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Institute
Partons traversing the strongly interacting medium produced in heavy-ion collisions are expected to lose energy depending on their color charge and mass. We measure the nuclear modification factors for charm- and bottom-decay electrons, defined as the ratio of yields, scaled by the number of binary nucleon-nucleon collisions, in sNN−−−√ = 200 GeV Au+Au collisions to p+p collisions (RAA), or in central to peripheral Au+Au collisions (RCP). We find the bottom-decay electron RAA and RCP to be significantly higher than that of charm-decay electrons. Model calculations including mass-dependent parton energy loss in a strongly coupled medium are consistent with the measured data. These observations provide clear evidence of mass ordering of charm and bottom quark energy loss when traversing through the strongly coupled medium created in heavy-ion collisions.
Partons traversing the strongly interacting medium produced in heavy-ion collisions are expected to lose energy depending on their color charge and mass. We measure the nuclear modification factors for charm- and bottom-decay electrons, defined as the ratio of yields, scaled by the number of binary nucleon-nucleon collisions, in sNN−−−√ = 200 GeV Au+Au collisions to p+p collisions (RAA), or in central to peripheral Au+Au collisions (RCP). We find the bottom-decay electron RAA and RCP to be significantly higher than that of charm-decay electrons. Model calculations including mass-dependent parton energy loss in a strongly coupled medium are consistent with the measured data. These observations provide clear evidence of mass ordering of charm and bottom quark energy loss when traversing through the strongly coupled medium created in heavy-ion collisions.
Partons traversing the strongly interacting medium produced in heavy-ion collisions are expected to lose energy depending on their color charge and mass. We measure the nuclear modification factors for charm- and bottom-decay electrons, defined as the ratio of yields, scaled by the number of binary nucleon-nucleon collisions, in sNN−−−√ = 200 GeV Au+Au collisions to p+p collisions (RAA), or in central to peripheral Au+Au collisions (RCP). We find the bottom-decay electron RAA and RCP to be significantly higher than that of charm-decay electrons. Model calculations including mass-dependent parton energy loss in a strongly coupled medium are consistent with the measured data. These observations provide clear evidence of mass ordering of charm and bottom quark energy loss when traversing through the strongly coupled medium created in heavy-ion collisions.
Azimuthal anisotropy (v2) and two-particle angular correlations of high pT charged hadrons have been measured in Au+Au collisions at sqrt[sNN]=130 GeV for transverse momenta up to 6 GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high pT partons. The monotonic rise of v2(pT) for pT<2 GeV/c is consistent with collective hydrodynamical flow calculations. At pT>3 GeV/c, a saturation of v2 is observed which persists up to pT=6 GeV/c.
Azimuthal anisotropy (v2) and two-particle angular correlations of high pT charged hadrons have been measured in Au+Au collisions at sqrt[sNN]=130 GeV for transverse momenta up to 6 GeV/c, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high pT partons. The monotonic rise of v2(pT) for pT<2 GeV/c is consistent with collective hydrodynamical flow calculations. At pT>3 GeV/c, a saturation of v2 is observed which persists up to pT=6 GeV/c.
Elliptic flow holds much promise for studying the early-time thermalization attained in ultrarelativistic nuclear collisions. Flow measurements also provide a means of distinguishing between hydrodynamic models and calculations which approach the low density (dilute gas) limit. Among the effects that can complicate the interpretation of elliptic flow measurements are azimuthal correlations that are unrelated to the reaction plane (nonflow correlations). Using data for Au + Au collisions at sqrt[sNN]=130 GeV from the STAR time projection chamber, it is found that four-particle correlation analyses can reliably separate flow and nonflow correlation signals. The latter account for on average about 15% of the observed second-harmonic azimuthal correlation, with the largest relative contribution for the most peripheral and the most central collisions. The results are also corrected for the effect of flow variations within centrality bins. This effect is negligible for all but the most central bin, where the correction to the elliptic flow is about a factor of 2. A simple new method for two-particle flow analysis based on scalar products is described. An analysis based on the distribution of the magnitude of the flow vector is also described.
