Energy Heavy Ion Collisions Research Articles

Development of a 100 ps TDC based on a Kintex 7 FPGA for the high granular neutron time-of-flight detector for the BM@N experiment

The prototype of a TDC board has been developed for the new high granular time-of-flight neutron detector (HGND). The board is based on the standard LVDS 4x asynchronous oversampling using the xc7k160 FPGA with a 100 ps bin width. The HGND is being developed for the BM@N (Baryonic Matter at Nuclotron) experiment to identify neutrons and to measure their energies in heavy-ion collisions at ion beam energies up to 4 A GeV. The HGND consists of about 2000 scintillator detectors (cells) with a size of 40×40×25mm3 and light readout with EQR15 11-6060D-S photodetectors. To measure the time resolution of the scintillator cells, the two-channel FPGA TDC board prototype with two scintillator cells was tested with an electron beam of “Pakhra” synchrotron at the LPI institute (Moscow, Russia). The measured cell time resolution is 146 ps, which is in a good agreement with the 142 ps time resolution measured with a 12-bit @ 5 GS/s CAEN DT5742 digitizer. For the full HGND, the TDC readout board with three such FPGAs will read 250 channels. In total, eight such TDC boards will be used for the full HGND at the BM@N experiment.

Finding signatures of the nuclear symmetry energy in heavy-ion collisions with deep learning

A deep convolutional neural network (CNN) is developed to study symmetry energy (Esym(ρ)) effects by learning the mapping between the symmetry energy and the two-dimensional (transverse momentum and rapidity) distributions of protons and neutrons in heavy-ion collisions. Supervised training is performed with labeled data-set from the ultrarelativistic quantum molecular dynamics (UrQMD) model simulation. It is found that, by using proton spectra on event-by-event basis as input, the accuracy for classifying the soft and stiff Esym(ρ) is about 60% due to large event-by-event fluctuations, while by setting event-summed proton spectra as input, the classification accuracy increases to 98%. The accuracies for 5-label (5 different Esym(ρ)) classification task are about 58% and 72% by using proton and neutron spectra, respectively. For the regression task, the mean absolute errors (MAE) which measure the average magnitude of the absolute differences between the predicted and actual L (the slope parameter of Esym(ρ)) are about 20.4 and 14.8 MeV by using proton and neutron spectra, respectively. Fingerprints of the density-dependent nuclear symmetry energy on the transverse momentum and rapidity distributions of protons and neutrons can be identified by convolutional neural network algorithm.

Necessity of self-consistent calculations for the electromagnetic field in probing the nuclear symmetry energy using pion observables in heavy-ion collisions

Within an isospin- and momentum-dependent transport model, we investigate the necessity of selfconsistent calculations for the electromagnetic field in probing the nuclear symmetry energy using pion observables in heavy-ion collisions at intermediate energies. To this end, we perform the $^{96}$Ru + $^{96}$Ru collisions at 400 MeV/nucleon with two calculations scenarios for the electromagnetic field including the selfconsistent calculation and the most used Li\'{e}nard-Wiechert formula, while the latter is a simplified one of the complete Li\'{e}nard-Wiechert formula by neglecting the radiation field for practical calculations in heavy-ion collisions at intermediate and/or relativistic energies. As a comparison, we also consider the static Coulomb field formula for calculations of the electromagnetic field in heavy-ion collisions. It is shown that the most used simplified Li\'{e}nard-Wiechert formula is not enough for the electromagnetic field calculation because the absent radiation field in this formula also affects significantly the charged pions as well as their $\pi^{-}/\pi^{+}$ ratio. Moreover, we also examine effects of the electromagnetic field in these scenarios on the double $\pi^{-}/\pi^{+}$ ratio of two isobar reaction systems of $^{96}$Ru + $^{96}$Ru and $^{96}$Zr + $^{96}$Zr at 400 MeV/nucleon. It is shown that the double $\pi^{-}/\pi^{+}$ ratio of two reactions tends to be less affected by the electromagnetic field calculation scenario and thus can still be an effective probe of the nuclear symmetry energy in heavy-ion collisions. Therefore, according to these findings, it is suggested that the selfconsistent calculation for the electromagnetic field should be carefully taken into account when using the pion observables to probe the nuclear symmetry energy in heavy-ion collisions.

