arXiv subjects

C. M. Ko

Publications and source records attributed to C. M. Ko.

At least 91 records · Page 5Linked to original sources

arXiv2001

Charm meson scattering cross sections by pion and rho meson

Using the local flavor SU(4) gauge invariance in the limit of vanishing vector meson masses, we extend our previous study of charm meson scattering cross sections by pion and rho meson, which is based only on the pseudoscalar-pseudoscalar-vector meson couplings, to include also contributions from the couplings among three vector mesons and among four particles. We find that diagrams with light meson exchanges usually dominate the cross sections. For the processes considered previously, the additional interactions lead only to diagrams involving charm meson exchanges and contact interactions, and the cross sections for these processes are thus not much affected. Nevertheless, these additional interactions introduce new processes with light meson exchanges and increase significantly the total scattering cross sections of charm mesons by pion and rho meson.

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arXiv2001

The Role of Gluons in Dilepton Production from the Quark-Gluon Plasma

We study high mass dilepton production from gluon-induced processes, $g q \to q γ^*$, $g \bar q \to \bar q γ^*$, and $g g \to q \bar q γ^*$, in a thermally equilibrated but chemically under-saturated partonic matter that is expected to be created in the initial stage of ultra-relativistic heavy ion collisions. Regulating the divergence in these processes by the thermal quark mass, we find that gluon-induced processes are more important than the leading-order $q\bar q$ annihilation process as a result of the larger number of gluons than quarks in the partonic matter. The dependence of the thermal dilepton yield from the partonic stage of heavy ion collisions on the initial conditions for the partonic matter is also studied. We further discuss the feasibility of observing thermal dileptons from the quark-gluon plasma in heavy ion experiments.

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arXiv2000

Kinetic equation with exact charge conservation

We formulate the kinetic master equation describing the production of charged particles which are created or destroyed only in pairs due to the conservation of their Abelian charge.Our equation applies to arbitrary particle multiplicities and reproduces the equilibrium results for both canonical (rare particles) and grand canonical (abundant particles) systems. For canonical systems, the equilibrium multiplicity is much lower and the relaxation time is much shorter than the naive extrapolation from the grand canonical ensemble results. Implications for particle chemical equilibration in heavy-ion collisions are discussed.

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arXiv2000

Medium effects on the flow of strange particles in heavy ion collisions

Strange particles such as the kaon and lambda are useful in probing the properties of dense matter formed in heavy ion collisions. We review in this talk the theoretical understandings of their properties in nuclear medium based on both the effective chiral Lagrangian and the phenomenological hadronic model. We further review the effects due to changes in their in-medium properties on their collective flow in heavy ion collisions.

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arXiv2000

Antiflow of kaons in relativistic heavy ion collisions

We compare relativistic transport model calculations to recent data on the sideward flow of neutral strange K^0_s mesons for Au+Au collisions at 6 AGeV. A soft nuclear equation of state is found to describe very well the positive proton flow data measured in the same experiment. In the absence of kaon potential, the K^0 flow pattern is similar to that of protons. The kaon flow becomes negative if a repulsive kaon potential determined from the impulse approximation is introduced. However, this potential underestimates the data which exhibits larger antiflow. An excellent agreement with the data is obtained when a relativistic scalar-vector kaon potential, that has stronger density dependence, is used. We further find that the transverse momentum dependence of directed and elliptic flow is quite sensitive to the kaon potential in dense matter.

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arXiv2000

Effective hadronic Lagrangian for charm mesons

An effective hadronic Lagrangian including the charm mesons is introduced to study their interactions in hadronic matter. Using coupling constants that are determined either empirically or by the SU(4) symmetry, we have evaluated the absorption cross sections of $J/ψ$ and the scattering cross sections of $D$ and $D^*$ by $π$ and $ρ$ mesons.

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arXiv2000

Model for $J/ψ$ absorption in hadronic matter

The cross sections for $J/ψ$ absorption by $π$ and $ρ$ mesons are studied in a meson-exchange model that includes not only pseudoscalar-pseudoscalar-vector-meson couplings but also three-vector-meson and four-point couplings. We find that they are much larger than in a previous study where only pseudoscalar-pseudoscalar-vector-meson couplings were considered. Including form factors at interaction vertices, the $J/ψ$ absorption cross sections $σ_{πψ}$ and $σ_{ρψ}$ are found to have values on the order of 7 mb and 3 mb, respectively. Their thermal averages in hadronic matter at temperature T=150 MeV are, respectively, about 1 mb and 2 mb.

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arXiv2000

J/psi suppression in ultrarelativistic nuclear collisions

Using a multiphase transport model, we study the relative importance of J/psi suppression mechanisms due to plasma screening, gluon scattering, and hadron absorption in heavy ion collisions at the Relativistic Heavy Ion Collider. We find that for collisions between heavy nuclei such as Au+Au, both plasma screening and gluon scattering are important. As a result, the effect due to absorption by hadrons becomes relatively minor. The final J/psi survival probability in these collisions is only a few percent. In the case of collisions between light nuclei such as S+S, the effect of plasma screening is, however, negligible in spite of the initial high parton density. The final J/psi survival probability thus remains appreciable after comparable absorption effects due to gluons and hadrons.

