Publications (73)76.83 Total impact
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ABSTRACT: In this Report we explore the remarkable connections between lightfront dynamics, its holographic mapping to gravity in a higherdimensional antide Sitter (AdS) space, and conformal quantum mechanics. This approach provides new insights into the origin of a fundamental mass scale and the physics underlying confinement dynamics in QCD in the limit of massless quarks. The result is a relativistic lightfront wave equation for arbitrary spin with an effective confinement potential derived from a conformal action and its embedding in AdS space. This equation allows for the computation of essential features of hadron spectra in terms of a single scale. The lightfront holographic methods described here gives a precise interpretation of holographic variables and quantities in AdS space in terms of lightfront variables and quantum numbers. This leads to a relation between the AdS wave functions and the boostinvariant lightfront wavefunctions describing the internal structure of hadronic boundstates in physical spacetime. The pion is massless and the excitation spectra of relativistic lightquark meson and baryon bound states lie on linear Regge trajectories with identical slopes in the radial and orbital quantum numbers. In the lightfront holographic approach described here currents are expressed as an infinite sum of poles, and form factors as a product of poles. At large $q^2$ the form factor incorporates the correct powerlaw falloff for hard scattering independent of the specific dynamics and is dictated by the twist. At low $q^2$ the form factor leads to vector dominance. The approach is also extended to include small quark masses. We briefly review in this Report other holographic approaches to QCD, in particular topdown and bottomup models based on chiral symmetry breaking. We also include a discussion of open problems and future applications.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We describe a procedure to extend the lightfront holographic approach to hadronic physics to include lightquark masses. The proposed framework allows us to extend the formalism of de Alfaro, Fubini and Furlan to the frameindependent lightfront Hamiltonian theory in the approximation where the dynamics in the invariant transverse variable is unchanged to first order in the quark masses. The confinement potential follows from an effective theory which encodes the fundamental conformality of the classical QCD Lagrangian and leads to a semiclassical relativistic lightfront wave equation for arbitrary spin. In particular, the K meson spectrum is successfully described without modifying the emerging confinement scale. The Wilson loop criteria for confinement is maintained, since for light quark masses a harmonic potential and linear Regge trajectories in the lightfront form of dynamics corresponds to a linear potential in the usual instantform.05/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We describe a procedure to extend the lightfront holographic approach to hadronic physics to include lightquark masses. The proposed framework allows us to extend the formalism of de Alfaro, Fubini and Furlan to the frameindependent lightfront Hamiltonian theory in the approximation where the dynamics in the invariant transverse variable is unchanged to first order in the quark masses. The confinement potential follows from an effective theory which encodes the fundamental conformality of the classical QCD Lagrangian and leads to a semiclassical relativistic lightfront wave equation for arbitrary spin. In particular, the K meson spectrum is successfully described without modifying the emerging confinement scale. The Wilson loop criteria for confinement is maintained, since for light quark masses a harmonic potential and linear Regge trajectories in the lightfront form of dynamics corresponds to a linear potential in the usual instantform.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: One of the most fundamental problems in Quantum Chromodynamics is to understand the origin of the mass scale which controls the range of color confinement and the hadronic spectrum. We show that a mass gap and a fundamental color confinement scale arise when one extends the formalism of de Alfaro, Fubini and Furlan to frameindependent lightfront Hamiltonian theory. Remarkably, the resulting lightfront potential has a unique form of a harmonic oscillator in the lightfront invariant impact variable if one requires that the action remains conformally invariant. The result is a singlevariable relativistic equation of motion for $q \bar q$ bound states, a "LightFront Shr\"odinger Equation", analogous to the nonrelativistic radial Schr\"odinger equation, which incorporates color confinement and other essential spectroscopic and dynamical features of hadron physics, including a massless pion for zero quark mass and linear Regge trajectories with the same slope in the radial quantum number and orbital angular momentum. The same equations with the correct hadron spin dependence arise from the holographic mapping of modified AdS$_5$ space with a specific dilaton profile. A fundamental mass parameter $\kappa$ appears, determining the hadron masses and the length scale which underlies hadron structure. Quark masses can be introduced to account for the spectrum of strange hadrons. This LightFront Holographic approach predicts not only hadron spectroscopy successfully, but also hadronic form factors, the QCD running coupling at small virtuality, and the lightfront wavefunctions of hadrons. Thus the combination of lightfront dynamics, its holographic mapping to gravity in a higherdimensional space and the dAFF procedure provides new insight into the physics underlying color confinement, chiral invariance, and the QCD mass scale.