arXiv subjects

C. M. Ko

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

At least 73 records · Page 4Linked to original sources

arXiv2003

Charmonium mass in nuclear matter

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 $J/ψ$ in nuclear matter decreases only slightly, those of $ψ(3686)$ and $ψ(3770)$ 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.

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arXiv2002

Partonic Effects on Pion Interferometry at the Relativistic Heavy Ion Collider

Using a multiphase transport (AMPT) model that includes both initial partonic and final hadronic interactions, we study the pion interferometry at the Relativistic Heavy Ion Collider. We find that the two-pion correlation function is sensitive to the magnitude of the parton scattering cross section, which controls the parton density at which the transition from the partonic to hadronic matter occurs. Also, the emission source of pions is non-Gaussian, leading to source radii that can be more than twice larger than the radius parameters extracted from a Gaussian fit to the correlation function.

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arXiv2002

Charmonium Absorption in the Meson-exchange Model

We review the meson-exchange model for charmonium absorption by hadrons. This includes the construction of the interaction Lagrangians, the determination of the coupling constants, the introduction of form factors, and the predicted cross sections for $J/ψ$ absorption by both mesons and nucleons. We further discuss the effects due to anomalous parity interactions, uncertainties in form factors, constraints from chiral symmetry, and the change of charmed meson mass in medium on the cross sections for charmonium absorption in hadronic matter.

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arXiv2002

Cascade production from antikaon induced reactions on lambda and sigma

Using a gauged flavor SU(3) invariant hadronic Lagrangian, we study $Ξ$ production from $\bar K$ induced reactions on $Λ$ and $Σ$ in a coupled-channel approach. Including the four channels of $\bar KΛ$, $\bar KΣ$, $πΞ$, and $ηΞ$, we solve the Bethe-Salpeter equation in the $K$-matrix approximation by neglecting the off-shell contribution in the intermediate states. For the transition potential which drives the higher-order contribution, we consider all allowed Born diagrams in the $s$, $t$, and $u$ channels. With coupling constants determined from the SU(3) symmetry with empirical input, we find that the cross sections for the reactions $\bar KΛ\toπΞ$, $\bar KΣ\toπΞ$, $\bar KΛ\toηΞ$, and $\bar KΣ\toηΞ$ all have values of a few mb. In contrast to the results from the lowest-order Born approximation, the magnitude of these cross sections is less sensitive to the values of the cut-off parameters in the form factors. From the transition matrix in the coupled-channel approach, we have further evaluated the cross sections for the elastic process $\bar KΛ\to\bar KΛ$, $\bar KΣ\to\bar KΣ$, $πΞ\toπΞ$, and $ηΞ\toηΞ$ as well as for the inelastic processes $\bar KΛ\to\bar KΣ$ and $πΞ\toηΞ$. Implications of the reactions studied here in $Ξ$ production from relativistic heavy ion collisions are discussed.

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arXiv2002

Flavor ordering of elliptic flows at high transverse momentum

Based on the quark coalescence model for the parton-to-hadron phase transition in ultra-relativistic heavy ion collisions, we relate the elliptic flow ($v_2$) of high $\pt$ hadrons to that of high $\pt$ quarks. For high $\pt$ hadrons produced from an isospin symmetric and quark-antiquark symmetric partonic matter, magnitudes of their elliptic flows follow a flavor ordering as $(v_{2,π}=v_{2,N}) > (v_{2,Λ}=v_{2,Σ}) > v_{2,K} > v_{2,Ξ} > (v_{2,ϕ}=v_{2,Ω})$ if strange quarks have a smaller elliptic flow than light quarks. The elliptic flows of high $\pt$ hadrons further follow a simple quark counting rule if strange quarks and light quarks have same high $\pt$ spectrum and coalescence probability.

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arXiv2002

Partonic Effects in Heavy Ion Collisions at RHIC

Effects of partonic interactions in heavy ion collisions at RHIC are studied in a multiphase transport model (AMPT) that includes both initial partonic and final hadronic interactions.It is found that a large parton scattering cross section is needed to understand the measured elliptic flow of pions and two-pion correlation function.

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arXiv2002

Phi meson production in relativistic heavy ion collisions

Within a multiphase transport model we study phi meson production in relativistic heavy ion collisions from both superposition of initial multiple proton-proton interactions and the secondary collisions in the produced hadronic matter. The yield of phi mesons is then reconstructed from their decaying product of either the kaon-antikaon pairs or the dimuon pairs. Since the kaon-antikaon pairs at midrapidity with low transverse momenta are predominantly rescattered or absorbed in the hadronic medium, they can not be used to reconstruct the phi meson and lead thus to a smaller reconstructed phi meson yield than that reconstructed from the dimuon channel. With in-medium mass modifications of kaons and phi mesons, the phi yield from dimuons is further enhanced compared to that from the kaon-antikaon pairs. The model result is compared with the experimental data at the CERN/SPS and RHIC energies and its implications to quark-gluon plasma formation are discussed.

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arXiv2002

J/psi production in relativistic heavy ion collisions from a multi-phase transport model

Using A Multi-Phase Transport (AMPT) model, we study J/psi production from interactions between charm and anti-charm quarks in initial parton phase and between D and Dbar mesons in final hadron phase of relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). Including also the inverse reactions of J/psi absorption by gluons and light mesons, we find that the net number of J/psi from the parton and hadron phases is smaller than that expected from the superposition of initial nucleon-nucleon collisions, contrary to the J/psi enhancement predicted by the kinetic formation model. The production of J/psi is further suppressed if one includes the color screening effect in the parton phase. We have also studied the dependence of J/psi production on the charm quark mass and the effective charm meson mass.

