arXiv subjects

Chueng-Ryong Ji

Publications and source records attributed to Chueng-Ryong Ji.

At least 91 records · Page 5Linked to original sources

arXiv2011

Light-front zero-mode contribution to the tensor form factors for the exclusive rare $P\to V\ell^+\ell^-$ decays

We study the light-front zero-mode contribution to the tensor form factors $T_i(i=1,2,3)$ for the exclusive rare $P\to V\ell^+\ell^-$ decays using a covariant fermion field theory model in $(3+1)$ dimensions. While the zero-mode contribution in principle depends on the form of the vector meson vertex $Γ^μ=γ^μ- (2k-P_V)^μ/D$, the three tensor form factors $T_i(i=1,2,3)$ are found to be free from the zero mode if the denominator $D$ contains the term proportional to the light-front energy or the longitudinal momentum fraction factor $(1/x)^n$ of the struck quark with the power $n>0$. Since the denominator $D$ used in the light-front quark model (LFQM) has the power $n=1/2$, the three tensor form factors $T_i(i=1,2,3)$ can be computed in LFQM safely without involving any complicate zero-mode contribution. The lack of zero-mode contribution benefits the phenomenology with LFQM.

hep-ph↗

arXiv2010

Spin Filter in DVCS amplitudes

In deeply virtual Compton scattering (DVCS) it is found that in some kinematics angular momentum is not conserved if the amplitudes are calculated in terms of widely used reduced operators. Consequently, those kinematics will lead to the wrong analysis of experimental data in terms of generalized parton distributions. Contrary to common lore, the contribution of the longitudinally polarized virtual photon should not be neglected in the analysis of DVCS amplitudes.

hep-ph↗

arXiv2010

On flavor conservation in weak interaction decays involving mixed neutrinos

In the context of quantum field theory (QFT), we compute the amplitudes of weak interaction processes such as $ W^{+} \rightarrow e^{+} + ν_{e}$ and $ W^{+} \rightarrow e^{+} + ν_μ$ by using different representations of flavor states for mixed neutrinos. Analyzing the short time limit of the above amplitudes, we find that the neutrino states defined in QFT as eigenstates of the flavor charges lead to results consistent with lepton charge conservation. On the contrary, the Pontecorvo flavor states produce a violation of lepton charge in the vertex, which is in contrast with what expected at tree level in the Standard Model.

hep-ph↗

arXiv2009

Non-leptonic two-body decays of the Bc meson in light-front quark model and QCD factorization approach

We study exclusive non-leptonic two-body $B_c\to(D_{(s)},η_c,B_{(s)})+F$ decays with $F$(pseudoscalar or vector meson) being factored out in QCD factorization approach. The non-leptonic decay amplitudes are related to the product of meson decay constants and the form factors for semileptonic $B_c$ decays. As inputs in obtaining the branching ratios for a large set of non-leptonic $B_c$ decays, we use the weak form factors for the semileptonic $B_c\to(D_{(s)},η_c,B_{(s)})$ decays in the whole kinematical region and the unmeasured meson decay constants obtained from our previous light-front quark model. We compare our results of the branching ratios with those of other theoretical studies.

hep-ph↗

arXiv2009

Semileptonic and radiative decays of the B_c meson in light-front quark model

We investigate the exclusive semileptonic $B_c\to (D,η_c,B,B_s)\ellν_\ell$, $η_b\to B_c\ellν_\ell$($\ell=e,μ,τ$) decays using the light-front quark model constrained by the variational principle for the QCD motivated effective Hamiltonian. The form factors $f_+(q^2)$ and $f_-(q^2)$ are obtained from the analytic continuation method in the $q^+=0$ frame. While the form factor $f_+(q^2)$ is free from the zero-mode, the form factor $f_-(q^2)$ is not free from the zero-mode in the $q^+=0$ frame. We quantify the zero-mode contributions to $f_-(q^2)$ for various semileptonic $B_c$ decays. Using our effective method to relate the non-wave function vertex to the light-front valence wave function, we incorporate the zero-mode contribution as a convolution of zero-mode operator with the initial and final state wave functions. Our results are then compared to the available experimental data and the results from other theoretical approaches. Since the prediction on the magnetic dipole $B^*_c\to B_c+γ$ decay turns out to be very sensitive to the mass difference between $B^*_c$ and $B_c$ mesons, the decay width $Γ(B^*_c \to B_c γ)$ may help in determining the mass of $B^*_c$ experimentally. Furthermore, we compare the results from the harmonic oscillator potential and the linear potential and identify the decay processes that are sensitive to the choice of confining potential. From the future experimental data on these sensitive processes, one may obtain more realistic information on the potential between quark and antiquark in the heavy meson system.

