arXiv subjects

Che Ming Ko

Publications and source records attributed to Che Ming Ko.

At least 55 records · Page 3Linked to original sources

arXiv2020

Light nuclei production in relativistic heavy ion collisions from the AMPT model

Based on an improved multiphase transport (AMPT) model, which gives a good description of proton production with a smooth quark matter to hadronic matter transition in relativistic heavy ion collisions, we study deuteron and triton production from the coalescence of nucleons at the kinetic freezeout of these collisions. For Au+Au collisions at center-of-mass energies $\sqrt{s_\text{NN}}$ from 7.7 GeV to 200 GeV available at the Relativistic Heavy Ion Collider (RHIC), we find that the yield ratio $N_\text{t}N_\text{p}/N_\text{d}^2$ of proton, deuteron\com{,} and triton is essentially a constant as a function of collision energy. Our result confirms the expectation that without a first-order quark to hadronic matter phase transition in the produced matter or its approach to the critical point of the QCD matter, this yield ratio does not show any non-monotonic behavior in its collision energy dependence.

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arXiv2020

Enhanced yield ratio of light nuclei in heavy ion collisions with a first-order QCD phase transition

Using a transport model that includes a first-order chiral phase transition between the partonic and the hadronic matter, we study the development of density fluctuations in the matter produced in heavy ion collisions as it undergoes the phase transition, and their time evolution in later hadronic stage of the collisions. Using the coalescence model to describe the production of deuterons and tritons from nucleons at the kinetic freeze out, we find that the yield ratio $ N_\text{t}N_\text{p}/ N_\text{d}^2$, where $N_\text{p}$, $N_\text{d}$, and $N_\text{t}$ are, respectively, the proton, deuteron, and triton numbers, is enhanced if the evolution trajectory of the produced matter in the QCD phase diagram passes through the spinodal region of a first-order chiral phase transition.

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arXiv2020

Suppression of light nuclei production in collisions of small systems at the Large Hadron Collider

We show that the recently observed suppression of the yield ratio of deuteron to proton and of helium-3 to proton in p+p collisions compared to those in p+Pb or Pb+Pb collisions by the ALICE Collaboration at the Large Hadron Collider (LHC) can be explained if light nuclei are produced from the coalescence of nucleons at the kinetic freeze-out of these collisions. This suppression is attributed to the non-negligible sizes of deuteron and helium-3 compared to the size of the nucleon emission source in collisions of small systems, which reduces the overlap of their internal wave functions with those of nucleons. The same model is also used to study the production of triton and hypertriton in heavy-ion collisions at the LHC. Compared to helium-3 in events of low charged particle multiplicity, the triton is less suppressed due to its smaller size and the hypertriton is even more suppressed as a result of its much larger size.

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arXiv2020

Charmed hadron production in an improved quark coalescence model

We study the production of charmed hadrons $D^{0}$ and $Λ_c^+$ in relativistic heavy-ion collisions using an improved quark coalescence model. In particular, we extend the usual coalescence model by letting a produced hadron to have the same velocity as the center-of-mass velocity of coalesced constituent quarks during hadronization to take into account the effect of collective flow in produced quark-gluon plasma. This results in a shift of charmed resonances of higher masses to larger transverse momenta ($p_T^{}$). Requiring all charm quarks of very low $p_T^{}$ to be converted to hadrons via coalescence and letting charm quarks not undergoing coalescence to hadronize by independent fragmentation, we obtain a good description of the measured yield ratio $Λ_c^+/D^0$ as a function of $p_T^{}$ in $\text{Au} + \text{Au}$ collisions at $\sqrt{s_{NN}}^{}=200$~GeV by the STAR Collaboration at the Relativistic Heavy Ion Collider.

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arXiv2020

Yield ratio of hypertriton to light nuclei in heavy-ion collisions from $\rm \sqrt{s_{NN}}$ = 4.9 GeV to 2.76 TeV

We resolve the difference in the yield ratio $\rm S_3$ = $\rm \frac{N_{^3_ΛH}/N_Λ}{N_{^3He}/N_p}$ measured in Au+Au collisions at $\rm \sqrt{s_{NN}}$ = 200 GeV and in Pb-Pb collisions at $\rm \sqrt{s_{NN}}$ = 2.76 TeV by adopting a different treatment of the weak decay contribution to the proton yield in Au+Au collisions at $\rm \sqrt{s_{NN}}$ = 200 GeV. We then use the coalescence model to extract information on the $Λ$ and nucleon density fluctuations at the kinetic freeze-out of heavy ion collisions. We also show from available experimental data that the yield ratio $\rm S_2$ = $\rm \frac{N_{^3_ΛH}}{N_ΛN_ d}$ is a more promising observable than $\rm S_3$ for probing the local baryon-strangeness correlation in the produced medium.

