 200 GeV, respectively. The energy dependence of this ratio could reflect varying contributions from baryons to charged particles, while mesons are the dominant contributors to photon production in the given kinematic region. The photon pseudorapidity distributions normalized by average number of participating nucleon pairs, when plotted as a function of \eta - ybeam, are found to follow a longitudinal scaling independent of centrality and colliding ion species at both beam energies.
Fig. 1. (Color online.) Top panel: Photon reconstruction efficiency $\left(\epsilon_{\gamma}\right)$ (solid symbols) and purity of photon sample $\left(f_{\mathrm{p}}\right)$ (open symbols) for PMD as a function of pseudorapidity $(\eta)$ for minimum bias $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{s_{\mathrm{NN}}}=$ $200 \mathrm{GeV}$. Bottom panel: Comparison between estimated $\epsilon_{\gamma}$ and $f_{\mathrm{p}}$ for PMD as a function of $\eta$ for minimum bias $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{s_{\mathrm{NN}}}=62.4 \mathrm{GeV}$ using HIJING and AMPT models. The error bars on the AMPT data are statistical and those for HIJING are within the symbol size. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.
Fig. 1. (Color online.) Top panel: Photon reconstruction efficiency $\left(\epsilon_{\gamma}\right)$ (solid symbols) and purity of photon sample $\left(f_{\mathrm{p}}\right)$ (open symbols) for PMD as a function of pseudorapidity $(\eta)$ for minimum bias $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{s_{\mathrm{NN}}}=$ $200 \mathrm{GeV}$. Bottom panel: Comparison between estimated $\epsilon_{\gamma}$ and $f_{\mathrm{p}}$ for PMD as a function of $\eta$ for minimum bias $\mathrm{Au}+\mathrm{Au}$ at $\sqrt{s_{\mathrm{NN}}}=62.4 \mathrm{GeV}$ using HIJING and AMPT models. The error bars on the AMPT data are statistical and those for HIJING are within the symbol size. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.
Fig. 2. (Color online.) Event-by-event photon multiplicity distributions (solid circles) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ at $\sqrt{s_{\mathrm{NN}}}=62.4$ and $200 \mathrm{GeV} .$ The distributions for top $0-5 \%$ central $\mathrm{Au}+$ Au collisions and top $0-10 \%$ central $\mathrm{Cu}+\mathrm{Cu}$ collisions are also shown (open circles). The photon multiplicity distributions for central collisions are observed to be Gaussian (solid line). Only statistical errors are shown. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.
We report on the first measurement of elliptic flow $v_2(p_T)$ of multi-strange baryons $\Xi+\bar{Xi}$ and $\Omega+\bar{Omega} in heavy-ion collisions. In minimum bias Au+Au collisions at sqrt(s_NN) = 200 GeV, a significant amount of elliptic flow, comparable to other non-strange baryons, is observed for multi-strange baryons which are expected to be particularly sensitive to the dynamics of the partonic stage of heavy-ion collisions. The $p_T$ dependence of $v_2$ of the multi-strange baryons confirms the number of constituent quark scaling previously observed for lighter hadrons. These results support the idea that a substantial fraction of the observed collective motion is developed at the early partonic stage in ultra-relativistic nuclear collisions at RHIC.
$\Xi^{-} + \Xi^{+}$ invariant mass distribution from minimum bias (0–80%) Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
$\Omega^{-} + \Omega^{+}$ invariant mass distribution from minimum bias (0–80%) Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
Azimuthal distributions with respect to the event plane of the $\Xi^{-} + \Xi^{+}$ and $\Omega^{-} + \Omega^{+}$ raw yields.
We present the first measurement of multiplicity and pseudorapidity distributions of photons in the pseudorapidity region 2.3 $\le$ $\eta$ $\le$ 3.7 for different centralities in Au + Au collisions at $\sqrt{s_{NN}}$ = 62.4 GeV. We find that the photon yield in this pseudorapidity range scales with the number of participating nucleons at all collision centralities studied. The pseudorapidity distribution of photons, dominated by neutral pion decays, has been compared to those of identified charged pions, photons, and inclusive charged particles from heavy ion and nucleon-nucleon collisions at various energies. The photon production in the measured pseudorapidity region has been shown to be consistent with the energy and centrality independent limiting fragmentation scenario.
