Beam Energy Dependence Research Articles

Production of Kaon andΛin Nucleus-Nucleus Collisions at Ultrarelativistic Energy from a Blast-Wave Model

The particle production of Kaon andΛis studied in nucleus-nucleus collisions at relativistic energy based on a chemical equilibrium blast-wave model. The transverse momentum spectra of Kaon andΛat the kinetic freeze-out stage from our model are in good agreement with the experimental results. The kinetic freeze-out parameters of temperatureTkinand radial flow parameterρ0are presented for the FOPI, RHIC, and LHC energies. And the resonance decay effect is also discussed. The systematic study for beam energy dependence of the strangeness particle production will help us to better understand the properties of the matter created in heavy-ion collisions at the kinetic freeze-out stage.

Beam energy dependence of the expansion dynamics in relativistic heavy ion collisions: Indications for the critical end point?

The flow harmonic $v_{n}$ and the emission source radii $R_{\text{out}}$, $R_{\text{side}}$ and $R_{\text{long}}$ are studied for a broad range of centrality selections and beam collision energies in Au+Au ($\sqrt{s_{NN}}= 7.7 - 200$ GeV) and Pb+Pb ($\sqrt{s_{NN}}= 2.76$ TeV) collisions at RHIC and the LHC respectively. They validate the acoustic scaling patterns expected for hydrodynamic-like expansion over the entire range of beam energies studied. The combined data sets allow estimates for the \sqsn\ dependence of the mean expansion speed $\left<c_s\right>$, emission duration $\left<\Delta\tau\right>$ and the viscous coefficients $\left<\beta''\right>$ that encode the magnitude of the specific shear viscosity $\left<\eta/s\right>$. The estimates indicate initial-state model independent values of $\left<\eta/s\right>$ which are larger for the plasma produced at 2.76 TeV (LHC) compared to that produced at 200 GeV (RHIC) ($\left<4\pi\eta/s\right>_{\text{LHC}}=2.2\pm 0.2$ and $\left<4\pi\eta/s\right>_{\text{RHIC}}=1.3\pm 0.2$). They also show a non-monotonic \sqsn\ dependence for $\left<\beta''\right>$, $\left<c_s\right>$ and $\left<\Delta\tau\right>$, with minima for $\left<\beta''\right>$ and $\left<c_s\right>$, and a complimentary maximum for $\left<\Delta\tau\right>$. These dependencies signal a significant change in reaction dynamics in a narrow span of $\sqrt{s_{NN}}$, which may be linked to reaction trajectories close to the critical end point (CEP) in the phase diagram for nuclear matter.

Beam energy dependence of dielectron production in Au+Au collisions from STAR at RHIC

We present the energy-dependent study of dielectron production in 0-80% minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ energies of 19.6, 27, 39, and 62.4 GeV in STAR. Invariant mass ($M_{ee}$) and transverse momentum ($p_T$) differential measurements of dielectron yields are compared to cocktail simulations of known hadronic sources and semi-leptonic charmed decays. The enhancement (excess yield) prominent in the Low-Mass Region (LMR) over the cocktail at all energies, is further compared to calculations of $\rho$ in-medium modifications. Within statistical and systematic uncertainties, we find that the model consistently describes this enhancement from SPS up to top RHIC energies in its $M_{ee}$- as well as $p_T$-dependence. Dielectron measurements drive the statistics for the future BES Phase-II program, which promises to improve our understanding of the LMR enhancement's trend with total baryon densities.

Beam Energy Dependence of Clan Multiplicity at RHIC

In this paper, STAR's measurement of clan multiplicity is presented for AuAu collisions at = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV, for a variety of centrality classes. The mean number of particles per clan is found to decrease with decreasing centrality. Within the same centrality class, the mean number of particles per clan exhibits a reduction between 19.6 GeV and 62 GeV, with the minimum around 27 GeV. The structure is visible for most centralities, and most prominent for central collisions.

