23,422 research outputs found
Nuclear Structure Functions and Heavy Flavour Leptoproduction Off the Nucleus at Small x in Perturbative QCD
Nuclear structure functions and cross-sections for heavy flavour production
in lepton-nucleus collisions are investigated in the low region accessible
now or in the near future. The scattering on a heavy nucleus is described by
the sum of fan diagrams of BFKL pomerons, which is exact in the high-colour
limit. The initial condition for the evolution at is taken from a
saturation model, which reproduces the experimental data on the proton. The
dependence of the structure functions is well described by a power factor
, with reaching values as low as 1/2 at extremely low .
The total cross-sections for heavy flavour production reach values of the order
of mb, and the corresponding transverse momentum distributions are sizeable up
to transverse momenta larger than the initial large scale .Comment: LaTeX2e, 16 pages, 6 eps figures included using epsfig; final
version, some comments added, results and conclusions unchange
Dependence of boundary lubrication on the misfit angle between the sliding surfaces
Using molecular dynamics based on Langevin equations with a coordinate- and
velocity-dependent damping coefficient, we study the frictional properties of a
thin layer of "soft" lubricant (where the interaction within the lubricant is
weaker than the lubricant-substrate interaction) confined between two solids.
At low driving velocities the system demonstrates stick-slip motion. The
lubricant may or may not be melted during sliding, thus exhibiting either the
"liquid sliding" (LS) or the "layer over layer sliding" (LoLS) regimes. The
LoLS regime mainly operates at low sliding velocities. We investigate the
dependence of friction properties on the misfit angle between the sliding
surfaces and calculate the distribution of static frictional thresholds for a
contact of polycrystalline surfaces.Comment: 8 pages, 11 figure
Conformal symmetry of the Lange-Neubert evolution equation
The Lange-Neubert evolution equation describes the scale dependence of the
wave function of a meson built of an infinitely heavy quark and light antiquark
at light-like separations, which is the hydrogen atom problem of QCD. It has
numerous applications to the studies of B-meson decays. We show that the kernel
of this equation can be written in a remarkably compact form, as a logarithm of
the generator of special conformal transformation in the light-ray direction.
This representation allows one to study solutions of this equation in a very
simple and mathematically consistent manner. Generalizing this result, we show
that all heavy-light evolution kernels that appear in the renormalization of
higher-twist B-meson distribution amplitudes can be written in the same form.Comment: 8 page
Evolution equations beyond one loop from conformal symmetry
We study implications of exact conformal invariance of scalar quantum field
theories at the critical point in non-integer dimensions for the evolution
kernels of the light-ray operators in physical (integer) dimensions. We
demonstrate that all constraints due the conformal symmetry are encoded in the
form of the generators of the collinear sl(2) subgroup. Two of them, S_- and
S_0, can be fixed at all loops in terms of the evolution kernel, while the
generator of special conformal transformations, S_+, receives nontrivial
corrections which can be calculated order by order in perturbation theory.
Provided that the generator S_+ is known at the k-1 loop order, one can fix the
evolution kernel in physical dimension to the k-loop accuracy up to terms that
are invariant with respect to the tree-level generators. The invariant parts
can easily be restored from the anomalous dimensions. The method is illustrated
on two examples: The O(n)-symmetric phi^4 theory in d=4 to the three-loop
accuracy, and the su(n) matrix phi^3 theory in d=6 to the two-loop accuracy. We
expect that the same technique can be used in gauge theories e.g. in QCD.Comment: 19 pages, 3 figure
Evolution equation for the higher-twist B-meson distribution amplitude
We find that the evolution equation for the three-particle quark-gluon
B-meson light-cone distribution amplitude (DA) of subleading twist is
completely integrable in the large limit and can be solved exactly. The
lowest anomalous dimension is separated from the remaining, continuous,
spectrum by a finite gap. The corresponding eigenfunction coincides with the
contribution of quark-gluon states to the two-particle DA so
that the evolution equation for the latter is the same as for the leading-twist
DA up to a constant shift in the anomalous dimension. Thus,
``genuine'' three-particle states that belong to the continuous spectrum
effectively decouple from to the leading-order accuracy. In
turn, the scale dependence of the full three-particle DA turns out to be
nontrivial so that the contribution with the lowest anomalous dimension does
not become leading at any scale. The results are illustrated on a simple model
that can be used in studies of corrections to heavy-meson decays in the
framework of QCD factorization or light-cone sum rules.Comment: Extended version, includes new results on the large momentum limit
and a detailed study of the evolution effects in a simple mode
Two-loop evolution equations for light-ray operators
QCD in non-integer d=4-2 epsilon space-time dimensions possesses a nontrivial
critical point and enjoys exact scale and conformal invariance. This symmetry
imposes nontrivial restrictions on the form of the renormalization group
equations for composite operators in physical (integer) dimensions and allows
to reconstruct full kernels from their eigenvalues (anomalous dimensions). We
use this technique to derive two-loop evolution equations for flavor-nonsinglet
quark-antiquark light-ray operators that encode the scale dependence of
generalized hadron parton distributions and light-cone distribution amplitudes
in the most compact form.Comment: 13 pages, 1 figur
Transverse momentum distributions and their forward- backward correlations in the percolating colour string approach
The forward-backward correlations in the distributions, which present a
clear signature of non-linear effects in particle production, are studied in
the model of percolating colour strings. Quantitative predictions are given for
these correlations at SPS, RHIC and LHC energies. Interaction of strings also
naturally explains the flattening of distributions and increase of
with energy and atomic number for nuclear collisionsComment: 6 pages in LaTex, 3 figures in Postscrip
Runoff modelling in glacierized Central Asian catchments for present-day and future climate
A conceptual precipitationârunoff model was applied in five glacierized catchments in Central Asia. The model, which was first developed and applied in the Alps, works on a daily time step and yields good results in the more continental climate of the Tien Shan mountains for present-day climate conditions. Runoff scenarios for different climates (doubling of CO2) and glacierization conditions predict an increased flood risk as a first stage and a more complex picture after a complete glacier loss: a higher discharge during spring due to an earlier and more intense snowmelt is followed by a water deficiency in hot and dry summer periods. This unfavourable seasonal redistribution of the water supply has dramatic consequences for the Central Asian lowlands, which depend to a high degree on the glacier melt water for irrigation and already nowadays suffer from water shortages
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