Light-front quantization is the same as instant-time quantization
Philip D. Mannheim
备注：
7 pages. Prepared for proceedings of "Light Cone 2019 - QCD on the light cone: from hadrons to heavy ions". This is the conference version of arXiv:1909.03548 [hep-ph], with which it thus has substantial overlap
Commutation or anticommutation relations quantized at equal instant time and commutation or anticommutation relations quantized at equal light-front time cannot be transformed into each other. While they would thus appear to describe different theories, we show that this is not in fact the case. In instant-time quantization unequal instant-time commutation or anticommutation relations for free scalar, fermion, or gauge boson fields are c-numbers. We show that when these unequal instant-time commutation or anticommutation relations are evaluated at equal light-front time they are identical to the equal light-front time commutation or anticommutation relations. Light-front quantization and instant-time quantization are thus the same and thus describe the same physics.
We compare light-front quantization and instant-time quantization both at the level of operators and at the level of their Feynman diagram matrix elements. At the level of operators light-front quantization and instant-time quantization lead to equal light-front time commutation (or anticommutation) relations that appear to be quite different from equal instant-time commutation (or anticommutation) relations. Despite this we show that at unequal times instant-time and light-front commutation (or anticommutation) relations actually can be transformed into each other, with it only being the rest…
It is often stated that the vacuum is trivial when light-front (null-plane) quantization is applied to a quantum field theory, in contrast to the situation with equal-time quantization. In fact, it is has long been known that the statement is false, and that in certain cases the standard rules for light-front perturbation theory need modification. This paper gives an elementary review of these issues, including an explanation of how and when there is a failure of the elementary derivation of the rules for light-front perturbation theory.
We employ an effective Hamiltonian that includes the transverse and longitudinal confinement and the one-gluon exchange interaction with fixed coupling constant. By solving the eigenvalue equation in basis light-front quantization (BLFQ), we generate the light-front wavefunctions (LFWFs) for the nucleon in the valence quark Fock space. Fitting the model parameters, we obtain high-quality descriptions of electromagnetic form factors and radius for proton while the results deviate somewhat from experimental data for neutron.
Commutation or anticommutation relations quantized at equal instant time and commutation or anticommutation relations quantized at equal light-front time not only cannot be transformed into each other, they take completely different forms. While they would thus appear to describe different theories, we show that this is not in fact the case. By looking not at equal times but at unequal times, we show that unequal instant-time commutation or anticommutation relations are completely equivalent to unequal light-front time commutation or anticommutation relations. Light-front quantization and inst…
We review several aspects of Yang-Mills theory (YMT) in two dimensions, related to its perturbative and topological properties. Consistency between light-front and equal-time formulations is thoroughly discussed.
Light-front quantization has important advantages for describing relativistic statistical systems, particularly systems for which boost invariance is essential, such as the fireball created in a heavy ion collisions. In this paper we develop light-front field theory at finite temperature and density with special attention to quantum chromodynamics. We construct the most general form of the statistical operator allowed by the Poincare algebra and show that there are no zero-mode related problems when describing phase transitions. We then demonstrate a direct connection between densities in ligh…
Light-front quantization of the conformally gauge-fixed Polyakov D1 brane action with a scalar dilaton field is studied using the equal light-cone world-sheet time framework.
An introductory overview on Light-Front quantization, with some emphasis on recent achievements, is given. Light-Front quantization is the most promising and physical tool to study deep inelastic scattering on the basis of quark gluon degrees of freedom. The simplified vacuum structure (nontrivial vacuum effects can only appear in zero-mode degrees of freedom) and the physical basis allows for a description of hadrons that stays close to intuition. Recent progress has ben made in understanding the connection between effective LF Hamiltonians and nontrivial vacuum condesates. Discrete Light-Con…
In this work, I consider the light-front quantization of a class of Nielsen-Olesen (Bogomol'nyi) models in two-space one-time dimensions in the so-called symmetry phase using the Hamiltonian, path integral and BRST formulations.
In this paper, we study the question of quantization of quantum field theories in a general light-front frame. We quantize scalar, fermion as well as gauge field theories in a systematic manner carrying out the Hamiltonian analysis carefully. The decomposition of the fields into positive and negative frequency terms needs to be done carefully after which we show that the (anti) commutation relations for the quantum operators become frame independent. The frame dependence is completely contained in the functions multiplying these operators in the field decomposition. We derive the propagators f…
Using Dirac's method for the quantization of constrained systems QED is canonically quantized in the front-form in a gauge which is the light-front analog of the Weyl gauge. From the obtained vacuum wave functional the spatial Wilson loop is calculated. The result known from the canonical instant-form quantization or from the covariant path integral quantization is found only if the static limit $x^+ \to 0$ is taken in a specific order. The same ambiguity is also found in the static photon propagator in coordinate space.
