Light front holography

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A proton in AdS space. Different values of the radius (labeled z {\displaystyle z} ) correspond to different scales at which the proton is examined. Events at short distances happen in the four-dimensional AdS boundary (large circumference). The inner sphere represents large distance events. In the figure, a small proton created at the AdS boundary falls into AdS space pulled by the gravitational field up to its larger size allowed by confinement. Due to the warped geometry the proton size shrinks near the AdS boundary as perceived by an observer in Minkowski space. AdSproton.jpg
A proton in AdS space. Different values of the radius (labeled ) correspond to different scales at which the proton is examined. Events at short distances happen in the four-dimensional AdS boundary (large circumference). The inner sphere represents large distance events. In the figure, a small proton created at the AdS boundary falls into AdS space pulled by the gravitational field up to its larger size allowed by confinement. Due to the warped geometry the proton size shrinks near the AdS boundary as perceived by an observer in Minkowski space.

In strong interaction physics, light front holography or light front holographic QCD is an approximate version of the theory of quantum chromodynamics (QCD) which results from mapping the gauge theory of QCD to a higher-dimensional anti-de Sitter space (AdS) inspired by the AdS/CFT correspondence [1] (gauge/gravity duality) proposed for string theory. This procedure makes it possible to find analytic solutions (closed-form expression) in situations where strong coupling occurs (the "strongly coupled regime"), improving predictions of the masses of hadrons (such as protons, neutrons, and mesons) and their internal structure revealed by high-energy accelerator experiments. The most widely used approach to finding approximate solutions to the QCD equations, lattice QCD, has had many successful applications; however, it is a numerical approach formulated in Euclidean space rather than physical Minkowski space-time. [2] [3]

Contents

Motivation and background

One of the key problems in elementary particle physics is to compute the mass spectrum and structure of hadrons, such as the proton, as bound states of quarks and gluons. Unlike quantum electrodynamics (QED), the strong coupling constant of the constituents of a proton calculates hadronic properties, such as the proton mass and color confinement, a most difficult problem to solve. The most successful theoretical approach has been to formulate QCD as a lattice gauge theory [2] and employ large numerical simulations on advanced computers. Notwithstanding, important dynamical QCD properties in Minkowski space-time are not amenable to Euclidean numerical lattice computations. [3] An important theoretical goal is thus to find an initial approximation to QCD which is both analytically tractable and which can be systematically improved.

To address this problem, the light front holography approach maps a confining gauge theory quantized on the light front [4] to a higher-dimensional anti-de Sitter space (AdS) incorporating the AdS/CFT correspondence [1] as a useful guide. The AdS/CFT correspondence is an example of the holographic principle, since it relates gravitation in a five-dimensional AdS space to a conformal quantum field theory at its four-dimensional space-time boundary.

Light front quantization was introduced by Paul Dirac to solve relativistic quantum field theories. It is the ideal framework to describe the structure of the hadrons in terms of their constituents measured at the same light-front time, , the time marked by the front of a light wave. In the light front the Hamiltonian equations for relativistic bound state systems and the AdS wave equations have a similar structure, which makes the connection of QCD with gauge/gravity methods possible. [5] The interrelation of the AdS geometrical representation with light-front holography provides a remarkable first approximation for the mass spectra and wave functions of meson and baryon light-quark bound states. [6]

Light front holographic methods were originally found by Stanley J. Brodsky and Guy F. de Téramond in 2006 by mapping the electric charge [7] and inertia [8] distributions from the quark currents and the stress–energy tensor [9] of the fundamental constituents within a hadron in AdS [10] [11] to physical space time [12] [13] using light-front theory. A gravity dual of QCD is not known, but the mechanisms of confinement can be incorporated in the gauge/gravity correspondence by modifying the AdS geometry at large values of the AdS fifth-dimension coordinate , which sets the scale of the strong interactions. [14] [15] In the usual AdS/QCD framework [16] [17] fields in AdS are introduced to match the chiral symmetry of QCD, and its spontaneous symmetry breaking, but without explicit connection with the internal constituent structure of hadrons. [18]

Light front wave equation

In a semiclassical approximation to QCD the light-front Hamiltonian equation is a relativistic and frame-independent Schrödinger equation [5]

where is the orbital angular momentum of the constituents and the variable is the invariant separation distance between the quarks in the hadron at equal light-front time. The variable is identified with the holographic variable in AdS space [7] and the confining potential energy is derived from the warp factor which modifies the AdS geometry and breaks its conformal invariance. [6] Its eigenvalues give the hadronic spectrum, and its eigenvectors represent the probability distributions of the hadronic constituents at a given scale.

See also

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Light-front quantization applications

The light front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular, it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates, where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, is one Cartesian coordinate, and is the speed of light. The other two Cartesian coordinates, and , are untouched and often called transverse or perpendicular, denoted by symbols of the type . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory, but in practical calculations some choices may be more suitable than others. The basic formalism is discussed elsewhere.

Light-front computational methods Technique in computational quantum field theory

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References

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