37,082 research outputs found
New Results in Light-Front Phenomenology
The light-front quantization of gauge theories such as QCD in light-cone
gauge provides a frame-independent wavefunction representation of relativistic
bound states, simple forms for current matrix elements, explicit unitarity, and
a trivial vacuum. 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
and to define a kinematical definition of angular momentum. The AdS/CFT
correspondence of large 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 analyses 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 LightCone 2004, Amsterdam, The Netherlands, 16-20 August
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Novel QCD Phenomena
I discuss a number of novel topics in QCD, including the use of the AdS/CFT
correspondence between Anti-de Sitter space and conformal gauge theories to
obtain an analytically tractable approximation to QCD in the regime where the
QCD coupling is large and constant. In particular, there is an exact
correspondence between the fifth-dimension coordinate z of AdS space and a
specific impact variable zeta which measures the separation of the quark
constituents within the hadron in ordinary space-time. This connection allows
one to compute the analytic form of the frame-independent light-front
wavefunctions of mesons and baryons, the fundamental entities which encode
hadron properties and allow the computation of exclusive scattering amplitudes.
I also discuss a number of novel phenomenological features of QCD. Initial- and
final-state interactions from gluon-exchange, normally neglected in the parton
model, have a profound effect in QCD hard-scattering reactions, leading to
leading-twist single-spin asymmetries, diffractive deep inelastic scattering,
diffractive hard hadronic reactions, the breakdown of the Lam Tung relation in
Drell-Yan reactions, and nuclear shadowing and non-universal
antishadowing--leading-twist physics not incorporated in the light-front
wavefunctions of the target computed in isolation. I also discuss tests of
hidden color in nuclear wavefunctions, the use of diffraction to materialize
the Fock states of a hadronic projectile and test QCD color transparency, and
anomalous heavy quark effects. The presence of direct higher-twist processes
where a proton is produced in the hard subprocess can explain the large
proton-to-pion ratio seen in high-centrality heavy-ion collisions.Comment: Invited talk presented at the Workshop on High PT Physics at the LHC
(LHC07), Jyvaskyla, Finland, 23-27 March 2007; typos corrected; added
reference;resolution of figures improve
Perspectives on Exclusive Processes in QCD
Hard hadronic exclusive processes are now at the forefront of QCD studies,
particularly because of their role in the interpretation of exclusive hadronic
B decays. Perturbative QCD and its factorization properties at high momentum
transfer provide an essential guide to the phenomenology of exclusive
amplitudes at large momentum transfer--the leading power fall-off of form
factors and fixed-angle cross sections, the dominant helicity structures, and
their color transparency properties. The hard scattering subprocess amplitude
T_H controlling the leading-twist amplitude is evaluated in the perturbative
domain where the propagator virtualities are above the separation scale. A
critical question is the momentum transfer required such that leading-twist
perturbative QCD contributions dominate. I review some of the contentious
theoretical issues and empirical challenges to Perturbative QCD based analyses,
such as the magnitude of the leading-twist contributions, the role of soft and
higher twist QCD mechanisms, the effects of non-zero orbital angular momentum,
the possibility of single-spin asymmetries in deeply virtual Compton
scattering, the role of hidden color in nuclear wavefunctions, the behavior of
the ratio of Pauli and Dirac nucleon form factors, the apparent breakdown of
color transparency in quasi-elastic proton-proton scattering, and the
measurement of hadron and photon wavefunctions in diffractive dijet production.Comment: Invited Talk, presented at the Workshop on Exclusive Processes at
High Momentum Transfer, Jefferson Lab, Newport News, Virginia, May 15-18,
2002. Added reference
Novel QCD Effects from Initial and Final State Interactions
Initial-state and final-state interactions, which are conventionally
neglected in the parton model, have a profound effect in QCD hard-scattering
reactions. These effects, which arise from gluon exchange between the active
and spectator quarks, cause leading-twist single-spin asymmetries, diffractive
deep inelastic scattering, diffractive hard hadronic reactions, and the
breakdown of the Lam-Tung relation in Drell-Yan reactions. Diffractive deep
inelastic scattering also leads to nuclear shadowing and non-universal
antishadowing of nuclear structure functions through multiple scattering
reactions in the nuclear target. Factorization-breaking effects are
particularly important for hard hadron interactions since both initial-state
and final-state interactions appear. Related factorization breaking effects can
also appear in exclusive electroproduction reactions and in deeply virtual
Compton scattering. None of the effects of initial-state and final-state
interactions are incorporated in the light-front wavefunctions of the target
hadron computed in isolation.Comment: Invited talk, presented at the Workshop on Exclusive Reactions at
High Momentum Transfer 21-24 May 2007, Newport News, Virgini
Novel QCD Phenomenology at the LHeC
The proposed electron-proton/ion collider at CERN, the LHeC, can test
fundamental and novel aspects of QCD and electroweak interactions as well as
explore physics beyond the standard model over an exceptionally large kinematic
range.Comment: LHeC Contributio
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