725 research outputs found
Gluon Structure Function of a Color Dipole in the Light-Cone Limit of Lattice QCD
We calculate the gluon structure function of a color dipole in
near-light-cone SU(2) lattice QCD as a function of . The quark and
antiquark are external non-dynamical degrees of freedom which act as sources of
the gluon string configuration defining the dipole. We compute the color dipole
matrix element of transversal chromo-electric and chromo-magnetic field
operators separated along a direction close to the light cone, the Fourier
transform of which is the gluon structure function. As vacuum state in the pure
glue sector, we use a variational ground state of the near-light-cone
Hamiltonian. We derive a recursion relation for the gluon structure function on
the lattice similar to the perturbative DGLAP equation. It depends on the
number of transversal links assembling the Schwinger string of the dipole.
Fixing the mean momentum fraction of the gluons to the "experimental value" in
a proton, we compare our gluon structure function for a dipole state with four
links with the NLO \emph{MRST} 2002 and the \emph{CTEQAB-0} parameterizations
at . Within the systematic uncertainty we find rather
good agreement. We also discuss the low behavior of the gluon structure
function in our model calculation.Comment: 44 pages, 10 figures, to be in accordance with the variant submitted
to Phys. Rev.
Large distance behaviour of light cone operator product in perturbative and nonperturbative QCD regimes
We evaluate the coordinate space dependence of the matrix elements of the
commutator of the electromagnetic and gluon currents in the vicinity of the
light-cone but at large distances within the parton model, DGLAP, the
resummation approaches to the small x behaviour of DIS processes, and for the
Unitarity Bound. We find that an increase of the commutator with relative
distance as is the generic property of QCD
at small but fixed space-time interval in perturbative and
nonperturbative QCD regimes. We explain that the factor follows within the
dipole model (QCD factorization theorem) from the properties of Lorents
transformation. The increase of disappeares at central impact parameters
if cross section of DIS may achieve the Unitarity Limit. We argue that such
long range forces are hardly consistent with thermodynamic equilibrium while a
Unitarity Limit may signal equilibration. Possible implications of this new
long range interaction are briefly discussed.Comment: 23 page
Prompt neutrino fluxes from atmospheric charm
We calculate the prompt neutrino flux from atmospheric charm production by
cosmic rays, using the dipole picture in a perturbative QCD framework, which
incorporates the parton saturation effects present at high energies. We compare
our results with the next-to-leading order perturbative QCD result and find
that saturation effects are large for neutrino energies above 10^6 GeV, leading
to a substantial suppression of the prompt neutrino flux. We comment on the
range of prompt neutrino fluxes due to theoretical uncertainties.Comment: 13 pages with 11 figures; expanded discussion, added references,
version to be published in Phys. Rev.
Evolution Equation for Generalized Parton Distributions
The extension of the method [arXiv:hep-ph/0503109] for solving the leading
order evolution equation for Generalized Parton Distributions (GPDs) is
presented. We obtain the solution of the evolution equation both for the flavor
nonsinglet quark GPD and singlet quark and gluon GPDs. The properties of the
solution and, in particular, the asymptotic form of GPDs in the small x and \xi
region are discussed.Comment: REVTeX4, 34 pages, 3 figure
Spectroscopy of phonons and spin torques in magnetic point contacts
Phonon spectroscopy is used to investigate the mechanism of current-induced
spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization
excitations observed in the magneto-conductance of the point contacts are
pronounced for diffusive and thermal contacts, where the electrons experience
significant scattering in the contact region. We find no magnetic excitations
in highly ballistic contacts. Our results show that impurity scattering at the
N/F interface is the origin of the new single-interface spin torque effect.Comment: 4 pages, 5 figs., accepted for publication in PR
Ultra-High Energy Neutrino-Nucleon Scattering and Parton Distributions at Small
The cross section for ultra-high energy neutrino-nucleon scattering is very
sensitive to the parton distributions at very small values of Bjorken x (. We numerically investigate the effects of modifying the
behavior of the gluon distribution function at very small in the DGLAP
evolution equation. We then use the Color Glass Condensate formalism to
calculate the neutrino-nucleon cross section at ultra-high energies and compare
the result with those based on modification of DGLAP evolution equation.Comment: 10 pages, 4 figures, INT-PUB-05-3
Simulation of Time-Resolved Vibronic Spectra and the possibility of analyzing molecules with similar apectral properties
The possibility of using time-resolved vibronic spectroscopy for spectral
analysis of mixtures of chemical compounds with similar optical properties,
when traditional methods are inefficient, is demonstrated by using the method
of computer simulation. The analysis is carried out by the example of molecules
of a series of polyenes (butadiene, hexatraene, octatetraene, decapentaene, and
decatetraene), their various cis- and trans-rotational isomers, and
phenyl-substituted polyenes. Ranges of relative concentrations of molecules
similar in their spectral properties, where reliable interpretation of
time-resolved spectra of mixtures and both qualitative and quantitative
analyses are possible, are determined. The use of computer simulation methods
for oprimizing full-scale experiments in femtosecond spectroscopy is shown to
hold much promise
Resummation of double logarithms in electroweak high energy processes
At future linear collider experiments in the TeV range, Sudakov
double logarithms originating from massive boson exchange can lead to
significant corrections to the cross sections of the observable processes.
These effects are important for the high precision objectives of the Next
Linear Collider. We use the infrared evolution equation, based on a gauge
invariant dispersive method, to obtain double logarithmic asymptotics of
scattering amplitudes and discuss how it can be applied, in the case of broken
gauge symmetry, to the Standard Model of electroweak processes. We discuss the
double logarithmic effects to both non-radiative processes and to processes
accompanied by soft gauge boson emission. In all cases the Sudakov double
logarithms are found to exponentiate. We also discuss double logarithmic
effects of a non-Sudakov type which appear in Regge-like processes.Comment: 26 pages, 3 figures, Latex2
The Color Dipole Picture of low-x DIS: Model-Independent and Model-Dependent Results
We present a detailed examination of the color-dipole picture (CDP) of
low- deep inelastic scattering. We discriminate model-independent results,
not depending on a specific parameterization of the dipole cross section, from
model-dependent ones. The model-independent results include the ratio of the
longitudinal to the transverse photoabsorption cross section at large , or
equivalently the ratio of the longitudinal to the unpolarized proton structure
function, , as well as the low- scaling
behavior of the total photoabsorption cross section as for
, and as for .
Here, denotes the low- scaling variable, with being
the saturation scale. The model-independent analysis also implies
at any for asymptotically
large energy, . Consistency with pQCD evolution determines the underlying
gluon distribution and the numerical value of in the expression
for the saturation scale, . In the
model-dependent analysis, by restricting the mass of the actively contributing
fluctuations by an energy-dependent upper bound, we extend the
validity of the color-dipole picture to . The
theoretical results agree with the world data on DIS for .Comment: 77 pages, 30 figure
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