Au Central Collisions Research Articles

Possible origin of HADES data on proton number fluctuations in Au+Au collisions

Recent data of the HADES Collaboration in Au+Au central collisions at sNN=2.4 GeV indicate large proton number fluctuations inside one unit of rapidity around midrapidity. This can be a signature of critical phenomena due to the strong attractive interactions between baryons. We study an alternative hypothesis that these large fluctuations are caused by the event-by-event fluctuations of the number of bare protons, and no interactions between these protons are assumed. The proton number fluctuations in five symmetric rapidity intervals Δy inside the region ΔY=1 are calculated using the binomial acceptance procedure. This procedure assumes the independent (uncorrelated) emission of protons, and it appears to be in agreement with the HADES data. To check this simple picture we suggest to calculate the correlation between proton multiplicities in non-overlapping rapidity intervals Δy1 and Δy2 placed inside ΔY=1.

Higher cumulants of baryon number in critical QCD

We study the higher moments of the baryon number in the immediate neighbourhood of the QCD critical endpoint within the framework of Ising-QCD thermodynamics of a finite-size critical system (N. G. Antoniou et al., Phys. Rev. D 97, 034015, 2018). We show that the kurtosis, as a function of the freeze-out baryon chemical potential, becoming negative attains a sharp minimum very close to the critical point. We argue that the sharpness of this minimum is due to the narrowness of the critical region in the chemical potential direction. Our analysis reveals that the broad minimum of the kurtosis (with a positive kurtosis value) observed in Au+Au central collisions at STAR (in RHIC-BES I) in the colliding energy region 17 GeV <s<39 GeV is apparently only a precursor of the critical point and not a signature of its location.

Rapidity Distribution of Charged Pions From Pb+Pb and Au+Au Central Collisions

Rapidity distributions for charged pions produced from central collisions Pb+Pb and Au+Au at (b ≤ 3.4 fm.) in range of energy 2 GeV up to  = 200 GeV are investigated. The experimental results are studied  in terms of the Ultra-relativistic Quantum Molecular Dynamic Model Ur-QMD. In general, the model can give suitable predictions of rapidity distributions for production of charged pions from interactions with energies below 160 A GeV. This model is supported the assumption that the onset of de-confinement is located at low energies. It treats the initial nucleon-nucleon interactions within a string-hadronic framework. In addition, it includes effects such as string-string interactions and hadronic re-scattering that expected to be relevant in A+A collisions. There is a kind of similarity for mechanism responsible for production of the two charge states of pions and it can extended to other hadrons. Ultra-relativistic quantum molecular dynamic model give good description on energy dependence for charged pion productions and points of maximum rapidity.Â

Improvement on Fermionic properties and new isotope production in molecular dynamics simulations

By considering momentum transfer in the Fermi constraint procedure, the stability of the initial nuclei and fragments produced in heavy-ion collisions can be further improved in quantum molecular dynamics simulations. The case of a phase-space occupation probability larger than one is effectively reduced with the proposed procedure. Simultaneously, the energy conservation can be better described for both individual nuclei and heavy-ion reactions. With the revised version of the improved quantum molecular dynamics model, the fusion excitation functions of 16O+186W and the central collisions of Au+Au at 35 AMeV are re-examined. The fusion cross sections at sub-barrier energies and the charge distribution of fragments are relatively better reproduced due to the reduction of spurious nucleon emission. The charge and isotope distribution of fragments in Xe+Sn, U+U and Zr+Sn at intermediate energies are also predicted. More unmeasured extremely neutron-rich fragments with Z = 16–28 are observed in the central collisions of 238U+238U than that of 96Zr+124Sn, which indicates that multi-fragmentation of U+U may offer a fruitful pathway to new neutron-rich isotopes.

Effect of partonic cascade on two-pion HBT parameters of the core-halo source in relativistic heavy-ion collisions

Using a multi-phase transport (AMPT) model, we investigate the effect of partonic cascade on two-pion HBT parameters of the core–halo source in relativistic heavy-ion collisions. The evolution time as a function of the partonic cross section for Au+Au central collisions at GeV shows that the evolution time of the pion emission source is decreased with an increasing partonic cross section, and the drop is due to the evolution time of the core. The partonic cascade with a larger cross section leads to a shorter lifetime of the source.

Shear Viscosity to Entropy Density Ratio in Au+Au Central Collisions

The shear viscosity (η) and entropy density (s) are studied for the central Au+Au collisions with the help of a microscopic transport model, namely the isospin-dependent quantum molecular dynamic model (IQMD). Employ the formula given in Ref.[1], η is calculated as a time dependent variable for different incident energies from 40 MeV/u to 120 MeV/u and the energy dependence of η/s of nuclear matter in the most compressed stage in collision process is displayed.

Study of High p T pion production with CBM at SIS 100 energies

Transverse momentum spectra of π± and the π+/π−—ratios are obtained for the Au + Au central collisions at the energies of 6, 8 and 10 AGeV. The particles identification has been done using the time-of-flight information from RPCs. The pions with large values of the transverse momentum can provide the information of the coexistence phase even in the energy domain of SIS100.