We report the first observation of K*(892)0--> pi K in relativistic heavy ion collisions. The transverse momentum spectrum of (K*0+K*0)/2 from central Au+Au collisions at sqrt[sNN]=130 GeV is presented. The ratios of the K*0 yield derived from these data to the yields of negative hadrons, charged kaons, and phi mesons have been measured in central and minimum bias collisions and compared with model predictions and comparable e+e-, pp, and p-barp results. The data indicate no dramatic reduction of K*0 production in relativistic heavy ion collisions despite expected losses due to rescattering effects.
Inclusive transverse momentum distributions of charged hadrons within 0.2<pT<6.0 GeV/c have been measured over a broad range of centrality for Au+Au collisions at sqrt[sNN]=130 GeV. Hadron yields are suppressed at high pT in central collisions relative to peripheral collisions and to a nucleon-nucleon reference scaled for collision geometry. Peripheral collisions are not suppressed relative to the nucleon-nucleon reference. The suppression varies continuously at intermediate centralities. The results indicate significant nuclear medium effects on high-pT hadron production in heavy-ion collisions at high energy.
We report the first measurement of strange ( Lambda ) and antistrange ( Lambda -bar) baryon production from sqrt[sNN]=130 GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Rapidity density and transverse mass distributions at midrapidity are presented as a function of centrality. The yield of Lambda and Lambda -bar hyperons is found to be approximately proportional to the number of negative hadrons. The production of Lambda -bar hyperons relative to negative hadrons increases very rapidly with transverse momentum. The magnitude of the increase cannot be described by existing hadronic string fragmentation models alone.
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 Au+Au collisions at sqrt[sNN]=130 GeV 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.
The STAR Collaboration reports the first observation of exclusive rho 0 photoproduction, AuAu-->AuAu rho 0, and rho 0 production accompanied by mutual nuclear Coulomb excitation, AuAu-->Au [star] Au [star] rho 0, in ultraperipheral heavy-ion collisions. The rho 0 have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at sqrt[sNN]=130 GeV agree with theoretical predictions treating rho 0 production and Coulomb excitation as independent processes.
We report STAR results on the azimuthal anisotropy parameter v2 for strange particles K0S, Lambda , and Lambda -bar at midrapidity in Au+Au collisions at sqrt[sNN]=130 GeV at the Relativistic Heavy Ion Collider. The value of v2 as a function of transverse momentum, pt, of the produced particle and collision centrality is presented for both particles up to pt~3.0 GeV/c. A strong pt dependence in v2 is observed up to 2.0 GeV/c. The v2 measurement is compared with hydrodynamic model calculations. The physics implications of the pt integrated v2 magnitude as a function of particle mass are also discussed.
Measurements of mass and Λ binding energy of 4ΛH and 4ΛHe in Au+Au collisions at sNN−−−√=3 GeV are presented, with an aim to address the charge symmetry breaking (CSB) problem in hypernuclei systems with atomic number A = 4. The Λ binding energies are measured to be 2.22±0.06(stat.)±0.14(syst.) MeV and 2.38±0.13(stat.)±0.12(syst.) MeV for 4ΛH and 4ΛHe, respectively. The measured Λ binding-energy difference is 0.16±0.14(stat.)±0.10(syst.) MeV for ground states. Combined with the γ-ray transition energies, the binding-energy difference for excited states is −0.16±0.14(stat.)±0.10(syst.) MeV, which is negative and comparable to the value of the ground states within uncertainties. These new measurements on the Λ binding-energy difference in A = 4 hypernuclei systems are consistent with the theoretical calculations that result in ΔB4Λ(1+exc)≈−ΔB4Λ(0+g.s.)<0 and present a new method for the study of CSB effect using relativistic heavy-ion collisions.
The transverse mass spectra and midrapidity yields for Xi s and Omega s are presented. For the 10% most central collisions, the Xi -bar+/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.
