Light-front quantization applications
Standard scattering theory in Hamiltonian frameworks can provide valuable guidance for developing a LFQCD-based analysis of high-energy reactions.
"Hard exclusive processes" refer to reactions in which at least one hadron scatters to large angles with a significant change in its transverse momentum.
At leading twist, the bound-state physics is encoded in terms of universal "distribution amplitudes",[23] the fundamental theoretical quantities which describe the valence quark substructure of hadrons as well as nuclei.
A basic feature of the gauge theory formalism is color transparency",[24] the absence of initial and final-state interactions of rapidly moving compact color-singlet states.
One can also distinguish experimentally whether the spin orientation (helicity) of a hadron such as the spin-1/2 proton changes during the scattering or remains the same, as in the Pauli (spin-flip) and Dirac (spin-conserving) form factors.
The elastic and inelastic form factors can then be expressed[25] as integrated overlaps of the light-front Fock eigenstate wave functions
is chosen since it eliminates off-diagonal contributions where the number of initial and final state particles differ; it was originally discovered by Drell and Yan[26] and by West.
[36][37][38][39][40] The deuteron distribution amplitude has five components corresponding to the five different color-singlet combinations of six color triplet quarks, only one of which is the standard nuclear physics product
A fundamental feature of gauge theory is that soft gluons decouple from the small color-dipole moment of the compact fast-moving color-singlet wave function configurations of the incident and final-state hadrons.
Thus, if we study hard quasi elastic processes in a nuclear target, the outgoing and ingoing hadrons will have minimal absorption - a novel phenomenon called "color transparency".
"Light-Front Holography" refers to the remarkable fact that dynamics in AdS space in five dimensions is dual to a semiclassical approximation to Hamiltonian theory in physical
in this frame-independent "light-front Schrödinger equation" systematically incorporates the effects of higher quark and gluon Fock states.
The result is a nonperturbative relativistic light-front quantum mechanical wave equation which incorporates color confinement and other essential spectroscopic and dynamical features of hadron physics.
These recent developments concerning AdS/CFT duality provide new insights about light-front wave functions which may form first approximations to the full solutions that one seeks in LFQCD, and be considered as a step in building a physically motivated Fock-space basis set to diagonalize the LFQCD Hamiltonian, as in the basis light-front quantization (BLFQ) method.
If the universe is described by an effective local quantum field theory down to the Planck scale, then we would expect a cosmological constant of the order of
High-intensity laser facilities offer prospects for directly measuring previously unobserved processes in QED, such as vacuum birefringence, photon-photon scattering and, still some way in the future, Schwinger pair production.
Perturbation theory has its place in QCD also, but only at large values of the transferred energy or momentum where it exhibits the property of asymptotic freedom.
The field of perturbative QCD is well developed and many phenomena have been described using it, such as factorization, parton distributions, single-spin asymmetries, and jets.
There is a wealth of data in this strong interaction regime that is waiting for explanation in terms of calculations proceeding directly from the underlying theory.
As one prominent application of an ab initio approach to QCD, many extensive experimental programs either measure directly, or depend upon the knowledge of, the probability distributions of the quark and gluon components of the hadrons.
Although lattice QCD can estimate some observables directly, it does not provide the wave functions that are needed for the description of the structure and dynamics of hadrons.
In many cases, though not always, one can expect that a finite number of degrees of freedom dominates, that is, the decomposition in the Fock components converges enough quickly.
In the perturbative approach, for a renormalizable field theory, in any fixed order of coupling constant, this cancellation is obtained as a by-product of the renormalization procedure.
The numerical results for the anomalous magnetic moment of fermion in the truncation keeping three Fock sectors are stable relative to increase of the cutoff.
Experiments that need a conceptually and mathematically precise theoretical description of hadrons at the amplitude level include investigations of: the structure of nucleons and mesons, heavy quark systems and exotics, hard processes involving quark and gluon distributions in hadrons, heavy ion collisions, and many more.
For example, LFQCD will offer the opportunity for an ab initio understanding of the microscopic origins of the spin content of the proton and how the intrinsic and spatial angular momenta are distributed among the partonic components in terms of the wave functions.
Generalized parton distributions (GPDs) were introduced to quantify each component of the spin content and have been used to analyze the experimental measurements of deeply virtual Compton scattering (DVCS).
There are major programs at accelerator facilities such as GSI-SIS, CERN-LHC, and BNL-RHIC to investigate the properties of a new state of matter, the quark–gluon plasma, and other features of the QCD phase diagram.
[90][91][92][93][94][95][96] Light-front quantization leads to new definitions of the partition function and temperature which can provide a frame-independent description of thermal and statistical systems.
[91][92] The goal is to establish a tool comparable in power to lattice QCD but extending the partition function to finite chemical potentials where experimental data are available.