hep-ph↗

arXiv2008

Conformal Symmetry and Pion Form Factor: Space- and Time-like Region

We extend a recent analysis of the pion electromagnetic form factor constrained by the conformal symmetry to explore the time-like region. We show explicitly that the time-like form factor obtained by the analytic continuation of the space-like form factor correctly satisfies the dispersion relation. Our results indicate that the quark spin and dynamical mass effects are crucial to yield the realistic features of the vector meson dominance phenomena.

hep-ph↗

arXiv2007

Perturbative QCD analysis of exclusive J/ψ+η_c production in e^+e^- annihilation

We analyze the exclusive charmonium $J/ψ+η_c$ pair production in $e^+e^-$ annihilation using the nonfactorized perturbative QCD and the light-front quark model(LFQM) that goes beyond the peaking approximation. We effectively include all orders of higher twist terms in the leading order of QCD coupling constant and compare our nonfactorized analysis with the usual factorized analysis in the calculation of the cross section. We also calculate the quark distribution amplitudes, the Gegenbauer moments, and the decay constants for $J/ψ$ and $η_c$ mesons using our LFQM. Our nonfactorized result enhances the NRQCD result by a factor of $3\sim4$ at $\sqrt{s}=10.6$ GeV.

hep-ph↗

arXiv2007

Zero-Mode Contribution in Nucleon-Delta Transition

We investigate the transition form factors between nucleon and $Δ$(1232) particles by using a covariant quark-spectator-diquark field theory model in (3+1) dimensions. Performing a light-front calculation in parallel with the manifestly covariant calculation in light-front helicity basis, we examine the light-front zero-mode contribution to the helicity components of light-front good ("+") current matrix elements. Choosing the light-front gauge ($ε^+_{h=\pm}=0$) with circular polarization in Drell-Yan-West frame, we find that only the helicity components $({1\over 2}, {1\over 2})$ and $({1\over 2},-{1\over 2})$ of the good current receive the zero-mode contribution. Taking into account the zero-mode, we find the prescription independence in obtaining the light-front solution of form factors from any three helicity matrix elements with smeared light-front wavefunctions. The angular condition, which guarantees the full covariance of different schemes, is recovered.

hep-ph↗

arXiv2007

Distribution amplitudes and decay constants for $(π,K,ρ,K^*)$ mesons in light-front quark model

We present a calculation of the quark distribution amplitudes(DAs), the Gegenbauer moments, and decay constants for $π,ρ,K$ and $K^*$ mesons using the light-front quark model. While the quark DA for $π$ is somewhat broader than the asymptotic one, that for $ρ$ meson is very close to the asymptotic one. The quark DAs for $K$ and $K^*$ show asymmetric form due to the flavor SU(3)-symmetry breaking effect. The decay constants for the transversely polarized $ρ$ and $K^*$ mesons($f^T_ρ$ and $f^T_{K^*}$) as well as the longitudinally polarized ones($f_ρ$ and $f_{K^*}$) are also obtained. Our averaged values for $f^T_V/f_V$, i.e. $(f^T_ρ/f_ρ)_{\rm av}=0.78$ and $(f^T_{K^*}/f_{K^*})_{\rm av}=0.84$, are found to be consistent with other model predictions. Especially, our results for the decay constants are in a good agreement with the SU(6) symmetry relation, $f^T_{ρ(K^*)}=(f_{π(K)}+f_{ρ(K^*)})/2$.

hep-ph↗

arXiv2006

New Developments in Treacherous Points of Light-Front Dynamics

Light-front dynamics(LFD) plays an important role in hadron phenomenology. Last few years, however, it has been emphasized that treacherous points such as zero-mode contributions should be taken into account for successful LFD applications to hadron phenomenology. We discuss examples of treacherous points and present new progresses made last few years to handle them correctly.

hep-ph↗

arXiv2006

The box diagram in Yukawa theory

We present a light-front calculation of the box diagram in Yukawa theory. The covariant box diagram is finite for the case of spin-1/2 constituents exchanging spin-0 particles. In light-front dynamics, however, individual time-ordered diagrams are divergent. We analyze the corresponding light-front singularities and show the equivalence between the light-front and covariant results by taming the singularities.

hep-th↗

arXiv2006

Higher Fock State Contributions to the Generalized Parton Distribution of Pion

We discuss the higher Fock state (q \bar q g) contributions to the nonzero value of the pion GPD at the crossover point x = zeta between the DGLAP and ERBL regions. Using the phenomenological light-front constituent quark model, we confirm that the higher Fock state contributions indeed give a nonzero value of the GPD at the crossover point. Iterating the light-front quark model wave function of the lowest q \bar q Fock state with the Bethe-Salpeter kernel corresponding to the one-gluon-exchange, we include all possible time-ordered q \bar q g Fock state contributions and obtain the pion GPD satisfying necessary sum rules and continuity conditions.