We report measurements of the longitudinal double-spin asymmetry, ALL, for inclusive jet and dijet production in polarized proton-proton collisions at midrapidity and center-of-mass energy s√ = 510 GeV, using the high luminosity data sample collected by the STAR experiment in 2013. These measurements complement and improve the precision of previous STAR measurements at the same center-of-mass energy that probe the polarized gluon distribution function at partonic momentum fraction 0.015 ≲x≲ 0.25. The dijet asymmetries are separated into four jet-pair topologies, which provide further constraints on the x dependence of the polarized gluon distribution function. These measurements are in agreement with previous STAR measurements and with predictions from current next-to-leading order global analyses. They provide more precise data at low dijet invariant mass that will better constraint the shape of the polarized gluon distribution function of the proton.
We report on the first measurements of J/ψ production at very low transverse momentum (pT< 0.2 GeV/c) in hadronic Au+Au collisions at √sNN = 200 GeV and U+U collisions at √sNN = 193 GeV. Remarkably, the inferred nuclear modification factor of J/ψ at mid-rapidity in Au+Au (U+U) collisions reaches about 24 (52) for pT< 0.05 GeV/c in the 60-80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the dN/dt distribution of J/ψ for the very low pT range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semi-central collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of J/ψ yield observed at extremely low pT originates from coherent photon-nucleus interactions. In particular, coherently produced J/ψ's in violent hadronic collisions may provide a novel probe of the quark-gluon-plasma.
We present measurements of the differential cross sections of inclusive J/ψ meson production as a function of transverse momentum (pJ/ψT) using the μ+μ− 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 μ+μ− channel is for 0 <pJ/ψT< 9 GeV/c and rapidity range |yJ/ψ|< 0.4, and that from the e+e− channel is for 4 <pJ/ψT< 20 GeV/c and |yJ/ψ|< 1.0. The ψ(2S) to J/ψ ratio is also measured for 4 <pmesonT< 12 GeV/c through the e+e− decay channel. Model calculations, which incorporate different approaches toward the J/ψ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties.
We report on the first measurements of J/ψ production at very low transverse momentum (pT< 0.2 GeV/c) in hadronic Au+Au collisions at √sNN = 200 GeV and U+U collisions at √sNN = 193 GeV. Remarkably, the inferred nuclear modification factor of J/ψ at mid-rapidity in Au+Au (U+U) collisions reaches about 24 (52) for pT< 0.05 GeV/c in the 60-80% collision centrality class. This noteworthy enhancement cannot be explained by hadronic production accompanied by cold and hot medium effects. In addition, the dN/dt distribution of J/ψ for the very low pT range is presented for the first time. The distribution is consistent with that expected from the Au nucleus and shows a hint of interference. Comparison of the measurements to theoretical calculations of coherent production shows that the excess yield can be described reasonably well and reveals a partial disruption of coherent production in semi-central collisions, perhaps due to the violent hadronic interactions. Incorporating theoretical calculations, the results strongly suggest that the dramatic enhancement of J/ψ yield observed at extremely low pT originates from coherent photon-nucleus interactions. In particular, coherently produced J/ψ's in violent hadronic collisions may provide a novel probe of the quark-gluon-plasma.
Data from the first physics run at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory, Au+Au collisions at sqrt[sNN]=130 GeV, have been analyzed by the STAR Collaboration using three-pion correlations with charged pions to study whether pions are emitted independently at freeze-out. We have made a high-statistics measurement of the three-pion correlation function and calculated the normalized three-particle correlator to obtain a quantitative measurement of the degree of chaoticity of the pion source. It is found that the degree of chaoticity seems to increase with increasing particle multiplicity.
Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics. This can lead to electric charge separation along the strong magnetic field in relativistic heavy-ion collisions – the chiral magnetic effect (CME). We report measurements by the STAR collaboration of a CME-sensitive observable in p + Au and d + Au collisions at 200 GeV, where the CME is not expected, using charge-dependent pair correlations relative to a third particle. We observe strong charge-dependent correlations similar to those measured in heavy-ion collisions. This bears important implications for the interpretation of the heavy-ion data.