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arXiv2000

Baryonic contributions to the dilepton spectra in relativistic heavy ion collisions

We investigate the baryonic contributions to the dilepton yield in high energy heavy ion collisions within the context of a transport model. The relative contribution of the baryonic and mesonic sources are examined. It is observed that most dominant among the baryonic channels is the decay of N*(1520) and mostly confined in the region below the rho peak. In a transport theory implementation we find the baryonic contribution to the lepton pair yield to be small.

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arXiv2000

Dilepton Production and Chiral Symmetry

We discuss how dilepton production is related to chiral symmetry and its restoration. We then analyse presently available data by the CERES collaboration in this context. We find that the present data do not support any conclusions concerning the restoration of chiral symmetry. We finally provide a prediction for the dilepton spectrum for the just completed low energy (40 GeV) run at the SPS.

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arXiv1999

Hadronic scattering of charmed mesons

The scattering cross sections of charm mesons with hadrons such as the pion, rho meson, and nucleon are studied in an effective Lagrangian. In heavy ion collisions, rescattering of produced charm mesons by hadrons affects the invariant mass spectra of both charm meson pairs and dileptons resulting from their decays. These effects are estimated for heavy ion collisions at SPS energies and are found to be significant.

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arXiv1999

A Multi-Phase Transport model for nuclear collisions at RHIC

To study heavy ion collisions at energies available from the Relativistic Heavy Ion Collider, we have developed a multi-phase transport model that includes both initial partonic and final hadronic interactions. Specifically, the parton cascade model ZPC, which uses as input the parton distribution from the HIJING model, is extended to include the quark-gluon to hadronic matter transition and also final-state hadronic interactions based on the ART model. Predictions of the model for central Au on Au collisions at RHIC are reported.

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arXiv1999

Elliptic flow in heavy ion collisions near the balance energy

The proton elliptic flow in collisions of Ca on Ca at energies from 30 to 100 MeV/nucleon is studied in an isospin-dependent transport model. With increasing incident energy, the elliptic flow shows a transition from positive to negative flow. Its magnitude depends on both the nuclear equation of state (EOS) and the nucleon-nucleon scattering cross section. Different elliptic flows are obtained for a stiff EOS with free nucleon-nucleon cross sections and a soft EOS with reduced nucleon-nucleon cross sections, although both lead to vanishing in-plane transverse flow at the same balance energy. The study of both in-plane and elliptic flows at intermediate energies thus provides a means to extract simultaneously the information on the nuclear equation of state and the nucleon-nucleon scattering cross section in medium.

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arXiv1999

Directed flow of neutral strange particles at AGS

Directed flow of neutral strange particles in heavy ion collisions at AGS is studied in the ART transport model. Using a lambda mean-field potential which is 2/3 of that for a nucleon as predicted by the constituent quark model, lambdas are found to flow with protons but with a smaller flow parameter as observed in experiments. For kaons, their repulsive potential, which is calculated from the impulse approximation using the measured kaon-nucleon scattering length, leads to a smaller anti-flow than that shown in the preliminary E895 data. Implications of this discrepancy are discussed.

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arXiv1999

Kaon differential flow in relativistic heavy-ion collisions

Using a relativistic transport model, we study the azimuthal momentum asymmetry of kaons with fixed transverse momentum, i.e., the differential flow, in heavy-ion collisions at beam momentum of 6 GeV/c per nucleon, available from the Alternating Gradient Synchrotron (AGS) at the Brookhaven National Laboratory (BNL). We find that in the absence of kaon potential the kaon differential flow is positive and increases with transverse momentum as that of nucleons. The repulsive kaon potential as predicted by theoretical models, however, reduces the kaon differetnial flow, changing it to negative for kaons with low momenta. Cancellation between the negative differential flow at low mementa and the positive one at high momenta is then responsible for the experimentally observed nearly vanishing in-plane transverse flow of kaons in heavy ion experiments.

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arXiv1999

Hadronic Scatterings of Charm Mesons and Enhancement of Intermediate Mass Dileptons

The scattering effects of charm mesons by hadrons such as the pion, rho meson and nucleon are studied in an effective Lagrangian, and are found to be important in heavy ion collisions. At the CERN-SPS energies, hadronic rescatterings are shown to harden the charm meson $m_T$ spectra, leading to a significant enhancement of the yield of intermediate-mass dimuons from charm meson decays.

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arXiv1999

Excitation function of nucleon and pion elliptic flow in relativistic heavy-ion collisions

Within a relativistic transport (ART) model for heavy-ion collisions, we show that the recently observed characteristic change from out-of-plane to in-plane elliptic flow of protons in mid-central Au+Au collisions as the incident energy increases is consistent with the calculated results using a stiff nuclear equation of state (K=380 MeV). We have also studied the elliptic flow of pions and the transverse momentum dependence of both the nucleon and pion elliptic flow in order to gain further insight about the collision dynamics.

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arXiv1998

Low-mass dileptons and dropping rho meson mass

Using the transport model, we have studied dilepton production from heavy-ion collisions at Bevalac energies. It is found that the enhanced production of low-mass dileptons observed in the experiment by the DLS collaboration cannot be explained by the dropping of hadron masses, in particular the $ρ$-meson mass, in dense matter.

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