hep-ph↗

arXiv2006

Light-front wavefunction dependence of the quark recombination

We present an extension of the recombination formalism to analyze the effects from the variation of the hadron wavefunctions. The hadron spectra are sensitive to the shape of the wavefunctions. However, when we fit the wavefunction parameters to the physical observables, such as the average charge radius, the final spectra are very similar each other. We discuss our numerical results in comparison with the published PHENIX and STAR data at RHIC. In the hadron spectra, the recombination of thermal partons dominates at intermediate transverse momentum ($P_{T}$ = 2 $\sim$ 5 GeV), and the fragmentation dominates at high $P_{T}$ ($>$ 5 GeV). The yield ratios and the nuclear modification factors for various hadron species are also estimated and compared to the experimental data. We present a new prediction on $\bar{p}/p$ and $K^{-}/K^{+}$ ratios, including the jet quenching effects to the fragmentation mechanism.

hep-ph↗

arXiv2006

Real and Virtual Nucleon Compton Scattering in the Perturbative Limit

We present the results of calculations analyzing nucleon Compton scattering to lowest order using perturbative QCD (pQCD) methods. Two scenarios are considered: (1) the incoming photon is real; and (2) the incoming photon is virtual. The case of a real photon has been previously analyzed at least 5 times using pQCD, but no two results are in agreement. Here it is shown that our result agrees with that of Brooks and Dixon published in 2000. The case of a virtual photon has been previously analyzed only once using pQCD. However, doubt has been cast on the validity of that result. The results presented here for virtual photon are believed to be more reliable. Some consideration is given of how to compare these results with experiment. Following the lead of Brooks and Dixon, for the proton, this involves normalizing the cross section using the Dirac proton form factor, which we also calculate. Finally, there is a comparison of our results with recent experiments.

hep-ph↗

arXiv2005

Instanton-inspired Model of QCD Phase Transition and Bubble Dynamics

We have reinvestigated the collision of gluonic bubbles in a SU(2) model of QCD which was studied by Johnson, Choi and Kisslinger in the context of the instanton-inspired model of QCD phase transition bubbles with plane wave approximation. We discuss treacherous points of the instanton-inspired model that cause the violation of causality due to the presence of imaginary gluon fields. By constructing a new slightly modified Lorentzian model where we have three independent real gluon fields, we reanalyzed the process of bubble collisions. Our numerical results show some indication of forming a bubble wall in colliding region.

hep-ph↗

arXiv2005

Revealing treacherous points for successful light-front phenomenological applications

Light-front dynamics(LFD) plays an important role in hadron phenomenology as evidenced from recent development of generalized parton distributions and other quantities involving hadrons. For successful LFD applications to hadron phenomenology, however, treacherous points such as zero-mode contributions should be taken into account. For a concrete example of zero-mode contribution, we present Standard Model analysis of vector anomaly in the CP-even form factors of W^{\pm} gauge bosons. Main distinguished features of LFD is discussed in comparison with other Hamiltonian dynamics. We also present a power counting method to correctly pin down which hadron form factors receive the zero-mode contribution and which ones do not. Indications from our analysis to hadron phenomenology are discussed.

hep-ph↗

arXiv2005

Possibility of determinig the parity of the pentaquark $Θ^+$ from photoproduction near threshold

We discuss the possibility of determining the parity of the $Θ^+$ baryon from photoproduction $γN\to KΘ^+$ process near threshold. We utilize the conservation laws of parity and angular momentum for the analysis of angular distributions and spin observables near threshold. Since the discussion is in essence a partial wave analysis of the production mechanism the result should be less dependent on the model parameters. Our analysis shows that the angular distribution and photon polarization asymmetry for the process of neutron target are sensitive to the parity of the $Θ^+$, but not for the case of proton target. In the case of proton target, the polarization asymmetries of target and recoiled $Θ^+$ are preferred for parity determination.

nucl-th↗

arXiv2004

A novel variational approach for Quantum Field Theory: example of study of the ground state and phase transition in Nonlinear Sigma Model

We discuss a novel form of the variational approach in Quantum Field Theory in which the trial quantum configuration is represented directly in terms of relevant expectation values rather than, e.g., increasingly complicated structure from Fock space. The quantum algebra imposes constraints on such expectation values so that the variational problem is formulated here as an optimization under constraints. As an example of application of such approach we consider the study of ground state and critical properties in a variant of nonlinear sigma model.

quant-ph↗