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Cheuk-Yin Wong

Publications and source records attributed to Cheuk-Yin Wong.

At least 127 records · Page 7Linked to original sources
arXiv2000

Cross Sections for pi- and rho-induced Dissociation of J/psi and psi'

We evaluate the cross sections for the dissociation of $J/ψ$ and $ψ'$ by $π$ and $ρ$ at low collision energies, using the quark-interchange model of Barnes and Swanson. The dissociation cross section for $J/ψ$ by $π$ is found to be relatively small with a maximum of about 1 mb and a kinetic energy threshold of 0.65 GeV. The pion-induced $ψ'$ dissociation cross section is found to be much larger, with a maximum of about 5 mb and a lower threshold. Dissociation cross sections for $J/ψ$ and $ψ'$ by $ρ$ mesons are also evaluated and are found to be large near threshold.

hep-ph↗
arXiv2000

Cross Sections for the Dissociation of J/psi and psi' by pi and rho at low energies

Using the quark-interchange model of Barnes and Swanson, we evaluate the dissociation cross sections for $J/ψ$ and $ψ'$ by $π$ and $ρ$ at low collision energies. In collisions with $π$ mesons near threshold the $π+ J/ψ$ dissociation cross section is predicted to be small, but the $π+ψ'$ cross section is found to be quite large, in qualitative agreement with experimental data on $J/ψ$ and $ψ'$ production in high-energy heavy-ion collisions. The $ρ+ J/ψ$ and $ρ+ ψ'$ dissociation cross sections are both predicted to be large numerically and are divergent near threshold. These $ρ+ J/ψ$ and $ρ+ ψ'$ cross sections show considerable sensitivity to the mass of the $ρ$ meson if it is off energy shell.

nucl-th↗
arXiv1999

Relativistic Modification of the Gamow Factor

In processes involving Coulomb-type initial- and final-state interactions, the Gamow factor has been traditionally used to take into account these additional interactions. The Gamow factor needs to be modified when the magnitude of the effective coupling constant increases or when the velocity increases. For the production of a pair of particles under their mutual Coulomb-type interaction, we obtain the modification of the Gamow factor in terms of the overlap of the Feynman amplitude with the relativistic wave function of the two particles. As a first example, we study the modification of the Gamow factor for the production of two bosons. The modification is substantial when the coupling constant is large.

nucl-th↗
arXiv1999

The Soft Gluon Emission Process in the Color-Octet Model for Heavy Quarkonium Production

The Color-Octet Model has been used successfully to analyze many problems in heavy quarkonium production. We examine some of the conceptual and practical problems of the soft gluon emission process in the Color-Octet Model. We use a potential model to describe the initial and final states in the soft gluon emission process, as the emission occurs at a late stage after the production of the heavy quark pair. It is found in this model that the soft gluon M1 transition, 1S0(8)->3S1(1), dominates over the E1 transition, 3PJ(8)->3S1(1), for J/psi and psi' production. Such a dominance may help resolve the questions of isotropic polarization and color-octet matrix element universality in the Color-Octet Model.

hep-ph↗
arXiv1998

Drell-Yan Cross Section and J/psi Production in High-Energy Nucleus-Nucleus Collisions

We use the Drell-Yan differential distribution dN_{DY}^{AB}/dE_T in high-energy nucleus-nucleus collisions to obtain a relation between the transverse energy E_T and the impact parameter b. Such a relation is then utilized to study the transverse-energy dependence of J/psi production in Pb-Pb collisions, using the J/psi absorption model presented previously. The anomalous Pb-Pb suppression data at 158A GeV can be explained if one assumes the occurrence of a new phase of strong J/psi absorption when the energy density exceeds 4.2 GeV/fm^3. The results are extended to make predictions for J/psi production at higher collision energies. It is found that J/psi survival probabilities reach the lowest survival limit when the nucleon-nucleon center-of-mass energies is greater than about 35 GeV.

hep-ph↗
arXiv1998

Unresolved Questions in J/psi Production and Propagation in Nuclei

In order to understand the J/psi suppression arising from the possible occurrence of the quark-gluon plasma in high-energy heavy-ion collisions, it is necessary to have a comprehensive picture how the J/psi and its precursors are produced, what their properties after production are, and how the J/psi and its precursors propagate inside nuclear matter. There are unresolved questions in the descriptions of J/psi production and propagation. We outline some of these questions and discuss the approaches for their resolution.

hep-ph↗
arXiv1998

Effects of Parton Intrinsic Transverse Momentum on Photon Production in Hard-Scattering Processes

