OAKTrust :: Browsing by Author "Ko, Che Ming."

Browsing by Author "Ko, Che Ming."

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Anisotropic flow in Cu plus Au collisions at root s(N N)=200GeV RID A-2398-2009
(American Physical Society, 2006) Chen, LW; Ko, Che Ming.
The anisotropic flow of charged hadrons in asymmetric Cu+Au collisions at the Relativistic Heavy Ion Collider is studied in a multiphase transport model. Compared with previous results for symmetric Au+Au collisions, charged hadrons produced around midrapidity in asymmetric collisions are found to have a stronger directed flow v(1) and their elliptic flow v(2) is also more sensitive to the parton scattering cross section. Although higher order flows v(3) and v(4) are small at all rapidities, both v(1) and v(2) in these collisions are appreciable and show an asymmetry in forward and backward rapidities.
Antikaon flow in heavy-ion collisions: Effects of absorption and mean-field potential
(American Physical Society, 1996) Li, GQ; Ko, Che Ming.
We study antikaon flow in heavy-ion collisions at SIS energies based on the relativistic transport model (RVUU 1.0). The production of antikaons from both baryon-baryon and pion-baryon collisions are included. Taking into account only elastic and inelastic collisions of the antikaon with nucleons and neglecting its mean-field potential as in the cascade model, a strong antiflow or anticorrelation of antikaons with respect to nucleons is seen as a result of the strong absorption of antikaons by nucleons. However, the antiflow of antikaons disappears after including also their propagation in the attractive mean-field potential. The experimental measurement of antikaon flow in heavy-ion collision will be very useful in shedding light on the relative importance of antikaon absorption versus its mean-field potential.
Antiproton Production in Ni+ni Collisions at 1.85 Gev/nucleon
(American Physical Society, 1994) LI, GQ; Ko, Che Ming.
Antiproton production in Ni+Ni collisions at 1.85 GeV/nucleon is studied in the relativistic Vlasov-Uehling-Uhlenbeck model. The self-energies of the antiproton are determined from the nucleon self-energies by the G-parity transformation. Also, the final-state interactions of the antiproton including both rescattering and annihilation are explicitly treated. With a soft nuclear equation of state, the calculated antiproton momentum spectrum is in good agreement with recent experimental data from the heavy-ion synchrotron at Gesellschaft, fur Schwerionenforschung Darmstadt. The effect due to the reduced nucleon and antinucleon masses in a medium is found to be more appreciable than in earlier Bevalac experiments with lighter systems and at higher energies.
Baryon number fluctuation and the quark-gluon plasma
(American Physical Society, 2001) Lin, ZW; Ko, Che Ming.
We show that omega (B) or omega ((B) over bar), the squared baryon or antibaryon number fluctuation per baryon or antibaryon, is a possible signature for the quark-gluon plasma that is expected to be created in relativistic heavy ion collisions, as it is a factor of 3 smaller than in an equilibrated hadronic matter due to the fractional baryon number of quarks. Using kinetic equations with exact baryon number conservation, we find that their values in an equilibrated matter are half of those expected from a Poisson distribution. Effects due to finite acceptance and nonzero net baryon number are also studied.
Charm elliptic flow in relativistic heavy-ion collisions RID A-2398-2009
(American Physical Society, 2005) Zhang, B.; Chen, LW; Ko, Che Ming.
Charm elliptic flow in heavy-ion collisions at the Relativistic Heavy Ion Collider is studied in a multiphase transport model. Assuming that the cross section for charm quark scattering with other light quarks is the same as that between light quarks, we find that both charm and light quark elliptic flows are sensitive to the value of the cross section. Compared to that of light quarks, the elliptic flow of charm quarks is smaller at low transverse momentum but approaches comparable values at high transverse momentum. Similar features are seen in the elliptic flow of charmed mesons as well as that of the electrons from their semileptonic decays when the charmed mesons are produced from quark coalescence during hadronization of the partonic matter. To describe the large electron elliptic flow observed in available experimental data requires a charm quark-scattering cross section that is much larger than that given by the perturbative quantum chromodynamics.
Charmonium mass in nuclear matter
(American Physical Society, 2003) Lee, S. H.; Ko, Che Ming.
The mass shift of charmonium states in nuclear matter is studied in the perturbative QCD approach. The leading-order effect due to the change of gluon condensate in nuclear matter is evaluated using the leading-order QCD formula, while the higher-twist effect due to the partial restoration of chiral symmetry is estimated using a hadronic model. We find that while the mass of J/psi in nuclear matter decreases only slightly, those of psi(3686) and psi(3770) states are reduced appreciably. Experimental study of the mass shift of charmonium states in nuclear matter can thus provide valuable information on the changes of the QCD vacuum in nuclear medium.
Contribution of the Reaction Ny-]Nnk to Antikaon Production in Relativistic Heavy-Ion Collisions
(American Physical Society, 1984) Ko, Che Ming.