Calculation of ET from single particle spectra

In high energy heavy ion collisions, measurements of transverse energy (ET ) can constrain the initial energy density and therefore provide insight into the formation of the Quark Gluon Plasma (QGP). This is particularly interesting in regions where it is unclear if the QGP is formed. The ET in a collision can be measured either from a calorimeter or calculated from the single particle momentum spectra. We use spectra measured by the STAR collaboration to calculate the transverse energy in heavy ion collisions in Au+Au collisions at . These calculations are compared to PHENIX measurements using a calorimeter.

Particle Production in XeXe Collisions at the LHC within the Integrated Hydrokinetic Model

The paper is devoted to the theoretical study of particle production in the Large Hadron Collider (LHC) Xe+Xe collisions at the energy s N N = 5 . 44 TeV. The description of common bulk observables, such as mean charged particle multiplicity, particle number ratios, and p T spectra, is obtained within the integrated hydrokinetic model, and the simulation results are compared to the corresponding experimental points. The comparison shows that the model is able to adequately describe the measured data for the considered collision type, similarly as for the cases of Pb+Pb LHC collisions and top Relativistic Heavy Ion Collider (RHIC) energy Au+Au collisions, analyzed in our previous works.

Role of the Surface Energy in Heavy-Ion Collisions

The surface energy is one of the fundamental properties of nuclei, appearing in the simplest form of the semi-empirical mass formula. The surface energy has an influence on e.g. the shape of a nucleus and its ability to deform. This in turn could be expected to have an effect in fusion reactions around the Coulomb barrier where dynamical effects such as the formation of a neck is part of the fusion process. Frozen Hartree-Fock and Time-Dependent Hartree-Fock calculations are performed for a series of effective interactions in which the surface energy is systematically varied, using 40Ca + 48Ca as a test case. The dynamical lowering of the barrier is greatest for the largest surface energy, contrary to naive expectations, and we speculate that this may be due to the variation in other nuclear matter properties for these effective interactions.

Nuclear modification of jet shape for inclusive jets and γ-jets at the LHC energies

With our coupled jet-fluid model, we study the nuclear modifications of full jets and jet structures for single inclusive jets and γ-jets in Pb+Pb collisions at 5.02 ATeV and 2.76 ATeV. The in-medium evolution of full jet shower is described by a set of coupled transport equations including the effects of collisional energy loss, transverse momentum broadening and medium-induced splitting process. The dynamical evolution of bulk medium is simulated by solving relativistic hydrodynamic equation with source term which accounts for the energy and momentum deposited by hard jet shower to soft medium. Our study demonstrates that the hydrodynamic medium response to jet propagation significantly enhances the broadening of jet shape at large angles and is essential for the cone-size dependence of jet energy loss and nuclear modification factor of inclusive jet production. It is also found that the nuclear modification pattern of jet shape is sensitive to jet energy but has weak dependence on the flavor of the parton that initiates the jet. Our result can naturally explain the different nuclear modification patterns of jet shape functions for single inclusive jet and γ-jet events as observed by the CMS Collaboration, and can be tested in the future by measuring the jet shape function over a wider range of jet energies in heavy-ion collisions.

A complete set of in-medium splitting functions to any order in opacity

In this Letter we report the first calculation of all O(αs) medium-induced branching processes to any order in opacity. Our splitting functions results are presented as iterative solutions to matrix equations with initial conditions set by the leading order branchings in the vacuum. The flavor and quark mass dependence of the in-medium q→qg, g→gg, q→gq, g→qq¯ processes is fully captured by the light-front wavefunction formalism and the color representation of the parent and daughter partons. We include the explicit solutions to second order in opacity as supplementary material and present numerical results in a realistic strongly-interacting medium produced in high center-of-mass energy heavy ion collisions at the Large Hadron Collider. Our numerical simulations show that the second order in opacity corrections can change the energy dependence of the in-medium shower intensity. We further find corrections to the longitudinal and angular distributions of the in-medium splitting kernels that may have important implications for jet substructure phenomenology.