Recent measurements of quarkonium production in p+p and A + A collisions from the STAR experiment

Abstract We report the measurements by the STAR Collaboration of J / ψ invariant yields as a function of transverse momentum at mid-rapidity ( | y | 1.0 ) in p + p collisions at s = 500 GeV , in Au + Au collisions at s NN = 39 , 62.4 and 200 GeV, and in U + U collisions at s NN = 193 GeV . The centrality, beam energy and collision system dependences of J / ψ production and nuclear modification factors are discussed. We also report the ratio of ψ ( 2 S ) to J / ψ yields in p + p collisions at s = 500 GeV and ϒ production in U + U collisions at s NN = 193 GeV .

Beam energy dependence of moments of the net-charge multiplicity distributions in Au+Au collisions at RHIC.

We report the first measurements of the moments--mean (M), variance (σ(2)), skewness (S), and kurtosis (κ)--of the net-charge multiplicity distributions at midrapidity in Au+Au collisions at seven energies, ranging from sqrt[sNN]=7.7 to 200 GeV, as a part of the Beam Energy Scan program at RHIC. The moments are related to the thermodynamic susceptibilities of net charge, and are sensitive to the location of the QCD critical point. We compare the products of the moments, σ(2)/M, Sσ, and κσ(2), with the expectations from Poisson and negative binomial distributions (NBDs). The Sσ values deviate from the Poisson baseline and are close to the NBD baseline, while the κσ(2) values tend to lie between the two. Within the present uncertainties, our data do not show nonmonotonic behavior as a function of collision energy. These measurements provide a valuable tool to extract the freeze-out parameters in heavy-ion collisions by comparing with theoretical models.

On moments of the multiplicity events of slow target fragments in relativistic Sulfur-ion collisions

A detailed study on the multiplicity characteristics of the slow target fragments emitted in relativistic heavy-ion collisions has been carried out at E Lab = 3.7 A and 200A GeV using 32 S projectile. The beam energy dependence of the black particles produced in the full phase space of 32 S -emulsion (32 S -Em) interactions on the target size in terms of their moments (mean, variance, skewness and kurtosis) is investigated. The various order moments of target fragments emitted in the interactions of 32 S beams with the heavy ( AgBr ) target nuclei are estimated in the forward (FHS) and backward (BHS) hemispheres. The investigated values of ratio of variance to mean at both energies show that the multiplicity distributions (MDs) are not Poissonian and the strongly correlated emission of target fragments are in the forward directions. The degree of anisotropic fragment emission and nature of correlation among the emitted fragments are investigated. The energy dependence of entropy is examined in both hemispheres. The entropy values normalized to average multiplicity are found to be energy independent. Scaling of MD of black particles produced in these interactions has been studied to verify the validity of scaling hypothesis via two scaling (Koba–Nielsen–Olesen (KNO)-scaling and Hegyi-scaling) functions. A simplified universal function has been used in each scaling to display the experimental data.

Beam-energy dependence of charge separation along the magnetic field in Au+Au collisions at RHIC.

Local parity-odd domains are theorized to form inside a quark-gluon plasma which has been produced in high-energy heavy-ion collisions. The local parity-odd domains manifest themselves as charge separation along the magnetic field axis via the chiral magnetic effect. The experimental observation of charge separation has previously been reported for heavy-ion collisions at the top RHIC energies. In this Letter, we present the results of the beam-energy dependence of the charge correlations in Au+Au collisions at midrapidity for center-of-mass energies of 7.7, 11.5, 19.6, 27, 39, and 62.4GeV from the STAR experiment. After background subtraction, the signal gradually reduces with decreased beam energy and tends to vanish by 7.7GeV. This implies the dominance of hadronic interactions over partonic ones at lower collision energies.

SU‐E‐T‐526: On the Linearity, Stability and Beam Energy Dependence of CdSe Quantum Dots as Scintillating Probes