${dN_{\gamma}}\over{d\eta}$ for various event centrality classes compared to HIJING and AMPT model calculations.
(Color Online) Estimated ${dN_{\pi^{0}}}\over{dy}$ from ${dN_{\gamma}}\over{dy}$ normalized to $N_{part}$, as compared to ${dN_{\pi^{\pm}}}\over{dy}$ normalized to $N_{part}$, as a function of $y-y_{beam}$ for central collisions at various collision energies.
We present first measurements of the $\phi$-meson elliptic flow ($v_{2}(p_{T})$) and high statistics $p_{T}$ distributions for different centralities from $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions at RHIC. In minimum bias collisions the $v_{2}$ of the $\phi$ meson is consistent with the trend observed for mesons. The ratio of the yields of the $\Omega$ to those of the $\phi$ as a function of transverse momentum is consistent with a model based on the recombination of thermal $s$ quarks up to $p_{T}\sim 4$ GeV/$c$, but disagrees at higher momenta. The nuclear modification factor ($R_{CP}$) of $\phi$ follows the trend observed in the $K^{0}_{S}$ mesons rather than in $\Lambda$ baryons, supporting baryon-meson scaling. Since $\phi$-mesons are made via coalescence of seemingly thermalized $s$ quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC.
The elliptic flow, $v_{2}$($p_{T}$), for the $\phi$-meson as a function of centrality. The vertical error bars represent the statistical errors while the shaded bands represent the systematic uncertainties. For clarity, data points are shifted slightly.
(color online) Transverse momentum distributions of $\phi$-mesons from Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. For clarity, distributions for different centralities are scaled by factors of ten. Dashed lines represent the exponential fits to the distributions and the dotted lines are Levy function fits. Error bars represent statistical errors only.
(color online) The $N(\Omega)/N(\phi)$ ratio vs. $p_{T}$ for three centrality bins in $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions. The solid and dashed lines represent recombination model predictions for central collisions [21] for total and thermal contributions, respectively.
Charged hadrons in 0.15 < pt < 4 GeV/c associated with particles of pt^trig > 4 GeV/c are reconstructed in pp and Au+Au collisions at sqrt(s_NN)=200 GeV. The associated multiplicity and pt magnitude sum are found to increase from pp to central Au+Au collisions. The associated pt distributions, while similar in shape on the near side, are significantly softened on the away side in central Au+Au relative to pp and not much harder than that of inclusive hadrons. The results, consistent with jet quenching, suggest that the away-side fragments approach equilibration with the medium traversed.
Dphi correlation functions for 0.15 < pT < 4 GEV/c and 4 < p_T^trig < 6 GEV/c.
Dphi correlation functions for 0.15 < pT < 4 GEV/c and 4 < p_T^trig < 6 GEV/c.
Dphi correlation functions for 2 < pT < 4 GEV/c and 4 < p_T^trig < 6 GEV/c.
Results on high transverse momentum charged particle emission with respect to the reaction plane are presented for Au+Au collisions at $\sqrt{s_{_{NN}}}$= 200 GeV. Two- and four-particle correlations results are presented as well as a comparison of azimuthal correlations in Au+Au collisions to those in $p+p$ at the same energy. Elliptic anisotropy, $v_2$, is found to reach its maximum at $p_t \sim 3$ GeV/c, then decrease slowly and remain significant up to $p_t\approx 7$ -- 10 GeV/c. Stronger suppression is found in the back-to-back high-$p_t$ particle correlations for particles emitted out-of-plane compared to those emitted in-plane. The centrality dependence of $v_2$ at intermediate $p_t$ is compared to simple models based on jet quenching.
Azimuthal correlations in Au+Au col- lisions (squares) as a function of centrality (peripheral to cen- tral from left to right) compared to minimum bias azimuthal correlations in p + p collisions (circles). Errors are statistical only.
$v_{2}$ of charged particles as a function of transverse momentum from the two-particle cumu- lant method (triangles) and four-particle cumulant method (stars). Open circles show the 2-particle correlation results after subtracting the correlations measured in p + p collisions. Only statistical errors are shown.
Upper panel, Azimuthal distributions of associated particles for trigger particles in-plane (squares) and out-of-plane (triangles) for Au+Au collisions at centrality 20-60%. Open symbols are reflections of solid symbols around $\Delta \phi$ = 0 and $\Delta \phi$ = $\pi$. Elliptic flow contribution is shown by dashed lines. Lower panel, Distributions after substracting elliptic flow, and the corresponding measurement in p + p collisions (histogram).
