High transverse momentum ($p_T$) particle production is suppressed due to parton (jet) energy loss in the hot dense medium created in relativistic heavy-ion collisions. Redistribution of energy at low-to-modest $p_T$ has been elusive to measure because of large anisotropic backgrounds. We report a data-driven method for background evaluation and subtraction, exploiting the away-side pseudorapidity gaps, to measure the jetlike correlation shape in Au+Au collisions at $\sqrt{s_{_\mathrm{NN}}}=200$ GeV with the STAR experiment. The correlation shapes, for trigger particle $p_T>3$ GeV/$c$ and various associated particle $p_T$ ranges within $0.5<p_T<10$ GeV/$c$, are consistent with Gaussians and their widths are found to increase with centrality. The results indicate jet broadening in the medium created in central heavy-ion collisions.
Distributions of the recoil momentum within 0.5<\eta<1 (Px|_{0.5}^{1}) from high-pT trigger particles of 3<pT_Trig<10 GeV/c in 50-80% peripheral collisions
Distributions of the recoil momentum within 0.5<\eta<1 (Px|_{0.5}^{1}) from high-pT trigger particles of 3<pT_Trig<10 GeV/c in 0-10% central collisions
Dihadron azimuthal correlations in close-region and far-region for 3<pT_Trig<10 GeV/c and 1<pT_Assoc<2 GeV/c in 10-30% Au+Au collisions at \sNN=200 GeV
The $\Lambda$ ($\bar{\Lambda}$) hyperon polarization along the beam direction has been measured for the first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The polarization dependence on the hyperons' emission angle relative to the second-order event plane exhibits a sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum ($p_T$) dependence at $p_T>1$ GeV/$c$. The magnitude of the signal is about five times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, blast-wave model calculations reproduce the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the signal once the model is required to reproduce the azimuthal dependence of the Gaussian source radii measured via the Hanbury-Brown and Twiss intensity interferometry technique.
$\langle \cos\theta_p* \rangle$ of $\Lambda$ and $\bar{\Lambda}$ hyperons as a function of azimuthal angle $\phi$ relative to the second-order event plane $\Psi_2$ for 20%–60% centrality bin in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
The second Fourier sine coefficient $\langle P_Z \sin(2\phi-2\Psi_2) \rangle$ of the polarization of $\Lambda$ and $\bar{\Lambda}$ along the beam direction as a function of the collision centrality in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
The second Fourier sine coefficient $\langle P_Z \sin(2\phi-2\Psi_2) \rangle$ of the polarization of $\Lambda$ and $\bar{\Lambda}$ along the beam direction as a function of the collision centrality in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. Results updated with $\alpha_{\Lambda} = -\alpha_{\bar{\Lambda}} = 0.732$.
We report the first measurement of rapidity-odd directed flow ($v_{1}$) for $D^{0}$ and $\overline{D^{0}}$ mesons at mid-rapidity ($|y| < 0.8$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200\,GeV using the STAR detector at the Relativistic Heavy Ion Collider. In 10--80\% Au+Au collisions, the slope of the $v_{1}$ rapidity dependence ($dv_{1}/dy$), averaged over $D^{0}$ and $\overline{D^{0}}$ mesons, is -0.080 $\pm$ 0.017 (stat.) $\pm$ 0.016 (syst.) for transverse momentum $p_{\rm T}$ above 1.5~GeV/$c$. The absolute value of $D^0$-meson $dv_1/dy$ is about 25 times larger than that for charged kaons, with 3.4$\sigma$ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.
Directed flow $v_1$ as a function of rapidity for $D^0$ and $\bar{D^0}$ mesons at $p_T>1.5$ GeV/c for 10–80% centrality Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
Directed flow $\langle v_1 \rangle$ for the combined samples of $D^0$ and $\bar{D^0}$ mesons at $p_T>1.5$ GeV/c for 10–80% centrality Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
Difference in $v_1(y)$ $(\Delta v_1)$ between $D^0$ and $\bar{D^0}$ mesons at $p_T>1.5$ GeV/c for 10–80% centrality Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.