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arXiv2020

Local spin polarizations in relativistic heavy ion collisions

Based on a generalized side-jump formalism for massless chiral fermions, which naturally takes into account the spin-orbit coupling in the scattering of two chiral fermions and the chiral vortical effect in a rotating chiral fermion matter, we have developed a covariant and total angular momentum conserved chiral transport model to study both the global and local polarizations of this matter. For a system of massless quarks of random spin orientations and finite vorticity in a box, we have demonstrated that the model can exactly conserve the total angular momentum of the system and dynamically generate the quark spin polarization expected from a thermally equilibrated quark matter. Using this model to study the spin polarization in relativistic heavy-ion collision, we have found that the local quark spin polarizations depend strongly on the reference frame where they are evaluated as a result of the nontrivial axial charge distribution caused by the chiral vortical effect. We have further shown that because of the anomalous orbital or side-jump contribution to the quark spin polarization, the local quark polarizations calculated in the medium rest frame are qualitatively consistent with the local polarizations of Lambda hyperons measured in experiments.

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arXiv2020

Properties of strange quark stars with isovector interactions

We study the properties of strange quark stars by employing a 3-flavor Nambu-Jona-Lasinio model with both scalar-isovector and vector-isovector interactions. Using the constraint on the vector-isoscalar interaction strength obtained from the elliptic flow splitting between particles and their antiparticles in relativistic heavy-ion collisions, we investigate the dependence of the properties of strange quark stars on the vector-isovector and the scalar-isovector interactions, and compare the results with the state-of-art astrophysical constraints on the compact star radius and mass as well as its tidal deformability from the GW170817 event. Results from our study reinforce the prospect of using both heavy-ion collisions and astrophysical observations to provide constraints on the isovector coupling strength in quark matter and thus the quark matter equation of state as well as the QCD phase structure at finite isospin chemical potentials.

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arXiv2020

Number of constituent quark scaling of elliptic flows in high multiplicity p-Pb collisions at $\sqrt{s_{NN}}=$ 5.02 TeV

We briefly summarize our recent study on the number of constituent quark (NCQ) scaling of hadron elliptic flows in high multiplicity p-Pb collisions at $\sqrt{s_{NN}}=$ 5.02 TeV. With the inclusion of hadron production via the quark coalescence model at intermediate $p_T$, the viscous hydrodynamics at low $p_T$, and jet fragmentation at high $p_T$, our $Hydro-Coal-Frag$ model provides a nice description of the $p_T$-spectra and differential elliptic flow $v_2(p_T)$ of pions, kaons and protons over the $p_T$ range from 0 to 6 GeV. Our results demonstrate that including the quark coalescence is essential for reproducing the observed approximate NCQ scaling of hadron $v_2$ at intermediate $p_T$ in experiments, indicating strongly the existence of partonic degrees of freedom and the formation of quark-gluon plasma in high multiplicity p--Pb collisions at the LHC.

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arXiv2019

Probing QCD critical fluctuations from the yield ratio of strange hadrons in relativistic heavy-ion collisions

By analyzing the available data on strange hadrons in central Pb+Pb collisions from the NA49 Collaboration at the Super Proton Synchrotron (SPS) and in central Au+Au collisions from the STAR Collaboration at the Relativistic Heavy-Ion Collider (RHIC) in a wide collision energy range from $\sqrt{s_{\rm NN}}$ = 6.3 GeV to 200 GeV, we find a possible non-monotonic behavior in the ratio $\mathcal{O}_\text{K-$Ξ$-$ϕ$-$Λ$}$= $\frac{N(K^+)N(Ξ^-)}{N(ϕ)N(Λ)}$ of $K^+$, $Ξ^-$, $ϕ$, and $Λ$ yields as a function of $\sqrt{s_{\rm NN}}$. Based on the quark coalescence model, which can take into account the effect of quark density fluctuations on hadron production, a possible non-monotonic behavior in the dependence of the strange quark density fluctuation on $\sqrt{s_{NN}}$ is obtained. This is in contrast to the coalescence model that does not include quark density fluctuations and also to the statistical hadronization model as both fail to describe even qualitatively the collision energy dependence of the ratio $\mathcal{O}_\text{K-$Ξ$-$ϕ$-$Λ$}$. Our findings thus suggest that the signal and location of a possible critical endpoint in the QCD phase diagram, which is expected to result in large quark density fluctuations, can be found in the on-going Bean Energy Scan program at RHIC.

