64,195 research outputs found
Global behavior of cosmological dynamics with interacting Veneziano ghost
In this paper, we shall study the dynamical behavior of the universe
accelerated by the so called Veneziano ghost dark energy component locally and
globally by using the linearization and nullcline method developed in this
paper. The energy density is generalized to be proportional to the Hawking
temperature defined on the trapping horizon instead of Hubble horizon of the
Friedmann-Robertson-Walker (FRW) universe. We also give a prediction of the
fate of the universe and present the bifurcation phenomenon of the dynamical
system of the universe. It seems that the universe could be dominated by dark
energy at present in some region of the parameter space.Comment: 8 pages, 7 figures, accepted for publication in JHE
Small-Recoil Approximation
In this review we discuss a technique to compute and to sum a class of
Feynman diagrams, and some of its applications. These are diagrams containing
one or more energetic particles that suffer very little recoil in their
interactions. When recoil is completely neglected, a decomposition formula can
be proven. This formula is a generalization of the well-known eikonal formula,
to non-abelian interactions. It expresses the amplitude as a sum of products of
irreducible amplitudes, with each irreducible amplitude being the amplitude to
emit one, or several mutually interacting, quasi-particles. For abelian
interaction a quasi-particle is nothing but the original boson, so this
decomposition formula reduces to the eikonal formula. In non-abelian situations
each quasi-particle can be made up of many bosons, though always with a total
quantum number identical to that of a single boson. This decomposition enables
certain amplitudes of all orders to be summed up into an exponential form, and
it allows subleading contributions of a certain kind, which is difficult to
reach in the usual way, to be computed. For bosonic emissions from a heavy
source with many constituents, a quasi-particle amplitude turns out to be an
amplitude in which all bosons are emitted from the same constituent. For
high-energy parton-parton scattering in the near-forward direction, the
quasi-particle turns out to be the Reggeon, and this formalism shows clearly
why gluons reggeize but photons do not. The ablility to compute subleading
terms in this formalism allows the BFKL-Pomeron amplitude to be extrapolated to
asymptotic energies, in a unitary way preserving the Froissart bound. We also
consider recoil corrections for abelian interactions in order to accommodate
the Landau-Pomeranchuk-Migdal effect.Comment: 21 pages with 4 figure
Proton Spin Structure from Measurable Parton Distributions
We present a systematic study of the proton spin structure in terms of
measurable parton distributions. For a transversely-polarizedproton, we derive
a polarization sum rule from the leading generalized parton distributions
appearing in hard exclusive processes. For a longitudinally-polarized proton,
we obtain a helicity decomposition from well-known quark and gluon helicity
distributions and orbital angular-momentum contributions. The latter is shown
to be related to measurable subleading generalized parton distributions and
quantum-phase space Wigner distributions.Comment: 7 pages, title change
Generalized Background-Field Method
The graphical method discussed previously can be used to create new gauges
not reachable by the path-integral formalism. By this means a new gauge is
designed for more efficient two-loop QCD calculations. It is related to but
simpler than the ordinary background-field gauge, in that even the triple-gluon
vertices for internal lines contain only four terms, not the usual six. This
reduction simplifies the calculation inspite of the necessity to include other
vertices for compensation. Like the ordinary background-field gauge, this
generalized background-field gauge also preserves gauge invariance of the
external particles. As a check of the result and an illustration for the
reduction in labour, an explicit calculation of the two-loop QCD
-function is carried out in this new gauge. It results in a saving of
45% of computation compared to the ordinary background-field gauge.Comment: 17 pages, Latex, 18 figures in Postscrip
Collins-Soper Equation for the Energy Evolution of Transverse-Momentum and Spin Dependent Parton Distributions
The hadron-energy evolution (Collins and Soper) equation for all the
leading-twist transverse-momentum and spin dependent parton distributions is
derived in the impact parameter space. Based on the result, we present
resummation formulas for the spin structure functions in the semi-inclusive
deep inelastic scattering.Comment: 16 pages, 4 figures included, revised versio
Probing Parton Orbital Angular Momentum in Longitudinally Polarized Nucleon
While the total orbital angular momentum (OAM) of a definite quark flavor in
a longitudinally-polarized nucleon can be obtained through a sum rule involving
twist-two generalized parton distribution (GPDs), its distribution as a
function of parton momentum in light-front coordinates is more complicated to
define and measure because it involves intrinsically twist-three effects. In
this paper, we consider two different parton OAM distributions. The first is
manifestly gauge invariant, and its moments are local operators and calculable
in lattice QCD. We show that it can potentially be measured through twist-three
GPDs. The second is the much-debated canonical OAM distribution natural in
free-field theory and light-cone gauge. We show the latter in light-cone gauge
can also be related to twist-three GPDs as well as quantum phase-space Wigner
distributions, both being measurable in high-energy experiments.Comment: 14 pages, no figur
- …