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.
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.
Flow harmonics ($v_n$) in the Fourier expansion of the azimuthal distribution of particles are widely used to quantify the anisotropy in particle emission in high-energy heavy-ion collisions. The symmetric cumulants, $SC(m,n)$, are used to measure the correlations between different orders of flow harmonics. These correlations are used to constrain the initial conditions and the transport properties of the medium in theoretical models. In this Letter, we present the first measurements of the four-particle symmetric cumulants in Au+Au collisions at $\sqrt{s_{NN}}$ = 39 and 200 GeV from data collected by the STAR experiment at RHIC. We observe that $v_{2}$ and $v_{3}$ are anti-correlated in all centrality intervals with similar correlation strengths from 39 GeV Au+Au to 2.76 TeV Pb+Pb (measured by the ALICE experiment). The $v_{2}$-$v_{4}$ correlation seems to be stronger at 39 GeV than at higher collision energies. The initial-stage anti-correlations between second and third order eccentricities are sufficient to describe the measured correlations between $v_{2}$ and $v_{3}$. The best description of $v_{2}$-$v_{4}$ correlations at $\sqrt{s_{NN}}$ = 200 GeV is obtained with inclusion of the system's nonlinear response to initial eccentricities accompanied by the viscous effect with $\eta/s$ $>$ 0.08. Theoretical calculations using different initial conditions, equations of state and viscous coefficients need to be further explored to extract $\eta/s$ of the medium created at RHIC.
New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient \vone, are presented for transverse momenta $\mathrm{p_T}$, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range $\mathrm{\sqrt{s_{_{NN}}}} = 7.7 - 200$ GeV. The measurements underscore the importance of momentum conservation and the characteristic dependencies on $\mathrm{\sqrt{s_{_{NN}}}}$, centrality and $\mathrm{p_T}$ are consistent with the expectations of geometric fluctuations generated in the initial stages of the collision, acting in concert with a hydrodynamic-like expansion. The centrality and $\mathrm{p_T}$ dependencies of $\mathrm{v^{even}_{1}}$, as well as an observed similarity between its excitation function and that for $\mathrm{v_3}$, could serve as constraints for initial-state models. The $\mathrm{v^{even}_{1}}$ excitation function could also provide an important supplement to the flow measurements employed for precision extraction of the temperature dependence of the specific shear viscosity.
We present two-particle $p_{\rm t}$ correlations as a function of event centrality for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV at the Relativistic Heavy Ion Collider using the STAR detector. These results are compared to previous measurements from CERES at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization. The relative dynamical correlations for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV show a power law dependence on the number of participant nucleons and agree with the results for Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 2.76~ {\rm TeV}$ from ALICE. As the collision energy is lowered from $\sqrt{s_{\rm NN}}$ = 200 GeV to 7.7 GeV, the centrality dependence of the relative dynamical correlations departs from the power law behavior observed at the higher collision energies. In central collisions, the relative dynamical correlations increase with collision energy up to $\sqrt{s_{\rm NN}}$ = 200 GeV in contrast to previous measurements that showed little dependence on the collision energy.
Global polarization of $\Lambda$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV, with no significant difference between $\Lambda$ and $\bar{\Lambda}$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity $\eta^H$ within $|\eta^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2\sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.
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.
We report systematic measurements of bulk properties of the system created in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 14.5 GeV recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC).The transverse momentum spectra of $\pi^{\pm}$, $K^{\pm}$ and $p(\bar{p})$ are studied at mid-rapidity ($|y| < 0.1$) for nine centrality intervals. The centrality, transverse momentum ($p_T$),and pseudorapidity ($\eta$) dependence of inclusive charged particle elliptic flow ($v_2$), and rapidity-odd charged particles directed flow ($v_{1}$) results near mid-rapidity are also presented. These measurements are compared with the published results from Au+Au collisions at other energies, and from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. The results at $\sqrt{s_{\mathrm{NN}}}$ = 14.5 GeV show similar behavior as established at other energies and fit well in the energy dependence trend. These results are important as the 14.5 GeV energy fills the gap in $\mu_B$, which is of the order of 100 MeV,between $\sqrt{s_{\mathrm{NN}}}$ =11.5 and 19.6 GeV. Comparisons of the data with UrQMD and AMPT models show poor agreement in general.