Direct photons in nuclear collisions at fair energies

Using the extrapolation of existing data, estimations of prompt-photon production at FAIR energies have been made. At y = y c.m. the rapidity density of prompt photons with p t > 1.5 GeV/c per central Au + Au event at 25 A GeV is estimated as ∼10−4. With the planned beam intensity 109 per second and 1% interaction probability, for 10% of most central events one can expect the prompt-photon rate ∼102 photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings πρ → πγ, ππ → ργ have been analyzed. Photons from short-living resonances (e.g., ω → π 0 γ) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the directphoton spectrum at p t = 0.5–1 GeV/c. At the FAIR energy 25 A GeV in Au + Au central collisions the HSD generator predicts, as a lower estimate, γ direct/\( \gamma _{\pi ^0 } \) ≃ 0.5% in the region p t = 0.5–1 GeV/c. At p t = 1.5–2 GeV/c γ prompt/\( \gamma _{\pi ^0 } \) ≃ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct-photon spectrumis very sensitive to the initial temperature parameter T 0 of the model. The 10-MeV increase in the T 0 value leads to ∼2 times higher photon yield.

Successes and failures with hard probes

The two major pillars of searches for the Quark Gluon Plasma have been: J/$\Psi$ suppression, proposed in 1986, and observed at both SPS fixed target energies and at RHIC; and, more recently, the suppression of $\pi^0$ with $p_T\geq 3$ GeV/c by a factor $\sim 5$ in Au+Au central collisions, observed at RHIC in 2001, which had been predicted in advance as a consequence of Landau-Pomeranchuck-Migdal coherent (gluon) bremsstrahlung by the outgoing hard-scattered partons traversing the medium. However, new effects were discovered and the quality of the measurements greatly improved so that the clarity of the original explanations has become obscured. For instance: J/$\Psi$ suppression is the same at SpS and RHIC. Is it the QGP, comovers, something else? QCD provides beautiful explanations of $\pi^0$ and direct $\gamma$ measurements in p-p collisions but precision fits of the best theories of $\pi^0$ suppression barely agree with the Au+Au data. Better data are needed for $10< p_T <20$ GeV/c, systematic errors are needed in theory calculations, the values of parameters of the medium such as $\mean{\hat{q}}$ derived from precision fits are the subject of controversy. ... These and other issues will be discussed with a view to identify which conclusions are firm and where further progress towards real understanding is required.

CENTRALITY, TRANSVERSE MOMENTUM AND PSEUDORAPIDITY DEPENDENCES OF “MACH-LIKE” CORRELATIONS IN A PARTONIC TRANSPORT MODEL

It has been observed a “Mach-like” structure of di-hadron azimuthal correlations in Au + Au central collisions at [Formula: see text] in the framework of a multi-phase transport model (AMPT) with both partonic and hadronic interactions. In this paper, we will show centrality, transverse momentum (pT) and pseudorapidity (η) dependences of “Mach-like” structure. The splitting parameter D, i.e. half distance between two splitting peaks on away side, decreases with the impact parameter b (corresponding to centrality) and slightly increases with transverse momentum of associated hadrons [Formula: see text], which from string melting AMPT version is consistent with experimental results. And the splitting parameter D is flat in mid-pseudorapidity region and rapidly decreases with the increasing of high-|η assoc |.

Hard scattering and jets: from pp collisions in the 1970’s to Au + Au collisions at RHIC

Hard scattering in pp collisions, discovered at the CERN ISR in 1972 by the method of leading particles, proved that the partons of Deeply Inelastic Scattering strongly interacted with each other. Further ISR measurements utilizing inclusive single or pairs of hadrons established that high $p_{\rm T}$ particles are produced from states with two roughly back-to-back jets which are the result of scattering of constituents of the nucleons as described by Quantum Chromodynamics (QCD), which was developed during the course of these measurements. These techniques, which are the only practical method to study hard-scattering and jet phenomena in Au + Au central collisions at RHIC energies, are reviewed, with application to present RHIC measurements.

From the ISR to RHIC—measurements of hard-scattering and jets using inclusive single particle production and 2-particle correlations

Hard scattering in p-p collisions, discovered at the CERN ISR in 1972 by the method of leading particles, proved that the partons of Deeply Inelastic Scattering strongly interacted with each other. Further ISR measurements utilizing inclusive single or pairs of hadrons established that high pT particles are produced from states with two roughly back-to-back jets which are the result of scattering of constituents of the nucleons as described by Quantum Chromodynamics (QCD), which was developed during the course of these measurements. These techniques, which are the only practical method to study hard-scattering and jet phenomena in Au+Au central collisions at RHIC energies, are reviewed, as an introduction to present RHIC measurements.

Experimental signals of phase transition

The connection between the thermodynamics of charged finite nuclear systems and the asymptotically measured partitions is presented. Some open questions, concerning in particular equilibrium partitions are discussed. We show a detailed comparison of the decay patterns in Au + C, Cu, Au central collisions and in Au quasi-projectile events. Observation of abnormally large fluctuations in carefully selected samples of data is reported as an indication of a first order phase transition (negative heat capacity) in the nuclear equation of state.

