Light-front QED Research Articles

Transverse structure of electron in momentum space in basis light-front quantization

We investigate the leading-twist transverse momentum-dependent distribution functions (TMDs) for a physical electron, a spin-1/2 composite system consisting of a bare electron and a photon, using the Basis Light-front Quantization (BLFQ) framework. The light-front wave functions of the physical electron are obtained from the eigenvectors of the light-front QED Hamiltonian. We evaluate the TMDs using the overlaps of the light-front wave functions. The BLFQ results are found to be in excellent agreement with those TMDs calculated using lowest-order perturbation theory.

Double parton distributions for a positroniumlike bound state using light-front wave functions

We investigate the double parton distributions (DPDs) for a positronium-like bound state using light-front QED. We incorporate the higher Fock three particle component of the state, that includes a photon. We obtain the overlap representation of the DPDs in terms of the three-particle light-front wave functions (LFWFs). Our calculation explores the correlations between the momentum fractions of the particles probed and the transverse distance between them, without any assumption of factorization between them. We also investigate the behavior of the DPDs near the kinematical boundary when the sum of the momentum fractions is close to one.

Light-front QED, Stueckelberg field and infrared divergence

Stueckelberg mechanism introduces a scalar field, known as Stueckelberg field, so that gauge symmetry is preserved in the massive Abelian gauge theory. In this work, we show that the role of the Stueckelberg field is similar to the Kulish and Faddeev coherent state approach to handle infrared (IR) divergences. We expect that the light-front quantum electrodynamics (LFQED) with Stueckelberg field must be IR finite in the massless limit of the gauge boson. We have explicitly shown the cancellation of IR divergences in the relevant diagrams contributing to self-energy and vertex correction at leading order.

Equivalence of One Loop Diagrams in Covariant and Light Front QED Revisited

We revisit the proof of equivalence of one loop expressions for fermion self-energy and vertex correction in light-front QED and Covariant QED at the Feynman diagram level and generalize, to all components, the proof of equivalence for the one loop vertex correction diagram which was presented earlier by us only for the $+$ component of $\Lambda^\mu$. We demonstrate, in the general case also, that the equivalence cannot be established without the third term in the three-term photon propagator in light cone gauge.

Revisiting the equivalence of light-front and covariant QED in the light-cone gauge

We discuss the equivalence between light-front time-ordered-perturbation theory and covariant quantum field theory in light-front quantization, in the case of quantum electrodynamics at one-loop level. In particular, we review the one-loop calculation of the vertex correction, fermion self-energy and vacuum polarization. We apply the procedure of integration by residue over the light-front energy in the loop to show how the perturbative expansion in covariant terms can be reduced to a sum of propagating and instantaneous diagrams of light-front time-ordered perturbation theory. The detailed proof of equivalence between the two formulations of the theory resolves the controversial question on which form should be used for the gauge-field propagator in the light-cone gauge in the covariant approach.

Self Energy Correction in Light Front QED And Coherent State Basis

We discuss the calculation of fermion self energy correction in Light Front QED using a coherent state basis. We show that if one uses coherent state basis instead of fock basis to calculate the transition matrix elements, the true infrared divergences in electron mass renormalization δm2 get canceled up to O(e4) in Light Front gauge. We have also verified this cancellation in Feynman gauge up to O(e2).

Cancellation of infrared divergences to all orders in light front QED

Coherent state approach has been proposed as an alternate way to deal with the true infrared divergences in light front field theory. We show that infrared divergences in fermion mass renormalization are eliminated to all orders in light front time ordered perturbation theory if one uses coherent state basis instead of the usual Fock basis to calculate the Hamiltonian matrix elements.

Quantum and Classical Radiation Reaction from Lightfront QED

We examine dynamical radiation reaction in lightfront QED. We take the classical limit and compare with predictions from various classical equations of motion.

