18,842 research outputs found

### Hadron Spectroscopy and Structure from AdS/CFT

The AdS/CFT correspondence between conformal field theory and string states
in an extended space-time has provided new insights into not only hadron
spectra, but also their light-front wavefunctions. We show that there is an
exact correspondence between the fifth-dimensional coordinate of anti-de Sitter
space and a specific impact variable which measures the separation of the
constituents within the hadron in ordinary space-time. This connection allows
one to predict the form of the light-front wavefunctions of mesons and baryons,
the fundamental entities which encode hadron properties and scattering
amplitudes. A new relativistic Schrodinger light-front equation is found which
reproduces the results obtained using the fifth-dimensional theory. Since they
are complete and orthonormal, the AdS/CFT model wavefunctions can be used as an
initial ansatz for a variational treatment or as a basis for the
diagonalization of the light-front QCD Hamiltonian. A number of applications of
light-front wavefunctions are also discussed.Comment: Invited talk, presented at the 4th International Conference On Quarks
And Nuclear Physics (QNP06), 5-10 June 2006, Madrid, Spai

### Applications of Light-Front QCD

Light-front Fock state wavefunctions encode the bound state properties of
hadrons in terms of their quark and gluon degrees of freedom at the amplitude
level. The freedom to choose the light-like quantization four-vector provides
an explicitly covariant formulation of light-front quantization and can be used
to determine the analytic structure of light-front wave functions. The AdS/CFT
correspondence of large N_C supergravity theory in higher-dimensional anti-de
Sitter space with supersymmetric QCD in 4-dimensional space-time has
interesting implications for hadron phenomenology in the conformal limit,
including an all-orders demonstration of counting rules for exclusive
processes. String/gauge duality also predicts the QCD power-law behavior of
light-front Fock-state hadronic wavefunctions with arbitrary orbital angular
momentum at high momentum transfer. The form of these near-conformal
wavefunctions can be used as an initial ansatz for a variational treatment of
the light-front QCD Hamiltonian. I also briefly review recent work which shows
that some leading-twist phenomena such as the diffractive component of deep
inelastic scattering, single spin asymmetries, nuclear shadowing and
antishadowing cannot be computed from the LFWFs of hadrons in isolation.Comment: Presented at QCD DOWN UNDER, 10--13 March 2004 in the Barossa Valley,
15--19 March 2004 at CSSM, Adelaide, Australi

### Exact solutions to Pauli-Villars-regulated field theories

We present a new class of quantum field theories which are exactly solvable.
The theories are generated by introducing Pauli-Villars fermionic and bosonic
fields with masses degenerate with the physical positive metric fields. An
algorithm is given to compute the spectrum and corresponding eigensolutions. We
also give the operator solution for a particular case and use it to illustrate
some of the tenets of light-cone quantization. Since the solutions of the
solvable theory contain ghost quanta, these theories are unphysical. However,
we also discuss how perturbation theory in the difference between the masses of
the physical and Pauli-Villars particles could be developed, thus generating
physical theories. The existence of explicit solutions of the solvable theory
also allows one to study the relationship between the equal-time and light-cone
vacua and eigensolutions.Comment: 20 pages, REVTeX; minor corrections to normalization

### Dynamic versus Static Hadronic Structure Functions

"Static" structure functions are the probabilistic distributions computed
from the square of the light-front wavefunctions of the target hadron. In
contrast, the "dynamic" structure functions measured in deep inelastic
lepton-hadron scattering include the effects of rescattering associated with
the Wilson line. Initial- and final-state rescattering, neglected in the parton
model, can have a profound effect in QCD hard-scattering reactions, producing
single-spin asymmetries, diffractive deep inelastic scattering, diffractive
hard hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan
reactions, nuclear shadowing, and non-universal nuclear antishadowing--novel
leading-twist physics not incorporated in the light-front wavefunctions of the
target computed in isolation. I also review how "direct" higher-twist processes
-- where a proton is produced in the hard subprocess itself -- can explain the
anomalous proton-to-pion ratio seen in high centrality heavy ion collisions.Comment: Invited talk presented at the International Conference on Particles
and Nuclei (PANIC08), Eilat, Israel, November 9-14, 200

### Light-Front Holography, AdS/QCD, and Hadronic Phenomena

AdS/QCD, the correspondence between theories in a modified five-dimensional
anti-de Sitter space and confining field theories in physical space-time,
provides a remarkable semiclassical model for hadron physics. 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
with a confining potential 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. 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: 11 pages, 3 figures. Talk presented by SJB at Light Cone 2009:
Relativistic Hadronic And Particle Physics, 8-13 Jul 2009, Sao Jose dos
Campos, Brazi

