We report measurements of Upsilon meson production in p+p, d+Au, and Au+Au collisions using the STAR detector at RHIC. We compare the Upsilon yield to the measured cross section in p+p collisions in order to quantify any modifications of the yield in cold nuclear matter using d+Au data and in hot nuclear matter using Au+Au data separated into three centrality classes. Our p+p measurement is based on three times the statistics of our previous result. We obtain a nuclear modification factor for Upsilon(1S+2S+3S) in the rapidity range |y|<1 in d+Au collisions of R_dAu = 0.79 +/- 0.24 (stat.) +/- 0.03 (sys.) +/- 0.10 (pp sys.). A comparison with models including shadowing and initial state parton energy loss indicates the presence of additional cold-nuclear matter suppression. Similarly, in the top 10% most-central Au+Au collisions, we measure a nuclear modification factor of R_AA=0.49 +/- 0.1 (stat.) +/- 0.02 (sys.) +/- 0.06 (pp sys.), which is a larger suppression factor than that seen in cold nuclear matter. Our results are consistent with complete suppression of excited-state Upsilon mesons in Au+Au collisions. The additional suppression in Au+Au is consistent with the level expected in model calculations that include the presence of a hot, deconfined Quark-Gluon Plasma. However, understanding the suppression seen in d+Au is still needed before any definitive statements about the nature of the suppression in Au+Au can be made.
Comparison of our d+Au measurements to the pA measurements from E772. Ratio of $\Upsilon$ production in pA to pp scaled by mass number as a function of mass number. Shown are the 1S and 2S+3S $\Upsilon$ measurements from E772 and our 1S measurement.
We report measurements of single- and double- spin asymmetries for $W^{\pm}$ and $Z/\gamma^*$ boson production in longitudinally polarized $p+p$ collisions at $\sqrt{s} = 510$ GeV by the STAR experiment at RHIC. The asymmetries for $W^{\pm}$ were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the proton's polarized quark distributions at the scale of the $W$ mass. The results are compared to theoretical predictions, constrained by recent polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range $0.05<x<0.2$.
$E_T^e$ distribution of $W^{\pm}$ candidate events, background contributions, and sum of backgrounds and W -> ev MC signal. This plot is for Electron |eta|<0.5.
$E_T^e$ distribution of $W^{\pm}$ candidate events, background contributions, and sum of backgrounds and W -> ev MC signal. This plot is for Electron 0.5<|eta|<1.1.
$E_T^e$ distribution of $W^{\pm}$ candidate events, background contributions, and sum of backgrounds and W -> ev MC signal. This plot is for Positron |eta|<0.5.
In this erratum we report changes on the $D^0$ $p_T$ spectra and nuclear modification factor ($R_{AA}$) in Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 200 GeV by fixing the errors in the efficiency and selection criteria that affected the Au+Au results. The p+p reference spectrum has changed as well and is updated with new fragmentation parameters.
$D^0$ $p_{\rm T}$ differential invariant yield in p+p collisions (open circles), which has been updated with the latest global analysis of charm fragmentation ratios from Ref and also taking into account the $p_{\rm T}$ dependence of the fragmentation ratio between $D^0$ and $D^{*{\pm}}$ from PYTHIA 6.4. The systematic uncertainties are shown as square brackets.
Centrality dependence of the $D^0$ $p_{\rm T}$ differential invariant yield in Au+Au collisions (solid symbols). The curves are number-of-binary-collision-scaled Levy functions from fitting to the p+p result (open circles), which has been updated with the latest global analysis of charm fragmentation ratios from Ref and also taking into account the $p_{\rm T}$ dependence of the fragmentation ratio between $D^0$ and $D^{*{\pm}}$ from PYTHIA 6.4. The arrow denotes the upper limit with 90% confidence level of the last data point for 10$-$40% collisions. The systematic uncertainties are shown as square brackets.
Panels (ab), $D^0$ $R_{\rm AA}$ for peripheral 40$-$80% and semi a central 10$-$40% collisions; Panel (c), $D^0$ $R_{\rm AA}$ for 0$-$10% most central events (blue circles) compared with model calculations from the TAMU (solid curve), SUBATECH (dashed curve), Torino (dot-dashed curve), Duke (long-dashed and long-dot-dashed curves), and LANL groups (filled band). The open symbol indicates the result with the extrapolated p+p reference. The vertical lines and brackets around the data points denote the statistical and systematic uncertainties respectively. The vertical bars around unity denote the overall normalization uncertainties in the Au+Au and p+p data, respectively. The $R_{\rm AA}$ probability distribution for the 0$-$0.7 GeV/$c$ data point is largely skewed. The uncertainty we report is the 68.3% probability range with respect to the measured central value assuming Gaussian distribution.
