Particles In Au Research Articles

Nuclear Modification Factor of Inclusive Charged Particles in Au+Au Collisions at sNN = 27 GeV with the STAR Experiment

The Beam Energy Scan (BES) program at RHIC aims to explore the QCD phase diagram, including the search for the evidence of the 1st order phase transition from hadronic matter to Quark-Gluon Plasma (QGP) and the location of the QCD critical point. One of the features previously observed in the study of QGP is the effect of suppression of particle production with high transverse momenta pT (>2 GeV/c) at energies sNN = 62.4–200 GeV, which was deduced from the charged-particle nuclear modification factor (RCP) measured using the data from Beam Energy Scan Program Phase I (BES-I) of STAR experiment. In 2018, STAR has collected over 500 million events from Au+Au collisions at sNN = 27 GeV as a part of the STAR BES-II program, which is about a factor of 10 higher than BES-I 27 GeV data size. In this report, we present new measurements of charged particle production and the nuclear modification factor RCP, from this new 27 GeV data set and compare them with the BES-I results. The new measurements extend the previous BES-I results to higher transverse momentum range, which allows better exploration of the jet quenching effects at low RHIC energies, and may help to understand the effects of the formation and properties of QGP at these energies.

Directed flow and global polarization in Au+Au collisions across energies covered by the beam energy scan at RHIC

We study the directed flow of identified particles in Au+Au collisions at $\sqrt{s_\text{NN}}=7.7$ to 62.4 GeV. The Glauber model is extended to include both a tilted deformation of the QGP fireball with respect to the longitudinal direction and a non-zero longitudinal flow velocity gradient in the initial state. By combining this improved initial condition with a (3+1)-dimensional viscous hydrodynamic model calculation, we obtain a satisfactory description of the transverse momentum spectra and the rapidity dependent directed flow coefficient of different hadron species. Our calculation indicates the sensitivity of the hadron directed flow, especially its splitting between protons and antiprotons, to both the initial geometry and the initial longitudinal flow velocity. Therefore, the combination of directed flow of different hadrons can provide a tight constraint on the initial condition of nuclear matter created in heavy-ion collisions. The initial condition extracted from the directed flow is further tested with the global polarization of $\Lambda$ and $\bar{\Lambda}$ within the same theoretical framework, where we obtain a reasonable description of these hyperon polarization observed at different collision energies at RHIC.

Study of Bulk Properties of Strange Particles in Au+Au Collisions at sNN = 54.4 GeV.

We analyzed the transverse momentum pT spectra of various strange hadrons KS0, Λ(Λ¯) and Ξ-(Ξ¯+) at mid-rapidity (y) in different centrality intervals from Au+Au collisions at sNN= 54.4 GeV. The pT spectra of these strange hadrons are investigated by the Tsallis-like distribution, which satisfactorily fits the experimental data. The bulk properties of the medium produced in ultra-relativistic heavy-ion collisions at the kinetic freeze-out are reflected by measuring the hadron spectra. The effective temperature T, transverse flow velocity βT, and mean pT along with other parameters that are strongly dependent on centrality and particle specie are extracted. The effective temperature of multi-strange particle (Ξ-(Ξ¯+)) is larger as compared to singly-strange particles Λ(Λ¯) and KS0. Furthermore, the kinetic freeze-out temperature T, transverse flow velocity βT. and mean pT (⟨pT⟩) show a decreasing trend towards lower centrality, while the entropy parameter q increases from central to peripheral collisions. In addition, a positive correlation of ⟨pT⟩ and T and a negative correlation of q and T are also reported.

Measurement of Intermittency for Charged Particles in Au + Au Collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7-200 GeV from STAR

Local density fluctuations near the QCD critical point can be probed by intermittency analysis of scaled factorial moments in relativistic heavy-ion collisions. We report the first measurement of intermittency for charged particles in Au + Au collisions at \sqrt{s_\mathrm{NN}}sNN = 7.7-200 GeV from the STAR experiment at RHIC. We observe scaling behaviors in central Au + Au collisions, with the extracted scaling exponent decreasing from mid-central to the most central Au + Au collisions. Furthermore, the scaling exponent exhibits a non-monotonic energy dependence with a minimum around \sqrt{s_\mathrm{NN}}sNN = 20-30 GeV in central Au + Au collisions.