Heavy flavor hadron measurement at RHIC

This paper reviews recent progress of the heavy flavor hadron production measurement at RHIC energy in heavy-ion collisions. Experimental results of the nuclear modification factor and elliptic flow of heavy flavor hadrons and electrons from heavy flavor decays are reported. Significant suppressions of the nuclear modification factors of D0 meson and heavy flavor decay electron at high transverse momentum, similar as light hadrons, have been observed, which indicates significant energy loss of charm quark in the Quark-Gluon Plasma created in the heavy-ion collisions. In the mean time, the elliptic flows of D0 meson and heavy flavor decay electron have been observed, and also similar as light hadrons, consisting with hydrodynamics. Above two evidences indicate strong interactions between charm quark and the nuclear medium created in the collisions. Comparison between data and models provides constraints on the key parameters of the QCD matter, such as the charm quark diffusion coefficient.

Blind spots of probing the high-density symmetry energy in heavy-ion collisions

The nuclear symmetry energy, especially at suprasaturation densities, plays crucial roles in many astrophysical studies. However, nowadays the high-density behavior of the symmetry energy is still very controversial in nuclear community. To constrain the high-density behavior of the symmetry energy, neutron-rich nuclei collisions at medium energies are considered to be one of the most effective methods. While probing the high-density symmetry energy by using heavy-ion collisions, blind spots may exist. In the framework of the Isospin-dependent Boltzmann–Uehling–Uhlenbeck (IBUU) transport model, the blind spots of probing the high-density symmetry energy by the n/p ratio in the central Au + Au reaction at 300MeV/nucleon are demonstrated. It is found that the nucleon observable neutron to proton ratio n/p in heavy-ion collisions cannot effectively probe the high-density symmetry energy when the high-density symmetry energy is less density-dependent.

The High-Density Symmetry Energy in Heavy-Ion Collisions and Compact Stars

High-density nuclear symmetry energy is of crucial importance in astrophysics. Information on such energy has been obtained from mass–radius determinations of neutron stars (NSs), and in the future NS mergers will increasingly contribute. In the laboratory, the symmetry energy can be studied in heavy-ion collisions (HICs) at different incident energies over a large range, from very low to several times higher saturation density. Transport theory is necessary to extract the symmetry energy from the typically non-equilibrated nuclear collisions. In this contribution, we first review the transport approaches, their differences, and recent studies of their reliability. We then discuss several prominent observables, which have been used to determine the symmetry energy at high density: collective flow, light cluster emission, and particle production. It is finally argued that the results of the symmetry energy from microscopic many-body calculations, nuclear structure, nuclear reactions, and astrophysics begin to converge but still need considerable improvements in terms of accuracy.

Effects of retarded electrical fields on observables sensitive to the high-density behavior of the nuclear symmetry energy in heavy-ion collisions at intermediate energies

Within the isospin- and momentum-dependent transport model IBUU11, we examine the relativistic retardation effects of electrical fields on the $\pi^{-}/\pi^{+}$ ratio and neutron-proton differential transverse flow in heavy-ion collisions at intermediate energies. Compared to the static Coulomb fields, the retarded electric fields of fast-moving charges are known to be anisotropic and the associated relativistic corrections can be significant. They are found to increase the number of energetic protons in the participant region at the maximum compression by as much as 25\% but that of energetic neutrons by less than 10\% in $^{197}$Au+$^{197}$Au reactions at a beam energy of 400 MeV/nucleon. Consequently, more $\pi^{+}$ and relatively less $\pi^{-}$ mesons are produced, leading to an appreciable reduction of the $\pi^{-}/\pi^{+}$ ratio compared to calculations with the static Coulomb fields. Also, the neutron-proton differential transverse flow, as another sensitive probe of high-density symmetry energy, is also decreased appreciably due to the stronger retarded electrical fields in directions perpendicular to the velocities of fast-moving charges compared to calculations using the isotropic static electrical fields. Moreover, the retardation effects on these observables are found to be approximately independent of the reaction impact parameter.