Purpose:Structure and energy transfer mechanisms confer colloidal quantum dots (cQDs) interesting properties, among them their potential as scintillators. CdSe multi‐shell cQDs in powder were investigated under photons irradiation. The purpose of this work is to characterize signal to dose linearity, stability with time and to quantify the dependence of their light output with beam energy.Methods:The cQDs are placed at the extremity of a non‐scintillating plastic collecting fiber, with the other extremity connected to an Apogee U2000C CCD camera. The CCD camera collects the fluorescence light from irradiated cQDs from which the delivered dose is extracted. This signal is corrected for Cerenkov contamination at MV energies using the chromatic technique. The detector was irradiated with two devices: Xstrahl 200 orthovoltage unit for 120, 180 and 220 kVp and a Varian Clinac iX for 6 and 23 MV.Results:Linear output response with varying dose is observed for all beam energies with R2 factors &gt; 0,999. Reproducibility measurements were performed at 120 kVp: the same set‐up was irradiated at different time intervals (one week and three months). The results showed only a small relative decrease of light output of 3,2 % after a combine deposited dose of approximately 95 Gy. CdSe nanocrystals response has been studied as a function of beam energy. The output increases with decreasing energy from 120 kVp to 6 MV and increase again for 23 MV. This behavior could be explained in part by the cQDs high‐Z composition.Conclusion:The fluorescence light output of CdSe cQDs was found to be linear as a function of dose. The results suggest stability of the scintillation output of cQDs over time. The specific composition of cQDs is the main cause of the observed energy dependence. We will further look into particle beam dependence of the cQDs. Bourse d' excellence aux etudes graduees du CRC (Centre de Recherche sur le Cancer, Universite Laval)Bourse d' excellence aux etudes graduees du CRC (Centre de Recherche sur le Cancer, Universite Laval)

Beam Energy Dependence of Strange Hadron Production from STAR at RHIC

We report the STAR measurements of Ks, Λ, Ξ, and Ω at mid-rapidity from Au+Au collisions at the beam energies of 7.7, 11.5, 19.6, 27 and 39 GeV per nucleon from the RHIC Beam Energy Scan (BES) program. Nuclear modification factors and ratios of baryon to meson yields are discussed. We also investigate the strangeness enhancement and ratios of anti-baryon to baryon yields as a function of beam energy at RHIC, and compare the results with the statistical thermal model.

Beam-energy dependence of the directed flow of protons, antiprotons, and pions in Au+Au collisions.

Rapidity-odd directed flow (v1) measurements for charged pions, protons, and antiprotons near midrapidity (y=0) are reported in sNN=7.7, 11.5, 19.6, 27, 39, 62.4, and 200GeV Au+Au collisions as recorded by the STAR detector at the Relativistic Heavy Ion Collider. At intermediate impact parameters, the proton and net-proton slope parameter dv1/dy|y=0 shows a minimum between 11.5 and 19.6GeV. In addition, the net-proton dv1/dy|y=0 changes sign twice between 7.7 and 39GeV. The proton and net-proton results qualitatively resemble predictions of a hydrodynamic model with a first-order phase transition from hadronic matter to deconfined matter, and differ from hadronic transport calculations.

Spectroscopic study of bismuth ions with an electron beam ion trap

We present the electron beam energy dependence of the emission spectra of multiply charged Bi ions measured with an electron beam ion trap in the wavelength range of the water window. Experimental results are compared to the theoretical ones calculated with the flexible atomic code.

Nonisotropic and nonsingle explosion in central129Xe +120Sn collisions at 50–125 MeV/nucleon

The expanding dynamics in central ${}^{129}$ Xe + ${}^{120}$Sn collisions at 50 MeV/nucleon is investigated within the framework of the isospin-dependent quantum molecular dynamics model in combination with the statistical decay code gemini. The nonisotropic distributions in the fragment size and average kinetic energy are reproduced. The collective expansion energy and the Coulomb explosion energy of different fragments are extracted. We focus on the nonisotropic explosion of the colliding system and the difference of explosion energies of H, He, Li, and Be fragments. The radial flow energies are extracted at the polar angle ${\ensuremath{\theta}}_{c.m.}$ = 90${}^{\ensuremath{\circ}}$. From studying the beam energy dependence at 50--125 MeV/nucleon, the threshold energies of the radial flow are investigated. The time evolutions of the density and the collective velocity are investigated. It is found that the radial flow causes the system to expand in the isotropic direction, whereas the residual incident energy causes the system to extend along the longitudinal direction. The Pauli blocking is responsible for the nonisotropic nature of the collective velocity.

Energy Dependence of Moments of Net-Proton Multiplicity Distributions at RHIC

We report the beam energy (sqrt[sNN]=7.7-200 GeV) and collision centrality dependence of the mean (M), standard deviation (σ), skewness (S), and kurtosis (κ) 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 phase diagram. The products of the moments, Sσ and κσ2, 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 antiproton 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.