PACIAE 2.2.1: An updated issue of the parton and hadron cascade model PACIAE 2.2

The parton and hadron cascade model PACIAE 2.2 (cf. Yan et al. (2018) [11]) has been upgraded to the new issue of PACIAE 2.2.1 with the chiral magnetic effect (CME). The charge correlation parameter, γss and γos, which is typically used to “detect” the CME, is calculated by PACIAE 2.2.1 in Au+Au and Pb+Pb collisions, and the result is consistent with the STAR and ALICE data. As an example, PACIAE 2.2.1 model is employed to mimic the pseudorapidity distribution (dNch/dη) and transverse momentum spectra (pT) of charged particles in non-central Au+Au collisions at RHIC energy and Pb+Pb collisions at LHC energy. The chiral magnetic effect is found to partly affect the final-state basic observables in relativistic nuclear collisions. By this new issue a more realistic dynamical evolution of relativistic nuclear collisions can be depicted. Program summaryProgram Title: PACIAE version 2.2.1CPC Library link to program files:https://doi.org/10.17632/w3g68dj4d9.3Code Ocean capsule:https://codeocean.com/capsule/3695338Licensing provisions: CC By 4.0Programming language: FORTRAN 77 or GFORTRANJournal reference of previous version: Comput. Phys. Comm. 224 (2018) 417Does the new version supersede the previous version?: No.Reasons for the new version: In order that PACIAE 2.2.1 model is now able to simulate the CME-related physics and a more realistic dynamical evolution scenario in the relativistic nucleus-nucleus collisions.Summary of revisions: In PACIAE 2.2.1 model the chiral magnetic effect is introduced. The CME-induced charge separation mechanism [7] into the initial partonic states is artificially added by switching py values for a fraction f of the downward moving partons with those of the corresponding upward moving antipartons.Nature of problem: PACIAE is based on PYTHIA, where it has no any mechanisms to generate the chiral magnetic effect (CME). The CME is a novel transport phenomenon arising from the interaction between quantum anomalies and strong magnetic fields in the ultra-relativistic nuclear collisions. Charge separation is an important consequence of the CME. In order to depict the CME, the corresponding upgrade about introducing a global charge separation scheme into the initial partonic states is highly required.Solution method: The global CME-induced charge separation mechanism [7] into the initial partonic states is introduced by switching py values for a fraction f of the downward moving partons with those of the corresponding upward moving antipartons.Additional comments including restrictions and unusual features: PACIAE 2.2.1 has three versions of PACIAE 2.2.1a, 2.2.1b, and 2.2.1c. PACIAE 2.2.1a is for the elementary collision (here no upgraded from PACIAE 2.2a). PACIAE 2.2.1b and PACIAE 2.2.1c are for the nucleus-nucleus collision of A + A with input file of usu.dat. PACIAE 2.2.1b and PACIAE 2.2.1c are similar in the physical contents but are different in the topological structure [8].•Using the attached input file of usux.dat to run 1000 events for the s = 200 GeV Non Single Diffractive pp collision by PACIAE 2.2.1a takes ≈0.5 min.•Using the attached input file of usu.dat to run 10 events for the 35%-45% non-central Au+Au collisions at sNN = 200 GeV by PACIAE 2.2.1b takes ≈6 min.•Using the attached input file of usu.dat to run 10 events for the 35%-45% non-central Au+Au collisions at sNN = 200 GeV by PACIAE 2.2.1c takes ≈8 min.

Particle species and collision energy dependencies of the midrapidity average transverse momenta of identified charged particles in Au+Au and Pb+Pb collisions in (Snn)1/2 = 62-5020GeV energy range at RHIC and LHC

