4,055 research outputs found
Hadron Spin Dynamics
Spin effects in exclusive and inclusive reactions provide an essential new
dimension for testing QCD and unraveling hadron structure. Remarkable new
experiments from SLAC, HERMES (DESY), and the Jefferson Laboratory present many
challenges to theory, including measurements at HERMES and SMC of the single
spin asymmetries in pion electroproduction, where the proton is polarized
normal to the scattering plane. This type of single spin asymmetry may be due
to the effects of rescattering of the outgoing quark on the spectators of the
target proton, an effect usually neglected in conventional QCD analyses. Many
aspects of spin, such as single-spin asymmetries and baryon magnetic moments
are sensitive to the dynamics of hadrons at the amplitude level, rather than
probability distributions. I illustrate the novel features of spin dynamics for
relativistic systems by examining the explicit form of the light-front
wavefunctions for the two-particle Fock state of the electron in QED, thus
connecting the Schwinger anomalous magnetic moment to the spin and orbital
momentum carried by its Fock state constituents and providing a transparent
basis for understanding the structure of relativistic composite systems and
their matrix elements in hadronic physics. I also present a survey of
outstanding spin puzzles in QCD, particularly the double transverse spin
asymmetry A_{NN} in elastic proton-proton scattering, the J/psi to rho-pi
puzzle, and J/psi polarization at the Tevatron.Comment: Concluding theory talk presented at SPIN2001, the Third
Circum-Pan-Pacific Symposium on High Energy Physics, October, 2001, Beijin
Light-Front QCD in Light-Cone Gauge
The light-front (LF) quantization of QCD in light-cone (l.c.) gauge is
discussed. The Dirac method is employed to construct the LF Hamiltonian and
theory quantized canonically. The Dyson-Wick perturbation theory expansion
based on LF-time ordering is constructed. The framework incorporates in it
simultaneously the Lorentz gauge condition as an operator equation as well. The
propagator of the dynamical part of the free fermionic propagator is
shown to be causal while the gauge field propagator is found to be transverse.
The interaction Hamiltonian is re-expressed in the form closely resembling the
one in covariant theory, except for additional instantaneous interactions,
which can be treated systematically. Some explicit computations in QCD are
given.Comment: Presented at VII Hadron Physics 2000, Caraguatatuba, Sao Paulo,
Brazil, 10-15 April 200
Illuminating the 1/x moment of parton distribution functions
The Weisberger relation, an exact statement of the parton model, elegantly
relates a high-energy physics observable, the 1/x moment of parton distribution
functions, to a nonperturbative low-energy observable: the dependence of the
nucleon mass on the value of the quark mass or its corresponding quark
condensate. We show that contemporary fits to nucleon structure functions fail
to determine this 1/x moment; however, deeply virtual Compton scattering can be
described in terms of a novel F_{1/x}(t) form factor which illuminates this
physics. An analysis of exclusive photon-induced processes in terms of the
parton-nucleon scattering amplitude with Regge behavior reveals a failure of
the high Q^2 factorization of exclusive processes at low t in terms of the
Generalized Parton-Distribution Functions which has been widely believed to
hold in the past. We emphasize the need for more data for the DVCS process at
large t in future or upgraded facilities.Comment: 11 pages, 3 figures, invited contribution to the 11th International
Conference on Meson-Nucleon Physics and the Structure of the Nucleon, sept.
