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Within an isospin- and momentum-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, we investigate the hard photons emission from neutron-proton bremsstrahlung in reaction system of $^{208}Pb+^{208}Pb$ around Fermi energy. Effects of neutron skins in coordinate ($r$) space and proton skins in momentum ($k$) space on the time evolution, the angular distribution, and the transverse momentum spectra of hard photons with different energies are studied. It is shown that the emission of direct hard photons is sensitive to the neutron skin, which has larger effects for more energetic hard photons. Meanwhile, we find that the proton skins have an important influence on the emission of direct hard photons, and its effect is actually even larger than that of neutron skins. It needs to take the effect of proton skins into account when we determine the size of neutron skins by comparing transport mode predictions of hard photons with the corresponding experiment measurements.

Based on a multi-phase transport model for relativistic heavy-ion collisions, effects of the parton scatterings on the production of strangeness in relativistic heavy-ion collisions are studied. It is found that the distributions of strange quark and strange baryon, especially for the double strangeness $\Xi^{-}$, are significantly affected by the parton scatterings in heavy-ion collisions below $\sqrt{s_{NN}}\sim$ 10 GeV. Given parton scatterings as a signal of the formation of quark matter, the transverse momentum distribution of the ratio of single and double strangeness $(\Lambda+\Sigma^{0})/\Xi^{-}$ produced in heavy-ion collisions may serve as a potential probe of the emergence of quark matter, or equivalently, the occurrence of hadron-quark phase transition in relativistic heavy-ion collisions., Comment: 5 pages, 5 figures, Physics Letters B, in production

Within a transport model coupled with a microscopic coalescence model, the directed and elliptic flows of protons and deuterons as well as their scalling properties are studied in the centrality of 20-30% Au+Au collisions at $\sqrt{s_{\rm NN}}=2.4$ GeV. It is found that the flows as well as their scaling properties simulated with the isospin- and momentum-dependent nuclear mean field with an incompressibility $K_{0}=230$ MeV fit fairly the HADES data, while those simulated with the commonly used momentum-independent nuclear mean field with an incompressibility $K_{0}=380$ MeV can only fit partially the HADES data. Moreover, by checking the rapidity distributions of both protons and deuterons in the centrality of 0-10% Au+Au collisions at $\sqrt{s_{\rm NN}}=2.4$ GeV, we find that the rapidity distributions of deuterons are underestimated while those of protons are overestimated by the simulations with the momentum-independent nuclear mean field. In contrast, the rapidity distributions of both protons and deuterons simulated with the isospin- and momentum-dependent nuclear mean field are in good agreement with the HADES data. Our findings imply that the momentum dependence of nuclear mean field is an unavoidable feature for a fundamental understanding of nuclear matter properties and for the successful interpretation of the HADES data., 7 pages, 5 figures. Accepted for publication in Physics Letters B

In the standard model of particle physics, the strong force is described by the theory of quantum chromodynamic (QCD). Based on QCD properties, it is commonly believed that at sufficiently high temperatures or densities, the hadrons are melted into their constituent quarks, i.e., undergo a deconfinement transition to a new phase of quarks and gluons. Such phase is frequently called quark matter or quark-gluon plasma (QGP). Although QGP has been observed in relativistic heavy-ion collisions, it is currently an open question about the onset of deconfinement. By comparing simulations from a reliable hadron and quark relativistic transport model and recent STAR experimental data, we infer that the onset of hadron-quark phase transition occurs at about fivefold nuclear compression corresponding temperature $T\sim$ 112 MeV and baryon chemical potential $\mu_{B}\sim$ 586 MeV in nuclear matter phase diagram. This finding has important implications for the studies of early and present universe, such as relevant to the fraction of dark matter formed in early universe and structure of neutron stars as well as the dynamics of neutron star mergers., Comment: 12 pages, 4 figures

Based on a relativistic transport model coupled with hadronic mean-field potentials for heavy-ion collisions, the productions of doubly strange hyperon $Ξ^-$ and trebly strange hyperon $Ω^-$ in relativistic Au+Au collisions at $\sqrt{s_{NN}}$ = 4.2 GeV are investigated. Compared with the double strangeness sensitive observable $Ξ^-$, the treble strangeness $Ω^-$, whether its yields or its collective flows, is extremely sensitive to the equation of state (EoS) of dense matter, making it a valuable observable for probing the properties of dense matter formed in relativistic heavy-ion collisions., 5 pages, 5 figures

