arXiv subjects

C. M. Ko

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

At least 55 records · Page 3Linked to original sources

arXiv2004

Pentaquark baryon production in nuclear reactions

Using a hadronic model with empirical coupling constants and form factors, we have evaluated the cross sections for the production of exotic pentaquark $Θ^+$ and/or $Ξ_5^+$ and $Ξ_5^{--}$ in reactions induced by photons, nucleons, pions, and kaons on nucleon targets. We have also predicted the $Θ^+$ yield in relativistic heavy ion collisions using a kinetic model that takes into account both $Θ^+$ production from the coalescence of quarks and antiquarks in the quark-gluon plasma and the effects due to subsequent hadronic absorption and regeneration.

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arXiv2004

Quark Coalescence for Charmed Mesons in Ultrarelativistic Heavy-Ion Collisions

We investigate effects of charm-quark interactions in a Quark-Gluon Plasma on the production of $D$ and $J/ψ$ mesons in high-energy heavy-ion collisions. Employing a previously constructed coalescence model that successfully reproduces the transverse momentum ($p_T$) spectra and elliptic flow ($v_2(p_T)$) of light hadrons at RHIC from underlying light-quark distributions at the phase transition temperature $T_c$, $D$-meson and $J/ψ$ $p_T$ spectra are evaluated. For the charm-quark distributions, we consider two limiting scenarios: (i) {\em no} rescattering, corresponding to perturbative QCD spectra and (ii) {\em complete} thermalization including transverse expansion. With the $D$-meson spectra acquiring a minimal $v_2$ inherited from their light-quark content, the corresponding semileptonic decay spectra of single electrons are found to practically preserve the $v_2$ of the parent particles, exhibiting marked differences between the pQCD and thermal scenarios for $p_T\ge 1$ GeV. Likewise, the $p_T$-spectra and yields of $J/ψ$'s differ appreciably in the two scenarios.

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arXiv2004

Effect of resonance decays on hadron elliptic flows

The influence of resonance decays on the elliptic flows of stable hadrons is studied in the quark coalescence model. Although difference between the elliptic flow of pions from resonance decays, except the rho meson, and that of directly produced pions is appreciable, those for other stable hadrons are small. Since there are more pions from the decays of rho mesons than from other resonances, including resonance decays can only account partially the deviation of final pion elliptic flow from the observed scaling of hadron elliptic flows, i.e., the hadron elliptic flow per quark is the same at same transverse momentum per quark. The remaining deviation can be explained by including the effect due to the quark momentum distribution inside hadrons.

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arXiv2004

Pentaquark baryon production at the Relativistic Heavy Ion Collider

Production of pentaquark $Θ^+$ baryons in central relativistic heavy ion collisions is studied in a kinetic model. Assuming that a quark-gluon plasma is produced in the collisions, we first determine the number of $Θ^+$ produced from the quark-gluon plasma using a parton coalescence model, and then take into consideration its production and absorption in subsequent hadronic matter via the reactions $KN\leftrightarrowΘ$, $KN\leftrightarrowπΘ$, and $\piN\leftrightarrow\bar KΘ$. We find that although the final $Θ^+$ number is affected by hadronic interactions, it remains sensitive to the initial number of $Θ^+$ produced from the quark-gluon plasma, particularly in the case of a small $Θ^+$ width as imposed by the $K^+N$ and $K^+d$ scattering data. Because of small baryon chemical potential in the hot dense matter produced in these collisions, the number of produced anti-$Θ$ is only slightly smaller than that of $Θ^+$.

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arXiv2004

Xi production at AGS energies

A relativistic transport model is used to study Xi- production in 2-11A GeV Au+Au collisions. Introducing the strangeness-exchange reactions between antikaons and hyperons as the sources for Xi-, we find that the cascade yield in these collisions is in reasonable agreement with the data. Although the Xi- abundance does not reach chemical equilibrium unless the cross section for strangeness-exchange reactions is enhanced by six times, it exhibits the strongest enhancement with increasing centrality of collision and with increasing beam energy.

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arXiv2004

Hadronization via Coalescence

We review the quark coalescence model for hadronization in relativistic heavy ion collisions and show how it can explain the observed large baryon to meson ratio at intermediate transverse momentum and scaling of the elliptic flows of identified hadrons. We also show its predictions on higher-order anisotropic flows and discuss how quark coalescence applied to open- and hidden-charm mesons can give insight to charm quark interactions in the quark-gluon plasma and $J/Ψ$ production in heavy ion collisions.