We report the first measurements of a complete second-order cumulant matrix of net-charge, net-proton, and net-kaon multiplicity distributions for the first phase of the beam energy scan program at RHIC. This includes the centrality and, for the first time, the pseudorapidity window dependence of both diagonal and off-diagonal cumulants in Au+Au collisions at \sNN~= 7.7-200 GeV. Within the available acceptance of $|\eta|<0.5$, the cumulants grow linearly with the pseudorapidity window. Relative to the corresponding measurements in peripheral collisions, the ratio of off-diagonal over diagonal cumulants in central collisions indicates an excess correlation between net-charge and net-kaon, as well as between net-charge and net-proton. The strength of such excess correlation increases with the collision energy. The correlation between net-proton and net-kaon multiplicity distributions is observed to be negative at \sNN~= 200 GeV and change to positive at the lowest collision energy. Model calculations based on non-thermal (UrQMD) and thermal (HRG) production of hadrons cannot explain the data. These measurements will help map the QCD phase diagram, constrain hadron resonance gas model calculations, and provide new insights on the energy dependence of baryon-strangeness correlations. An erratum has been added to address the issue of self-correlation in the previously considered efficiency correction for off-diagonal cumulant measurement. Previously considered unidentified (net-)charge correlation results ($\sigma^{11}_{Q,p}$ and $\sigma^{11}_{Q,k})$ are now replaced with identified (net-)charge correlation ($\sigma^{11}_{Q^{PID},p}$ and $\sigma^{11}_{Q^{PID},k}$)
The dependence of efficiency corrected second-order diagonal and off-diagonal cumulants on the width of the η-window. The filled and open circles represent 0-5% and 70-80% central collisions respectively. The shaded band represents the systematic uncertainty. The statistical uncertainties are within the marker size and solid lines are UrQMD calculations.
The dependence of efficiency corrected second-order diagonal and off-diagonal cumulants on the width of the η-window. The filled and open circles represent 0-5% and 70-80% central collisions respectively. The shaded band represents the systematic uncertainty. The statistical uncertainties are within the marker size and solid lines are UrQMD calculations.
Centrality dependence of efficiency corrected second-order diagonal cumulants of net-proton, net-kaon and net-pion (top to bottom) of the multiplicity distributions for Au+Au collisions at GeV (left to right) within kinematic range of |η| < 0.5 and 0.4 < pT < 1.6 GeV/c. The boxes represent the systematic error. The statistical error bars are within the marker size. The dashed lines represent scaling predicted by central limit theorem and the solid lines are UrQMD calculations.
We report a new measurement of $D^0$-meson production at mid-rapidity ($|y|$\,$<$\,1) in Au+Au collisions at ${\sqrt{s_{\rm NN}} = \rm{200\,GeV}}$ utilizing the Heavy Flavor Tracker, a high resolution silicon detector at the STAR experiment. Invariant yields of $D^0$-mesons with transverse momentum $p_{T}$ $\lesssim 9$\,GeV/$c$ are reported in various centrality bins (0--10\%, 10--20\%, 20--40\%, 40--60\% and 60--80\%). Blast-Wave thermal models are used to fit the $D^0$-meson $p_{T}$ spectra to study $D^0$ hadron kinetic freeze-out properties. The average radial flow velocity extracted from the fit is considerably smaller than that of light hadrons ($\pi,K$ and $p$), but comparable to that of hadrons containing multiple strange quarks ($\phi,\Xi^-$), indicating that $D^0$ mesons kinetically decouple from the system earlier than light hadrons. The calculated $D^0$ nuclear modification factors re-affirm that charm quarks suffer large amount of energy loss in the medium, similar to those of light quarks for $p_{T}$\,$>$\,4\,GeV/$c$ in central 0--10\% Au+Au collisions. At low $p_{T}$, the nuclear modification factors show a characteristic structure qualitatively consistent with the expectation from model predictions that charm quarks gain sizable collective motion during the medium evolution. The improved measurements are expected to offer new constraints to model calculations and help gain further insights into the hot and dense medium created in these collisions.