J/ψ 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/ψ production at mid-rapidity through the dimuon decay channel in Au+Au collisions at √sNN = 200 GeV with the STAR experiment. These measurements became possible after the installation of the Muon Telescope Detector was completed in 2014. The J/ψ yields are measured in a wide transverse momentum (pT) 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/ψ yield is suppressed by a factor of approximately 3 for pT > 5 GeV/c relative to that in p + p collisions scaled by the number of binary nucleon-nucleon collisions. The J/ψ nuclear modification factor displays little dependence on pT in all centrality bins. Model calculations can qualitatively describe the data, providing further evidence for the color-screening effect experienced by J/ψ mesons in the QGP.
The polarization of Λ and Λ¯ hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at √sNN = 200 GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild pT dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagree with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and pT dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.
Energy spectra for p, d, t, 3He, 4He, and 6He from the reaction 12C+197Au at 35 MeV/nucleon are presented. A common intermediate rapidity source is identified using a moving source fit to the spectra that yields cross sections which are compared to analogous data at other bombarding energies and to several different models. The excitation function of the composite to proton ratios is compared with quantum statistical, hydrodynamic, and thermal models.
Measurement of complex fragments and clues to the entropy production from 42-137-MeV/nucleon Ar + Au
(1983)
Intermediate-rapidity fragments with A=1-14 emitted from 42-137-MeV/nucleon Ar + Au have been measured. Evidence is presented that these fragments arise from a common moving source. Entropy values are extracted from the mass distributions by use of quantum statistical and Hauser-Feshbach theories. The extracted entropy values of S/A≈2-2.4 are much smaller than the values expected from measured deuteron-to-proton ratios, but are still considerably higher than theoretically predicted values.
Proton emission in relativistic nuclear collisions is examined for events of low and high multiplicity, corresponding to large and small impact parameters. Peripheral reactions exhibit distributions of protons in agreement with spectator-participant decay modes. Central collisions of equal-size nuclei are dominated by the formation and decay of a fireball system. Central collisions of light projectiles with heavy targets exhibit an enhancement in sideward emission which is predicted by recent hydrodynamical calculations.
Inclusive energy spectra of protons, deuterons, and tritons were measured with a telescope of silicon and germanium detectors with a detection range for proton energies up to 200 MeV. Fifteen sets of data were taken using projectiles ranging from protons to 40Ar on targets from 27Al to 238U at bombarding energies from 240 MeV/nucleon to 2.1 GeV/nucleon. Particular attention was paid to the absolute normalization of the cross sections. For three previously reported reactions, He fragment cross sections have been corrected and are presented. To facilitate a comparison with theory the sum of nucleonic charges emitted as protons plus composite particles was estimated and is presented as a function of fragment energy per nucleon in the interval from 15 to 200 MeV/nucleon. For low-energy fragments at forward angles the protons account for only 25% of the nucleonic charges. The equal mass 40Ar plus Ca systems were examined in the center of mass. Here at 0.4 GeV/nucleon 40Ar plus Ca the proton spectra appear to be nearly isotropic in the center of mass over the region measured. Comparisons of some data with firestreak, cascade, and fluid dynamics models indicate a failure of the first and a fair agreement with the latter two. In addition, associated fast charged particle multiplicities (where the particles had energies larger than 25 MeV/nucleon) and azimuthal correlations were measured with an 80 counter array of plastic scintillators. It was found that the associated multiplicities were a smooth function of the total kinetic energy of the projectile. NUCLEAR REACTIONS U(20Ne,X), E / A=240 MeV/nucleon; U(40Ar,X), Ca(40Ar,X), U(20Ne,X), Au(20Ne,X), Ag(20Ne,X), Al(20Ne,X), U(4He,X), Al(4He,X), E / A=390 MeV/nucleon; U(40Ar,X), Ca(40Ar,X), U(20Ne,X), U(4He,X), U(p,X), E / A=1.04 GeV/nucleon; U(20Ne,X), E / A=2.1 GeV/nucleon; measured sigma (E, theta ), X=p,d,t.
Pion-production cross sections have been measured for the reaction 40Ar+40Ca--> pi ++X at a laboratory energy of 1.05 GeV/nucleon. A maximum in the pi + cross section occurs at mid-rapidity, which is anomalous relative to p+p and p+nucleus reactions and compared to many other heavy-ion reactions. Calculations based on cascade and thermal models fail to fit the data.