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arXiv2004

Partonic effects on higher-order anisotropic flows in relativistic heavy-ion collisions

Higher-order anisotropic flows $v_{4}$ and $v_{6}$ in heavy ion collisions at the Relativistic Heavy Ion Collider are studied in a multiphase transport model that has previously been used successfully for describing the elliptic flow $v_2$ in these collisions. We find that the same parton scattering cross section of about 10 \textrm{mb} used in explaining the measured $v_2$ can also reproduce the recent data on $v_{4}$ and $v_{6}$ from Au + Au collisions at $\sqrt{s}=200$ \textrm{AGeV}. It is further found that the $% v_{4}$ is a more sensitive probe of the initial partonic dynamics in these collisions than $v_{2}$. Moreover, higher-order parton anisotropic flows are nonnegligible and satisfy the scaling relation $v_{n,q}(p_{T})\sim v_{2,q}^{n/2}(p_{T})$, which leads naturally to the observed similar scaling relation among hadron anisotropic flows when the coalescence model is used to describe hadron production from the partonic matter.

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arXiv2004

Effects of momentum-dependent nuclear potential on two-nucleon correlation functions and light cluster production in intermediate energy heavy-ion collisions

Using an isospin- and momentum-dependent transport model, we study the effects due to the momentum dependence of isoscalar nuclear potential as well as that of symmetry potential on two-nucleon correlation functions and light cluster production in intermediate energy heavy-ion collisions induced by neutron-rich nuclei. It is found that both observables are affected significantly by the momentum dependence of nuclear potential, leading to a reduction of their sensitivity to the stiffness of nuclear symmetry energy. However, the t/$^{3}$He ratio remains a sensitive probe of the density dependence of nuclear symmetry energy.

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arXiv2004

Nonlinear Parker Instability with the Effect of Cosmic-Ray Diffusion

We present the results of linear analysis and two-dimensional local magnetohydrodynamic (MHD) simulations of the Parker instability, including the effects of cosmic rays (CRs), in magnetized gas disks (galactic disks). As an unperturbed state for both the linear analysis and the MHD simulations, we adopted an equilibrium model of a magnetized two-temperature layered disk with constant gravitational acceleration parallel to the normal of the disk. The disk comprises a thermal gas, cosmic rays and a magnetic field perpendicular to the gravitational accelerartion. Cosmic ray diffusion along the magnetic field is considered; cross field-line diffusion is supposed to be small and is ignored. We investigated two cases in our simulations. In the mechanical perturbation case we add a velocity perturbation parallel to the magnetic field lines, while in the explosional perturbation case we add cosmic ray energy into a sphere where the cosmic rays are injected. Linear analysis shows that the growth rate of the Parker instability becomes smaller if the coupling between the CR and the gas is stronger (i.e., the CR diffusion coefficient is smaller). Our MHD simulations of the mechanical perturbation confirm this result. We show that the falling matter is impeded by the CR pressure gradient, this causes a decrease in the growth rate. In the explosional perturbation case, the growth of the magnetic loop is faster when the coupling is stronger in the early stage. However, in the later stage the behavior of the growth rate becomes similar to the mechanical perturbation case.

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arXiv2003

Photoproduction of pentaquark cascades from nucleons

The cross sections for production of pentaquark $Ξ^+_5$ from the reaction $γp\to K^0K^0Ξ^+_5$ and $Ξ^{--}_5$ from the reaction $γn\to K^+K^+Ξ^{--}_5$ are evaluated in a hadronic model that includes their couplings to both $Σ\bar K$ and $Σ\bar K^*$. With these coupling constants determined from the empirical $πNN(1710)$ and $ρNN(1710)$ coupling constants by assuming that $Ξ^+_5$, $Ξ^{--}_5$, and N(1710) belong to the same antidecuplet of spin 1/2 and positive parity, and using form factors at strong interaction vertices similar to those for pentaquark $Θ^+$ production in photonucleon reactions, we obtain a cross section of about 0.03-0.6 nb for the reaction $γp\to K^0K^0Ξ^+_5$ and about 0.1-0.6 nb for the reaction $γn\to K^+K^+Ξ^{--}_5$ at photon energy $E_γ=4.5$ GeV, depending on the value of the coupling constant $g_{K^*ΣΞ_5}$.

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arXiv2003

Light cluster production in intermediate energy heavy-ion collisions induced by neutron-rich nuclei

The coalescence model based on nucleon distribution functions from an isospin-dependent transport model is used to study the production of light clusters such as deuteron, triton, and $^{3}$He from heavy-ion collisions induced by neutron-rich nuclei at intermediate energies. It is found that the emission time of light clusters depends on their masses. For clusters with the same momentum per nucleon, heavier ones are emitted earlier. Both the yield and energy spectrum of light clusters are sensitive to the density dependence of nuclear symmetry energy, with more light clusters produced in the case of a stiff symmetry energy. On the other hand, effects due to the stiffness of the isoscalar part of nuclear equation of state and the medium dependence of nucleon-nucleon cross sections on light cluster production are unimportant. We have also studied the correlation functions of clusters, and they are affected by the density dependence of nuclear symmetry energy as well, with the stiff symmetry energy giving a stronger anti-correlation of light clusters, particularly for those with large kinetic energies. Dependence of light cluster production on the centrality and incident energy of heavy ion collisions as well as the mass of the reaction system is also investigated.