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
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.
We present the first measurements of the longitudinal double-spin asymmetry $A_{LL}$ for dijets with at least one jet reconstructed within the pseudorapidity range $0.8 < \eta < 1.8$. The dijets were measured in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s}$ = 200 GeV. Values for $A_{LL}$ are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction $x$ down to $x \sim$ 0.01. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on $\Delta g(x)$ in this poorly constrained region when included in future global analyses.
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.
The transverse spin transfer from polarized protons to $\Lambda$ and $\bar{\Lambda}$ hyperons is expected to provide sensitivity to the transversity distribution of the nucleon and to the transversely polarized fragmentation functions. We report the first measurement of the transverse spin transfer to $\Lambda$ and $\bar{\Lambda}$ along the polarization direction of the fragmenting quark, $D_\mathrm{TT}$, in transversely polarized proton-proton collisions at $\sqrt{s}=200\,\mathrm{GeV}$ with the STAR detector at RHIC. The data correspond to an integrated luminosity of $18\,\mathrm{pb}^{-1}$ and cover the pseudorapidity range $\left|\eta\right| < 1.2$ and transverse momenta $p_{\mathrm{T}}$ up to $8\,\mathrm{GeV}/c$. The dependence on $p_\mathrm{T}$ and $\eta$ are presented. The $D_\mathrm{TT}$ results are found to be comparable with a model prediction, and are also consistent with zero within uncertainties.
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.
We present a measurement of inclusive $J/\psi$ production at mid-rapidity ($|y|<1$) in $p+p$ collisions at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The differential production cross section for $J/\psi$ as a function of transverse momentum ($p_T$) for $0
$J/\psi$ suppression has long been considered a sensitive signature of the formation of the Quark-Gluon Plasma (QGP) in relativistic heavy-ion collisions. In this letter, we present the first measurement of inclusive $J/\psi$ production at mid-rapidity through the dimuon decay channel in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV with the STAR experiment. These measurements became possible after the installation of the Muon Telescope Detector was completed in 2014. The $J/\psi$ yields are measured in a wide transverse momentum ($p_{\rm{T}}$) range of 0.15 GeV/$c$ to 12 GeV/$c$ from central to peripheral collisions. They extend the kinematic reach of previous measurements at RHIC with improved precision. In the 0-10% most central collisions, the $J/\psi$ yield is suppressed by a factor of approximately 3 for $p_{\rm{T}}>5$ GeV/$c$ relative to that in p+p collisions scaled by the number of binary nucleon-nucleon collisions. The $J/\psi$ nuclear modification factor displays little dependence on $p_{\rm{T}}$ in all centrality bins. Model calculations can qualitatively describe the data, providing further evidence for the color-screening effect experienced by $J/\psi$ mesons in the QGP.
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.
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.
We present measurements of the differential production cross sections of the inclusive $J/\psi$ meson as a function of transverse momentum ($p_{T}^{J/\psi}$) using the $\mu^{+}\mu^{-}$ and $e^{+}e^{-}$ decay channels in proton+proton collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the $\mu^{+}\mu^{-}$ channel is for 0 $< p_{T}^{J/\psi} <$ 9 GeV/$c$ and rapidity range $|y^{J/\psi}| < $ 0.4, and that from the $e^{+}e^{-}$ channel is for 4 $< p_{T}^{J/\psi} <$ 20 GeV/$c$ and $|y^{J/\psi}| < $ 1.0. The $\psi(2S)$ to $J/\psi$ ratio is also measured for 4 $< p_{T}^{\rm meson} <$ 12 GeV/$c$ through the $e^{+}e^{-}$ decay channel. Model calculations, which incorporate different approaches toward the $J/\psi$ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties. A more discriminating comparison to theoretical models at low $p_T$ can be performed in the future, if the calculations are carried out within our fiducial volume, eliminating the uncertainty due to the $J/\psi$ polarization.
Forum