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arXiv2002

Charm meson production from meson-nucleon scattering

Using an effective hadronic Lagrangian with physical hadron masses and coupling constants determined either empirically or from SU(4) flavor symmetry, we study the production cross sections of charm mesons from pion and rho meson interactions with nucleons. With a cutoff parameter of 1 GeV at interaction vertices as usually used in studying the cross sections for $J/ψ$ absorption and charm meson scattering by hadrons, we find that the cross sections for charm meson production have values of a few tenth of mb and are dominated by the s channel nucleon pole diagram. Relevance of these reactions to charm meson production in relativistic heavy ion collisions is discussed.

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arXiv2001

Partonic effects on the elliptic flow at relativistic heavy ion collisions

The elliptic flow in heavy ion collisions at RHIC is studied in a multiphase transport model. By converting the strings in the high energy density regions into partons, we find that the final elliptic flow is sensitive to the parton scattering cross section. To reproduce the large elliptic flow observed in Au+Au collisions at $\sqrt s=130A$ GeV requires a parton scattering cross section of about 6 mb. We also study the dependence of the elliptic flow on the particle multiplicity, transverse momentum, and particle mass.

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arXiv2001

Baryon Number Fluctuation and the Quark-Gluon Plasma

We show that $ω_B$ or $ω_{\bar B}$, 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 three 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 non-zero net baryon number are also studied.

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arXiv2001

Charmonium absorption cross section by nucleon

The $J/ψ$ absorption cross section by nucleon is studied using a gauged SU(4) hadronic Lagrangian but with empirical particle masses, which has been used previously to study the $J/ψ$ absorption cross section by pion and rho meson. Including both two-body and three-body final states, we find that with a cutoff parameter of 1 GeV at interaction vertices involving charm hadrons, the $J/ψ-N$ absorption is at most 5 mb and is consistent with that extracted from $J/ψ$ production from both photo-nuclear and proton-nucleus reactions.

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arXiv2001

Diomega production in relativistic heavy ion collisions

Using a multiphase transport model, we study the production of a new strange dibaryon (ΩΩ)_{0+} in dense hadronic matter formed in relativistic heavy ion collisions. The (multi-)strange baryons (Ξand Ω) are produced by strangeness-exchange reactions between antikaons and hyperons in the pure hadronic phase. The rescattering between the omegas at midrapidity leads to a production probability of \simeq 3x10^{-7} (ΩΩ)_{0+} per event at the RHIC energy of \sqrt s=130A GeV. The production probability would be enhanced by two orders of magnitude if (ΩΩ)_{0+} and omega reach chemical equilibrium during heavy ion collisions. We further find that the yield of (ΩΩ)_{0+} increases continuously from SPS to the highest RHIC energy.

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arXiv2001

Multistrange baryon production in relativistic heavy ion collisions

Using a multiphase transport model, we study the production of multistrange baryons from the hadronic matter formed in relativistic heavy ion collisions. The mechanism we introduce is the strangeness-exchange reactions between antikaons and hyperons. We find that these reactions contribute significantly to the production of multistrange baryons in heavy ion collisions at SPS energies, which has been found to be appreciably enhanced. We have also made predictions for multistrange baryon production in heavy ion collisions at RHIC and found a similar enhancement.

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arXiv2001

Multiphase transport model for heavy ion collisions at RHIC

Using a multiphase transport model (AMPT) with both partonic and hadronic interactions, we study the multiplicity and transverse momentum distributions of charged particles such as pions, kaons and protons in central Au+Au collisions at RHIC energies. Effects due to nuclear shadowing and jet quenching on these observables are also studied. We further show preliminary results on the production of multistrange baryons from the strangeness-exchange reactions during the hadronic stage of heavy ion collisions.

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arXiv2001

Strangeness equilibration in heavy ion collisions

Using a relativistic transport model for heavy ion collisions at energies that are below the threshold for kaon and antikaon production in nucleon-nucleon collisions, we study how their abundances approach the canonical equilibrium during the collisions. We find that kaons are far from chemical equilibrium at the initial and high density stage, and they approach equilibrium only during the expansion stage of the collisions when their production rate is small and becomes comparable to their annihilation rate. In contrast, antikaons approach chemical equilibrium much earlier but eventually fall out of equilibrium again as a result of their large annihilation cross sections in nuclear matter.

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arXiv2001

Charged particle rapidity distributions at RHIC

Using a multiphase transport model (AMPT), which includes both initial partonic and final hadronic interactions, we study the rapidity distributions of charged particles such as protons, antiprotons, pions, and kaons in heavy ion collisions at RHIC. The theoretical results for the total charged particle multiplicity at midrapidity are consistent with those measured by the PHOBOS collaboration in central Au+Au collisions at $\sqrt s$ = 56 and 130 AGeV. We find that these hadronic observables are much more sensitive to the hadronic interactions than the partonic interactions.

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arXiv2001

$Υ$ absorption in hadronic matter

The cross sections of $Υ$ absorption by $π$ and $ρ$ mesons are evaluated in a meson-exchange model. Including form factors with a cutoff parameter of 1 or 2 GeV, we find that due to the large threshold of these reactions the thermal average of their cross sections is only about 0.2 mb at a temperature of 150 MeV. Our results thus suggest that the absorption of directly produced $Υ$ by hadronic comovers in high energy heavy ion collisions is unimportant.

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