Nuclear dynamics and particle production near threshold energies in heavy-ion collisions

Recent progress of the quantum molecular dynamics model for describing the dynamics of heavy-ion collisions is viewed, in particular the nuclear fragmentation, isospin physics, particle production and in-medium effect, hadron-induced nuclear reactions, hypernucleus etc. The neck fragmentation in Fermi-energy heavy-ion collisions is investigated for extracting the symmetry energy at subsaturation densities. The isospin effects, in-medium properties and the behavior of high-density symmetry energy in medium and high energy heavy-ion collisions are thoroughly discussed. The hypernuclide dynamics formed in heavy-ion collisions and in hadron induced reactions is analyzed and addressed in the future experiments at the High-Intensity heavy-ion Accelerator Facility (HIAF).

Influence of the nuclear symmetry energy on the collective flows of charged pions

Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, we studied charged pion transverse and elliptic flows in semicentral $^{197}\mathrm{Au}+^{197}\mathrm{Au}$ collisions at 600 MeV/nucleon. It is found that ${\ensuremath{\pi}}^{+}\ensuremath{-}{\ensuremath{\pi}}^{\ensuremath{-}}$ differential transverse flow and the difference of ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ transverse flows almost show no effects of the symmetry energy. Their corresponding elliptic flows are largely affected by the symmetry energy, especially at high transverse momenta. The isospin-dependent pion elliptic flow at high transverse momenta thus provides a promising way to probe the high-density behavior of the symmetry energy in heavy-ion collisions at the Facility for Antiproton and Ion Research (FAIR) at GSI, Darmstadt or at the Cooling Storage Ring (CSR) at HIRFL, Lanzhou.

Comprehending particle production at RHIC and LHC energies using global measurements

The centrality dependence of the charged-particle multiplicity densities [Formula: see text] and transverse energy densities [Formula: see text] are investigated using the two-component Glauber approach for broad range of energies in heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC). A comprehensive study shows that the data is well-described within the framework of two-component model which includes the contribution of “soft processes” and “hard processes” for different centrality classes and energies. The data at two different energies are compared by means of the ratio of [Formula: see text] (and [Formula: see text]) to see the interplay of energy scaling and relative contribution of hard processes.

Extracting temperature and transverse flow by fitting transverse mass spectra and HBT radii together

Single particle transverse mass spectra and HBT radii of identical pion and identical kaon are analyzed with a blast-wave parametrization under the assumptions of local thermal equilibrium and transverse expansion. Under the assumptions, temperature parameter [Formula: see text] and transverse expansion rapidity [Formula: see text] are sensitive to the shapes of transverse mass [Formula: see text] spectrum and HBT radius [Formula: see text]. Negative and positive correlations between [Formula: see text] and [Formula: see text] are observed by fitting [Formula: see text] spectrum and HBT radius [Formula: see text], respectively. For a Monte Carlo simulation using the blast-wave function, [Formula: see text] and [Formula: see text] are extracted by fitting [Formula: see text] spectra and HBT radii together utilizing a combined optimization function [Formula: see text]. With this method, [Formula: see text] and [Formula: see text] of the Monte Carlo sources can be extracted. Using this method for A Multi-Phase Transport (AMPT) model at Relativistic Heavy Ion Collider (RHIC) energy, the differences of [Formula: see text] and [Formula: see text] between pion and kaon are observed obviously, and the tendencies of [Formula: see text] and [Formula: see text] versus collision energy [Formula: see text] are similar with the results extracted directly from the AMPT model.

Collisions of Small Nuclei in the Thermal Model

An analysis is presented of the expectations of the thermal model for particle production in collisions of small nuclei. The maxima observed in particle ratios of strange particles to pions as a function of beam energy in heavy ion collisions, are reduced when considering smaller nuclei. Of particular interest is the $\Lambda/\pi^+$ ratio shows the strongest maximum which survives even in collisions of small nuclei.

Thermal model for small systems

An analysis is presented of the expectations for particle production in collisions of small nuclei using the thermal model. The maxima observed in particle ratios of strange particles to pions as a function of beam energy in heavy ion collisions, are reduced when considering smaller nuclei. Of particular interest is the Λ/π + ratio which shows a strong maximum surviving even in collisions of small nuclei.

The Density Dependence of the Nuclear Symmetry Energy in Heavy-ion Collisions

The Density Dependence of the Nuclear Symmetry Energy in Heavy-ion Collisions

Equation of State and Symmetry Energy in Heavy-Ion Collisions

Equation of State and Symmetry Energy in Heavy-Ion Collisions