$D^0$ (in terms of (D0 +D0)/2)) invariant yield at mid-rapidity ($|y| < 1$) vs transverse momentum for different centrality classes. Error bars indicate statistical uncertainties and brackets depict systematic uncertainties. Global systematic uncertainties in B.R. are not plotted. Solid and dashed lines depict Levy function fits.
$D^0$ (in terms of (D0 +D0)/2)) spectra in pp collisions. Note, the $\sigma_{NSD}$ = 30 $m$b for p+p was used in the calculations.
Integrated $D^0$ cross section per nucleon-nucleon collision at mid-rapidity for $p_T >0$ (a) and $p_T >4$ GeV/c (b) as a function of centrality $N_{part}$. The statistical and systematic uncertainties are shown as error bars and brackets on the data points. The green boxes on the data points depict the overall normalization uncertainties in p+p and Au+Au data respectively.
We report new STAR measurements of the single-spin asymmetries $A_L$ for $W^+$ and $W^-$ bosons produced in polarized proton--proton collisions at $\sqrt{s}$ = 510 GeV as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250 pb$^{-1}$. The results are combined with previous results obtained with 86 pb$^{-1}$. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton--proton data suggests a difference between the $\bar{u}$ and $\bar{d}$ quark helicity distributions for $0.05 < x < 0.25$. In addition, we report new results for the double-spin asymmetries $A_{LL}$ for $W^\pm$, as well as $A_L$ for $Z/\gamma^*$ production and subsequent decay into electron--positron pairs.
Distribution of the product of Q, the TPC recon-structed charge-sign, and $E_T/p_T$ in the BEMC region.
Distribution of the product of Q, the TPC recon-structed charge-sign, and $E_T/p_T$ in the EEMC region.
$E_T^e$ distributions of electron candidate events, background contributions, and sum of backgrounds and $W \rightarrow e\nu$ MC signal in the BEMC region for $-1.1 < \eta_e < -0.5$.
The longitudinal spin transfer $D_{LL}$ to $\Lambda$ and $\bar{\Lambda}$ hyperons produced in high-energy polarized proton--proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and anti-quarks of the proton, and to longitudinally polarized fragmentation functions. We report an improved measurement of $D_{LL}$ from data obtained at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure-of-merit than prior results and cover $|\eta|<$ 1.2 in pseudo-rapidity with transverse momenta $p_T$ up to 6 GeV/c. In the forward scattering hemisphere at largest $p_T$, the longitudinal spin transfer is found to be $D_{LL}$ = -0.036 $\pm$ 0.048 (stat) $\pm$ 0.013(sys) for $\Lambda$ hyperons and $D_{LL}$ = 0.032 $\pm$ 0.043\,(stat) $\pm$ 0.013\,(sys) for $\bar{\Lambda}$ anti-hyperons. The dependences on $\eta$ and $p_T$ are presented and compared with model evaluations.
The invariant mass distribution for $\Lambda$ and $\bar{\Lambda}$ candidates with 3 < p_T < $ 4 GeV/c in this analysis
The raw spin transfer $D _{LL}^{raw}$ versus cos$\theta^*$ for a) $\Lambda$ and b) $\bar{\Lambda}$ hyperons and c) the spin asymmetry $\delta_{LL}$ for the control sample of $K_S^0$ mesons versus cos$\theta^*$ for $3<p_T<4$ GeV/c for JP1 triggered sample.
