221 research outputs found
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
Pion Form Factor in the Factorization Formalism
Based on the light-cone (LC) framework and the factorization formalism,
the transverse momentum effects and the different helicity components'
contributions to the pion form factor are recalculated. In
particular, the contribution to the pion form factor from the higher helicity
components (), which come from the spin-space Wigner
rotation, are analyzed in the soft and hard energy regions respectively. Our
results show that the right power behavior of the hard contribution from the
higher helicity components can only be obtained by fully keeping the
dependence in the hard amplitude, and that the dependence in LC wave
function affects the hard and soft contributions substantially. As an example,
we employ a model LC wave function to calculate the pion form factor and then
compare the numerical predictions with the experimental data. It is shown that
the soft contribution is less important at the intermediate energy region.Comment: 21 pages, 4 figure
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Factorization of the Drell-Yan cross section
We state the weak and strong factorization theorems for the Drell-Yan cross section and outline the ingredients involved in their proof. We also discuss the physical picture implied by the factorization results and its phenomenological consequences
Large corrections to asymptotic and in the light-cone perturbative QCD
The large- behavior of - and -
transition form factors, and
are analyzed in the framework of light-cone perturbative QCD with the heavy
quark ( and ) mass effect, the parton's transverse momentum dependence
and the higher helicity components in the light-cone wave function are
respected. It is pointed out that the quark mass effect brings significant
modifications to the asymptotic predictions of the transition form factors in a
rather broad energy region, and this modification is much severer for
than that for due to the
-quark being heavier than the -quark. The parton's transverse momentum
and the higher helicity components are another two factors which decrease the
perturbative predictions. For the transition form factor
, they bring sizable corrections in the present
experimentally accessible energy region (). For the
transition form factor , the corrections coming from
these two factors are negligible since the -quark mass is much larger than
the parton's average transverse momentum. The coming collider (LEP2)
will provide the opportunity to examine these theoretical predictions.Comment: 8 pages, RevTex, 5 PostScript figure
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
Fixing the renormalisation scheme in NNLO perturbative QCD using conformal limit arguments
We discuss how the renormalisation scheme ambiguities in QCD can be fixed,
when two observables are related, by requiring the coefficients in the
perturbative expansion relating the two observables to have their conformal
limit values, i.e. to be independent of the -function of the
renormalised coupling. We show how the next-to-leading order BLM automatic
scale fixing method can be extended to next-to-next-to-leading order to fix
both the renormalisation scale and in a unique way. As an example we
apply the method to the relation between Bjorken's sum rule and and
compare with experimental data as well as other scheme fixing methods.Comment: 14 pages LaTeX, uses revtex.sty, 1 encapsulated PostScript figur
Optimal Renormalization Scale and Scheme for Exclusive Processes
We use the BLM method to fix the renormalization scale of the QCD coupling in
exclusive hadronic amplitudes such as the pion form factor and the
photon-to-pion transition form factor at large momentum transfer.
Renormalization-scheme-independent commensurate scale relations are established
which connect the hard scattering subprocess amplitudes that control exclusive
processes to other QCD observables such as the heavy quark potential and the
electron-positron annihilation cross section. The commensurate scale relation
connecting the heavy quark potential, as determined from lattice gauge theory,
to the photon-to-pion transition form factor is in excellent agreement with
data assuming that the pion distribution amplitude is
close to its asymptotic form . We also reproduce the
scaling and normalization of the data at large
momentum transfer. Because the renormalization scale is small, we argue that
the effective coupling is nearly constant, thus accounting for the nominal
scaling behavior of the data. However, the normalization of the space-like pion
form factor obtained from electroproduction experiments is
somewhat higher than that predicted by the corresponding commensurate scale
relation. This discrepancy may be due to systematic errors introduced by the
extrapolation of the electroproduction data to the
pion pole.Comment: 22 pages, Latex, 7 Latex figures. Several references added,
discussion of scale fixing revised for clarity. Final version to appear in
Phys. Rev.
Consistent Analysis of O(\alpha_s) Corrections to Pion Elastic Form Factor
We examine the role of O(\alpha_s) perturbative corrections to the pion
elastic form factor F_\pi(Q^2). We express the quark current three-point
function in terms of light cone variables and use Borel transformation to
simultaneously model the Feynman mechanism contribution determined by the soft
part of the pion light cone wave function and the hard term involving one gluon
exchange. We find that for Q^2 \sim 5 GeV^2 the total one gluon exchange
contribution may reach 40% of the soft contribution, even though its hard,
factorization scale dependent part remains relatively small.Comment: 15 pages; version to appear in Phys. Rev.
Gluon Virtuality and Heavy Sea Quark Contributions to the Spin-Dependent g_1 Structure Function
We analyze the quark mass dependence of photon gluon fusion in polarized deep
inelastic scattering for both the intrinsic and extrinsic gluon distributions
of the nucleon. We calculate the effective number of flavors for each of the
heavy and light quark photon gluon fusion contributions to the first moment of
the spin-dependent structure function .Comment: LaTex, 19 page
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