### The Impact of QCD and Light-Cone Quantum Mechanics on Nuclear Physics

We discuss a number of novel applications of Quantum Chromodynamics to
nuclear structure and dynamics, such as the reduced amplitude formalism for
exclusive nuclear amplitudes. We particularly emphasize the importance of
light-cone Hamiltonian and Fock State methods as a tool for describing the
wavefunctions of composite relativistic many-body systems and their
interactions. We also show that the use of covariant kinematics leads to
nontrivial corrections to the standard formulae for the axial, magnetic, and
quadrupole moments of nucleons and nuclei.Comment: 25 pages, uuencoded postscript file---To obtain a hard copy of this
paper, send e-mail to [email protected] and ask fo

### Final-State Interactions and Single-Spin Asymmetries in Semi-Inclusive Deep Inelastic Scattering

Recent measurements from the HERMES and SMC collaborations show a remarkably
large azimuthal single-spin asymmetries A_{UL} and A_{UT} of the proton in
semi-inclusive pion leptoproduction. We show that final-state interactions from
gluon exchange between the outgoing quark and the target spectator system lead
to single-spin asymmetries in deep inelastic lepton-proton scattering at
leading twist in perturbative QCD; i.e., the rescattering corrections are not
power-law suppressed at large photon virtuality Q^2 at fixed x_{bj}. The
existence of such single-spin asymmetries requires a phase difference between
two amplitudes coupling the proton target with J^z_p = + 1/2 and -1/2 to the
same final state, the same amplitudes which are necessary to produce a nonzero
proton anomalous magnetic moment. We show that the exchange of gauge particles
between the outgoing quark and the proton spectators produces a Coulomb-like
complex phase which depends on the angular momentum L_z of the proton's
constituents and is thus distinct for different proton spin amplitudes. The
single-spin asymmetry which arises from such final-state interactions does not
factorize into a product of distribution function and fragmentation function,
and it is not related to the transversity distribution delta q(x,Q) which
correlates transversely polarized quarks with the spin of the transversely
polarized target nucleon.Comment: Version to appear in Physics Letters B. Typographical errors
corrected in Eqs. (13) and (14

### Photoproduction of charm near threshold

Charm and bottom production near threshold is sensitive to the multi-quark,
gluonic, and hidden-color correlations of hadronic and nuclear wavefunctions in
QCD since all of the target's constituents must act coherently within the small
interaction volume of the heavy quark production subprocess. Although such
multi-parton subprocess cross sections are suppressed by powers of $1/m^2_Q$,
they have less phase-space suppression and can dominate the contributions of
the leading-twist single-gluon subprocesses in the threshold regime. The small
rates for open and hidden charm photoproduction at threshold call for a
dedicated facility.Comment: 5 pages 5 figures Changes: 1- Added refs 24,25; 2- Added two
sentences, top of column 2 of page 3, on the definition of x, its range and
the domain of validity of the mode

### Two-boson truncation of Pauli-Villars-regulated Yukawa theory

We apply light-front quantization, Pauli-Villars regularization, and
numerical techniques to the nonperturbative solution of the dressed-fermion
problem in Yukawa theory in 3+1 dimensions. The solution is developed as a
Fock-state expansion truncated to include at most one fermion and two bosons.
The basis includes a negative-metric heavy boson and a negative-metric heavy
fermion in order to provide the necessary cancellations of ultraviolet
divergences. The integral equations for the Fock-state wave functions are
solved by reducing them to effective one-boson--one-fermion equations for
eigenstates with J_z=1/2. The equations are converted to a matrix equation with
a specially tuned quadrature scheme, and the lowest mass state is obtained by
diagonalization. Various properties of the dressed-fermion state are then
computed from the nonperturbative light-front wave functions. This work is a
major step in our development of Pauli-Villars regularization for the
nonperturbative solution of four-dimensional field theories and represents a
significant advance in the numerical accuracy of such solutions.Comment: 32 pages, 17 figures; requires elsart.cl

### Initial-State Interactions and Single-Spin Asymmetries in Drell-Yan Processes

We show that the initial-state interactions from gluon exchange between the
incoming quark and the target spectator system lead to leading-twist
single-spin asymmetries in the Drell-Yan process. The QCD initial-state
interactions produce a $T-$odd spin-correlation between the target spin and the
virtual photon production plane which is not power-law suppressed in the
Drell-Yan scaling limit. The origin of the single-spin asymmetry in $\pi
p^\uparrow \to \ell^+ \ell^- X$ is a phase difference between two amplitudes
coupling the proton target with $J^z_p = \pm {1\over 2}$ to the same
final-state, the same amplitudes which are necessary to produce a nonzero
proton anomalous magnetic moment. The calculation requires the overlap of
target light-front wavefunctions differing by one unit of orbital angular
momentum projection $L_z;$ thus the SSA in the Drell-Yan reaction provides a
direct measure of orbital angular momentum in the QCD bound state. The
single-spin asymmetry predicted for the Drell-Yan process $\pi p^\uparrow \to
\ell^+ \ell^- X$ is similar to the single-spin asymmetries in deep inelastic
semi-inclusive leptoproduction $\ell p^\uparrow \to \ell' \pi X$ which arises
from the final-state rescattering of the outgoing quark.Comment: LaTex, 15 pages, 2 figure

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