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We observe that the effective potentials obtained from lattice gauge theory, derived from string models of hadrons, and determined from models using frontform dynamics and lightfront holography agree with each other at leading approximation, not only in their shape  which depends on the form of dynamics  but also in their numerical strength.03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We briefly review the remarkable connections between lightfront QCD, gravity in AdS space, and conformal quantum mechanics. We discuss, in particular, the group theoretical and geometrical aspects of the underlying onedimensional quantum field theory. The resulting effective theory leads to a phenomenologically successful confining interaction potential in the relativistic lightfront wave equation which incorporates relevant nonperturbative dynamical aspects of hadron physics.01/2014;  [Show abstract] [Hide abstract]
ABSTRACT: To a first semiclassical approximation one can reduce the multiparton lightfront problem in QCD to an effective onedimensional quantum field theory, which encodes the fundamental conformal symmetry of the classical QCD Lagrangian. This procedure leads to a relativistic lightfront wave equation for arbitrary spin which incorporates essential spectroscopic and nonperturbative dynamical features of hadron physics. The mass scale for confinement and higher dimensional holographic mapping to AdS space are also emergent properties of this framework.01/2014;  [Show abstract] [Hide abstract]
ABSTRACT: LightFront Hamiltonian theory provides a rigorous frameindependent framework for solving nonperturbative QCD. The valence Fockstate wavefunctions of the lightfront QCD Hamiltonian satisfy a singlevariable relativistic equation of motion, analogous to the nonrelativistic radial Schr\"odinger equation, with an effective confining potential U which systematically incorporates the effects of higher quark and gluon Fock states. Remarkably, the potential U has a unique form of a harmonic oscillator potential if one requires that the chiral QCD action remains conformally invariant. A mass gap arises when one extends the formalism of de Alfaro, Fubini and Furlan to lightfront Hamiltonian theory. The valence LF meson wavefunctions for zero quark mass satisfy a singlevariable relativistic equation of motion in the invariant variable $\zeta^2=b^2_\perp x(1x)$, which is conjugate to the invariant mass squared. The result is a nonperturbative relativistic lightfront quantum mechanical wave equation which incorporates color confinement and other essential spectroscopic and dynamical features of hadron physics, including a massless pion for zero quark mass and linear Regge trajectories with the same slope in the radial quantum number n and orbital angular momentum L. The corresponding lightfront Dirac equation provides a model of nucleons. The same lightfront equations arise from the holographic mapping of the softwall model modification of AdS_5 space with a unique dilaton profile to QCD (3+1) at fixed lightfront time. Lightfront holography thus provides a precise relation between amplitudes in the fifth dimension of AdS space and lightfront wavefunctions. We also discuss the implications of the underlying conformal template of QCD for renormalization scalesetting, and the implications of lightfront quantization for the value of the cosmological constant.10/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Antide Sitter space in five dimensions provides an exact geometrical representation of the conformal group. Remarkably, gravity in AdS$_5$ space is holographically dual to frameindependent lightfront Hamiltonian theory, derived from the quantization of the QCD Lagrangian at fixed lightfront time $\tau = x^0+x^3$. Lightfront holography also leads to a precise relation between the boundstate amplitudes in the fifth dimension $z$ of AdS space and the variable $\zeta$, where $\zeta^2 = b^2_\perp x(1x)$ is the argument of the boostinvariant lightfront wavefunctions describing the internal structure of hadrons in physical spacetime. The holographic mapping of AdS space with a specific "softwall" dilaton yields a confining potential $U(\zeta^2)$ for the lightfront Schr\"odinger equation for hadrons with arbitrary spin $J$. Remarkably, $U(\zeta^2)$ has a unique form of a harmonic oscillator potential if one requires that the chiral QCD action remains conformally invariant. One thus obtains an effective lightfront effective theory for general spin which respects the conformal symmetry of the fourdimensional classical QCD Lagrangian. The predictions of the LF equations of motion include a zeromass pion in the chiral $m_q\to 0$ limit, and linear Regge trajectories $M^2(n,L) \propto n+L$ with the same slope in the radial quantum number $n$ and the orbital angular momentum $L$. The lightfront AdS/QCD holographic approach gives a frameindependent representation of colorconfining dynamics, Regge spectroscopy, as well as the excitation spectra of relativistic lightquark meson and baryon bound states in QCD in terms of a single mass parameter. We also briefly discuss the implications of the underlying conformal template of QCD for renormalization scalesetting, and the implications of lightfront quantization for the value of the cosmological constant.09/2013;  [Show abstract] [Hide abstract]