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arXiv2003

Pentaquark $Θ^+$ production from the reaction $γp\toπ^+K^-Θ^+$

The cross section for $Θ^+$ production from the reaction $γp\toπ^+K^-Θ^+$, which was observed in the CLAS experiment at the Jefferson National Laboratory, is evaluated in a hadronic model that includes couplings of $Θ^+$ to both $KN$ and $K^*N$. With their coupling constants determined from the empirical $πNN(1710)$ and $ρNN(1710)$ coupling constants using the SU(3) symmetry, the cross section for this reaction has been evaluated by taking $Θ^+$ to have spin 1/2 and isospin 0 but either positive or negative parity. We find that the cross section is 10-15 nb if $Θ^+$ has positive parity as predicted by the chiral soliton model. The cross section is reduced by more than a factor of 10 if $Θ^+$ has negative parity as given by lattice QCD studies. For both parities, the differential distribution peaks at small negative four momentum transfer as expected from the dominating $t$-channel kaon-exchange diagram that involves only the coupling of $Θ^+$ to $KN$.

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arXiv2003

Pentaquark baryon production from photon-neuteron reactions

Extending the hadronic Lagrangians that we recently introduced for studying pentaquark $Θ^+$ baryon production from meson-proton, proton-proton, and photon-proton reactions near threshold to include the anomalous interaction between $γ$ and $K^*K$, we evaluate the cross section for $Θ^+$ production from photon-neutron reactions, in which the $Θ^+$ was first detected in the SPring-8 experiment in Japan and the CLAS experiment at Thomas Jefferson National Laboratory. With empirical coupling constants and form factors, and assuming that the decay width of $Θ^+$ is 20 MeV, the predicted cross section is found to have a peak value of about 280 nb, which is substantially larger than that for $Θ^+$ production from photon-proton reactions.

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arXiv2003

Cross sections for pentaquark baryon production from protons in reactions induced by hadrons and photons

Using hadronic Lagrangians that include the interaction of pentaquark $Θ^+$ 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 $μ$b in proton-proton reactions, and 40 nb in photon-proton reactions.

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arXiv2003

Isospin effects on two-nucleon correlation functions in heavy-ion collisions at intermediate energies

Using an isospin-dependent transport model, we study isospin effects on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei at intermediate energies. We find that these correlation functions are sensitive to the density dependence of nuclear symmetry energy, but not to the incompressibility of symmetric nuclear matter and the medium dependence of nucleon-nucleon cross sections. This sensitivity is mainly due to effects of nuclear symmetry energy on the emission times of neutrons and protons as well as their relative emission sequence. We also study the variations of the symmetry energy effects on nucleon-nucleon correlations with respect to the impact parameter, incident energy, and mass number of heavy ion collisions.

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arXiv2003

Kaon Interferometry at RHIC from AMPT Model

The two-kaon interferometry at RHIC is studied in a multi-phase transport model. Similar to the pion case, we find strong space-time correlation at freeze-out for the kaon emission source, which results in a large positive $R_{\rm out}-t$ term and tends to reduce the $R_{\rm out}/R_{\rm side}$ ratio. Unlike the pion case, the source radii for kaons determined from the emission function are close to the radius parameters extracted from a Gaussian fit to the correlation function.

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arXiv2003

Parton coalescence at RHIC

Using a covariant coalescence model, we study hadron production in relativistic heavy ion collisions from both soft partons in the quark-gluon plasma and hard partons in minijets. Including transverse flow of soft partons and independent fragmentation of minijet partons, the model is able to describe available experimental data on pion, kaon, and antiproton spectra. The resulting antiproton to pion ratio is seen to increase at low transverse momenta and reaches a value of about one at intermediate transverse momenta, as observed in experimental data at RHIC. A similar dependence of the antikaon to pion ratio on transverse momentum is obtained, but it reaches a smaller value at intermediate transverse momenta. At high transverse momenta, the model predicts that both the antiproton to pion and the antikaon to pion ratio decrease and approach those given by the perturbative QCD. Both collective flow effect and coalescence of minijet partons with partons in the quark-gluon plasma affect significantly the spectra of hadrons with intermediate transverse momenta. Elliptic flows of protons, Lambdas, and Omegas have also been evaluated from partons with elliptic flows extracted from fitting measured pion and kaon elliptic flows, and they are found to be consistent with available experimental data.

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arXiv2003

Parton coalescence and antiproton/pion anomaly at RHIC

Coalescence of minijet partons with the partons from the quark-gluon plasma formed in relativistic heavy ion collisions is suggested as the mechanism for production of hadrons with intermediate transverse momentum. The resulting enhanced antiproton and pion yields at intermediate transverse momentum gives a plausible explanation for the observed large antiproton to pion ratio. With further increasing momentum, the ratio is predicted to decrease and approach the small value given by independent fragmentations of minijet partons after their energy loss in the quark-gluon plasma.

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