5,244 research outputs found
Quantum Chromodynamics and Other Field Theories on the Light Cone
We discuss the light-cone quantization of gauge theories as a calculational
tool for representing hadrons as QCD bound-states of relativistic quarks and
gluons, and also as a novel method for simulating quantum field theory on a
computer. The light-cone Fock state expansion of wavefunctions provides a
precise definition of the parton model and a general calculus for hadronic
matrix elements. We present several new applications of light-cone Fock
methods, including calculations of exclusive weak decays of heavy hadrons, and
intrinsic heavy-quark contributions to structure functions. Discretized
light-cone quantization, is outlined and applied to several gauge theories. We
also discuss the construction of the light-cone Fock basis, the structure of
the light-cone vacuum, and outline the renormalization techniques required for
solving gauge theories within the Hamiltonian formalism on the light cone.Comment: 206 pages Latex, figures included, Submitted to Physics Report
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
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
Perturbative QCD relations inspired by hypothetical tau leptons
We review our recent works on tests of perturbative QCD, inspired by the
relation between the hadronic decay of the tau lepton and the e+ e-
annihilation into hadrons. First, we present a set of commensurate scale
relations that probe the self-consistency of leading-twist QCD predictions for
any observable which defines an effective charge. These tests are independent
of the renormalization scheme and scale, and are applicable over wide data
ranges. As an example we apply this approach to R_{e+ e-}. Second, using a
differential form of these conmensurate scale relations, we present a method to
measure the QCD Gell-Mann--Low Psi function.Comment: To appear in the proceedings of the QCD 99 Euroconference, 7-13th
July 1999 Montpellier, France 4 pages, uses espcrc2.sty (included
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]
Photon-meson transition form factors of light pseudoscalar mesons
The photon-meson transition form factors of light pseudoscalar mesons , , and are systematically calculated in a
light-cone framework, which is applicable as a light-cone quark model at low
and is also physically in accordance with the light-cone pQCD approach
at large . The calculated results agree with the available experimental
data at high energy scale. We also predict the low behaviors of the
photon-meson transition form factors of , and , which are measurable in process via Primakoff
effect at JLab and DESY.Comment: 22 Latex pages, 7 figures, Version to appear in PR
Jet Asymmetry in High Energy Diffractive Production
We propose the asymmetry in the fractional energy of charm versus anticharm
jets produced in high energy diffractive photoproduction as a sensitive test of
the interference of the Odderon (C = -) and Pomeron (C = +) exchange amplitudes
in QCD. If measured at HERA, this asymmetry could provide the first
experimental evidence of the Odderon.Comment: Contribution to the Proceedings of the International Euroconference
in Quantum Chromodynamics: 15 Years of the QCD - Montpellier Conference (QCD
00), Montpellier, France, 6-12 July 200
Direct Estimation of Sizes of Higher-Order Graphs
With the aid of simple examples we show how to make simple estimates of the
sizes of higher-order Feynman graphs. Our methods enable appropriate values of
renormalization and factorization scales to be made. They allow the diagnosis
of the source of unusually large corrections that are in need of resummation.Comment: 22 pages Revtex with epsf, postscript figures. Replacement is due to
author error. Version is same as origina
Confinement contains condensates
Dynamical chiral symmetry breaking and its connection with the generation of
hadron masses has historically been viewed as a vacuum phenomenon. We argue
that confinement makes such a position untenable. If quark-hadron duality is a
reality in QCD, then condensates, those quantities that were commonly viewed as
constant empirical mass-scales that fill all spacetime, are instead wholly
contained within hadrons; viz., they are a property of hadrons themselves and
expressed, e.g., in their Bethe-Salpeter or light-front wave functions. We
explain that this paradigm is consistent with empirical evidence, and
incidentally expose misconceptions in a recent Comment.Comment: 10 pages, 2 figure
Gluon mass and freezing of the QCD coupling
Infrared finite solutions for the gluon propagator of pure QCD are obtained
from the gauge-invariant non-linear Schwinger-Dyson equation formulated in the
Feynman gauge of the background field method. These solutions may be fitted
using a massive propagator, with the special characteristic that the effective
mass employed drops asymptotically as the inverse square of the momentum
transfer, in agreement with general operator-product expansion arguments. Due
to the presence of the dynamical gluon mass the strong effective charge
extracted from these solutions freezes at a finite value, giving rise to an
infrared fixed point for QCD.Comment: 3 pages, 2 figures, based on talk given at the 2007 Europhysics
Conference on High Energy Physics, Manchester, 19-25 Jul
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