ABSTRACT: LightFront Hamiltonian theory, derived from the quantization of the QCD Lagrangian at fixed lightfront time \tau = t+z/c, provides a rigorous frameindependent framework for solving nonperturbative QCD. The eigenvalues of the lightfront QCD Hamiltonian predict the hadronic mass spectrum, and the eigensolutions provide the lightfront wavefunctions describing hadron structure. The valence Fockstate wavefunctions of the lightfront QCD Hamiltonian satisfy a singlevariable relativistic equation of motion, analogous to the nonrelativistic radial Schrodinger equation, with an effective confining potential U which systematically incorporates the effects of higher quark and gluon Fock states. The potential U has a unique form if one requires that the action for zero quark mass remains conformally invariant. The holographic mapping of gravity in AdS space to QCD with a specific softwall dilaton yields the same lightfront Schrodinger equation. It also gives a precise relation between the boundstate amplitudes in the fifth dimension z of AdS space and the boostinvariant lightfront wavefunctions describing the internal structure of hadrons in physical spacetime. The elastic and transition form factors of the pion and the nucleons are found to be well described in this framework. The predictions include a zeromass pion in the chiral limit, and linear Regge trajectories with the same slope in the radial quantum number n and orbital angular momentum L. The lightfront AdS/QCD holographic approach thus gives a frameindependent representation of colorconfining dynamics and the excitation spectra of lightquark hadrons in terms of a single mass parameter. We also discuss the implications of the underlying conformal template of QCD for renormalization scalesetting and the implications of lightfront quantization for the value of the cosmological constant.Nuclear Physics B  Proceedings Supplements. 08/2013; s 251–252.  [Show abstract] [Hide abstract]
ABSTRACT: We review some outstanding puzzles and experimental anomalies in hadron physics that appear to challenge conventional wisdom and, in some cases, the foundations of QCD. We also discuss possible solutions and propose new tests and experiments that could illuminate the underlying physics and novel phenomenological features of QCD. In some cases, new perspectives for QCD physics have emerged.Annual Review of Nuclear and Particle Science 02/2013; 62. · 7.40 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that (a) the conformal properties of Antide Sitter (AdS) space, (b) the properties of a field theory in one dimension under the full conformal group found by de Alfaro, Fubini and Furlan, and (c) the frameindependent singlevariable lightfront Schr\"odinger equation for bound states all lead to the same result: a relativistic nonperturbative model which successfully incorporates salient features of hadronic physics, including confinement, linear Regge trajectories, and results which are conventionally attributed to spontaneous chiral symmetry breaking.02/2013;  [Show abstract] [Hide abstract]
ABSTRACT: A complementary approach, derived from (a) higher dimensional antide Sitter (AdS) space, (b) lightfront quantization and (c) the invariance properties of the full conformal group in one dimension leads to a nonperturbative relativistic lightfront wave equation which incorporates essential spectroscopic and dynamical features of hadron physics. The fundamental conformal symmetry of the classical QCD Lagrangian in the limit of massless quarks is encoded in the resulting effective theory. The mass scale for confinement emerges from the isomorphism between the conformal group and SO(2,1). This scale appears in the lightfront Hamiltonian by mapping to the evolution operator in the formalism of de Alfaro, Fubini and Furlan, which retains the conformal invariance of the action. Remarkably, the specific form of the confinement interaction and the corresponding modification of AdS space are uniquely determined in this procedure.02/2013;  [Show abstract] [Hide abstract]
ABSTRACT: A remarkable holographic feature of dynamics in AdS space in five dimensions is that it is dual to Hamiltonian theory in physical spacetime, quantized at fixed lightfront time {\tau} = t+z/c. This lightfront holographic principle provides a precise relation between the boundstate amplitudes in AdS space and the boostinvariant lightfront wavefunctions describing the internal structure of hadrons in physical spacetime. The fifth dimension coordinate z is dual to the light front variable {\zeta} describing the invariant separation of the quark constituents. The resulting valence Fockstate wavefunction eigensolutions of the lightfront QCD Hamiltonian satisfy a singlevariable relativistic equation of motion, analogous to the nonrelativistic radial Schr\"odinger equation. The softwall dilaton profile exp ({\kappa}^2 {\zeta}^2) provides a model for the lightfront potential which is colorconfining and reproduces well the linear Regge behavior of the lightquark hadron spectrum in both L, the orbital angular momentum, and n, the radial node number. The pion mass vanishes in the chiral limit and other features of chiral symmetry are satisfied. The resulting running QCD coupling displays an infrared fixed point. The elastic and transition form factors of the pion and the nucleons are