A concept of \emph{isospin blocking} in the process of isospin diffusion in heavy-ion collisions is raised. Generally, in the process of isospin diffusion, isospin asymmetry would diffuse from the place with large asymmetry to the place with small asymmetry. However, our study shows that the isospin diffusion could be blocked in case the local value of the symmetry energy is larger. We dub this phenomenon ``isospin blocking''. To check this behavior, in the framework of an Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, isospin diffusions in the isotope Sn+Sn reactions at 270 MeV/nucleon are studied. It is shown that the value of the after-diffusion asymmetry is distinctly blocked if the local symmetry energy is large. The effects of the isospin blocking on the isospin asymmetry of dilute and dense matter and the final $\pi^-/\pi^+$ ratio in heavy ion collisions are demonstrated., Comment: 4 pages, 4 figures, PRC-Letter in production

In terrestrial laboratory with existing accelerators, the maximum multinucleon system produced so far is limited to that with total nucleon number being less than the sum of two colliding nuclei. Such situation may hinder our studies on the properties of nuclear many-body system. Here a way of producing large multinucleon system via multi-nucleus collision device Collider Plus in terrestrial laboratory is proposed. It is shown that large chunk of nucleonic matter can be produced through nuclear ternary fusion reactions at lower beam energies and also large block of dense matter can be formed via ternary collisions of heavy nuclei at intermediate energies. These investigations may shed lights on the studies of the synthesis of superheavy elements and the properties of bulk superdense nuclear matter., Comment: 5 pages, 5 figures, Phys. Lett. B, in production

Recent beam energy scan (BES) experiments at RHIC by the STAR Collaboration (PLB {\bf 827},137003 (2022) and PRL {\bf 128}, 202303 (2022)) found that hadronic interactions dominate the collective flow and the proton cumulant ratios are driven by baryon number conservation in a region of high baryon density in $\sqrt{s_{NN}}$ = 3 GeV Au+Au reactions, indicating the dense medium formed in such collisions is likely hadronic matter. Within an updated ART (A Relativistic Transport) model with momentum dependent isoscalar and isovector single-nucleon mean-field potentials corresponding to different symmetry energies at suprasaturation densities, the $n/p$, $π^{-}/π^{+}$, $K_{s}^{0}/K^{+}$, $Σ^{-}/Σ^{+}$ and $Ξ^{-}/Ξ^{0}$ ratios are studied for central Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV where the maximum central density reaches about $(3.6\sim 4.0)ρ_0$. The doubly strange $Ξ^{-}/Ξ^{0}$ ratio is found to have the strongest sensitivity to the variation of high-density nuclear symmetry energy. Thus, the $Ξ^{-}/Ξ^{0}$ ratio in relativistic heavy-ion reactions at $\sqrt{s_{NN}} \sim 3$ GeV may help probe sensitively the poorly known symmetry energy of dense neutron-rich matter critically important for understanding various properties of neutron stars., With minor revisions. Phys. Rev. C in press

Double strangeness $��^{-}$ production in Au+Au collisions at 2, 4, and 6 GeV/nucleon incident beam energies is studied with the pure hadron cascade version of a multi-phase transport model. It is found that due to larger nuclear compression, the model with the soft equation of state (EoS) gives larger yields of both single strangeness ($K^{+}$ and $��+��^{0}$) and double strangeness $��^{-}$. The sensitivity of the double strangeness $��^{-}$ to the EoS is evidently larger than that of $K^{+}$ or $��+��^{0}$ since the phase-space distribution of produced $��^{-}$ is more compact compared to those of the single strangeness. The larger sensitivity of the yields ratio of $��^{-}$ to the EoS from heavy and light systems is kept compared to that of the single strangeness. The study of $��^{-}$ production in relativistic heavy-ion collisions provides an alternative for the ongoing heavy-ion collision program at facilities worldwide for identifying the EoS at high densities, which is relevant to the investigation of the phase boundary and onset of deconfinement of dense nuclear matter., 5 pages, 5 figures, PLB, in production

Based on the Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, the in-medium nucleon-nucleon inelastic scattering ( which is dominated by pion production at low and intermediate energies) is explored. It is found that the in-medium modification of nucleon-nucleon inelastic scatterings appears to reduce the neutron to proton ratio n/p at higher kinetic energies. Although the in-medium modification of nucleon-nucleon inelastic scatterings, as expected, affects the value of $\pi^-/\pi^+$ ratio, considering a series of undetermined properties of delta resonance and $\pi$ in medium, the energetic neutron to proton ratio n/p is more suitable to be used to probe the in-medium correction of nucleon-nucleon inelastic scatterings., Comment: 6 pages, 4 figures, PRC in production

