[en] Even though the Drell-Yan-West formulation is the most rigorous and well-established framework to compute the exclusive processes, its utility has been limited only to the space-like region because of the intrinsic kinematic constraint q⁺=0. We present an explicit example demonstrating how one may obtain the necessary information (i.e. nonvalence or so called Z-graph contribution) in the time-like region of exclusive process without encountering a formidable task of direct calculation that has hindered so far the progress in this area. In the analysis of qQbar bound state form factors using an exactly solvable model of (3+1)-dimensional scalar field theory interacting with gauge fields, the results analytically continued from the space-like region coincide exactly with the direct results in the time-like region. This example verifies that the method of analytic continuation is capable of yielding the effect of complicate nonvalence contributions. The meson peaks analogous to the vector meson dominance (VMD) phenomena are also generated at the usual VMD positions

[en] We discuss the soft contribution to the elastic pion form factor with the mass evolution from current to constituent quark being taken into account using a light-front Bethe-Salpeter (LFBS) model, which is a light-front quark model (LFQM) with a running mass. It is shown that partial conservation of the axial-vector current (PCAC) is satisfied with a running quark mass. We examine the sensitivity of the pion form factor using two different functional forms of the quark propagator. The Ball-Chiu ansatz is used to maintain local gauge invariance of the quark-photon vertex. The extension of our model to the hard contribution is also discussed

[en] The divergences appearing in the (3+1)-dimensional fermion-loop calculations are often regulated by smearing the vertices in a covariant manner. Performing a parallel light-front calculation, we corroborate the similarity between the vertex-smearing technique and the Pauli-Villars regularization. In the light-front calculation of the electromagnetic meson current, we find that the persistent end-point singularity that appears in the case of point vertices is removed even if the smeared vertex is taken to the limit of the point vertex. Recapitulating the current conservation, we substantiate the finiteness of both valence and nonvalence contributions in all components of the current with the regularized bound-state vertex. However, we stress that each contribution, valence or nonvalence, depends on the reference frame even though the sum is always frame independent. The numerical taxonomy of each contribution including the instantaneous contribution and the zero-mode contribution is presented in the π, K, and D-meson form factors

[en] Starting from the QCD action in an instanton-like SU(2) Yang-Mills field theory, we derive equations of motion in Minkowski space. Possible bubble collisions are studied in a 1+1 dimension reduction. We find gluonic structures which might give rise to CMBR effects

[en] We investigate the magnetic dipole decays V→Pγ of various heavy-flavored mesons such as (D,D*,D_s,D_s*,η_c,J/ψ) and (B,B*,B_s,B_s*,η_b,Υ) using the light-front quark model constrained by the variational principle for the QCD-motivated effective Hamiltonian. The momentum dependent form factors F_VP(q²) for V→Pγ* decays are obtained in the q⁺=0 frame and then analytically continued to the timelike region by changing q_{perpendicular} to iq_{perpendicular} in the form factors. The coupling constant g_VPγ for real photon case is then obtained in the limit as q²→0, i.e. g_VPγ=F_VP(q²=0). The weak decay constants of heavy pseudoscalar and vector mesons are also calculated. Our numerical results for the decay constants and radiative decay widths for the heavy-flavored mesons are overall in good agreement with the available experimental data as well as other theoretical model calculations

[en] We investigate space- and timelike form factors for ω→πγ* and K*→Kγ* decays using the light-front quark model constrained by the variational principle for the QCD-motivated effective Hamiltonian. The momentum dependent spacelike form factors are obtained in the q⁺=0 frame and then analytically continued to the timelike region. Our prediction for the timelike form factor F_ωπ(q²) is in good agreement with the experimental data. We also find that the spacelike form factor F_K*^±K^±(Q²) for charged kaons encounters a zero because of the negative interference between the two currents to the quark and the antiquark.

[en] We investigate the exclusive rare B_c→D_(s)ν_lν_l and B→D_(s)l⁺l^- (l=e, μ, τ) decays within the standard model and the light-front quark model constrained by the variational principle for the QCD-motivated effective Hamiltonian. The form factors f_±(q²) and f_T(q²) are obtained from the analytic continuation method in the q⁺=0 frame. While the form factors f₊(q²) and f_T(q²) are free from the zero mode, the form factor f_-(q²) is not free from the zero mode in the q⁺=0 frame. We discuss the covariance (i.e. frame independence) of our model calculation and quantify the zero-mode contributions to f_-(q²) for B_c→D_(s) decays. The branching ratios and the longitudinal lepton polarization asymmetries are calculated with and without the long-distance contributions. Our numerical results for the nonresonant branching ratios for and B_c→D(D_s)l⁺l^- are in the order of 10^-8 (10^-7) and 10^-9 (10^-8), respectively. The averaged values of the lepton polarization asymmetries obtained from the linear (harmonic oscillator) potential parameters are found to be -0.99 (-0.99) for B_c→Dμ⁺μ^- and -0.16 (-0.15) for B_c→Dτ⁺τ^-, and -0.98 (-0.98) for B_c→D_sμ⁺μ^- and -0.14 (-0.12) for B_c→D_sτ⁺τ^-, respectively.

[en] Motivated by the recent lattice QCD results indicating that the topological charge contribution to the flavor singlet axial vector current can be traded off by the constituent quark masses, we investigate the radiative decays of pseudoscalar (π, K, η, η'), vector (ρ, K*, ω, φ) and axial vector (A₁) mesons using a simple relativistic constituent quark model. For both simplicity and relativity, we take advantage of the distinguished features in the light-cone quantization method: (1) the Fock-state expansion of meson wavefunctions are not contaminated by the vacuum fluctuation, (2) the assignment of meson quantum numbers are given by the Melosh transformation. The computed decay widths of ρ, ω → π(η)γ, K* → Kγ, A₁ → πγ and the transition form factor of π⁰ → γ*γ at 0 ≤ Q² ≤ 8 GeV² are in a remarkably good agreement with the experimental data. (author)

[en] We investigate the form factors and decay rates of exclusive 0^-→0^- semileptonic meson decays using the constituent quark model based on the light-front quantization. Our model is constrained by the variational principle for the linear plus Coulomb interaction motivated by QCD. Our numerical results are in a good agreement with the available experimental data

[en] We analyzed two twist-3 distribution amplitudes of pion, i.e. pseudoscalar ϕ"P_3_;_π(x) and pseudo tensor ϕ"σ_3_;_π(x), within the LFQM. Our LFQM descriptions both for twist-3 ϕ"P_3_;_π and ϕ"σ_3_;_π obtained from the Gaussian radial wave function not only satisfy the fundamental constraint required from the isospin symmetry, but also reproduce exactly the asymptotic forms anticipated from QCD’s conformal limit. (author)