Cross sections for pentaquark baryon production from protons in reactions induced by hadrons and photons
(American Physical Society, 2003) Liu, W.; Ko, Che Ming.
Using hadronic Lagrangians that include the interaction of pentaquark Theta(+) baryon with K and N, we evaluate the cross sections for its production from meson-proton, proton-proton, and photon-proton reactions near threshold.,With empirical coupling constants and form factors, the predicted cross sections are about 1.5 mb in kaon-proton reactions, 0.1 mb in rho-nucleon reactions, 0.05 mb in pion-nucleon reactions, 20 mub in proton-proton reactions, and 40 nb in photon-proton reactions.
Density matrix expansion for the isospin- and momentum-dependent MDI interaction
(American Physical Society, 2010) Xu, Jun; Ko, Che Ming.
By assuming that the isospin- and momentum-dependent MDI interaction has a form similar to the Gogny-like effective two-body interaction with a Yukawa finite-range term and the momentum dependence originates only from the finite-range exchange interaction, we determine its parameters by comparing the predicted potential energy density functional in uniform nuclear matter with what has been usually given and used extensively in transport models for studying isospin effects in intermediate-energy heavy-ion collisions as well as in investigating the properties of hot asymmetric nuclear matter and neutron star matter. We then use the density matrix expansion to derive from the resulting finite-range exchange interaction an effective Skyrme-like zero-range interaction with density-dependent parameters. As an application, we study the transition density and pressure at the inner edge of neutron star crusts using the stability conditions derived from the linearized Vlasov equation for the neutron star matter.
Deuteron production and elliptic flow in relativistic heavy ion collisions
(American Physical Society, 2009) Oh, Yongseok; Lin, Zi-Wei; Ko, Che Ming.
The hadronic transport model ART is extended to include the production and annihilation of deuterons via the reactions BB dM, where B and M stand for baryons and mesons, respectively, as well as their elastic scattering with mesons and baryons in the hadronic matter. This new hadronic transport model is used to study the transverse momentum spectrum and elliptic flow of deuterons in relativistic heavy ion collisions, with the initial hadron distributions after hadronization of the produced quark-gluon plasma taken from a blast wave model. The results are compared with those measured by the PHENIX and STAR Collaborations for Au+Au collisions at s(NN)=200 GeV and also with those obtained from the coalescence model based on freeze-out nucleons in the transport model.
Deuteron-nucleus collisions in a multiphase transport model
(American Physical Society, 2003) Lin, ZW; Ko, Che Ming.
Using a multiphase transport model, we study pseudorapidity distributions and transverse momentum spectra in deuteron-gold collisions at RHIC. We find that final-state partonic and hadronic interactions affect the transverse momentum spectrum of protons more than those of kaons or pions. Relative to p+p collisions at same center-of-mass energy per nucleon pair, the effect of final-state interactions on the charged particle transverse momentum spectra in d+Au collisions is much smaller than observed in experimental data, indicating that initial-state effects such as the Cronin effect are important.
Dilepton production from resonance scattering in hot hadronic matter
(American Physical Society, 1996) Song, C.; Ko, Che Ming.
Dilepton production from resonance scattering in hot hadronic matter is studied. Including the widths of these resonances, which enhance the phase space for dilepton production, we find that the production rate is significantly increased if a resonance appears in the extended phase space. For the reaction pi+rho-->l(+)+l(-) , the finite rho meson width extends the invariant mass below the omega-meson mass, so a peak at the omega-meson mass is seen in the dilepton spectrum. Similarly, a rho-meson peak appears in the reaction rho+rho-->l(+)+l(-). On the other hand, the effect of particle widths in the reaction pi+a(1)-->l(+)+l(-) is small since the extended phase space does not include any resonance.
Dilepton production in schematic causal viscous hydrodynamics
(American Physical Society, 2011) Song, Taesoo; Han, Kyong Chol; Ko, Che Ming.
Assuming that in the hot dense matter produced in relativistic heavy-ion collisions, the energy density, entropy density, and pressure as well as the azimuthal and space-time rapidity components of the shear tensor are uniform in the direction transversal to the reaction plane, we derive a set of schematic equations from the Isreal-Stewart causal viscous hydrodynamics. These equations are then used to describe the evolution dynamics of relativistic heavy-ion collisions by taking the shear viscosity to entropy density ratio of 1/4 pi for the initial quark-gluon plasma (QGP) phase and of 10 times this value for the later hadron-gas (HG) phase. Using the production rate evaluated with particle distributions that take into account the viscous effect, we study dilepton production in central heavy-ion collisions. Compared with results from the ideal hydrodynamics, we find that although the dilepton invariant mass spectra from the two approaches are similar, the transverse momentum spectra are significantly enhanced at high transverse momenta by the viscous effect. We also study the transverse momentum dependence of dileptons produced from QGP for a fixed transverse mass, which is essentially absent in the ideal hydrodynamics, and find that this so-called transverse mass scaling is violated in the viscous hydrodynamics, particularly at high transverse momenta.