Infrared Divergences in Light Front Field Theory and Coherent State Formalism

We discuss fermion self energy correction in light front QED using a coherent state basis. We show that if one uses coherent state basis instead of fock basis to calculate the transition matrix elements the true infrared divergences in $\delta m^2$ get canceled up to $O(e^4)$ . We show this in Light-front as well as in Feynman gauge.

Radiation reaction from QED: Lightfront perturbation theory in a plane wave background

We derive dynamical, real time radiation reaction effects from lightfront QED. Combining the Hamiltonian formalism with a plane wave background field, the calculation is performed in the Furry picture for which the background is treated exactly while interactions between quantum fields are treated in perturbation theory as normal. We work to a fixed order in perturbation theory, but no other approximation is made. The literature contains many proposals for the correct classical equation describing a radiating particle; we take the classical limit of our results and identify which equations are consistent with QED.

Fermion self-energy correction in light-front QED using coherent state basis

We discuss IR divergences in lepton mass renormalization in Light-Front Quantum Electrodynamics (LFQED) in Feynman gauge. We consider LFQED with Pauli-Villars fields and using old-fashioned time ordered perturbation theory (TOPT), we show that our earlier result regarding cancellation of true IR divergences upto $O(e^4)$ in coherent state basis holds in Feynman gauge also.

Infrared divergences in light-front QED and coherent state basis

We present a next to leading order calculation of electron mass renormalization in Light-Front Quantum Electrodynamics (LFQED) using old-fashioned time ordered perturbation theory (TOPT). We show that the true infrared divergences in $\delta m^2$ get canceled up to $O(e^4)$ if one uses coherent state basis instead of fock basis to calculate the transition matrix elements.

First nonperturbative calculation in light-front QED for an arbitrary covariant gauge

This work is the first check of gauge invariance for nonperturbative calculations in light-front QED. To quantize QED in an arbitrary covariant gauge, we use a light-front analog of the equal-time Stueckelberg quantization. Combined with a Pauli-Villars regularization, where massive, negative-metric photons and fermions are included in the Lagrangian, we are then able to construct the light-front QED Hamiltonian and the associated mass eigenvalue problem in a Fock-space representation. The formalism is applied to the dressed-electron state, with a Fock-space truncation to include at most one photon. From this eigenstate, we compute the anomalous magnetic moment. The result is found to be gauge independent, to an order in $\alpha$ consistent with the truncation.

Equivalence of covariant and light front QED: Generating instantaneous diagrams

One loop expressions for fermion self-energy, vacuum polarization, and vertex correction in light-front time ordered perturbation theory can be obtained from respective covariant expressions by performing ${k}^{\ensuremath{-}}$ integration. In an earlier work, we have shown that the third term in the doubly transverse gauge propagator is necessary to generate the diagrams involving instantaneous photon exchange both in the case of fermion self-energy as well as vertex correction. In this work, using the two-term photon propagator, we show that the instantaneous photon exchange diagrams in fermion self-energy as well as the IR singular terms in the propagating diagrams can be generated by taking the asymptotic limit of the covariant expression. It is further shown that this method reproduces the IR singular terms in propagating diagrams of vacuum polarization also.

Nonperturbative Pauli-Villars regularization of vacuum polarization in light-front QED

We continue the development of a nonperturbative light-front Hamiltonian method for the solution of quantum field theories by considering the one-photon eigenstate of Lorentz-gauge QED. The photon state is computed nonperturbatively for a Fock basis with a bare photon state and electron-positron pair states. The calculation is regulated by the inclusion of Pauli-Villars (PV) fermions, with one flavor to make the integrals finite and a second flavor to guarantee a zero mass for the physical photon eigenstate. We compute in detail the constraints on the PV coupling strengths that this zero mass implies. As part of this analysis, we provide the complete Lorentz-gauge light-front QED Hamiltonian with two PV fermion flavors and two PV photon flavors, which will be useful for future work. The need for two PV photons was established previously; the need for two PV fermions is established here.