We present $\Lambda\Lambda$ correlation measurements in heavy-ion collisions for Au+Au collisions at $\sqrt{s_{NN}}= 200$ GeV using the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). The Lednick\'{y}-Lyuboshitz analytical model has been used to fit the data to obtain a source size, a scattering length and an effective range. Implications of the measurement of the $\Lambda\Lambda$ correlation function and interaction parameters for di-hyperon searches are discussed.
The invariant mass distribution for $\Lambda$ and $\bar{\Lambda}$ produced in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, for 0-80% centrality. The $\Lambda$ and $\bar{\Lambda}$ candidates lying in the mass range 1.112 to 1.120 GeV/c^2 were selected for the correlation measurement.
The $\Lambda\Lambda$ and $\bar{\Lambda}\bar{\Lambda}$ correlation function in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, for 0-80% centrality.
The combined $\Lambda\Lambda$ and $\bar{\Lambda}\bar{\Lambda}$ correlation function for 0-80% centrality Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
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.
We report a new high-precision measurement of the mid-rapidity inclusive jet longitudinal double-spin asymmetry, $A_{LL}$, in polarized $pp$ collisions at center-of-mass energy $\sqrt{s}=200$ 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 for positive gluon polarization in the Bjorken-$x$ region $x>0.05$.
Jet neutral energy fraction (NEF) comparing data with simulations, where both are calculated with pT subtraction. This plot shows 8.4 < $p_T$ < 9.9 GeV/c.
Jet neutral energy fraction (NEF) comparing data with simulations, where both are calculated with pT subtraction. This plot shows 26.8 < $p_T$ < 31.6 GeV/c.
Inclusive jet $A_{LL}$ vs. parton jet $p_T$ for |eta|<0.5.
We report the STAR measurements of dielectron ($e^+e^-$) production at midrapidity ($|y_{ee}|<$1) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200\,GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30-0.76 ($\rho$-like), 0.76-0.80 ($\omega$-like), and 0.98-1.05 ($\phi$-like) GeV/$c^{2}$. The spectrum in the $\omega$-like and $\phi$-like regions can be well described by the hadronic cocktail simulation. In the $\rho$-like region, however, the vacuum $\rho$ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77$\pm$0.11(stat.)$\pm$0.24(sys.)$\pm$0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the $\rho$ meson. The excess yield in the $\rho$-like region increases with the number of collision participants faster than the $\omega$ and $\phi$ yields. Theoretical models with broadened $\rho$ contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies.
Integrated dielectron yields in the mass regions of 0.30-0.76 (rho-like), 0.76-0.80 (omega-like), and 0.98-1.05 (phi-like) GeV/c^2 compared to hadronic cocktails within the STAR acceptance as a function of centrality.
We report the beam energy (\sqrt s_{NN} = 7.7 - 200 GeV) and collision centrality dependence of the mean (M), standard deviation (\sigma), skewness (S), and kurtosis (\kappa) of the net-proton multiplicity distributions in Au+Au collisions. The measurements are carried out by the STAR experiment at midrapidity (|y| < 0.5) and within the transverse momentum range 0.4 < pT < 0.8 GeV/c in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the Quantum Chromodynamic (QCD) phase diagram. The products of the moments, S\sigma and \kappa\sigma^{2}, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and anti-proton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation, and also to a hadron resonance gas model.
Collision energy and centrality dependence of the net-proton $S\sigma$ and $\kappa\sigma^2$ from Au+Au and p+p collisions at RHIC.
We report on the first measurement of the azimuthal anisotropy ($v_2$) of dielectrons ($e^{+}e^{-}$ pairs) at mid-rapidity from $\sqrt{s_{_{NN}}} = 200$ GeV Au+Au collisions with the STAR detector at RHIC, presented as a function of transverse momentum ($p_T$) for different invariant-mass regions. In the mass region $M_{ee}\!<1.1$ GeV/$c^2$ the dielectron $v_2$ measurements are found to be consistent with expectations from $\pi^{0}$, $\eta$, $\omega$ and $\phi$ decay contributions. In the mass region $1.1\!<M_{ee}\!<2.9$ GeV/$c^2$, the measured dielectron $v_2$ is consistent, within experimental uncertainties, with that from the $c\bar{c}$ contributions.
The $p_T$-integrated dielectron $v_2$ as a function of $M_{ee}$ in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
The $v_2$ of hadrons $\pi$, $K$, $p$, $\phi$ and $\Lambda$ in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV for the $\pi^0$ mass region.
The $p_T$-integrated dielectron $v_2$ as a function of $M_{ee}$ in minimum-bias Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, expected from the decays of $\pi^0$, $\eta$, $\omega$ and $\phi$ and from the $car{c}$ contribution.
Event-by-event fluctuations of the ratio of inclusive charged to photon multiplicities at forward rapidity in Au+Au collision at $\sqrt{s_{NN}}$=200 GeV have been studied. Dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidences of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as baselines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A non-zero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation due to particle decay. Results are compared to the expectations based on the generic production mechanism of pions due to isospin symmetry, for which no significant (<1%) deviation is observed.
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