We report on the observed differences in production rates of strange and multi-strange baryons in Au+Au collisions at sqrts = 200 GeV compared to pp interactions at the same energy. The strange baryon yields in Au+Au collisions, then scaled down by the number of participating nucleons, are enhanced relative to those measured in pp reactions. The enhancement observed increases with the strangeness content of the baryon, and increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at lower collision energy sqrts =17.3 GeV. The previous observations are for the bulk production, while at intermediate pT, 1 < pT< 4 GeV/c, the strange baryons even exceed binary scaling from pp yields.
Midrapidity E(i) as a function of $<N_{part}>$ for $\Lambda$, $\bar{\Lambda}$ ($|y| < 1.0$), $\Xi^{-}$, $\bar{\Xi}^{+}$ ($|y| < 0.75$). Error bars on the data points represent those from the heavy ions. Stat. and syst. errors added in quadrature. Grand Canonical Model arrows(values in brackets), for $\Lambda$ E(2.6) and T(165 MeV) for $\bar{\Lambda}$ E(2.2) and T(170 MeV), for $\Xi$ E(10.7) and T(165 MeV), for anti-$\Xi$ E(7.5) and T(170 MeV).
Midrapidity E(i) as a function of $<N_{part}>$ for $\Omega^{-}+\bar{\Omega}^{+}$ ($|y| < 0.75$). Error bars on the data points represent those from the heavy ions. Stat. and syst. errors added in quadrature.
Midrapidity E(i) as a function of $<N_{part}>$ for inclusive $p$ ($|y| < 0.5$). Error bars on the data points represent those from the heavy ions. Stat. and syst. errors added in quadrature.
In ultra-peripheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a $\rho^0$. The $\rho^0$ production occurs in two well-separated (median impact parameters of 20 and 40 fermi for the cases considered here) nuclei, so the system forms a 2-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing $\rho^0$ production. Since the $\rho^0$ decay before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled non-local wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is $87% \pm 5% {\rm (stat.)}\pm 8%$ (syst.) of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors, of 23% at the 90% confidence level.
Rapidity (left) and $M_{\pi\pi}$ (right) of the $\pi^{+}\pi^{-}$ distributions for the topology (exclusive $\rho^0$, top) and MB (Coulomb breakup, bottom) samples. The points with statistical error bars are the data, and the histograms are the simulations. The ’notch’ in the topology data around y = 0 is due to the explicit rapidity cut to remove cosmic-ray backgrounds.
Rapidity (left) and $M_{\pi\pi}$ (right) of the $\pi^{+}\pi^{-}$ distributions for the topology (exclusive $\rho^0$, top) and MB (Coulomb breakup, bottom) samples. The points with statistical error bars are the data, and the histograms are the simulations. The ’notch’ in the topology data around y = 0 is due to the explicit rapidity cut to remove cosmic-ray backgrounds.
Raw (uncorrected) ρ0 $t_{\perp}$-spectrum in the range 0.0 < |y| < 0.5 for the MB data. The points are data, with statistical errors. The dashed (filled) histogram is a simulation with an interference term (“Int”), while the solid histogram is a simulation without interference (“NoInt”). The handful of events histogrammed at the bottom of the plot are the wrong-sign ($\pi^{+}\pi^{+}+\pi^{-}\pi^{-}$) events, used to estimate the combinatorial background.
The STAR collaboration at RHIC presents measurements of \Jpsi$\to{e^+e^-}$ at mid-rapidity and high transverse momentum ($p_T>5$ GeV/$c$) in \pp and central \cucu collisions at \sNN = 200 GeV. The inclusive \Jpsi production cross section for \cucu collisions is found to be consistent at high $p_T$ with the binary collision-scaled cross section for \pp collisions, in contrast to previous measurements at lower $p_T$, where a suppression of \Jpsi production is observed relative to the expectation from binary scaling. Azimuthal correlations of $J/\psi$ with charged hadrons in \pp collisions provide an estimate of the contribution of $B$-meson decays to \Jpsi production of $13% \pm 5%$.
J/psi differential production cross section in sqrt(s).
J/psi transverse momentum distribution in sqrt(s).
J/psi transverse momentum distribution in sqrt(s).
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