We calculate the photon production cross section arising from the hard scattering of partons in nucleon-nucleon collisions by taking into account the intrinsic parton transverse momentum distribution and the next-to-leading-order contributions. As first pointed out by Owens, the inclusion of the intrinsic transverse momentum distribution of partons leads to an enhancement of photon production cross section in the region of photon transverse momenta of a few GeV/c for nucleon-nucleon collisions at a center-of-mass energy of a few tens of GeV. The enhancement increases as $\sqrt{s}$ decreases. Such an enhancement is an important consideration in the region of photon momenta under investigation in high-energy heavy-ion collisions.

hep-ph↗
arXiv1998

Possible Mechanism of Formation and Stability of Anomalous States of Water

We examine the physical processes which are involved in the formation and stability of the anomalous states of water reported recently. The initial step of adding a small amount of ionic compound XY to pure water leads to the formation of water clusters (X^+)(H2O)_n and (Y^-)(H2O)_n with n>>1. The structure of the cluster around the ion depends strongly on the equation of state. We explore the consequences of possible polymorphic states of H2O in the liquid phase at room temperature. If there are low-lying polymorphic states, the local dipole moment and the local density will change discontinuously as a function of the radial distance from the ion, and regions of different polymorphic states will be found at different separations from the ion. Fragmentation of the cluster by vigorous shaking may break up the cluster into small domains to allow subsequent coalescence of these domains or the growth of the domains as seeds to form greater domains of polymorphic states. Further experimental and theoretical analyses are needed to study these pictures.

physics.chem-ph↗
arXiv1997

J/psi Suppression as a Signal for the Quark-Gluon Plasma

We review the search for the quark-gluon plasma using the signal of the suppression of J/psi production in high-energy heavy-ion collisions. Recent observations of anomalous J/psi suppression in high-energy Pb-Pb collisions observed by the NA50 Collaboration are examined and compared with earlier results from pA and nucleus-nucleus collisions with heavy ions of smaller mass numbers. The anomalous suppression of J/psi production in Pb-Pb collisions can be explained as due to the occurrence of a new phase of strong J/psi absorption, which sets in when the number of nucleon-nucleon collisions at a spatial point exceeds about 4 and corresponds to a local energy density of about 3.4 GeV/fm**3.

hep-ph↗
arXiv1997

Heavy Quarkonium Production and Propagation in Nuclei

We describe a precursor in heavy quarkonium production in terms of a coherent admixture of states of different color, spin, and angular momentum quantum numbers, and obtain the production amplitudes for different quarkonium bound states by projecting out this precursor state onto these bound states. The precursor is absorbed in its passage through a nucleus in a pA reaction, and the total cross section between this precursor with a nucleon can be calculated with the two-gluon model of the Pomeron. Such a description of coherent precursors and their subsequent interactions with nucleons can explain many salient features of J/psi and psi' production in pA collisions.

hep-ph↗
arXiv1997

Signatures of Absorption Mechanisms for J/psi and psi' Production in High-Energy Heavy-Ion Collisions

J/psi and psi' produced in high-energy heavy-ion collisions are absorbed by their collisions with nucleons and produced soft particles, leading to two distinct absorption mechanisms. The signature of absorption by produced soft particles, as revealed by psi' production data, consists of a gap and a change of the slope in going from the pA line to the nucleus-nucleus line when we make a semi-log plot of the survival probability as a function of the path length. Using this signature, we find from the J/psi production data in pA, O-Cu, O-U, and S-U collisions that the degree of J/psi absorption by produced soft particles is small and cannot account for the J/psi data in Pb-Pb collisions. The anomalous suppression of J/psi production in Pb-Pb collisions can be explained as due to the occurrence of a new phase of strong J/psi absorption, which sets in when the local energy density exceeds about 3.4 GeV/fm^3. To probe the chemical content of the new phase, we propose to study the abundance of open-charm mesons and charm hyperons which depends sensitively on the quark chemical potential.

hep-ph↗
arXiv1997

Color-Octet Fraction in J/Psi Production and Absorption

The cross section between a $c \bar c$ pair and a nucleon is small and sensitive to the $c - \bar c$ separation if the pair is in a color-singlet state, but very large and insensitive to the separation if it is in a color-octet state. We use this property in an absorption model involving both color components to deduce the color structure of $c \bar c$ pairs produced in $p(B)A \to ψX$ reactions. Our analysis shows that the NA3, NA38 and E772 data are not inconsistent with the theoretical picture that color-octet and color-singlet precursors are produced in roughly equal proportions if the produced color-singlet precursors are pointlike and transparent. However, if the color-singlet precursors are not transparent but have a cross section of a few mb, these data do show a definite preference for a larger fraction of color-singlet precursors. In either case, the color-octet fraction increases with $x_F$, approaching unity as $x_F$ becomes large.