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arXiv2019

Isospin splitting of pion elliptic flow in relativistic heavy-ion collisions

Based on the framework of an extended multiphase transport model with mean-field potentials in both the partonic phase and the hadronic phase, we explain the elliptic flow difference between $π^+$ and $π^-$ in the beam-energy scan program at the relativistic heavy-ion collider by incorporating the vector-isovector potential for quarks and antiquarks with different isospins. It is found that the isospin splitting of pion elliptic flow favors a strong vector-isovector interaction, and thus serves as a probe of the quark matter equation of state as well as the QCD phase structure at finite baryon and isospin chemical potentials.

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arXiv2019

Comparison of heavy-ion transport simulations: Collision integral with pions and $Δ$ resonances in a box

We compare ten transport codes for a system confined in a box, aiming at improved handling of the production of $Δ$ resonances and pions, which is indispensable for constraining high-density symmetry energy from observables such as the $π^-/π^+$ yield ratio in heavy-ion collisions. The system in a box is initialized with nucleons at saturation density and at 60 MeV temperature. The reactions $NN\leftrightarrow NΔ$ and $Δ\leftrightarrow Nπ$ are implemented, but the Pauli blocking and the mean-field potential are deactivated in the present comparison. Results are compared to those from the two reference cases of a chemically equilibrated ideal gas mixture and of the rate equation. In the results of the numbers of $Δ$ and $π$, deviations from the reference values are observed in many codes, and they depend significantly on the size of the time step. These deviations are tied to different ways in ordering the sequence of collisions and decays, that take place in the same time step. Better agreements are seen in the reaction rates and the number ratios among the isospin species of $Δ$ and $π$. These are, however, affected by the correlations, which are absent in the Boltzmann equation, but are induced by the way particle scatterings are treated in transport calculations. The uncertainty in the transport-code predictions of the $π^-/π^+$ ratio for the system initialized at n/p = 1.5, after letting the existing $Δ$ resonances decay, is found to be within a few percent, which is sufficiently small so that it does not strongly impact constraining the high-density symmetry energy from heavy-ion collisions. Most of the sources of uncertainties have been understood, and individual codes may be further improved. This investigation will be extended in the future to heavy-ion collisions to ensure the problems identified here remain under control.

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arXiv2019

Nuclear matter properties at finite temperatures from effective interactions

We study if commonly used nucleon-nucleon effective interactions, obtained from fitting the properties of cold nuclear matter and of finite nuclei, can properly describe the hot dense nuclear matter produced in intermediate-energy heavy-ion collisions. We use two representative effective interactions, i.e., an improved isospin- and momentum-dependent interaction with its isovector part calibrated by the results from the \emph{ab initio} non-perturbative self-consistent Green's function (SCGF) approach with chiral forces, and a Skyme-type interaction fitted to the equation of state of cold nuclear matter from chiral effective many-body perturbation theory and the binding energy of finite nuclei. In the mean-field approximation, we evaluate the equation of state and the single-nucleon potential for nuclear matter at finite temperatures and compare them to those from the SCGF approach. We find that the improved isospin- and momentum-dependent interaction reproduces reasonably well the SCGF results due to its weaker momentum dependence of the mean-field potential than in the Skyrme-type interaction. Our study thus indicates that effective interactions with the correct momentum dependence of the mean-filed potential can properly describe the properties of hot dense nuclear matter and are thus suitable for use in transport models to study heavy-ion collisions at intermediate energies.

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arXiv2018

Azimuthal angle dependence of the longitudinal spin polarization in relativistic heavy ion collisions

The azimuthal angle dependence of quark spin polarization in the longitudinal beam direction of non-central relativistic heavy ion collisions is studied in the chiral kinetic approach. Contrary to the prediction from models based on the assumption of thermal equilibrium of spin degrees of freedom that the quark spin polarization always points along the direction of local vorticity field, we find the two can have opposite directions due to the effect from the transversal component of vorticity field, which can lead to a redistribution of axial charges in the produced matter. Our finding is consistent with the azimuthal angle dependence of the longitudinal spin polarization of $Λ$ hyperons, which is mainly determined by that of the strange quark, recently measured in the experiments by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC).