We find a correspondence between semiclassical QCD quantized on the light-front and a dual gravity model in anti--de Sitter (AdS) space, thus providing an initial approximation to QCD in its strongly coupled regime. This correspondence -- light-front holography -- leads to a light-front Hamiltonian and relativistic bound-state wave equations that are functions of an invariant impact variable $ζ$ which measures the separation of the quark and gluonic constituents within hadrons at equal light-front time. The eigenvalues of the resulting light-front Schrödinger and Dirac equations are consistent…
It is shown that the quantization of the unphysical degrees of freedom, which leads to the Mandelstam--Leibbrandt prescription for the infrared spurious singularities in the continuum light cone gauge, does indeed suggest some quite natural recipe to treat the zero modes in the Discretized Light Front Quantization of gauge theories.
Recently, we proposed a new front-form quantization which treated both the $x^{+}$ and the $x^{-}$ coordinates as front-form 'times.' This quantization was found to preserve parity explicitly. In this paper we extend this construction to fermion fields in the context of the Yukawa theory. We quantize this theory using a method proposed originally by Faddeev and Jackiw . We find that $P^-$ {\it and} $P^+$ become dynamical and that the theory is manifestly invariant under parity.
Convergence in charmonium structure: light-front wave functions from basis light-front quantization and Dyson-Schwinger equations
University of Science & Technology of China（中国科学技术大学）；Anhui Center for Fundamental Sciences in Theoretical Physics（安徽基础前沿与交叉科学研究中心）；Nanjing University of Aeronautics and Astronautics（南京航空航天大学）；Iowa State University（爱荷华州立大学）；Anhui University of Science and Technology（安徽理工大学）
We study the structure of scalar field light front quantization vacuum graphs. In instant time quantization both non-vacuum and vacuum graphs can equivalently be described by either the off-shell four-dimensional Feynman diagram approach or the on-shell three-dimensional Fock space approach, with this being the case since the relevant Feynman diagrams are given entirely by pole terms. This is also the case for light front quantization non-vacuum graphs. However this is the not the case for light front vacuum sector diagrams, since then there are also circle at infinity contributions to Feynman…
We revisit the problem of quantizing field theories on noncommutative Moyal spacetime with \emph{light-like} noncommutativity. To tackle the issues arising from noncommuting and hence nonlocal time, we argue that for this case light-front quantization procedure should be employed. In this appropriate quantization scheme we perform the non-planar loop analysis for the light-like noncommutative field theories. One of the important and peculiar features of light-front quantization is that the UV cutoff of the light-cone Hamiltonian manifests itself as an IR cutoff for the light-cone momentum, $p^…
Transverse force tomography inside a proton from Basis Light-front Quantization
Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所）；School of Nuclear Science and Technology, University of Chinese Academy of Sciences（中国科学技术大学核科学与技术学院）
Recently, we have proposed a new front-form quantization which treated both the $x^{+}$ and the $x^{-}$ coordinates as front-form 'times.' This quantization was found to preserve parity explicitly. In this paper we extend this construction to local Abelian gauge fields . We quantize this theory using a method proposed originally by Faddeev and Jackiw . We emphasize here the feature that quantizing along both $x^+$ and $x^-$ , gauge theories does not require extra constraints (also known as 'gauge conditions') to determine the solution uniquely.
By performing the canonical quantization of the Abelian Chern-Simons model on the light-front (as suggested by Dirac), we clarify some controversies appearing in recent papers that discuss the relation between the existence of excitations carrying fractional spin and statistics (anyons) and this model. Properties of the Chern-Simons model on the light-front are investigated in detail, following the Dirac method for constrained dynamical systems, both for a coupled complex scalar field as well as for a spinor field.
The Dirac procedure for dealing with constraints is applied to the quantization of gauge theories on the light front. The light cone gauge is used in conjunction with the first class constraints that arise and the resulting Dirac brackets are found. These gauge conditions are not used to eliminate degrees of freedom from the action prior to applying the Dirac constraint procedure. This approach is illustrated by considering Yang-Mills theory and the superparticle in a 2 + 1 dimensional target space.