The nuclear symmetry energy, which describes the energy difference of per proton and neutron in nuclear matter, has been extensively studied within the last two decades. Around saturation density, both the value and the slope of the nuclear symmetry energy have been roughly constrained, its high-density behavior is now still in argument. Probing high-density symmetry energy at terrestrial laboratories is being carried out at facilities that offer radioactive beams worldwide. While relevant experiments are being conducted, we theoretically developed more advanced isospin-dependent transport model including new physics such as nucleon-nucleon short-range correlations and in-medium isospin-dependence of baryon-baryon scattering cross section. New sensitive probes of high-density symmetry energy are provided, such as squeezed-out neutron to proton ratio, photon and light cluster as well as the production of mesons with strangeness or hidden strangeness. The blind spots of probing the high-density symmetry energy by sensitive observable are demonstrated. Model dependence of frequently used sensitive probes of the symmetry energy has been studied thoroughly based on different transport models. A qualitative observable of neutron to proton ratio at high emitting energy is proposed to probe the high-density symmetry energy qualitatively. The probed density regions of the symmetry energy are carefully studied. Effects of nucleon-nucleon short-range correlations on the some sensitive observables of the symmetry energy in heavy-ion collisions are explored carefully. Probing the curvature of the symmetry energy by involving the slope information of the symmetry energy at saturation point in the transport model is proposed., Comment: 13 pages, 25 figures

Based on the Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, the isotope Sn+Sn reactions at 270 MeV/nucleon are investigated. It is shown that nucleon and meson observables in the Sn+Sn reactions at 270 MeV/nucleon cannot effectively probe the high-density symmetry energy. These observables, however, are sensitive to the curvature of the symmetry energy. It thus sheds light on the study of the high-density behavior of the nuclear symmetry energy by extracting the curvature of the symmetry energy through the interpretation of forthcoming RIBF/RIKEN related data., Comment: 5 pages, 8 figures

By involving the constraints of the slope of nuclear symmetry energy $L$ into the question of determination of the high-density symmetry energy, one needs to probe the curvature of nuclear symmetry energy $K_{\rm{sym}}$. Based on the Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, effects of the curvature of nuclear symmetry energy on the squeezed-out nucleons are demonstrated in the semi-central Au+Au reaction at 400 and 600 MeV/nucleon. It is shown that the squeezed-out isospin-dependent nucleon emissions at high transverse momenta are sensitive to the curvature of nuclear symmetry energy. The curvature of nuclear symmetry energy at saturation density thus can be determined by the high momentum squeezed-out isospin-dependent nucleon emissions experiments from the semi-central Au+Au reaction at 400 or 600 MeV/nucleon., 6 pages, 4 figures, Physical Review C, in press

In this article we investigate the beam energy dependence of relativistic retardation effects of electrical fields on the single and double $��^{-}/��^{+}$ ratios in three heavy-ion reactions with an isospin- and momentum-dependent transport model IBUU11. With the beam energy increasing from 200 to 400 MeV/nucleon, effects of the relativistically retarded electrical fields on the $��^{-}/��^{+}$ ratio are found to increase gradually from negligibly to considerably significant as expectedly; it is however, the interesting observation is the relativistic retardation effects of electrical fields on the $��^{-}/��^{+}$ ratio are becoming gradually insignificant as the beam energy further increasing from 400 to 800 MeV/nucleon. Moreover, we also investigate the isospin dependence of relativistic retardation effects of electrical fields on the $��^{-}/��^{+}$ ratio in two isobar reaction systems of $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr at the beam energies from 200 to 800 MeV/nucleon. It is shown that the relativistic retardation effects of electrical fields on the $��^{-}/��^{+}$ ratio are independent of the isospin of reaction. Furthermore, we also examine the double $��^{-}/��^{+}$ ratio in reactions of $^{96}$Zr+$^{96}$Zr over $^{96}$Ru+$^{96}$Ru at the beam energies from 200 to 800 MeV/nucleon with the static field and retarded field, respectively. It is shown the double $��^{-}/��^{+}$ ratio from two reactions is still an effective observable of symmetry energy without the interference of electrical field due to using the relativistic calculation compared to the nonrelativistic calculation., 8 pages, 7 figures. Abbreviated abstract to meet the criterion of arXiv platform. Accepted for publication in Physical Review C. An extended but necessary complementary study to arXiv:1709.09127

