@article{ji_li_ma_suzuki_2018, title={Interpolating Quantum Electrodynamics}, volume={59}, ISSN={["1432-5411"]}, DOI={10.1007/s00601-018-1397-4}, abstractNote={We discuss the interpolation between the instant form dynamics (IFD) and the light-front dynamics (LFD) proposed by Dirac in 1949 and present its application to quantum electrodynamics (QED). Our analysis clarifies any conceivable confusion in the prevailing notion of the equivalence between the IFD at infinite momentum frame and the LFD. Entwining the fermion propagator interpolation with our previous works of the interpolating helicity spinors and the electromagnetic gauge field interpolation, we fasten the bolts and nuts necessary to launch the interpolating QED at least in the tree level.}, number={5}, journal={FEW-BODY SYSTEMS}, author={Ji, Chueng-Ryong and Li, Ziyue and Ma, Bailing and Suzuki, Alfredo Takashi}, year={2018}, month={Sep} }
 @article{ji_li_ma_suzuki_2018, title={Interpolating quantum electrodynamics between instant and front forms}, volume={98}, ISSN={["2470-0029"]}, DOI={10.1103/PhysRevD.98.036017}, abstractNote={The instant form and the front form of relativistic dynamics proposed by Dirac in 1949 can be linked by an interpolation angle parameter $\delta$ spanning between the instant form dynamics (IFD) at $\delta =0$ and the front form dynamics which is now known as the light-front dynamics (LFD) at $\delta =\pi/4$. We present the formal derivation of the interpolating quantum electrodynamics (QED) in the canonical field theory approach and discuss the constraint fermion degree of freedom which appears uniquely in the LFD. The constraint component of the fermion degrees of freedom in LFD results in the instantaneous contribution to the fermion propagator, which is genuinely distinguished from the ordinary equal-time forward and backward propagation of relativistic fermion degrees of freedom. As discussed in our previous work, the helicity of the on-mass-shell fermion spinors in LFD is also distinguished from the ordinary Jacob-Wick helicity in the IFD with respect to whether the helicity depends on the reference frame or not. To exemplify the characteristic difference of the fermion propagator between IFD and LFD, we compute the helicity amplitudes of typical QED processes such as $e^+ e^- \to \gamma \gamma$ and $e \gamma \to e \gamma$ and present the whole landscape of the scattering amplitudes in terms of the frame dependence or the scattering angle dependence with respect to the interpolating angle dependence. Our analysis clarifies any conceivable confusion in the prevailing notion of the equivalence between the infinite momentum frame approach and the LFD.}, number={3}, journal={PHYSICAL REVIEW D}, author={Ji, Chueng-Ryong and Li, Ziyue and Ma, Bailing and Suzuki, Alfredo Takashi}, year={2018}, month={Aug} }
 @article{ji_li_suzuki_2015, title={Electromagnetic gauge field interpolation between the instant form and the front form of the Hamiltonian dynamics}, volume={91}, ISSN={["1550-2368"]}, DOI={10.1103/physrevd.91.065020}, abstractNote={We present the electromagnetic gauge field interpolation between the instant form and the front form of the relativistic Hamiltonian dynamics and extend our interpolation of the scattering amplitude presented in the simple scalar field theory to the case of the electromagnetic gauge field theory with the scalar fermion fields known as the sQED theory. We find that the Coulomb gauge in the instant form dynamics (IFD) and the light-front gauge in the front form dynamics, or the light-front dynamics (LFD), are naturally linked by the unified general physical gauge that interpolates between these two forms of dynamics and derive the spin-1 polarization vector for the photon that can be generally applicable for any interpolation angle. Corresponding photon propagator for an arbitrary interpolation angle is found and examined in terms of the gauge field polarization and the interpolating time ordering. Using these results, we calculate the lowest-order scattering processes for an arbitrary interpolation angle in sQED. We provide an example of breaking the reflection symmetry under the longitudinal boost, $P^z \leftrightarrow -P^z$, for the time-ordered scattering amplitude in any interpolating dynamics except the LFD and clarify the confusion in the prevailing notion of the equivalence between the infinite momentum frame (IMF) and the LFD. The particular correlation found in our previous analysis of the scattering amplitude in the simple scalar field theory, coined as the J-shaped correlation, between the total momentum of the system and the interpolation angle persists in the present analysis of the sQED scattering amplitude. We discuss the singular behavior of this correlation in conjunction with the zero-mode issue in the LFD.}, number={6}, journal={PHYSICAL REVIEW D}, author={Ji, Chueng-Ryong and Li, Ziyue and Suzuki, Alfredo Takashi}, year={2015}, month={Mar} }