10th-14th 2007, Juelich, German
Light-Front-Quantized QCD in Covariant Gauge
The light-front (LF) canonical quantization of quantum chromodynamics in
covariant gauge is discussed. The Dirac procedure is used to eliminate the
constraints in the gauge-fixed front form theory quantum action and to
construct the LF Hamiltonian formulation. The physical degrees of freedom
emerge naturally. The propagator of the dynamical part of the free
fermionic propagator in the LF quantized field theory is shown to be causal and
not to contain instantaneous terms. Since the relevant propagators in the
covariant gauge formulation are causal, rotational invariance---including the
Coulomb potential in the static limit---can be recovered, avoiding the
difficulties encountered in light-cone gauge. The Wick rotation may also be
performed allowing the conversion of momentum space integrals into Euclidean
space forms. Some explicit computations are done in quantum electrodynamics to
illustrate the equivalence of front form theory with the conventional covariant
formulation. LF quantization thus provides a consistent formulation of gauge
theory, despite the fact that the hyperplanes used to impose
boundary conditions constitute characteristic surfaces of a hyperbolic partial
differential equation.Comment: LaTex, 16 page
Light-cone QCD predictions for elastic ed-scattering in the intermediate energy region
The contributions of helicity-flip matrix elements to the deuteron form
factors are discussed in the light-cone frame. Normalized , ,
and are obtained in a simple QCD-inspired model. We find
that plays an important role in . Our numerical results
are consistent with the data in the intermediate energy region.Comment: 9 pages, REVTeX file, 5 figure
The running coupling method with next-to-leading order accuracy and pion, kaon elm form factors
The pion and kaon electromagnetic form factors are calculated at
the leading order of pQCD using the running coupling constant method. In
calculations the leading and next-to-leading order terms in
expansion in terms of are taken into
account. The resummed expression for is found. Results of numerical
calculations for the pion (asymptotic distribution amplitude) are presented.Comment: 9 pages, 1 figur
The hbar Expansion in Quantum Field Theory
We show how expansions in powers of Planck's constant hbar = h/2\pi can give
new insights into perturbative and nonperturbative properties of quantum field
theories. Since hbar is a fundamental parameter, exact Lorentz invariance and
gauge invariance are maintained at each order of the expansion. The physics of
the hbar expansion depends on the scheme; i.e., different expansions are
obtained depending on which quantities (momenta, couplings and masses) are
assumed to be independent of hbar. We show that if the coupling and mass
parameters appearing in the Lagrangian density are taken to be independent of
hbar, then each loop in perturbation theory brings a factor of hbar. In the
case of quantum electrodynamics, this scheme implies that the classical charge
e, as well as the fine structure constant are linear in hbar. The connection
between the number of loops and factors of hbar is more subtle for bound states
since the binding energies and bound-state momenta themselves scale with hbar.
The hbar expansion allows one to identify equal-time relativistic bound states
in QED and QCD which are of lowest order in hbar and transform dynamically
under Lorentz boosts. The possibility to use retarded propagators at the Born
level gives valence-like wave-functions which implicitly describe the sea
constituents of the bound states normally present in its Fock state
representation.Comment: 8 pages, 1 figure. Version to be published in Phys. Rev.
Hadron Optics in Three-Dimensional Invariant Coordinate Space from Deeply Virtual Compton Scattering
The Fourier transform of the deeply virtual Compton scattering amplitude
(DVCS) with respect to the skewness parameter \zeta= Q^2/ 2 p.q can be used to
provide an image of the target hadron in the boost-invariant variable \sigma,
the coordinate conjugate to light-front time \tau=t+ z/ c. As an illustration,
we construct a consistent covariant model of the DVCS amplitude and its
associated generalized parton distributions using the quantum fluctuations of a
fermion state at one loop in QED, thus providing a representation of the
light-front wavefunctions of a lepton in \sigma space. A consistent model for
hadronic amplitudes can then be obtained by differentiating the light-front
wavefunctions with respect to the bound-state mass. The resulting DVCS helicity
amplitudes are evaluated as a function of \sigma and the impact parameter \vec
b_\perp, thus providing a light-front image of the target hadron in a
frame-independent three-dimensional light-front coordinate space. Models for
the LFWFs of hadrons in (3+1) dimensions displaying confinement at large
distances and conformal symmetry at short distances have been obtained using
the AdS/CFT method. We also compute the LFWFs in this model in invariant three
dimensional coordinate space. We find that in the models studied, the Fourier
transform of the DVCS amplitudes exhibit diffraction patterns. The results are
analogous to the diffractive scattering of a wave in optics where the
distribution in \sigma measures the physical size of the scattering center in a
one-dimensional system.Comment: minor modification to text, preprint number update
Polarization as a Probe to the Production Mechanisms of Charmonium in Collisions
Measurements of the polarization of \jp produced in pion-nucleus collisions
are in disagreement with leading twist QCD prediction where \jp is observed
to have negligible polarization whereas theory predicts substantial
polarization. We argue that this discrepancy cannot be due to poorly known
structure functions nor the relative production rates of \jp and .
The disagreement between theory and experiment suggests important higher twist
corrections, as has earlier been surmised from the anomalous non-factorized
nuclear -dependence of the \jp cross section.Comment: 8 page
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