Study of the multiplicity distributions in relativistic nucleus - nucleus collisions using the multiplicity distribution moments method

In this work, we will present beam energy and system size dependence of the higher order moments (skewness and kurtosis) of net-kaon and total proton multiplicity distributions for Au+Au collisions simulated using the UrQMD code.

Hard Probes at RHIC

Measurements of jets and heavy flavour, the so called hard probes, play a crucial role in understanding properties of hot and dense nuclear matter created in high energy heavy-ion collisions. The measurements at the Relativistic Heavy Ion Collider (RHIC) showed that in central Au+Au collisons at RHIC energy ( √sNN = 200 GeV) the nuclear matter created has properties close to those of perfect liquid, manifests partonic degrees of freedom and is opaque to hard probes. In order to draw quantitative conclusions on properties of this hot and dense nuclear matter reference measurements in proton-proton collisions and d+Au collisions are essential to estimate cold nuclear matter effects. In this proceedings a review of recent results on hard probes measurements in p+p, d+Au and A+A collisions as well as of beam energy dependence of jet quenching from STAR and PHENIX experiments at RHIC is presented.

Hadronic freeze-out in an effective relativistic mean field model

We investigate an effective relativistic equation of state at finite values of temperature and baryon chemical potential with the inclusion of the full octet of baryons, the Delta-isobars and the lightest pseudoscalar and vector meson degrees of freedom. These last particles have been introduced within a phenomenological approach by taking into account of an effective chemical potential and mass depending on the self-consistent interaction between baryons. In this framework, we study of the hadron yield ratios measured in central heavy ion collisions over a broad energy range and present the beam energy dependence of underlying dynamic quantities like the net baryon density and the energy density.

Beam energy and centrality dependence of the statistical moments of the net-charge and net-kaon multiplicity distributions in Au + Au collisions at STAR

In part to search for a possible critical point (CP) in the phase diagram of hot nuclear matter, a Beam Energy Scan was performed at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory. The STAR experiment collected significant Au+Au data sets at beam energies, $\sqrt{{\rm s}_{\rm NN}}$, of 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV. Lattice and phenomenological calculations suggest that the presence of a CP might result in divergences of the thermodynamic susceptibilities and correlation length. The statistical moments of the multiplicity distributions of particles reflecting conserved quantities, such as net-charge and net-strangeness, are expected to depend sensitively on these correlation lengths, making them attractive tools in the search for a possible critical point. The centrality and beam-energy dependence of the statistical moments of the net-charge multiplicity distributions will be discussed. The observables studied include the lowest four statistical moments (mean, variance, skewness, kurtosis) and the products of these moments. The measured moments of the net-kaon multiplicity distributions will also be presented. These will be compared to the predictions from approaches lacking critical behavior, such as the Hadron Resonance Gas model and Poisson statistics.

Event anisotropy v2 in Au + Au collisions at sNN=7.7–62.4 GeV with STAR

We present the $v_2$ measurement at midrapidity from Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV for inclusive charged hadrons and identified hadrons ($\pi^{\pm}$, $K^{\pm}$, $K_{S}^{0}$, $p$, $\bar{p}$, $\phi$, $\Lambda$, $\bar{\Lambda}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$, $\bar{\Omega}^{+}$) up to 4 GeV/$c$ in $p_{T}$. The beam energy and centrality dependence of charged hadron $v_2$ are presented with comparison to higher energies at RHIC and LHC. The identified hadron $v_{2}$ are used to discuss the NCQ scaling for different beam energies. Significant difference in $v_{2}(p_{T})$ is observed between particles and corresponding anti-particles for $\sqrt{s_{NN}} <$ 39 GeV. These differences are more pronounced for baryons compared to mesons and they increase with decreasing energy.

Beam energy dependence of strange hadron production from STAR at RHIC

We present STAR measurements of K^{0}_{S}, \phi, \Lambda, \Xi, and \Omega at mid-rapidity from Au+Au collisions at \sqrt{s_{NN}} = 7.7, 11.5, 19.6, 27, and 39 GeV from the Beam Energy Scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC). Nuclear modification factors and baryon-to-meson ratios are measured to understand recombination and parton energy loss mechanisms. Implications on partonic versus hadronic dynamics at low beam energies are discussed.