The studies, from several independent methods, consistently show that $��^-/��^+$ ratio in the $^{132}$Sn+$^{124}$Sn reaction at 300 MeV/nucleon, which is being carried out at Radioactive Isotope Beam Facility (RIBF) in Japan, just probes the symmetry energy around saturation density., 6 pages, 6 figures, accepted by Journal of Physics G: Nuclear and Particle Physics

It is theoretically and experimentally argued that there may exist bubble or toroid-shaped configurations in some nucleus systems. Based on the nuclear transport model, it is shown that compared with the collision of normal nuclei, there is a depletion of central density of compression with bubble configurations in projectile and target nuclei. This depletion of central compression density may affect some observables in heavy-ion collisions., 5 pages, 6 figures, accepted by European Physical Journal A

Based on the recently updated Isospin-dependent Boltzmann-Uehling-Uhlenbeck (IBUU) transport model, we studied the effects of symmetry energy on the neutron to proton n/p ratio and the $\pi^{-}/\pi^{+}$ ratio in central $^{132}\rm {Sn}+^{124}\rm {Sn}$ reaction at 300 MeV/nucleon. It is found that the n/p ratio and the $\pi^{-}/\pi^{+}$ ratio in central $^{132}\rm {Sn}+^{124}\rm {Sn}$ reaction at 300 MeV/nucleon mainly probe the symmetry energy in the density region 1-1.5 times saturation density. However, the $\pi^{-}/\pi^{+}$ ratio may be able to probe the density-dependent symmetry energy above 1.5 times saturation density by making some kinematic restrictions such as the azimuthal angle and kinetic energy cuts of emitting pions., Comment: 7 pages, 8 figures, accepted version of PRC

Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU) transport model, effects of $\Delta$ resonance potential on the free n/p and $\pi^-/\pi^+$ ratios in the central collision of $^{197}Au+^{197}Au$ at beam energies of 200 and 400 MeV/A are studied. It is found that the effect of $\Delta$ potential on the ratio of pre-equilibrium free $n/p$ is invisible. The effect of $\Delta$ isovector potential on the kinetic energy integrating ratio of $\pi^-/\pi^+$ may be observable only at lower incident beam energies and with stiffer symmetry energy. The strength of the $\Delta$ isoscalar potential affects the height of the $\pi^-/\pi^+$ ratio around the Coulomb peak but does not affect the kinetic energy integrating ratio of $\pi^-/\pi^+$. In heavy-ion collisions at intermediate energies, relating to the question of non-conservation of energy on $\Delta$ or $\pi$ productions, one can replace the $\Delta$ potential by nucleon isoscalar potential especially a soft symmetry energy is employed., Comment: 5 pages, 9 figures, revised version, accepted by PRC

Within the framework of the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU) transport model, we studied the effects of the pion potential and the symmetry energy on the pion production in the central $^{197}Au+^{197}Au$ collisions around the pion production threshold. It is found that the pion potential has opposite effects on the value of $��^-/��^+$ ratio at low and high pion energies. The effect of the pion potential on the total $��^-/��^+$ ratio becomes larger in heavy-ion collisions at beam energies below the pion production threshold. And at beam energies below the pion production threshold, with the pion potential, the effect of the symmetry energy on the $��^-/��^+$ ratio becomes smaller compared with that above the pion production threshold., 13 pages, 10 figures, PRC in production

Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck(IBUU04) transport model, we explored effects of in-medium $NN$ elastic scattering cross section and nuclear symmetry energy on the sub-threshold pion production in $^{132}Sn+^{124}Sn$ reaction. We find that with decrease of incident beam energy, effects of in-medium $NN$ elastic scattering cross section on the $\pi^{-}/\pi^{+}$ ratio are larger than that of the symmetry energy although the latter may be also larger. While keeping the effect of symmetry energy, the double ratio of $\pi^{-}/\pi^{+}$ from neutron-rich and neutron-poor reaction systems (with the same mass number of system) $^{132}Sn+^{124}Sn$ and $^{128}Pm+^{128}Pm$ almost fully cancels out the effects of in-medium $NN$ elastic scattering cross section., Comment: 5 pages, 7 figures. added one figure and related discussions, accepted by PRC

In the framework of the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model, effect of the momentum dependence of nuclear symmetry potential on pion-/pion+ ratio in the neutron-rich reaction 132Sn+124Sn at a beam energy of 400 MeV/nucleon is studied. We find that the momentum dependence of nuclear symmetry potential affects the compressed density of colliding nuclei, numbers of produced pion- and pion+, as well as the value of pion-/pion+ ratio. The momentum dependent nuclear symmetry potential increases the compressed density of colliding nuclei, numbers of produced resonances delta(1232), N*(1440), pion- and pion+, as well as the value of pion-/pion+ ratio., 4 pages, 3 figures, accepted by PRC