The instant form and the front form of relativistic dynamics proposed by Dirac in 1949 can be linked by an interpolation angle parameter $δ$ spanning between the instant form dynamics (IFD) at $δ=0$ and the front form dynamics which is now known as the light-front dynamics (LFD) at $δ=π/4$. 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 co…
We show that the standard result for the Casimir force between conducting plates at rest in an inertial frame can be computed in light-front quantization. This is not the same as light-front analyses where the plates are at "rest" in an infinite momentum frame. In that case, Lenz and Steinbacher have shown that the result does not agree with the standard result for plates at rest. The two important ingredients in the present analysis are a careful treatment of the boundary conditions, inspired by the work of Almeida et al. on oblique light-front coordinates, and computation of the ordinary ene…
We address problems associated with compactification near and on the light front. In perturbative scalar field theory we illustrate and clarify the relationships among three approaches: (1) quantization on a space-like surface close to a light front; (2) infinite momentum frame calculations; and (3) quantization on the light front. Our examples emphasize the difference between zero modes in space-like quantization and those in light front quantization. In particular, in perturbative calculations of scalar field theory using discretized light cone quantization there are well-known ``zero-mode i…
Gaussian effective potential is obtained for $ϕ^4_{1+1}$ quantized on a light front. It coincides with the one obtained previously within the equal time quantization. The computation of the paper substantiates the claim that light front quantization reproduces the phase structure of the theory implied by the equal time quantization.
We use the light front ``machinery'' to study the behavior of a relativistic free particle and obtain the quantum commutation relations from the classical Poisson brackets. We argue that their usual projection onto the light-front coordinates from the covariant commutation relations show that there is an inconsistency in the expected correlation between canonically conjugate variables ``time'' and ``energy''. Moreover we show that this incompatibility originates from the very definition of the Poisson brackets that is employed and present a simple remedy to this problem and envisages a profoun…
Some basic topics in Light-Front (LF) quantized field theory are reviewed. Poincarè algebra and the LF Spin operator are discussed. The local scalar field theory of the conventional framework is shown to correspond to a non-local Hamiltonian theory on the LF in view of the constraint equations on the phase space, which relate the bosonic condensates to the non-zero modes. This new ingredient is useful to describe the spontaneous symmetry breaking on the LF. The instability of the symmetric phase in two dimensional scalar theory when the coupling constant grows is shown in the LF theory renorma…
There is a discrepancy between light-front and equal-time values for the critical coupling of two-dimensional $ϕ^4$ theory. A proposed resolution is to take into account the difference between mass renormalizations in the two quantizations. This distinction was first discussed by M. Burkardt. It prevents direct comparison of bare parameters; however, a method proposed here allows calculation of the difference and thereby resolves the discrepancy. We also consider the consequences of allowing a sector-dependent constituent mass.
The canonical quantization on a single light front is performed for the Abelian gauge fields with the Weyl gauge coupled with fermion field currents. The analysis is carried separately for 1+1 dimensions and for higher dimensions. The Gauss law, implemented weakly as the condition on states, selects physical subspace with the Poincare covariance recovered. The perturbative gauge field propagators are found with the ML prescription for their spurious poles. The LF Feynman rules are found and their equivalence with the usual equal-time perturbation for the S-matrix elements is studied for all or…
We use the light-front machinery to study the behavior of a relativistic free particle and obtain the quantum commutation relations from the classical Poisson brackets. We argue that the usual projection onto the light-front coordinates for these from the covariant commutation ralations does not reproduce the expected results.
Without a gauge fixing, canonical variables for the light-front SU(2) gluodynamics are determined. The Gauss law is written in terms of the canonical variables. The system is qualified as a generalized dynamical system with first class constraints. Abeliazation is a specific feature of the formulation (most of the canonical variables transform nontrivially only under the action of an Abelian subgroup of the gauge transformations). At finite volume, a discrete spectrum of the light-front Hamiltonian $P_+$ is obtained in the sector of vanishing $P_-$. We obtain, therefore, a quantized form of th…
In this paper, we present an approach to quantize singular systems. This is an extension of the constant integration method (Belhadi et al. (2014)) which is applicable only for the case of exactly solvable systems. In our approach, we determine Dirac brackets at the initial instant with the help of Taylor expansion, and using their covariance, we deduce the brackets at any time. We also illustrate our approach by studying two systems: the self-dual model and the Dirac spinorial field. Keywords : quantization, singular systems, integration constants, Liouville operator, Taylor expansion.