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arXiv2018

Probing the topological charge in QCD matter via multiplicity up-down asymmetry

Relativistic heavy ion collisions provide the possibility to study the topological charge in QCD matter through the event-by-event fluctuating net axial charge or nonequal numbers of left- and right-handed quarks they generate in the produced quark-gluon plasma. Based on the chiral kinetic approach for nearly massless quarks and antiquarks in the strong vorticity field produced along the normal direction of the reaction plane of non-central heavy ion collisions, we show that a unique signal for the topological charge in QCD matter can be identified from the asymmetric distribution of particles with momenta pointing in the upper and lower hemispheres of the reaction plane as a result of the fluctuating net axial charge.

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arXiv2018

Spectra and flow of light nuclei in relativistic heavy ion collisions at RHIC and the LHC

Within the framework of the coalescence model based on the phase-space distributions of protons and neutrons generated from the {\tt iEBE-VISHNU} hybrid model with {\tt AMPT} initial conditions, we study the spectra and elliptic flow of deuterons and helium-3 in relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Larger Hadron Collider (LHC). Results from our model calculations for Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV at RHIC and Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at the LHC are compared with available experimental data. Good agreements are generally seen between theoretical results and experimental data, except that the calculated yield of helium-3 in Pb + Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV underestimates the data by about a factor of two. Possible reasons for these discrepancies are discussed. We also make predictions on the spectra and elliptic flow of deuterons and helium-3 in Pb + Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV that are being studied at LHC.

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arXiv2018

Light nuclei production as a probe of the QCD phase diagram

It is generally believed that the quark-hadron transition at small values of baryon chemical potentials $μ_B$ is a crossover but changes to a first-order phase transition with an associated critical endpoint (CEP) as $μ_B$ increases. Such a $μ_B$-dependent quark-hadron transition is expected to result in a double-peak structure in the collision energy dependence of the baryon density fluctuation in heavy-ion collisions with one at lower energy due to the spinodal instability during the first-order phase transition and another at higher energy due to the critical fluctuations in the vicinity of the CEP. By analyzing the data on the $p$, d and $^3$H yields in central heavy-ion collisions within the coalescence model for light nuclei production, we find that the relative neutron density fluctuation $Δρ_n=\langle(δρ_n)^2\rangle/\langle ρ_n\rangle^2$ at kinetic freeze-out indeed displays a clear peak at $\sqrt{s_{NN}}=8.8$ GeV and a possible strong re-enhancement at $\sqrt{s_{NN}}=4.86$ GeV. Our findings thus provide a strong support for the existence of a first-order phase transition at large $μ_B$ and its critical endpoint at a smaller $μ_B$ in the temperature versus baryon chemical potential plane of the QCD phase diagram.

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arXiv2018

Chiral kinetic approach to the chiral magnetic effect in isobaric collisions

Based on the chiral kinetic approach using quarks and antiquarks from a multiphase transport model as initial conditions, we study the chiral magnetic effect, i.e., the magnetic field induced separation of charged particles in the transverse plane, in non-central isobaric collisions of Zr$+$Zr and Ru$+$Ru, which have the same atomic number but different proton numbers. For the observable $γ^{OS}-γ^{SS}$ related to the difference between the correlations of particles of opposite charges and of same charges, we find a difference between the two collision systems if the magnetic field has a long lifetime of 0.6 fm$/c$ and the observable is evaluated using the initial reaction plane. This signal of the chiral magnetic effect becomes smaller and comparable to the background contributions from elliptic flow if the event plane determined from particle emission angles is used. For the other observable given by the $R(ΔS)$ correlator related to the distribution of average charge separation in a collision, the signal due to the chiral magnetic effect is found to depend less on whether the reaction or event plane is used in the analysis, and their difference between the two isobaric collision systems is thus a more robust observable.

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arXiv2017

Nuclear dipole polarizability from mean-field modeling constrained by chiral effective field theory

We construct a new Skyrme interaction Sk$χ$m$^*$ by fitting the equation of state and nucleon effective masses in asymmetric nuclear matter from chiral two- and three-body forces as well as the binding energies of finite nuclei. Employing this interaction to study the electric dipole polarizabilities of $^{48}$Ca, $^{68}$Ni, $^{120}$Sn, and $^{208}$Pb in the random-phase approximation, we find that the theoretical predictions are in good agreement with experimentally measured values without additional fine tuning of the Skyrme interaction, thus confirming the usefulness of the new Skyrme interaction in studying the properties of nuclei. We further use this interaction to study the neutron skin thicknesses of $^{48}$Ca and $^{208}$Pb, and they are found to be consistent with the experimental data.

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