The raw spin transfer $D _{LL}^{raw}$ versus cos$\theta^*$ for a) $\Lambda$ and b) $\bar{\Lambda}$ hyperons and c) the spin asymmetry $\delta_{LL}$ for the control sample of $K_S^0$ mesons versus cos$\theta^*$ for $3<p_T<4$ GeV/c for JP1 triggered sample. Results updated with $\alpha_{\Lambda (\bar{\Lambda})} = 0.732$.
We present the first measurement of the proton-$\Omega$ correlation function in heavy-ion collisions for central (0-40$\%$) and peripheral (40-80$\%$) Au+Au collisions at \sqrtsNN\,\,=200 GeV by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Predictions for the ratio of peripheral collisions to central collisions for the proton-$\Omega$ correlation function are sensitive to the presence of a nucleon-$\Omega$ bound state. These predictions are based on the proton-$\Omega$ interaction extracted from (2+1)-flavor lattice QCD calculations at the physical point. The measured ratio of proton-$\Omega$ correlation function from peripheral (small system) to central (large system) collisions is less than unity for relative momentum smaller than 40 MeV/c. Comparison of our measured correlation ratio with the theoretical calculation slightly favors a proton-$\Omega$ bound system with a binding energy of $\sim$ 27~MeV.
Measured correlation function (C($k^{*}$)) for proton-$\Omega$ and antiproton-$\bar{\Omega}$ (P$\Omega$ + $\bar{P}$$\bar{\Omega}$) for (0-40)$\%$ and (40-80$\%$) Au+Au collisons at $\sqrt{s}_{NN}$ = 200 GeV. The errors are statistical and systematic.
Ratio of small system (40-80$\%$) to large system (0-40$\%$) for proton-$\Omega$ and antiproton-$\bar{\Omega}$ (P$\Omega$ + $\bar{P}$$\Omega$). The errors are statistical and symmetric.
We report first measurements of $e^{+}e^{-}$ pair production in the mass region 0.4 $
The centrality dependence of e+e− invariant mass spectra within the STAR acceptance from Au+Au collisions and U+U collisions for pair pT < 0.15 GeV/c. The vertical bars on data points depict the statistical uncertainties, while the systematic uncertainties are shown as gray boxes. The hadronic cocktail yields from U+U collisions are ∼5%–12% higher than those from Au+Au collisions in given centrality bins; thus only cocktails for Au+Au collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity.
The centrality dependence of e+e− invariant mass spectra within the STAR acceptance from Au+Au collisions and U+U collisions for pair pT < 0.15 GeV/c. The vertical bars on data points depict the statistical uncertainties, while the systematic uncertainties are shown as gray boxes. The hadronic cocktail yields from U+U collisions are ∼5%–12% higher than those from Au+Au collisions in given centrality bins; thus only cocktails for Au+Au collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity.
The centrality dependence of e+e− invariant mass spectra within the STAR acceptance from Au+Au collisions and U+U collisions for pair pT < 0.15 GeV/c. The vertical bars on data points depict the statistical uncertainties, while the systematic uncertainties are shown as gray boxes. The hadronic cocktail yields from U+U collisions are ∼5%–12% higher than those from Au+Au collisions in given centrality bins; thus only cocktails for Au+Au collisions are shown here as solid lines, with shaded bands representing the systematic uncertainties for clarity.
The STAR Collaboration reports measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for neutral pions produced at forward directions in polarized proton-proton collisions, at a center-of-mass energy of $510$ GeV. Results are given for transverse momenta in the range $2
Longitudinal Double-Spin Asymmetry, $A_{LL}$ vs $\pi^0$ transverse momentum in polarized pp collisions at $\sqrt{s}$ = 510 GeV. $\pm 6.7\%$ polarization scale uncertainty not shown.
Longitudinal Double-Spin Asymmetry, $A_{LL}$ vs $\pi^0$ transverse momentum in polarized pp collisions at $\sqrt{s}$ = 510 GeV. $\pm 6.7\%$ polarization scale uncertainty not shown.
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