4,928 research outputs found
A nonperturbative calculation of the electron's magnetic moment
In principle, the complete spectrum and bound-state wave functions of a
quantum field theory can be determined by finding the eigenvalues and
eigensolutions of its light-cone Hamiltonian. One of the challenges in
obtaining nonperturbative solutions for gauge theories such as QCD using
light-cone Hamiltonian methods is to renormalize the theory while preserving
Lorentz symmetries and gauge invariance. For example, the truncation of the
light-cone Fock space leads to uncompensated ultraviolet divergences. We
present two methods for consistently regularizing light-cone-quantized gauge
theories in Feynman and light-cone gauges: (1) the introduction of a spectrum
of Pauli-Villars fields which produces a finite theory while preserving Lorentz
invariance; (2) the augmentation of the gauge-theory Lagrangian with higher
derivatives. In the latter case, which is applicable to light-cone gauge (A^+ =
0), the A^- component of the gauge field is maintained as an independent degree
of freedom rather than a constraint. Finite-mass Pauli-Villars regulators can
also be used to compensate for neglected higher Fock states. As a test case, we
apply these regularization procedures to an approximate nonperturbative
computation of the anomalous magnetic moment of the electron in QED as a first
attempt to meet Feynman's famous challenge.Comment: 35 pages, elsart.cls, 3 figure
Perturbative QCD and factorization of coherent pion photoproduction on the deuteron
We analyze the predictions of perturbative QCD for pion photoproduction on
the deuteron, gamma D -> pi^0 D, at large momentum transfer using the reduced
amplitude formalism. The cluster decomposition of the deuteron wave function at
small binding only allows the nuclear coherent process to proceed if each
nucleon absorbs an equal fraction of the overall momentum transfer.
Furthermore, each nucleon must scatter while remaining close to its mass shell.
Thus the nuclear photoproduction amplitude, M_{gamma D -> pi^0 D}(u,t),
factorizes as a product of three factors: (1) the nucleon photoproduction
amplitude, M_{gamma N_1 -> pi^0 N_1}(u/4,t/4), at half of the overall momentum
transfer, (2) a nucleon form factor, F_{N_2}(t/4), at half the overall momentum
transfer, and (3) the reduced deuteron form factor, f_d(t), which according to
perturbative QCD, has the same monopole falloff as a meson form factor. A
comparison with the recent JLAB data for gamma D -> pi^0 D of Meekins et al.
[Phys. Rev. C 60, 052201 (1999)] and the available gamma p -> pi^0 p data shows
good agreement between the perturbative QCD prediction and experiment over a
large range of momentum transfers and center of mass angles. The reduced
amplitude prediction is consistent with the constituent counting rule, p^11_T
M_{gamma D -> pi^0 D} -> F(theta_cm), at large momentum transfer. This is found
to be consistent with measurements for photon lab energies E_gamma > 3 GeV at
theta_cm=90 degrees and \elab > 10 GeV at 136 degrees.Comment: RevTeX 3.1, 17 pages, 6 figures; v2: incorporates minor changes as
version accepted by Phys Rev
Hadron Spectroscopy and Wavefunctions in QCD and the AdS/CFT Correspondence
The AdS/CFT correspondence has led to important insights into the properties
of quantum chromodynamics even though QCD is a broken conformal theory. A
holographic model based on a truncated AdS space can be used to obtain the
hadronic spectrum of light and bound states. Specific
hadrons are identified by the correspondence of string modes with the dimension
of the interpolating operator of the hadron's valence Fock state, including
orbital angular momentum excitations. The predicted mass spectrum is linear at high orbital angular momentum. Since only one parameter, the QCD
scale , is introduced, the agreement with the pattern of
physical states is remarkable. In particular, the ratio of to nucleon
trajectories is determined by the ratio of zeros of Bessel functions. One can
also use the extended AdS/CFT space-time theory to obtain a model for hadronic
light-front wavefunctions, thus providing a relativistic description of hadrons
in QCD at the amplitude level. The model wavefunctions display confinement at
large inter-quark separation and conformal symmetry at short distances. In
particular, the scaling and conformal properties of the LFWFs at high relative
momenta agree with perturbative QCD. These AdS/CFT model wavefunctions could be
used as an initial ansatz for a variational treatment of the light-front QCD
Hamiltonian. Hadron form factors in both the space-like and time-like regions
are also predicted.Comment: Invited Talk, presented presented at the XI. International Conference
on Hadron Spectroscopy--HADRON 05,Rio de Janeiro, Brazil, 21-26 August 200
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
How the nuclear Fermi motion plus a simple statistical model explains the EMC effect
We present calculation of influence caused by nucleon Fermi motion on the
parton distributions in nuclei. Our approach is based on the model where
momenta of valence partons have some primordial distribution inside the hadron
at rest, which is either provided by a statistical considerations or calculated
using spherically symmetric Gaussian distribution with a width derived from the
Heisenberg uncertainty relation. The sea parton contribution emerges from the
