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C. M. Ko

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

At least 127 records · Page 7Linked to original sources
arXiv1996

Dilepton production in proton-nucleus and nucleus-nucleus collisions at SPS energies

Dilepton production in proton- and nucleus-induced reactions is studied in relativistic transport model using initial conditions determined by the string dynamics from RQMD. It is found that both the CERES and HELIOS-3 data for dilepton spectra in proton-nucleus reactions can be well described by the `conventional' mechanism of Dalitz decay and direct vector meson decay. However, to provide a quantitative explanation of the observed dilepton spectra in central S+Au and S+W collisions requires contributions other than these direct decays. Introducing a decrease of vector meson masses in hot and dense medium, we find that these heavy-ion data can also be satisfactorily explained. This agrees with our earlier conclusions based on a fire cylinder model. We also give predictions for Pb+Au collisions at 160 GeV/nucleon using current CERES mass resolution and acceptance.

nucl-th↗
arXiv1996

Medium effects in high energy heavy-ion collisions

The change of hadron properties in dense matter based on various theoretical approaches are reviewed. Incorporating these medium effects in the relativistic transport model, which treats consistently the change of hadron masses and energies in dense matter via the scalar and vector fields, heavy-ion collisions at energies available from SIS/GSI, AGS/BNL, and SPS/CERN are studied. This model is seen to provide satisfactory explanations for the observed enhancement of kaon, antikaon, and antiproton yields as well as soft pions in the transverse direction from the SIS experiments. In the AGS heavy-ion experiments, it can account for the enhanced $K^+/π^+$ ratio, the difference in the slope parameters of the $K^+$ and $K^-$ transverse kinetic energy spectra, and the lower apparent temperature of antiprotons than that of protons. This model also provides possible explanations for the observed enhancement of low-mass dileptons, phi mesons, and antilambdas in heavy-ion collisions at SPS energies. Furthermore, the change of hadron properties in hot dense matter leads to new signatures of the quark-gluon plasma to hadronic matter transition in future ultrarelativistic heavy-ion collisions at RHIC/BNL.

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arXiv1996

Antikaon flow in heavy-ion collisions: the effects of absorption and mean fields

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 anti-correlation 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 lights on the relative importance of antikaon absorption versus its mean-field potential.

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arXiv1996

$J/Ψ$ suppression in nuclear collisions at SPS energies

We have carried out a first calculation of $J/Ψ$ production from nuclear collisions within the covariant transport approach HSD, which has been very successful in describing both hadronic and electromagnetic observables from heavy-ion collisions at intermediate and high energies. The production of $J/Ψ$'s is based on the Lund string fragmentation model, while its interactions with hadrons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the $J/Ψ$-baryon cross sections and 3 mb for the $J/Ψ$-meson cross sections above the $D\bar{D}$ threshold, we find that data on $J/Ψ$ suppression from both proton-nucleus and nucleus-nucleus collisions (including Pb + Pb) can be explained without assuming the formation of a quark-gluon plasma in these collisions. Our microscopic studies thus confirm the suggestion of Gavin et al. that $J/Ψ$ suppression observed in nuclear collisions is largely due to absorption by comovers, i.e., the produced mesons.

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arXiv1996

Enhancement of low-mass dileptons in SPS heavy-ion collisions: possible evidence for dropping rho meson mass in medium

Dilepton production in proton- and nucleus-induced reactions at SPS energies is studied in the relativistic transport model using initial conditions determined by the string dynamics from RQMD. It is found that both the CERES and HELIOS-3 data for dilepton spectra in proton-nucleus reactions can be well described by the conventional mechanism of Dalitz decay and direct vector meson decay. However, to provide a quantitative explanation of the observed dilepton spectra in central S+Au and S+W collisions requires contributions other than these direct decays. Introducing a decrease of vector meson masses in hot dense medium, we find that these heavy-ion data can also be satisfactorily explained. We also give predictions for Pb+Au collisions at 160 GeV/nucleon using current CERES mass resolution and acceptance.