also found to be well described in this framework. The lightfront AdS/QCD holographic approach thus gives a frameindependent analytic first approximation of the colorconfining dynamics, spectroscopy, and excitation spectra of relativistic lightquark bound states in QCD.01/2013;  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we derive holographic wave equations for hadrons with arbitrary spin starting from an effective action in a higherdimensional space asymptotic to antide Sitter (AdS) space. Our procedure takes advantage of the local tangent frame, and it applies to all spins, including halfinteger spins. An essential element is the mapping of the higherdimensional equations of motion to the lightfront Hamiltonian, thus allowing a clear distinction between the kinematical and dynamical aspects of the holographic approach to hadron physics. Accordingly, the nontrivial geometry of pure AdS space encodes the kinematics, and the additional deformations of AdS space encode the dynamics, including confinement. It thus becomes possible to identify the features of holographic QCD which are independent of the specific mechanisms of conformal symmetry breaking. In particular, we account for some aspects of the striking similarities and differences observed in the systematics of the meson and baryon spectra.Physical review D: Particles and fields 01/2013; 87(7).  [Show abstract] [Hide abstract]
ABSTRACT: Studies of the structure of excited baryons are key to the N* program at Jefferson Lab. Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever yet achieved, with a kinematic reach up to Q2 = 12 GeV2. This highQ2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless barequark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with QCDbased approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the nonperturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.International Journal of Modern Physics E 12/2012; · 0.63 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: One of the most important nonperturbative methods for solving QCD is quantization at fixed lightfront time, \tau = t+z/c  Dirac's "Front Form". The eigenvalues of the lightfront QCD Hamiltonian predict the hadron spectrum and the eigensolutions provide the lightfront wavefunctions which describe hadron structure. More generally, we show that the valence Fockstate wavefunctions of the lightfront QCD Hamiltonian satisfy a singlevariable relativistic equation of motion, analogous to the nonrelativistic radial Schr\"odinger equation, with an effective confining potential U which systematically incorporates the effects of higher quark and gluon Fock states. We outline a method for computing the required potential from first principles in QCD. The holographic mapping of gravity in AdS space to QCD, quantized at fixed lightfront time, yields the same light front Schr\"odinger equation; in fact, the softwall AdS/QCD approach provides a model for the lightfront potential which is colorconfining and reproduces well the lighthadron spectrum. One also derives via lightfront holography a precise relation between the boundstate amplitudes in the fifth dimension of AdS space and the boostinvariant lightfront wavefunctions describing the internal structure of hadrons in physical spacetime. The elastic and transition form factors of the pion and the nucleons are found to be well described in this framework. The lightfront AdS/QCD holographic approach thus gives a frameindependent first approximation of the colorconfining dynamics, spectroscopy, and excitation spectra of relativistic lightquark bound states in QCD.08/2012; 
Article: Lightfront holography  A new approach to relativistic hadron dynamics and nonperturbative QCD
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ABSTRACT: The holographic mapping of gravity in AdS space to QCD, quantized at fixed lightfront time, provides a precise relation between the boundstate amplitudes in the fifth dimension of AdS space and the boostinvariant lightfront wavefunctions describing the internal structure of hadrons in physical spacetime. In particular, the elastic and transition form factors of the pion and the nucleons are well described in this framework. The lightfront AdS/QCD holographic approach thus gives a frameindependent first approximation of the colorconfining dynamics, spectroscopy, and excitation spectra of relativistic lightquark bound states in QCD. More generally, we show that the valence Fockstate wavefunctions of the eigensolutions of the lightfront QCD Hamiltonian satisfy a singlevariable relativistic equation of motion, analogous to the nonrelativistic radial Schr\"odinger equation, with an effective confining potential which systematically incorporates the effects of higher quark and gluon Fock states. The proposed method to compute the effective interaction thus resembles the twoparticleirreducible functional techniques used in quantum field theory.06/2012;  [Show abstract] [Hide abstract]
ABSTRACT: We show that the nonperturbative lightfront dynamics of relativistic hadronic bound states has a dual semiclassical gravity description on a higher dimensional warped AdS space in the limit of zero quark masses. This mapping of AdS gravity theory to the boundary quantum field theory, quantized at fixed lightfront time, allows one to establish a precise relation between holographic wave functions in AdS space and the lightfront wavefunctions describing the internal structure of hadrons. The resulting AdS/QCD model gives a remarkably good accounting of the spectrum, elastic and transition form factors of the lightquark hadrons in terms of one parameter, the QCD gap scale. The lightfront holographic approach described here thus provides a frameindependent first approximation to the lightfront Hamiltonian problem for QCD. This article is based on lectures at the Niccolo Cabeo International School of Hadronic Physics, Ferrara, Italy, May 2011.03/2012; 