The experimental spectra of [Formula: see text] versus the average pseudorapidity multiplicity density, [Formula: see text], and [Formula: see text] dependencies of the identified charged particles at midrapidity in Au[Formula: see text]Au and Pb[Formula: see text]Pb collisions at RHIC and LHC in (s[Formula: see text]-5020[Formula: see text]GeV energy range are described well with the proposed simple power model function. The degree of flattening of [Formula: see text] of the charged pions and kaons, protons and antiprotons in the analyzed heavy-ion collisions at RHIC and LHC in (s[Formula: see text]-5020[Formula: see text]GeV energy range are investigated analyzing the dependencies of the obtained exponent parameter [Formula: see text] of the simple power model function on the particle species and collision energy (s[Formula: see text]. The coincidence of the parameter [Formula: see text] for pions and kaons in Pb[Formula: see text]Pb collisions at (s[Formula: see text][Formula: see text]TeV, reflecting practically identical shapes of [Formula: see text] versus the average pseudorapidity multiplicity density, [Formula: see text], and [Formula: see text] spectra for pions and kaons in these collisions, is obtained. This important result is interpreted as due to creation of the highly thermalized QGP, where the difference between [Formula: see text], [Formula: see text] and [Formula: see text] flavors practically disappears, resulting in similar mechanisms of production of pions and kaons in Pb[Formula: see text]Pb collisions at (s[Formula: see text][Formula: see text]TeV.

Ion beam synthesis of Au-Ag alloy nanoparticles in TiN thin films

Formation of Au-Ag alloy nanoparticles in TiN thin films is demonstrated through sequential implantation of Au and Ag ions. The irradiations were done with 200 keV Au ions at fluence of 1.0 × 1016 ions/cm2 and 150 keV Ag ions to the fluences in the range of 0.7–1.3 × 1017 ions/cm2. It was found that irradiations introduce around 1.5 at.% of gold and up to ~20 at.% of silver species causing also the significant sputtering of the surface. TiN surface besides the topographic undergoes chemical changes. The presence of silver and gold is confirmed by photoemission spectroscopy showing that the implanted ions did not form any kind of compound with the surrounding matrix. Electron microscopy evidenced the formation of crystalline and quasi-spherical particles with dimensions below 10 nm. Elemental analysis revealed that the obtained nanoparticles present Au-Ag alloys, with two elements mixed over the whole volume of the particles in different Au:Ag ratios.

Systematic investigation of azimuthal anisotropy in Au+Au and U+U collisions at

We present a study of azimuthal anisotropy parameter (vn) of produced particles in Au+Au and U+U collisions at top RHIC collision energy. The U+U collisions have been studied with and without including the deformed shape of the uranium nuclei. For this purpose, we have used a multi-phase transport (AMPT) model that includes the implementation of deformed uranium nuclei. It is found that the elliptic flow (v2) is higher in central U+U collisions with deformed uranium nuclei compared to Au+Au and spherical U+U. On the other hand, other higher harmonics (v3, v4, and v5) are found to be almost similar in magnitude for all three systems (Au+Au, deformed U+U, and spherical U+U). We have shown that the magnitude of v2/ϵ2 is higher (similar) in Au+Au collisions compared to deformed (spherical) U+U collisions at most central collisions. However, we believe v2/ϵ2 reflect true collectivity and more collectivity is expected in large nuclei (U235) compared to the small Au197 nucleus. This indicates that in the case of deformed nuclei, eccentricity (ϵ2) scaled v2 may not reflect true collectivity. On the other hand, magnitudes of v3/ϵ3, v4/ϵ4 and v5/ϵ5 for a given centrality are the same for Au+Au, deformed U+U, and spherical U+U collisions. These are interesting observations and can be checked in experimental data at RHIC. We have also compared Au+Au and U+U results with that of Cu+Cu for a better understanding of our findings.

Thermodynamic and transport properties in Au + Au collisions at RHIC energies from the clustering of color strings

In this work, we have extracted the initial temperature from the transverse momentum spectra of charged particles in Au + Au collisions using STAR data at RHIC energies from [Formula: see text] = 7.7–200 GeV. The initial energy density [Formula: see text], shear viscosity to entropy density ratio [Formula: see text], trace anomaly [Formula: see text], the squared speed of sound [Formula: see text], entropy density, and bulk viscosity to entropy density ratio [Formula: see text] are obtained and compared with the lattice QCD calculations for (2 + 1) flavor. The initial temperatures obtained are compared with various hadronization and chemical freeze-out temperatures. The analysis of the data shows that the deconfinement-to-confinement transition possibly takes place between [Formula: see text] = 11.5 and 19.6 GeV.