On the nonperturbative solution of Pauli–Villars-regulated light-front QED: A comparison of the sector-dependent and standard parameterizations

We consider quantum electrodynamics quantized on the light front in Feynman gauge and regulated in the ultraviolet by the inclusion of massive, negative-metric Pauli–Villars (PV) particles in the Lagrangian. The eigenstate of the electron is approximated by a Fock-state expansion truncated to include one photon. The Fock-state wave functions are computed from the fundamental Hamiltonian eigenvalue problem and used to calculate the anomalous magnetic moment, as a point of comparison. Two approaches are considered: a sector-dependent parameterization, where the bare parameters of the Lagrangian are allowed to depend on the Fock sectors between which the particular Hamiltonian term acts, and the standard choice, where the bare parameters are the same for all sectors. Both methods are shown to require some care with respect to ultraviolet divergences; neither method can allow all PV masses to be taken to infinity. In addition, the sector-dependent approach suffers from an infrared divergence that requires a nonzero photon mass; due to complications associated with this divergence, the standard parameterization is to be preferred. We also show that the self-energy effects obtained from a two-photon truncation are enough to bring the standard-parameterization result for the anomalous moment into agreement with experiment within numerical errors. This continues the development of a method for the nonperturbative solution of strongly coupled theories, in particular quantum chromodynamics.

Restoration of the chiral limit in Pauli-Villars-regulated light-front QED

The dressed-electron eigenstate of Feynman-gauge QED is computed in light-front quantization with a Fock-space truncation to include at most the one-photon/one-electron sector. The theory is regulated by the inclusion of three massive Pauli-Villars (PV) particles, one PV electron, and two PV photons. In particular, the chiral-limit is investigated, and the correct limit is found to require two PV photons, not just one as previously thought. The renormalization and covariance of the electron current are also analyzed. We find that the plus component is well behaved and use its spin-flip matrix element to compute the electron's anomalous moment. The dependence of the moment on the regulator masses is shown to be slowly varying when the second PV photon is used to guarantee the correct chiral limit.

Equivalence of covariant and light front QED at one loop level

We consider the equivalence of covariant and light front quantum electrodynamics at one loop level. We show that the one loop expressions for fermion self-energy, vacuum polarization, and vertex correction in the covariant perturbation theory can be reduced to a sum of propagating and instantaneous diagrams of light front time-ordered perturbation theory by performing ${k}^{\ensuremath{-}}$ integration. We show that the third term in the doubly transverse gauge propagator is necessary to generate the diagrams involving instantaneous photon exchange both in the case of fermion self-energy as well as the vertex correction. We also show that the correspondence between the covariant and light front diagrams cannot be established if one removes the ${k}^{+}=0$ modes by an infrared cutoff before performing ${k}^{\ensuremath{-}}$ integration.

Weyl gauge, Schwinger terms and bosonization in light-front field theory

Abstract A systematic study of non-perturbative quantum structure of the massive light-front Schwinger model and QED(3+1) in the continuum formulation is outlined. The light-front Hamiltonian and field algebra are derived in the Weyl gauge using the Dirac-Bergmann constrained quantization. Unitary transformation to the light-cone gauge representation is performed and the gauge-invariant fermi field is constructed. The importance of the Schwinger term in the current-current commutation relations for the derivation of the fermionic vacuum structure and bosonization in two dimensions is indicated.

Twist four longitudinal structure function for a positronium-like bound state in light-front QED

To have an analytic understanding of the higher-twist structure functions, we calculate twist four longitudinal structure function for a positronium-like bound state in weak coupling light-front QED. We find that in the weakly coupled system, the fermionic part of $F_L$ is related to the kinetic energy of the fermions and not to the interaction. We verify a previously proposed sum rule in this limit, which in this case reduces to a relation connecting the kinetic and the potential energies to the binding energy of positronium. Using the analytic form of the wave function of positronium in this limit, we show that the constituent counting rule does not hold for $x \to 1$. The twist four $F_L$ in this limit is similar in form to a widely used phenomenological ansatz.