Light-Front Holography and AdS/QCD Correspondence

Light-Front Holography is a remarkable consequence of the correspondence between string theory in AdS space and conformal field theories in physical-space time. It allows string modes $\Phi(z)$ in the AdS fifth dimension to be precisely mapped to the light-front wavefunctions of hadrons in terms of a specific light-front impact variable $\zeta$ which measures the separation of the quark and gluonic constituents within the hadron. This mapping was originally obtained by matching the exact expression for electromagnetic current matrix elements in AdS space with the corresponding exact expression for the current matrix element using light-front theory in physical space-time. More recently we have shown that one obtains the identical holographic mapping using matrix elements of the energy-momentum tensor, thus providing an important consistency test and verification of holographic mapping from AdS to physical observables defined on the light-front. The resulting light-front Schrodinger equations predicted from AdS/QCD give a good representation of the observed meson and baryon spectra and give excellent phenomenological predictions for amplitudes such as electromagnetic form factors and decay constants.Comment: 14 pages. Presented at QCD Down Under II, Auckland, New Zealand, January 17-19, 200

Applications of AdS/QCD and Light-Front Holography to Baryon Physics

The correspondence between theories in anti-de Sitter space and field theories in physical space-time leads to an analytic, semiclassical model for strongly-coupled QCD which has scale invariance at short distances and color confinement at large distances. These equations, for both mesons and baryons, give a very good representation of the observed hadronic spectrum, including a zero mass pion. Light-front holography allows hadronic amplitudes in the AdS fifth dimension to be mapped to frame-independent light-front wavefunctions of hadrons in physical space-time, thus providing a relativistic description of hadrons at the amplitude level. The meson and baryon wavefunctions derived from light-front holography and AdS/QCD also have remarkable phenomenological features, including predictions for the electromagnetic form factors and decay constants. The approach can be systematically improved using light-front Hamiltonian methods. Some novel features of QCD for baryon physics are also discussed.Comment: Presented by SJB at the International Conference on the Structure of Baryons, BARYONS'10, December 7-11, 2010, Osaka, Japa

Nearly Conformal QCD and AdS/CFT

The AdS/CFT correspondence is a powerful tool to study the properties of conformal QCD at strong coupling in terms of a higher dimensional dual gravity theory. The power-law falloff of scattering amplitudes in the non-perturbative regime and calculable hadron spectra follow from holographic models dual to QCD with conformal behavior at short distances and confinement at large distances. String modes and fluctuations about the AdS background are identified with QCD degrees of freedom and orbital excitations at the AdS boundary limit. A description of form factors in space and time-like regions and the behavior of light-front wave functions can also be understood in terms of a dual gravity description in the interior of AdS.Comment: 8 pages, LateX, 3 figures. Presented at First Workshop on Quark-Hadron Duality and the Transition to pQCD, Frascati, Italy, 6-8 June 2005. AdS LFWF mapping reexamine

Excited Baryons in Holographic QCD

The light-front holographic QCD approach is used to describe baryon spectroscopy and the systematics of nucleon transition form factors.Comment: Invited talk presented by GdT at NSTAR 2011, the 8th International Workshop on the Physics of Excited Nucleons, Jefferson Laboratory, May 17 - 20, 201

AdS/CFT and Exclusive Processes in QCD

The AdS/CFT correspondence between string theory in AdS space and conformal field theories in physical space-time leads to an analytic, semi-classical model for strongly-coupled QCD which has scale invariance and dimensional counting at short distances and color confinement at large distances. One can use holography to map the amplitude describing the hadronic state in the fifth dimension of Anti-de Sitter space to the light-front wavefunctions of hadrons in physical space-time, thus providing a relativistic description of hadrons in QCD at the amplitude level. In particular, we show that there is an exact correspondence between the fifth-dimensional coordinate of AdS space z and a specific impact variable zeta which measures the separation of the quark and gluonic constituents within the hadron in ordinary space-time. New relativistic light-front equations in ordinary space-time can then be derived which reproduce the results obtained using the 5-dimensional theory. The effective light-front equations possess elegant algebraic structures and integrability properties. This connection between the AdS and the light-front representations allows one to compute the analytic form of the frame-independent light-front wavefunctions, the fundamental entities which encode hadron properties and allow the computation of decay constants, form factors, deeply virtual Compton scattering, exclusive heavy hadron decays and other exclusive scattering amplitudes. As specific examples we compute the pion coupling constant and study the behavior of the pion form factor in the space and time-like regions. We also determine the Dirac form factors of the proton and neutron in the space-like region.Comment: 29 pages, 7 figures. Invited talk, presented at the Workshop on Exclusive Reactions at High Momentum Transfer 21-24 May 2007, Newport News, Virgini

Light-Front Holography and Novel Effects in QCD

The correspondence between theories in anti-de Sitter space and conformal field theories in physical space-time leads to an analytic, semiclassical model for strongly-coupled QCD. Light-front holography allows hadronic amplitudes in the AdS fifth dimension to be mapped to frame-independent light-front wavefunctions of hadrons in physical space-time, thus providing a relativistic description of hadrons at the amplitude level. We identify the AdS coordinate $z$ with an invariant light-front coordinate $\zeta$ which separates the dynamics of quark and gluon binding from the kinematics of constituent spin and internal orbital angular momentum. The result is a single-variable light-front Schr\"odinger equation for QCD which determines the eigenspectrum and the light-front wavefunctions of hadrons for general spin and orbital angular momentum. The mapping of electromagnetic and gravitational form factors in AdS space to their corresponding expressions in light-front theory confirms this correspondence. Some novel features of QCD are discussed, including the consequences of confinement for quark and gluon condensates and the behavior of the QCD coupling in the infrared. The distinction between static structure functions such as the probability distributions computed from the square of the light-front wavefunctions versus dynamical structure functions which include the effects of rescattering is emphasized. A new method for computing the hadronization of quark and gluon jets at the amplitude level, an event amplitude generator, is outlined.Comment: 16 pages, 3 figures. Presented by SJB at the XIII Mexican School of Particles and Field

Gauge/Gravity Duality and Hadron Physics at the Light-Front

We discuss some remarkable features of the light-front holographic mapping of classical gravity in anti-de Sitter space modified by a confining dilaton background. In particular, we show that a positive-sign dilaton solution $\exp(+\kappa^2 z^2)$ has better chances to describe the correct hadronic phenomenology than the negative solution $\exp{(- \kappa^2 z^2)}$ extensively studied in the literature. We also show that the use of twist-scaling dimensions, instead of canonical dimensions, is required to give a good description of the spectrum and form factors of hadrons. Another key element is the explicit connection of AdS modes of total angular momentum $J$ with the internal structure of hadrons and the proper identification of the orbital angular momentum of the constituents.Comment: Invited talk presented by GdT at XI Hadron Physics, Maresias Beach, Sao Paulo, Brazil, 21-26 March 201