similar Gaussian distribution with a width dictated by the presence of virtual
pions in hadron. We show that the influence of Fermi motion changes
substantially the nucleonic structure function inside the nucleus in the right
direction and therefore should be considered seriously in all attempts devoted
to explain the experimentally observed EMC effect for .Comment: Contribution to PANIC 2002 conference, Sept. 30 - October 4, 2002,
Osaka, Japan. Some misprints correcte
Classical sum rules and spin correlations in photoabsorption and photoproduction processes
In this paper we study the possibility of generalizing the classical
photoabsorption () sum rules, to processes
and crossed helicity amplitudes. In the first case, using detailed balance, the
sum rule is written as where is a kinematical constant which depends only
on the mass of the particles and the center of mass energy. For other crossed
helicity amplitudes, we show that there is a range of values of and for
which the differential cross section for the process or in which the helicities of the photon and particle have
specific values, is equal to the differential cross section for the process in
which one of these two helicities is reversed (parallel-antiparallel spin
correlation).Comment: 9 pages, 2 figure
Structure Functions are not Parton Probabilities
The common view that structure functions measured in deep inelastic lepton
scattering are determined by the probability of finding quarks and gluons in
the target is not correct in gauge theory. We show that gluon exchange between
the fast, outgoing partons and target spectators, which is usually assumed to
be an irrelevant gauge artifact, affects the leading twist structure functions
in a profound way. This observation removes the apparent contradiction between
the projectile (eikonal) and target (parton model) views of diffractive and
small x_{Bjorken} phenomena. The diffractive scattering of the fast outgoing
quarks on spectators in the target causes shadowing in the DIS cross section.
Thus the depletion of the nuclear structure functions is not intrinsic to the
wave function of the nucleus, but is a coherent effect arising from the
destructive interference of diffractive channels induced by final state
interactions. This is consistent with the Glauber-Gribov interpretation of
shadowing as a rescattering effect.Comment: 35 pages, 8 figures. Discussion of physical consequences of final
state interactions amplified. Material on light-cone gauge choices adde
Light-Cone Quantization and Hadron Structure
In this talk, I review the use of the light-cone Fock expansion as a
tractable and consistent description of relativistic many-body systems and
bound states in quantum field theory and as a frame-independent representation
of the physics of the QCD parton model. Nonperturbative methods for computing
the spectrum and LC wavefunctions are briefly discussed. The light-cone Fock
state representation of hadrons also describes quantum fluctuations containing
intrinsic gluons, strangeness, and charm, and, in the case of nuclei, "hidden
color". Fock state components of hadrons with small transverse size, such as
those which dominate hard exclusive reactions, have small color dipole moments
and thus diminished hadronic interactions; i.e., "color transparency". The use
of light-cone Fock methods to compute loop amplitudes is illustrated by the
example of the electron anomalous moment in QED. In other applications, such as
the computation of the axial, magnetic, and quadrupole moments of light nuclei,
the QCD relativistic Fock state description provides new insights which go well
beyond the usual assumptions of traditional hadronic and nuclear physics.Comment: LaTex 36 pages, 3 figures. To obtain a copy, send e-mail to
[email protected]
Application of the Principle of Maximum Conformality to Top-Pair Production
A major contribution to the uncertainty of finite-order perturbative QCD
predictions is the perceived ambiguity in setting the renormalization scale
. For example, by using the conventional way of setting , one obtains the total production cross-section
with the uncertainty \Delta \sigma_{t \bar{t}}/\sigma_{t
\bar{t}}\sim ({}^{+3%}_{-4%}) at the Tevatron and LHC even for the present
NNLO level. The Principle of Maximum Conformality (PMC) eliminates the
renormalization scale ambiguity in precision tests of Abelian QED and
non-Abelian QCD theories. In this paper we apply PMC scale-setting to predict
the cross-section at the Tevatron and LHC
colliders. It is found that remains almost unchanged by
varying within the region of . The convergence
of the expansion series is greatly improved. For the -channel,
which is dominant at the Tevatron, its NLO PMC scale is much smaller than the
top-quark mass in the small -region, and thus its NLO cross-section is
increased by about a factor of two. In the case of the -channel, which is
dominant at the LHC, its NLO PMC scale slightly increases with the subprocess
collision energy , but it is still smaller than for
TeV, and the resulting NLO cross-section is increased by
. As a result, a larger is obtained in comparison
to the conventional scale-setting method, which agrees well with the present
Tevatron and LHC data. More explicitly, by setting GeV, we
predict pb,
pb and pb. [full abstract can be found in the
paper.]Comment: 15 pages, 11 figures, 5 tables. Fig.(9) is correcte
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