Spectra of negative particles in Au+Au collisions from BES-I at RHIC and z-scaling

Negative particle yields in Au + Au collisions obtained by the STAR Collaboration from BES-I at RHIC are analyzed in the framework of the z-scaling approach. The spectra were measured over a wide range of collision energy $\sqrt {{s_{NN}}} = 7.7 - 200\,{\rm{GeV}}$ and transverse momentum of produced particles for different centralities at $\left| \eta \right| \lt 0.5$. The scaling behavior of the spectra as a function of the collision energy and centrality in z-presentation is verified. The constituent energy loss in dependence on the energy and centrality of collision and transverse momentum of inclusive particle is estimated. New indication on self-similarity of fractal structure of nuclei and fragmentation process is found. A discontinuity of the model parameters - fractal and fragmentation dimensions and specific heat - is considered from the point of view of a phase transition in nuclear matter.

Chemical Potentials of Light Flavor Quarks from Yield Ratios of Negative to Positive Particles in Au+Au Collisions at RHIC

The transverse momentum spectra of π-, π+, K-, K+, p¯, and p produced in Au+Au collisions at center-of-mass energy sNN=7.7, 11.5, 19.6, 27, 39, 62.4, 130, and 200 GeV are analyzed in the framework of a multisource thermal model. The experimental data measured at midrapidity by the STAR Collaboration are fitted by the (two-component) standard distribution. The effective temperature of emission source increases obviously with the increase of the particle mass and the collision energy. At different collision energies, the chemical potentials of up, down, and strange quarks are obtained from the antiparticle to particle yield ratios in given transverse momentum ranges available in experiments. With the increase of logarithmic collision energy, the chemical potentials of light flavor quarks decrease exponentially.

Clustering of gold particles in Au implanted CrN thin films: The effect on the SPR peak position

We report on the formation of gold particles in 280nm thin polycrystalline CrN layers caused by Au+ ion implantation. The CrN layers were deposited at 150°C by d.c. reactive sputtering on Si(100) wafers and then implanted at room temperature with 150keV Au+ ions to fluences of 2×1016cm−2 to 4.1×1016cm−2. The implanted layers were analysed by the means of Rutherford backscattering spectrometry, X-ray diffraction, atomic force microscopy and spectroscopic ellipsometry measurements. The results revealed that the Au atoms are situated in the near-surface region of the implanted CrN layers. At the fluence of 2×1016cm−2 the formation of Au particles of ∼200nm in diameter has been observed. With increasing Au ion fluence the particles coalesce into clusters with dimensions of ∼1.7μm. The synthesized particles show a strong absorption peak associated with the excitation of surface plasmon resonances (SPR). The position of the SPR peak shifted in the range of 426.8–690.5nm when the Au+ ion fluence was varied from 2×1016cm−2 to 4.1×1016cm−2. A correlation of the shift in the peak wavelength caused by the change in the particles size and clustering has been revealed, suggesting that the interaction between Au particles dominate the surface plasmon resonance effect.

Pseudorapidity distributions of the produced charged particles in nucleus-nucleus collisions at low energies at the BNL relativistic heavy ion collider

The revised Landau hydrodynamic model is used to discuss the pseudorapidity distributions of the produced charged particles in Au+Au and Cu+Cu collisions at energies of √sNN = 19.6 and 22.4 GeV respectively at the BNL Relativistic Heavy Ion Collider. It is found that the revised Landau hydrodynamic model alone can give a good description of the experimental measurements. This is different from the result with the same collisions but at the maximum energy of √sNN = 200 GeV, where in addition to the revised Landau hydrodynamic model, the effects of leading particles have to be taken into account in order to explain the experimental observations. This can be attributed to the different degrees of transparency of participants at the different incident energies.

Collective flow of light particles in Au + Au collisions at intermediate energies

The Skyrme potential energy density functional is introduced into the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model and the updated version is applied to studying the directed and elliptic flows of light particles (protons, neutrons, deuterons, tritons, $^3$He and $^4$He) in $^{197}$Au+$^{197}$Au collisions at beam energies 150, 250 and 400 MeV/nucleon. The results are compared with the recent FOPI experimental data. It is found that the yields and collective flows of light particles can be described quite well. The influence of the equation of state (EoS), medium-modified nucleon-nucleon elastic cross sections (NNECS) and cluster recognition criteria on the directed and elliptic flows is studied in detail. It is found that the flows of light particles are sensitive to the medium-modified NNECS, but not sensitive to the isospin dependent cluster recognition criteria. It seems difficult, however, even with the new data and calculations, to obtain a more accurate constraint on the nuclear incompressibility $K_0$ than the interval 200-260 MeV.