Pion production and charged-particle multiplicity selection in relativistic nuclear collisions
(1982)
Spectra of positive pions with energies of 15-95 MeV were measured for high energy proton, 4He, 20Ne, and 40Ar bombardments of targets of 27Al, 40Ca, 107,109Ag, 197Au, and 238U. A Si-Ge telescope was used to identify charged pions by dE / dx-E and, in addition, stopped pi + were tagged by the subsequent muon decay. In all, results for 14 target-projectile combinations are presented to study the dependence of pion emission patterns on the bombarding energy (from E / A=0.25 to 2.1 GeV) and on the target and the projectile masses. In addition, associated charged-particle multiplicities were measured in an 80-paddle array of plastic scintillators, and used to make impact parameter selections on the pion-inclusive data. NUCLEAR REACTIONS U(20Ne, pi +), E / A=250 MeV; U(40Ar, pi +), Ca(40Ar, pi +), U(20Ne, pi +), Au(20Ne, pi +), Ag(20Ne, pi +), Al(20Ne, pi +), U(4He, pi +), Al(4He, pi +). E / A=400 MeV; Ca(40Ar, pi +), U(20Ne, pi +), U(4He, pi +), U(p, pi +), E / A=1.05), GeV; U(20Ne, pi +), E / A=2.1 GeV; measured sigma (E, theta ), inclusive and selected on associated charged-particle multiplicity.
Double-differential cross sections have been measured for high-energy p, d, t, 3He, and 4He particles emitted from uranium targets irradiated with 20Ne ions at energies of 250, 400, and 2100 MeV/nucleon and 4He ions at 400 MeV/nucleon. By using the shape and yield of the proton energy spectra, the shape and yield of the d, t, 3He, and 4He energy spectra can be deduced at all measured angles for all incident projectile energies by assuming that they are formed by a coalescence of cascade nucleons, using a model analogous to that of Butler and Pearson, and Schwarzschild and Zupancic-caron.
A simple model is proposed for the emission of nucleons with velocities intermediate between those of the target and projectile. In this model, the nucleons which are mutually swept out from the target and projectile form a hot quasiequilibrated fireball which decays as an ideal gas. The overall features of the proton-inclusive spectra from 250- and 400-MeV/nucleon 20Ne ions and 400-MeV/nucleon 4He ions interacting with uranium are fitted without any adjustable parameters.
The energy spectra of protons and light nuclei produced by the interaction of 4He and 20Ne projectiles with Al and U targets have been investigated at incident energies ranging from 0.25 to 2.1 GeV per nucleon. Single fragment inclusive spectra have been obtained at angles between 25° and 150°, in the energy range from 30 to 150 MeV/nucleon. The multiplicity of intermediate and high energy charged particles was determined in coincidence with the measured fragments. In a separate study, fragment spectra were obtained in the evaporation energy range from 12C and 20Ne bombardment of uranium. We observe structureless, exponentially decaying spectra throughout the range of studied fragment masses. There is evidence for two major classes of fragments; one with emission at intermediate temperature from a system moving slowly in the lab frame, and the other with high temperature emission from a system propagating at a velocity intermediate between target and projectile. The high energy proton spectra are fairly well reproduced by a nuclear fireball model based on simple geometrical, kinematical, and statistical assumptions. Light cluster emission is also discussed in the framework of statistical models. NUCLEAR REACTIONS U(20Ne,X), E=250 MeV/nucl.; U(20Ne,X), U(α,X) E=400 MeV/nucl.; U(20Ne,X), Al(20Ne,X), E=2.1 GeV/nucl.; measured σ(E,θ), X=p, d, t, 3He,4He. U(20Ne,X), U(α,X), E=400 MeV/nucl.; U(20Ne,X), E=2.1 GeV/nucl.; measured σ(E, θ), Li to O. U(20Ne,X), U(12C,X), E=2.1 GeV/nucl.; measured σ(E, 90°), 4He to B. Nuclear fireballs, coalescence, thermodynamics of light nuclei production.