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arXiv1996

Lambda flow in heavy-ion collisions: the role of final-state interactions

Lambda flow in Ni+Ni collisions at SIS energies is studied in the relativistic transport model (RVUU 1.0). It is found that for primordial lambdas the flow is considerably weaker than proton flow. The inclusion of final-state interactions, especially the propagation of lambdas in mean-field potential, brings the lambda flow close to that of protons. An accurate determination of lambda flow in heavy-ion experiments is shown to be very useful for studying lambda properties in dense matter.

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arXiv1996

Effects of in-medium vector meson masses on low-mass dileptons from SPS heavy-ion collisions

Using a relativistic transport model to describe the expansion of the fire-cylinder formed in the initial stage of heavy-ion collisions at SPS/CERN energies, we study the production of dileptons with mass below about 1 GeV from these collisions. The initial hadron abundance and their momentum distributions in the fire-cylinder are determined by following the general features of the results from microscopic models based on the string dynamics and further requiring that the final proton and pion spectra and rapidity distributions are in agreement with available experimental data. For dilepton production, we include the Dalitz decay of $π^0$, $η$, $η^\prime$, $ω$ and $a_1$ mesons, the direct decay of primary $ρ^0$, $ω$ and $ϕ$ mesons, and the pion-pion annihilation that proceeds through the $ρ^0$ meson, the pion-rho annihilation that proceeds through the $a_1$ meson, and the kaon-antikaon annihilation that proceeds through the $ϕ$ meson. We find that the modification of vector meson properties, especially the decrease of their mass due to the partial restoration of chiral symmetry, in hot and dense hadronic matter, provides a quantitative explanation of the recently observed enhancement of low-mass dileptons by the CERES collaboration in central S+Au collisions and by the HELIOS-3 collaboration in central S+W collisions.

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arXiv1996

From $K^+$ in heavy-ion collisions to $K^-$ in kaonic atoms

Kaon production and kaon flow in heavy-ion collisions at SIS energies are analyzed using the relativistic transport model. The experimental data are found to be consistent with the scenario that kaon scalar (before being cut down by the range term) and vector mean field potentials are about 1/3 of that of nucleons in the Walecka model. This also explains the strong attraction found for $K^-$ in kaonic atoms, and gives information on the possibility of $K^-$ condensation in dense matter.

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arXiv1996

Transverse momentum dependence of transverse flow in relativistic heavy-ion collisions

The strength of transverse flow is examined as a function of transverse momentum $p_t$ using a simple, transversely moving thermal model and a more realistic, relativistic transport model (ART). It is shown that the $p_t$ dependence reveals useful information about the collective flow that is complementary to that obtained from the standard in-plane transverse momentum analysis. Interesting features of using the $p_t$ dependence to study the equation of state of the superdense hadronic matter formed in relativistic heavy-ion collisions are demonstrated.

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arXiv1996

Isospin dependence of collective flow in heavy-ion collisions at intermediate energies

Within the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck (BUU) model using initial proton and neutron densities calculated from the nonlinear relativistic mean-field (RMF) theory, we compare the strength of transverse collective flow in reactions $^{48}Ca+^{58}Fe$ and $^{48}Cr+^{58}Ni$, which have the same mass number but different neutron/proton ratios. The neutron-rich system ($^{48}Ca+^{58}Fe$) is found to show significantly stronger negative deflection and consequently has a higher balance energy, especially in peripheral collisions. NOTE ADDED IN PROOF: The new phenomenon predicted in this work has just been confirmed by an experiment done by G.D. Westfall et al. using the NSCL/MSU radioactive beam facility and a spartan soccer. A paper by R. Pak et al. is submitted to PRL to report the experimental result.