We use the interpolating coordinates studied by Hornbostel to investigate a transition from equal-time quantization to light-front quantization, in the context of two-dimensional $ϕ^4$ theory. A consistent treatment is found to require careful consideration of vacuum bubbles, in a nonperturbative extension of the analysis by Collins. Numerical calculations of the spectrum at fixed box size are shown to yield results equivalent to those of equal-time quantization, except when the interpolating coordinates are pressed toward the light-front limit. In that regime, a fixed box size is inconsistent…
The spontaneous symmetry breaking (and Higgs) mechanism in the theory quantized on the light-front ({\it l.f.}), in the {\it discretized formulation}, is discussed. The infinite volume limit is taken to obtain the {\it continuum version}. The hamiltonian formulation is shown to contain a new ingredient in the form of nonlocal {\it constraint eqs.} which lead to a {\it nonlocal l.f. Hamiltonian}. The description of the broken symmetry here has the same physical content as in the conventional formulation though arrived at through a different mechanism.
Light-front wavefunctions provide a frame-independent representation of hadrons in terms of their physical quark and gluon degrees of freedom. The light-front Hamiltonian formalism provides new nonperturbative methods for obtaining the QCD spectrum and eigensolutions, including resolvant methods, variational techniques, and discretized light-front quantization. A new method for quantizing gauge theories in light-cone gauge using Dirac brackets to implement constraints is presented. In the case of the electroweak theory, this method of light-front quantization leads to a unitary and renormaliza…
Naive light-front quantization, carried out by a light-front energy integration of covariant amplitudes, is not guaranteed to generate the corresponding Feynman amplitudes. In an explicit example we show that the nonvalence contribution to the minus-component of the EM current of a meson with fermion constituents has a persistent end-point singularity. Only after this term is subtracted, the result is covariant and satisfies current conservation. If the spin-1/2 constituents are replaced by spin zero ones, the singularity does not occur and the result is, without any adjustment, identical to t…
Gravitational form factors of light mesons from Basis Light-Front Quantization
Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所）；School of Nuclear Physics, University of Chinese Academy of Sciences（中国科学院大学核科学与技术学院）
Within a scheme of light front quantization of $ϕ^4_{1+1}$, it is demonstrated that dynamics of zero modes implies phase transition, and that the critical value of the coupling coincides with the one of the equal time quantization.
We apply the basis light-front quantization framework to solve for the structures of mesons with light and strange valence quarks. Our approach treats mesons as relativistic bound states with quarks confined in both the transverse direction and the light-front longitudinal direction. The spin-orbit interactions of these confined quarks are further specified by the Nambu--Jona-Lasinio model. We address the $\mathrm{U}(1)_{\mathrm{A}}$ axial anomaly by including the Kobayashi-Maskawa-'t Hooft interaction regularized by our basis. We present the structures of the pion, the kaon, the eta meson, an…
The analogy between quantum chemistry and light-front quantum field theory, first noted by Kenneth G. Wilson, serves as motivation to develop light-front quantum simulation of quantum field theory. We demonstrate how calculations of hadron structure can be performed on Noisy Intermediate-Scale Quantum devices within the Basis Light-Front Quantization framework. We calculate the light-front wave functions of pions using an effective light-front Hamiltonian in a basis representation on a current quantum processor. We use the Variational Quantum Eigensolver to find the ground state energy and wav…
A genuine continuum treatment of the massive ϕ^4_{1+1}-theory in light-cone quantization is proposed. Fields are treated as operator valued distributions thereby leading to a mathematically well defined handling of ultraviolet and light cone induced infrared divergences and of their renormalization. Although non-perturbative the continuum light cone approach is no more complex than usual perturbation theory in lowest order. Relative to discretized light cone quantization, the critical coupling increases by 30% to a value r = 1.5. Conventional perturbation theory at the corresponding order yiel…
Twist-3 Generalized Parton Distribution for the Proton from Basis Light-Front Quantization
Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所）；School of Nuclear Science and Technology, University of Chinese Academy of Sciences（中国科学院大学核科学与技术学院）
We give an overview of the light-front holographic approach to strongly coupled QCD, whereby a confining gauge theory, quantized on the light front, is mapped to a higher-dimensional anti de Sitter (AdS) space. The framework is guided by the AdS/CFT correspondence incorporating a gravitational background asymptotic to AdS space which encodes the salient properties of QCD, such as the ultraviolet conformal limit at the AdS boundary at $z \to 0$, as well as modifications of the geometry in the large $z$ infrared region to describe confinement and linear Regge behavior. There are two equivalent p…
We study the semileptonic decay of the $B_c (0^-)$ to charmonia through the bottom-to-charm-quark electroweak current in the framework of basis light-front quantization. Explicitly, we calculate the weak transition form factors for processes of $B_c$ decaying into $η_c$ or $J/ψ$ based on the corresponding initial and final valence light-front wave functions both obtained from the basis light-front quantization. We also present the corresponding differential decay width and branching ratios, as well as the branching ratios for decays into the $η_c'$ and the $ψ'$. We observe unphysical frame dep…
Two different scenarios (light-front and equal-time) are possible for Yang-Mills theories in two dimensions. The exact $\bar q q$-potential can be derived in perturbation theory starting from the light-front vacuum, but requires essential instanton contributions in the equal-time formulation. In higher dimensions no exact result is available and, paradoxically, only the latter formulation (equal-time) is acceptable, at least in a perturbative context.