Experimental signals of phase transition

The connection between the thermodynamics of charged finite nuclear systems and the asymptotically measured partitions is presented. Some open questions, concerning in particular equilibrium partitions are discussed. We show a detailed comparison of the decay patterns in Au + C, Cu, Au central collisions and in Au quasi-projectile events. Observation of abnormally large fluctuations in carefully selected samples of data is reported as an indication of a first order phase transition (negative heat capacity) in the nuclear equation of state.

Open charm production in relativistic nucleus–nucleus collisions

We calculate excitation functions for open charm mesons in Au +Au reactions from AGS to RHIC energies within the HSD transport approach which is based on string, quark, diquark (q, q ̄ ,qq, q ̄ q ̄ ) and hadronic degrees of freedom. The open charm cross sections from pN and πN reactions are fitted to results from PYTHIA and scaled in magnitude to the available experimental data. From our dynamical calculations we find an approximate m T -scaling for pions, kaons, D mesons and J/Ψ — when discarding final-state elastic scattering of kaons and φ mesons with pions — in central collisions of Au +Au at 160 A GeV (with an apparent slope of 176 MeV) without employing the assumption of a Quark–Gluon Plasma (QGP). We demonstrate that this result is essentially due to a relative m T -scaling in pp collisions at s ≈17.3 GeV. At lower bombarding energies of 25 A GeV a suppression of D mesons by a factor of ∼10 relative to a global m T -scaling with slope 143 MeV is expected. However, when incorporating attractive D-meson self-energies as suggested by QCD sum rules, an approximate m T -scaling is regained even at 25 A GeV. The effects of D-meson rescattering and charmonium absorption are discussed, furthermore, with respect to rapidity and transverse mass distributions in central collisions of Au +Au at 25, 160 A GeV and 21.3 A TeV.

Inclusive and direct photons in S+Au collisions at200AGeV/c

A hadron and string cascade model, JPCIAE, which is based on LUND string model, PYTHIA event generator especially, is used to study both inclusive photon production and direct photon production in 200A GeV S + Au central collisions. The model takes into account the photon production from the partonic QCD scattering process, the hadronic final-state interaction, and the hadronic decay and deals with them consistently. The results of JPCIAE model reproduce successfully both the WA93 data of low p_T inclusive photon distribution and the WA80 data of transverse momentum dependent upper limit of direct photon. The photon production from different decay channels is investigated for both direct and inclusive photons. We have discussed the effects of the partonic QCD scattering and the hadronic final-state interaction on direct photon production as well.

Deuterons and space-momentum correlations in high energy nuclear collisions

Using a microscopic transport model together with a coalescence after-burner, we study the formation of deuterons in Au + Au central collisions at $\sqrt{s}=200 AGeV$. It is found that the deuteron transverse momentum distributions are strongly affected by the nucleon space-momentum correlations, at the moment of freeze-out, which are mostly determined by the number of rescatterings. This feature is useful for studying collision dynamics at ultrarelativistic energies.

Au + Au central collisions at 150, 250 and 400 A MeV energies in QMD with relativistic forces

Using the small acceleration approximation we derive a relativistic scalar-vector force from a modified Zimányi-Moszkowski Lagrangian based on σ, ω and ϱ meson exchanges. The momentum dependence of the force is fixed automatically by the theory. We present an application of such a force in a QMD calculation at intermediate energies comparing the results with the experimental ones published recently by the FOPI collaboration. For most of the quantities (number of intermediate mass fragments, ERAT, integrated side flow, central flow, charge distributions, etc.) we find agreement with the experimental results.

Antisymmetrized molecular dynamics with quantum branching processes for collisions of heavy nuclei

Antisymmetrized molecular dynamics (AMD) with quantum branching processes is reformulated so that it can be applicable to the collisions of heavy nuclei such as Au + Au multifragmentation reactions. The quantum branching process due to the wave packet diffusion effect is treated as a random term in a Langevin-type equation of motion, whose numerical treatment is much easier than the method of the previous papers. Furthermore a new approximation formula, called the triple-loop approximation, is introduced in order to evaluate the Hamiltonian in the equation of motion with much less computation time than the exact calculation. A calculation is performed for the Au + Au central collisions at 150 MeV/nucleon. The result shows that AMD almost reproduces the copious fragment formation in this reaction.

Two-particles correlations from central Au+Au collisions at the AGS at BNL

Bose-Einstein correlation functions are measured using the E877 apparatus in 11.4 A GeV/c Au+Au central collisions (0–4% σ/σgeom) at the AGS at BNL. π+π+ and π∔π∔ correlations are measured in varying pair averaged rapidity and transverse momentum bins while theK+K+ correlation is measured in only one bin, the pair’s full acceptance bin, due to limited statistics. Finite source size Coulomb corrections are applied to all measured correlations to isolate only the Bose-Einstein statistics effect. Additionally model correlations are calculated using RQMD(v2.3) cascade code coupled with the Koonin-Pratt code which imposes statistical and final state interaction weights to the paris and compared to data.