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arXiv1995

Dilepton Production at SPS Energies

We present a nonperturbative dynamical study of $e^+e^-$ production in proton- nucleus and nucleus-nucleus collisions at SPS energies on the basis of a covariant transport approach. For p + A reactions the dilepton yield for invariant masses m $\leq $ 1.2 GeV is found to be dominated by the decays of the $η, ρ, ω$ and $Φ$ mesons in line with the findings of the CERES collaboration. For S + Au collisions at 200 GeV/A the dilepton yield is, however, dominated by $π^+π^-$ annihilation due to the high pion densities achieved. Whereas for `free' meson masses and form factors the experimental cross section is slightly underestimated for 0.3 GeV $\leq$ m $\leq$ 0.45 GeV, different medium modifications of the $ρ$-meson appear compatible with current CERES data.

hep-ph↗
arXiv1995

Dilepton production from resonance scattering in hot hadronic matter

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 $π+ρ\to l^++l^-$, the finite $ρ$ meson width extends the invariant mass below the $ω$-meson mass, so a peak at the $ω$ meson mass is seen in the dilepton spectrum. Similarly, a $ρ$-meson peak appears in the reaction $ρ+ρ\to l^++l^-$. On the other hand, the effect of particle widths in the reaction $π+a_1\to l^++l^-$ is small since the extended phase space does not include any resonance.

hep-ph↗
arXiv1995

Thermal effects on dilepton production from $π-π$ annihilation

We study finite temperature effects on dilepton production from pion-pion annihilation in hot hadronic matter. The softening of the pion dispersion relation in a medium is found to enhance the production rate of dileptons with invariant masses in the region of $2m^*_π(T)<M<m_ρ$. On the other hand, the reduction of the pion electromagnetic form factor at finite temperature leads to a suppression of the dilepton production rate. Including both effects, we have found that the dilepton yield is slightly enhanced in the invariant mass region $M= 270\sim 600$ MeV but is suppressed around the vector meson resonance. We further discuss the relevance of our results to recent experimental data from the S+Au collisions at CERN/SPS energies by the CERES collaboration.

hep-ph↗
arXiv1995

Kaon production cross sections from baryon-baryon interactions

In a one-pion plus one-kaon exchange model, we calculate the kaon production cross sections in nucleon-nucleon, nucleon-delta and delta-delta interactions. We find that this model describes reasonably well the experimental data on kaon production in the proton-proton interaction. Near the kaon production threshold, the cross section obtained from this model is smaller than that from the linear parameterization of Randrup and Ko. For kaon production cross sections from the nucleon-delta and delta-delta interactions, the cross sections are singular in free space, so we calculate them in a nuclear medium by including the (complex) pion self-energy. The results are compared with the scaling ansatz of Randrup and Ko. The theoretical cross sections are then used in a transport model to study kaon production from Au+Au collisions at 1 GeV/nucleon.

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arXiv1995

Enhancement of low-mass dileptons in heavy-ion collisions

Using a relativistic transport model for the expansion stage of S+Au collisions at 200 GeV/nucleon, we show that the recently observed enhancement of low-mass dileptons by the CERES collaboration can be explained by the decrease of vector meson masses in hot and dense hadronic matter.

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arXiv1995

Mean field effects in heavy-ion collisions at AGS energies

The question whether mean field effects exist in heavy-ion collisions at AGS energies is studied in the framework of A Relativistic Transport (ART) model. It is found that in central collisions of Au+Au at $P_{beam}/A=$11.6 GeV/c a simple, Skyrme-type nuclear mean field satisfying the causality requirement reduces the maximum baryon and energy densities reached in the cascade model by about 30\% and 40\%, respectively. The mean field affects the inclusive, single particle observables of various hadrons by at most 20\%. We show, however, that the mean field causes a factor of 2.5 increase in the strength of the baryon transverse collective flow.

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arXiv1994

Subthreshold kaon production and the nuclear equation of state

We reexamine in the relativistic transport model the dependence of kaon yield on the nuclear equation of state in heavy ion collisions at energies that are below the threshold for kaon production from the nucleon-nucleon interaction in free space. For Au+Au collisions at 1 GeV/nucleon, we find that the kaon yield measured by the Kaos collaboration at GSI can be accounted for if a soft nuclear equation of state is used. We also confirm the results obtained in the non-relativistic transport model that the dependence of kaon yield on the nuclear equation of state is more appreciable in heavy ion collisions at lower incident energies. We further clarify the difference between the predictions from the relativistic transport model and the non-relativistic transport model with a momentum-dependent potential.

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