We develop a method by which vacuum transitions may be included in light-front calculations. This allows tadpole contributions which are important for symmetry-breaking effects and yet are missing from standard light-front calculations. These transitions also dictate a nontrivial vacuum and contributions from vacuum bubbles to physical states. In nonperturbative calculations these separate classes of contributions (tadpoles and bubbles) cannot be filtered; instead, we regulate the bubbles and subtract the vacuum energy from the eigenenergy of physical states. The key is replacement of momentum…
One loop expressions for fermion self energy,vacuum polarization and vertex correction in light front time ordered perturbation theory (LFTPOT) can be obtained from respective covariant expressions by performing k^- integration. In 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 case of fermion self-energy as well as vertex correction.In this work,we show that instantaneous photon exchange diagrams in fermion self-energy as well as th IR singular terms in the propagat…
A review is made on some recent studies which support the point of view that the relativistic field theory quantized on the light-front (LF) is more transparent compared to the conventional equal-time one. The discussion may be of relevance in the context of the quantization of gravitation theory. The LF quantization is argued to be equally appropriate as the conventional equal-time one. The description on the LF of the spontaneous symmetry breaking and the (tree level) Higgs mechanism, the emergence of the $θ$-vacua in the Schwinger model, the absence of such vacua in the Chiral SM, the BRS-B…
Motivated by an apparent puzzle of the light-front vacua incompatible with the axial anomaly, we have considered the two-dimensional massless Schwinger model for an arbitrary interpolating angle of the quantization surface. By examining spectral deformation of the Dirac sea under an external electric field semiclassically, we have found that the axial anomaly is quantization angle independent. This indicates an intricate nontrivial vacuum structure present even in the light-front limit.
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 particula…
The formulation of statistical physics using light-front quantization, instead of conventional equal-time boundary conditions, has important advantages for describing relativistic statistical systems, such as heavy ion collisions. We develop light-front field theory at finite temperature and density with special attention to quantum chromodynamics. First, we construct the most general form of the statistical operator allowed by the Poincare algebra. In light-front quantization, the Green's functions of a quark in a medium can be defined in terms of just 2-component spinors and does not lead to…
We introduce a nonperturbative, first principles numerical approach for solving time-dependent problems in quantum field theory, using light-front quantization. As a first application we consider QED in a strong background field, and the process of non-linear Compton scattering in which an electron is excited by the background and emits a photon. We track the evolution of the quantum state as a function of time. Observables, such as the invariant mass of the electron-photon pair, are first checked against results from perturbation theory, for suitable parameters. We then proceed to a test case…
We study the light-front Schwinger model at finite temperature following the recent proposal in \cite{alves}. We show that the calculations are carried out efficiently by working with the full propagator for the fermion, which also avoids subtleties that arise with light-front regularizations. We demonstrate this with the calculation of the zero temperature anomaly. We show that temperature dependent corrections to the anomaly vanish, consistent with the results from the calculations in the conventional quantization. The gauge self-energy is seen to have the expected non-analytic behavior at f…
We produce the light-front wave functions (LFWFs) of the nucleon from a basis light-front ap- proach in the leading Fock sector representation. We solve for the mass eigenstates from a light-front effective Hamiltonian, which includes a confining potential adopted from light-front holography in the transverse direction, a longitudinal confinement, and a one-gluon exchange interaction with fixed coupling. We then employ the LFWFs to obtain the electromagnetic and axial form factors, the par- ton distribution functions (PDFs) and the generalized parton distribution functions for the nucleon. The…