 @article{li_an_ji_2015, title={Interpolating helicity spinors between the instant form and the light-front form}, volume={92}, ISSN={["1550-2368"]}, DOI={10.1103/physrevd.92.105014}, abstractNote={We discuss the helicity spinors interpolating between the instant form dynamics (IFD) and the front form dynamics, or the light-front dynamics (LFD), and present the interpolating helicity amplitudes as well as their squares for the scattering of two fermions, and the annihilation of fermion and anti-fermion. We parametrize the interpolation between the two dynamics, IFD and LFD, by an interpolation angle and derive not only the generalized helicity spinors in the $(0,J)\oplus(J,0)$ chiral representation that links naturally the two typical IFD vs. LFD helicity spinors but also the generalized Melosh transformation that relates these generalized helicity spinors to the usual Dirac spinors. Analyzing the directions of the particle momentum and spin with the variation of the interpolation angle, we inspect the whole landscape of the generalized helicity intermediating between the usual Jacob-Wick helicity in the IFD and the light-front helicity in the LFD. Our analysis clarifies the characteristic difference of the helicity amplitudes between the IFD and the LFD. In particular, we find that the behavior of the angle between the momentum direction and the spin direction bifurcates at a critical interpolation angle and the IFD and the LFD separately belong to the two different branches bifurcated at this critical interpolation angle. This finding further clarifies any conceivable confusion in the prevailing notion of the equivalence between the infinite momentum frame and the LFD. The existence of the universal J-curve found in our previous works of scalar field theory and the sQED theory is confirmed in the present work of interpolating helicity amplitudes for the fermion scattering and annihilation processes. In conjunction with the bifurcation of branches, the two boundaries appear in the interpolating helicity amplitudes and interestingly the J-curve persists within these two boundaries.}, number={10}, journal={PHYSICAL REVIEW D}, author={Li, Ziyue and An, Murat and Ji, Chueng-Ryong}, year={2015}, month={Nov} }
 @article{choi_ji_li_ryu_2015, title={Variational analysis of mass spectra and decay constants for ground state pseudoscalar and vector mesons in the light-front quark model}, volume={92}, ISSN={["1089-490X"]}, DOI={10.1103/physrevc.92.055203}, abstractNote={Using the variational principle, we compute mass spectra and decay constants of ground state pseudoscalar and vector mesons in the light-front quark model (LFQM) with the QCD-motivated effective Hamiltonian including the hyperfine interaction. By smearing out the Dirac $\ensuremath{\delta}$ function in the hyperfine interaction, we avoid the issue of negative infinity in applying the variational principle to the computation of meson mass spectra and provide analytic expressions for the meson mass spectra. Our analysis with the smeared hyperfine interaction indicates that the interaction for the heavy meson sector including the bottom and charm quarks gets more point-like. We also consider the flavor mixing effect in our analysis and determine the mixing angles from the mass spectra of $(\ensuremath{\omega},\ensuremath{\phi})$ and $(\ensuremath{\eta},{\ensuremath{\eta}}^{\ensuremath{'}})$. Our variational analysis with the trial wave function including the two lowest order harmonic oscillator basis functions appears to improve the agreement with the data of meson decay constants and the heavy meson mass spectra over the previous computation handling the hyperfine interaction as perturbation.}, number={5}, journal={PHYSICAL REVIEW C}, author={Choi, Ho-Meoyng and Ji, Chueng-Ryong and Li, Ziyue and Ryu, Hui-Young}, year={2015}, month={Nov} }