hep-ph↗
arXiv1996

Effects of Final-State Interaction and Screening on Strange- and Heavy-Quark Production

Final-state interaction and screening have a great influence on $q \bar q$ production cross sections, which are important quantities in many problems in quark-gluon plasma physics. They lead to an enhancement of the cross section for a $q \bar q$ color-singlet state and a suppression for a color-octet state. The effects are large near the production threshold. The presence of screening gives rise to resonances for $q\bar q$ production just above the threshold at specific plasma temperatures. These resonances, especially $c\bar c$ and $b \bar b$ resonances, may be utilized to search for the quark-gluon plasma by studying the temperature dependence of heavy-quark pair production just above the threshold.

hep-ph↗
arXiv1996

Effects of Screening on Quark-Antiquark Cross Sections in Quark-Gluon Plasma

Lowest-order cross sections for $q\bar q$ production and annihilation can be approximately corrected for higher-order QCD effects by using a corrective $K$-factor. For energies where quark masses cannot be ignored, the $K$-factor is dominated by the wave function distortion arising from the initial- or final-state interaction between the quark and the antiquark. We evaluate this $K$-factor for $q \bar q$ production and annihilation in a quark-gluon plasma by taking into account the effects of Debye screening through a color-Yukawa potential. We present the corrective $K$-factor as a function of dimensionless parameters which may find applications in other systems involving attractive or repulsive Yukawa interactions. Prominent peaks of the $K$-factor occur for an attractive $q$-$\bar q$ color-Yukawa interaction with Debye screening lengths of 0.835 and 3.23 times the Bohr radius, corresponding to two lowest $s$-wave $q\bar q$ bound states moving into the continuum to become $q\bar q$ resonances as the Debye screening length decreases. These resonances, especially the $c\bar c$ and the $b \bar b$ resonances, may be utilized to search for the quark-gluon plasma by studying the systematics of the temperature dependence of heavy-quark pair production just above the threshold.

hep-ph↗
arXiv1996

Suppression of Psi' and J/Psi in High-Energy Heavy-Ion Collisions

The experimental ratio of $ψ'$ to $J/ψ$ is approximately a constant in $pA$ collisions, but decreases as the transverse energy increases in nucleus-nucleus collisions. These peculiar features can be explained as arising from approximately the same $c \bar c$-baryon absorption cross section for $ψ'$ and $J/ψ$ but greater disruption probabilities for $ψ'$ than for $J/ψ$ due to the interaction of the $c\bar c$ system with soft particles produced in baryon-baryon collisions.

hep-ph↗
arXiv1996

Singularity-Free Breit Equation from Constraint Two-Body Dirac Equations

We examine the relation between two approaches to the quantum relativistic two-body problem: (1) the Breit equation, and (2) the two-body Dirac equations derived from constraint dynamics. The Breit equation is known to be pathological when singularities appear at finite separations $r$ in the reduced set of coupled equations for attractive potentials even when the potentials themselves are not singular there. They then give rise to unphysical bound states and resonances. In contrast, the two-body Dirac equations of constraint dynamics do not have these pathologies in many nonperturbative treatments. To understand these marked differences, we first express these contraint equations in a hyperbolic form. These coupled equations are then re-cast into two equivalent equations: (1) a covariant Breit-like equation with potentials that are exponential functions of certain ``generator'' functions, and (2) a covariant orthogonality constraint on the relative momentum. This reduction enables us to show in a transparent way that finite-$r$ singularities do not appear as long as the the exponential structure is not tampered with and the exponential generators of the interaction are themselves nonsingular for finite $r$. These Dirac or Breit equations, free of the structural singularities which plague the usual Breit equation, can then be used safely under all circumstances, encompassing numerous applications in the fields of particle, nuclear, and atomic physics which involve highly relativistic and strong binding configurations.

hep-ph↗
arXiv1995

Effect of $q\bar q$ Initial-state Interaction on Dilepton Emission Rate from Quark-Gluon Plasma

We calculate the dilepton production rate from a thermalized quark-gluon plasma in heavy-ion collisions at RHIC energies. Higher-order QCD corrections are included by using an analytical correction factor $K^{(i)}$, which takes into account the $q\bar q$ initial-state interactions. We show that the analytic correction factor gives very good agreement with experimental Drell-Yan data and leads to large enhancement of the thermal dilepton emission rates. We compare the thermal dilepton yields with the expected production from open-charm decays and Drell-Yan background and assess the prospects of observing thermal dileptons from the quark-gluon-plasma at invariant masses of a few GeV.

hep-ph↗