Exploring Timelike Region of QCD Exclusive Processes in Relativistic Quark Model

We investigate the form factors and decay rates of exclusive $0^{-}\to0^{-}$ 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.Comment: 6 pages, Latex with 2 figures: Talk given at the 12th Nuclear Physics Summer School and Symposium and the 11th International Light-Cone Workshop "New Directions in QCD", June 21-25, 1999, Kyungju, Kore

Exploring timelike exclusive processes in the light-front approach

We discuss a necessary nonvalence contribution in timelike exclusive processes. Utilizing a Schwinger-Dyson type of approach, we relate the nonvalence contribution to an ordinary light-front wave function that has been extensively tested in the spacelike exclusive processes. An application to $K_{\ell3}$ decays provides encouraging results.Comment: Contribution to the third international conference on ``Perspectives in Hadronic Physics", 7-11 May 2001, Miramare-Trieste, Italy; 4 pages including 2 figure

Interpolating Scattering Amplitudes between Instant Form and Front Form of Relativistic Dynamics

Among the three forms of relativistic Hamiltonian dynamics proposed by Dirac in 1949, the instant form and the front form can be interpolated by introducing an interpolation angle between the ordinary time $t$ and the light-front time $(t+z/c)/\sqrt{2}$. Using this method, we introduce the interpolating scattering amplitude that links the corresponding time-ordered amplitudes between the two forms of dynamics and provide the physical meaning of the kinematic transformations as they allow the invariance of each individual time-ordered amplitude for an arbitrary interpolation angle. In particular, it exhibits that the longitudinal boost is kinematical only in the front form dynamics, or the light-front dynamics (LFD), but not in any other interpolation angle dynamics. It also shows that the disappearance of the connected contributions to the current arising from the vacuum occurs when the interpolation angle is taken to yield the LFD. Since it doesn't require the infinite momentum frame (IMF) to show this disappearance and the proof is independent of reference frames, it resolves the confusion between the LFD and the IMF. The well-known utility of IMF usually discussed in the instant form dynamics is now also extended to any other interpolation angle dynamics using our interpolating scattering amplitudes.Comment: 34 pages, 5 figure

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.Comment: talk presented at DPF2004 meeting in Riverside, CA; to appear in a supplement in International Journal of Modern Physics