Charged Particle Pseudorapidity Distributions for Pb-Pb and Au-Au Collisions Using Neural Network Model

The artificial neural network (ANN) approach is used to model the Pb–Pb and Au–Au collisions on the basis of the Levenberg–Marquardt learning algorithm. We simulate the rapidity distribution for п- and к+- produced in Pb–Pb collisions at different energies and the pseudorapidity distribution of charged particles in Au–Au collisions. Our functions obtained within the ANN model show a very good agreement with the experimental data for both types of collisions, which indicates that the trained network takes on the optimal generalization performance. Thus, the ANN model can be widely applied to the modeling of heavy-ion collisions.

Third harmonic flow of charged particles in Au+Au collisions atsNN=200 GeV

We report measurements of the third harmonic coefficient of the azimuthal anisotropy, v_3, known as triangular flow. The analysis is for charged particles in Au+Au collisions at sqrtsNN = 200 GeV, based on data from the STAR experiment at the Relativistic Heavy Ion Collider. Two-particle correlations as a function of their pseudorapidity separation are fit with narrow and wide Gaussians. Measurements of triangular flow are extracted from the wide Gaussian, from two-particle cumulants with a pseudorapidity gap, and also from event plane analysis methods with a large pseudorapidity gap between the particles and the event plane. These results are reported as a function of transverse momentum and centrality. A large dependence on the pseudorapidity gap is found. Results are compared with other experiments and model calculations.

RCP and RAA Measurements of Identified and Unidentified Charged Particles at High pT in Au + Au Collisions at sNN=7.7,11.5,19.6,27,39,and62.4GeV in STAR

Abstract The suppression of high p T hadrons in 200 GeV Au + Au collisions at RHIC has been seen as a signature for a partonic medium being formed. The evolution of this key QGP signature is a powerful tool for studying the QCD phase structure in the RHIC Beam Energy Scan. In these proceedings, we present measurements of identified π + , K + , and p, as well as unidentified charged particles in Au + Au collisions at s N N = 7.7 , 11.5 , 19.6 , 27 , 39 , and 62.4 GeV . We report nuclear modification factors R C P and R A A where published p + p references are available. These results offer insight into the s N N dependence of high p T suppression in nuclear collisions.

Third Harmonic Flow of Charged Particles in Au+Au Collisions at GeV

In this proceedings, we report measurements of the third harmonic coefficient of the azimuthal anisotropy, v3, known as triangular flow. The analysis is for charged particles near midrapidity in Au+Au collisions at GeV, based on data from the STAR experiment at the Relativistic Heavy Ion Collider. Triangular flow as a function of centrality, pseudorapidity and transverse momentum are reported using various methods, including a study of the signal for particle pairs as a function of their pseudorapidity separation. Results are compared with other experiments and model predictions.

Probing the QCD phase boundary with elliptic flow in relativistic heavy ion collisions at STAR

Abstract We present measurement of elliptic flow, v 2, for charged and identified particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } $ = 7.7–39 GeV. We compare the inclusive charged hadron v 2 to those from transport model calculations, such as the UrQMD model, AMPT default model and AMPT string-melting model. We discuss the energy dependence of the difference in v 2 between particles and anti-particles. The v 2 of ϕ meson is observed to be systematically lower than other particles in Au+Au collisions at $\sqrt {s_{NN} } $ = 11.5 GeV.

Strangeness Enhancement in Cu-Cu and Au-Au Collisions atsNN=200 GeV

We report new STAR measurements of midrapidity yields for the Λ, Λ[over ¯], K(S)(0), Ξ(-), Ξ[over ¯](+), Ω(-), Ω[over ¯](+) particles in Cu+Cu collisions at √S(NN)==200 GeV, and midrapidity yields for the Λ, Λ[over ¯], K(S)(0) particles in Au+Au at √S(NN)==200 GeV. We show that, at a given number of participating nucleons, the production of strange hadrons is higher in Cu+Cu collisions than in Au+Au collisions at the same center-of-mass energy. We find that aspects of the enhancement factors for all particles can be described by a parametrization based on the fraction of participants that undergo multiple collisions.