Article: LightFront Holographic QCD
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ABSTRACT: The relation between the hadronic shortdistance constituent quark and gluon particle limit and the longrange confining domain is yet one of the most challenging aspects of particle physics due to the strong coupling nature of Quantum Chromodynamics, the fundamental theory of the strong interactions. The central question is how one can compute hadronic properties from first principles; i.e., directly from the QCD Lagrangian. The most successful theoretical approach thus far has been to quantize QCD on discrete lattices in Euclidean spacetime. Lattice numerical results follow from computation of framedependent moments of distributions in Euclidean space and dynamical observables in Minkowski spacetime, such as the timelike hadronic form factors, are not amenable to Euclidean lattice computations. The DysonSchwinger methods have led to many important insights, such as the infrared fixed point behavior of the strong coupling constant, but in practice, the analyses are limited to ladder approximation in Landau gauge. Baryon spectroscopy and the excitation dynamics of nucleon resonances encoded in the nucleon transition form factors can provide fundamental insight into the strongcoupling dynamics of QCD. New theoretical tools are thus of primary interest for the interpretation of the results expected at the new mass scale and kinematic regions accessible to the JLab 12 GeV Upgrade Project. The AdS/CFT correspondence between gravity or string theory on a higherdimensional antide Sitter (AdS) space and conformal field theories in physical spacetime has led to a semiclassical approximation for stronglycoupled QCD, which provides physical insights into its nonperturbative dynamics. The correspondence is holographic in the sense that it determines a duality between theories in different number of spacetime dimensions. This geometric approach leads in fact to a simple analytical and phenomenologically compelling nonperturbative approximation to the full lightfront QCD Hamiltonian 'LightFront Holography'. LightFront Holography is in fact one of the most remarkable features of the AdS/CFT correspondence. The Hamiltonian equation of motion in the lightfront (LF) is frame independent and has a structure similar to eigenmode equations in AdS space. This makes a direct connection of QCD with AdS/CFT methods possible. Remarkably, the AdS equations correspond to the kinetic energy terms of the partons inside a hadron, whereas the interaction terms build confinement and correspond to the truncation of AdS space in an effective dual gravity approximation. One can also study the gauge/gravity duality starting from the boundstate structure of hadrons in QCD quantized in the lightfront. The LF Lorentzinvariant Hamiltonian equation for the relativistic boundstate system is P{sub }P{sup }(P)> = (P{sup +}P  P{sub }²)(P)> = M²(P)>, P{sup {+}} = P° {+} P³, where the LF time evolution operator P is determined canonically from the QCD Lagrangian. To a first semiclassical approximation, where quantum loops and quark masses are not included, this leads to a LF Hamiltonian equation which describes the boundstate dynamics of light hadrons in terms of an invariant impact variable which measures the separation of the partons within the hadron at equal lightfront time = x° + x³. This allows us to identify the holographic variable z in AdS space with an impact variable . The resulting Lorentzinvariant Schroedinger equation for general spin incorporates color confinement and is systematically improvable. Lightfront holographic methods were originally introduced by matching the electromagnetic current matrix elements in AdS space with the corresponding expression using LF theory in physical space time. It was also shown that one obtains identical holographic mapping using the matrix elements of the energymomentum tensor by perturbing the AdS metric around its static solution. A gravity dual to QCD is not known, but the mechanisms of confinement can be incorporated in the gauge/gravity correspondence by modifying the AdS geometry in the large infrared (IR) domain z 1 = {sub QCD}, which also sets the scale of the strong interactions. In this simplified approach we consider the propagation of hadronic modes in a fixed effective gravitational background asymptotic to AdS space, which encodes salient properties of the QCD dual theory, such as the ultraviolet (UV) conformal limit at the AdS boundary, as well as modifications of the background geometry in the large z IR region to describe confinement. The modified theory generates the pointlike hard behavior expected from QCD, instead of the soft behavior characteristic of extended objects.01/2012;
Publication Stats
1k  Citations  
76.83  Total Impact Points  
Top Journals
Institutions

2003–2013

Stanford University
 SLAC National Accelerator Laboratory
Palo Alto, California, United States


1982–2013

University of Costa Rica
 Escuela de Física
San José, San José, Costa Rica


1984

Harvard University
Cambridge, Massachusetts, United States