Based on a Relativistic Boltzmann-Uehling- Uhlenbeck transport model, proton and $^{3}$He induced reactions on $^{197}$Au target at beam energies of 2.8, 5, 10 and 16.587 GeV/nucleon are studied. It is found that compared with proton induced reactions, $^{3}$He induced reactions give larger cross sections of pion production, about 5 times those of the proton induced reactions. And more importantly, pion production from $^{3}$He induced reaction is more inclined to low-angle emission. Neutrino production via positively charged pion is also discussed accordingly., Comment: 5 pages, 11 figures, one figure(fig9) was added to show the scaling behavior of pion production, AMPT calculations were discussed, accepted by PRC

Based on the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model and the scaling model according to nucleon effective mass, effects of elastic and inelastic NN scattering cross sections on pion-/pion+ in the neutron-rich reaction of Ca48+Ca48 at a beam energy of 400 MeV/nucleon are studied. It is found that cross-section effects of both NN elastic and inelastic scatterings affect Delta_1232, pion- and pion+ production, as well as the value of pion-/pion+., 5 pages, 4 figures, in production

A systematic study of the $��^-/��^+$ ratio in heavy-ion collisions with the same neutron/proton ratio but different masses can help single out effects of the nuclear mean field on pion production. Based on simulations using the IBUU04 transport model, it is found that the $��^-$ / $��^+$ ratio in head-on collisions of $^{48}$Ca+$^{48}$Ca, $^{124}$Sn +$^{124}$Sn and $^{197}$Au+$^{197}$Au at beam energies from 0.25 to 0.6 \agev increases with increasing the system size or decreasing the beam energies. A comprehensive analysis of the dynamical isospin fractionation and the \rpi ratio as well as their time evolution and spatial distributions demonstrates clearly that the \rpi ratio is an effective probe of the high-density behavior of the nuclear symmetry energy., New results and discussions added. Version to appear in PRC (2009)

Using a transport model coupled with a phase-space coalescence after-burner we study the triton-$^3$He ratio, relative and differential transverse flows in semi-central $^{132}Sn+^{124}Sn$ reactions at a beam energy of 400 MeV/A. The neutron-proton ratio, relative and differential flows are also discussed as a reference. We find that similar to the neutron-proton pairs the triton-$^3$He pairs also carry interesting information about the density dependence of the nuclear symmetry energy. Moreover, the nuclear symmetry energy affects more strongly the t-$^3$He relative and differential flows than the $\pi^-/\pi^+$ ratio in the same reaction. The t-$^3$He relative flow can be used as a particularly powerful probe of the high-density behavior of the nuclear symmetry energy., Comment: Discussions added. Version accepted by PRC

Based on an isospin- and momentum-dependent hadronic transport model IBUU04, effects of the nuclear symmetry energy on the single and double pi^{-}/pi^{+}ratios in central reactions of ^{132}Sn+^{124}Sn and ^{112}Sn+^{112}Sn at a beam energy of 400 MeV/nucleon are studied. It is found that around the Coulomb peak of the single pi^{-}/pi^{+} ratio the double pi^{-}/pi^{+} ratio taken from the two isotopic reactions retains about the same sensitivity to the density dependence of nuclear symmetry energy. Because the double pi^{-}/pi^{+}ratio can reduce significantly the systematic errors, it is thus a more effective probe for the high-density behavior of the nuclear symmetry energy., 11 pages, 3 figures, to appear in Physics Review C

Using an isospin- and momentum-dependent transport model we study near-threshold pion production in heavy-ion collisions induced by radioactive beams at the planned Rare Isotope Accelerator (RIA). We revisit the question of probing the high density behavior of nuclear symmetry energy $E_{sym}(\rho)$ using the $\pi^-/\pi^+$ ratio. It is found that both the total and differential $\pi^-/\pi^+$ ratios remain sensitive to the $E_{sym}(\rho)$ when the momentum-dependence of both the isoscalar and isovector potentials are consistently taken into account. Moreover, the multiplicity and spectrum of $\pi^-$ mesons are found more sensitive to the $E_{sym}(\rho)$ than those of $\pi^+$ mesons. Finally, effects of the Coulomb potential on the pion spectra and $\pi^-/\pi^+$ ratio are also discussed., Comment: 22 pages including 10 figures, submitted to Phys. Rev. C