Effect of resonance decays on hadron elliptic flows
(American Physical Society, 2004) Greco, V.; Ko, Che Ming.
Within the quark coalescence model, we study effects of resonance decays, and of the quark momentum distribution in hadrons, on the elliptic flows of stable hadrons. We find that, with the exception of rho-meson decays, the resonance decays could have a significant effect on pion elliptic flow. However, most secondary pions stem from the rho-meson decays, resulting in a reduced effect of resonance decays on their flow. Proton and kaon flows as well as the lambda flow are, however, not much affected by resonance decays. The distribution of quark momentum in hadrons also influences their elliptic flows, leading to a better agreement with experimental flow data when compared with the naive quark coalescence model, which only allows quarks with equal momentum to form a hadron.
Effect of Tunneling on the One-Body Proximity Friction
(American Physical Society, 1979) Ko, Che Ming.
Effects of (N)over-Barn Polarization on Vector-Meson Masses at Finite-Temperature
(American Physical Society, 1995) Song, C. S.; Xia, P. W.; Ko, Che Ming.
Effects of (N) over bar N polarization on vector meson (rho and omega) masses at finite temperature are studied. Including a reduced nucleon mass in a hot matter, we find that the reduction of vector meson masses due to the vacuum effect is larger than the increase of their masses from interactions with nucleons and pions in the hot matter. The vector meson masses at finite temperature thus decrease with temperature.
Effects of triangular flow on di-hadron azimuthal correlations in relativistic heavy ion collisions
(American Physical Society, 2011) Xu, Jun; Ko, Che Ming.
Using the AMPT model for relativistic heavy ion collisions, we have studied the di-hadron azimuthal angular correlations triggered by emitted jets in Au + Au collisions at center-of-mass energy root s(NN) = 200 GeV and impact parameter b = 8 fm. A double-peak structure for the associated particles at the away side of trigger particles is obtained after subtracting background correlations due to the elliptic flow. Both the near-side peak and the away-side double peaks in the azimuthal angular correlations are, however, significantly suppressed (enhanced) in events of small (large) triangular flow or triangularity in the initial collision geometry as a result of fluctuations. After subtraction of background correlations due to the triangular flow, the away-side double peaks change into a single peak with broad shoulders on both sides. The away side of the di-hadron correlations becomes essentially a single peak after further subtraction of higher-order flows.
Elliptic flow of deuterons in relativistic heavy-ion collisions
(American Physical Society, 2007) Oh, Yongseok; Ko, Che Ming.
Using a dynamical model based on the NN -> d pi,NNN -> dN, and NN pi -> d pi reactions and measured proton and pion transverse momentum spectra and elliptic flows, we study the production of deuterons and their elliptic flow in heavy-ion collisions at RHIC. The results are compared with those from the coalescence model. The deviation of deuteron elliptic flow from the constituent nucleon number scaling expected from the coalescence model and the comparison with the experimental data are discussed in connection to the allowed nucleon phase space in these reactions.
Evidence for Statistical Fluctuation in the Fragmentation of O-16 at Intermediate Energies
(American Physical Society, 1980) Ko, Che Ming.
Formation of Superdense Hadronic Matter in High-Energy Heavy-Ion Collisions
(American Physical Society, 1995) Li, Ba0-An; Ko, Che Ming.
We present the detail of a newly developed relativistic transport model (ART 1.0) for high energy heavy-ion collisions. Using this model, we first study the general collision dynamics between heavy ions at the AGS energies. We then show that in central collisions there exists a large volume of sufficiently long-lived superdense hadronic matter whose local baryon and energy densities exceed the critical densities for the hadronic matter to quark-gluon plasma transition. The size and lifetime of this matter are found to depend strongly on the equation of state. We also investigate the degree and time scale of thermalization as well as the radial flow during the expansion of the superdense hadronic matter. The flow velocity profile and the temperature of the hadronic matter at freeze-out are extracted. The transverse momentum and rapidity distributions of protons, pions, and kaons calculated with and without the mean field are compared with each other and also with the preliminary data from the E866/E802 Collaboration to search for experimental observables that are sensitive to the equation of state. It is found that these inclusive, single particle observables depend weakly on the equation of state. The difference between results obtained with and without the nuclear mean field is only about 20%. The baryon transverse collective flow in the reaction plane is also analyzed. It is shown that both the flow parameter and the strength of the ''bounce-off'' effect are very sensitive to the equation of state. In particular, a soft equation of state with a compressibility of 200 MeV results in an increase of the flow parameter by a factor of 2.5 compared to the cascade case without the mean field. This large effect makes it possible to distinguish the predictions from different theoretical models and to detect the signatures of the quark-gluon plasma which is expected to significantly soften the equation of state.

Browsing by Author "Ko, Che Ming."

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