2,821 research outputs found

### Photoproduction of Vector Mesons at Large Transfer

At forward angles, the cross-sections of photoproduction of vector mesons
($\rho$, $\omega$, and $\phi$) are well accounted for by the exchange of the
Pomeron at high energies, while contributions of $t$ channel exchange of
Reggeons are significant at low energies. At large angles, the impact parameter
becomes small enough to prevent their constituents to build up the exchanged
Reggeons or Pomeron. Two gluon exchange appears to dominate above $-t\simeq 1$
GeV$^2$, especially in the $\phi$ channel.Comment: 4 pages, 3 figure

### Time Evolution of Jets and Perturbative Color Neutralization

In-medium production of leading hadrons in hard reactions, carrying the main
fraction of the jet momentum, involves two stages: (i) the parton originated
from the hard process propagates through the medium radiating gluons due to the
initial hard collision, as well as to multiple interactions in the medium; (ii)
perturbative color neutralization, e.g. picking up an anti-colored parton
produced perturbatively, followed by evolution and attenuation of the
(pre)hadron in the medium. The color neutralization (or production) length for
leading hadrons is controlled by coherence, energy conservation and Sudakov
suppression. The pT-broadening is a sensitive and model independent probe for
the production length. The color neutralization time is expected to shrink with
rising hard scale. In particular, we found a very fast energy dissipation by a
highly virtual parton: half of the jet energy is radiated during the first
Fermi. Energy conservation makes the production of leading hadrons at longer
times difficult.Comment: Based on talk given by B.K. at the Fifth International Conference on
Perspectives in Hadronic Physics, Trieste, May 200

### Resummation of nuclear enhanced higher twist in the Drell Yan process

We investigate higher twist contributions to the transverse momentum
broadening of Drell Yan pairs in proton nucleus collisions. We revisit the
contribution of matrix elements of twist-4 and generalize this to matrix
elements of arbitrary twist. An estimate of the maximal nuclear broadening
effect is derived. A model for nuclear enhanced matrix elements of arbitrary
twist allows us to give the result of a resummation of all twists in closed
form. Subleading corrections to the maximal broadening are discussed
qualitatively.Comment: 10 pages, 5 figures; v2: minor changes in text, acknowledgement
added; v3: mistake in fig. 1 correcte

### X-ray Investigation of the Critical Exponent η in Argon

Measurements were made of the small-angle x-ray scattering intensity from argon near the critical point. After the scattering curves are corrected for all known effects except those resulting from irrelevant variables, a value of the critical exponent η=0.09±0.02 is obtained, while if a correction for the estimated effect of irrelevant variables is also made, η=0.03±0.03. The scattering data for argon therefore show that if irrelevant variables can be neglected, the critical exponent η for argon is in good agreement with the value obtained by Warkulwiz, Mozer, and Green from small-angle neutron scattering data for neon. These values of η, however, are clearly greater than those obtained by calculations using high-temperature expansions and renormalizationgroup techniques. On the other hand, if the effects of irrelevant variables are given by the estimate used in correcting the scattering data, the η value computed from the scattering curves agrees with the theoretical results

### Confinement and Processing Can Alter the Morphology and Periodicity of Bottlebrush Block Copolymers in Thin Films

Bottlebrush block copolymers (BBCPs) are intriguing architectural variations on linear BCPs with highly tunable structure. Confinement can have a significant impact on polymer assembly, giving rise to changes in morphology, assembly kinetics, and properties like the glass transition. Given that confinement leads to significant changes in the persistence length of bottlebrush homopolymers, it is reasonable to expect that BBCPs will see significant changes in their structure and periodicity relative to the bulk morphology. Understanding how confinement influences assembly will be important for designing BBCPs for thin film applications including membranes, integrated photonic structures, and potentially BCP lithography. In order to study the effects of confinement on BBCP conformation and morphology, a blade coating was used to prepare films with continuous variation in film thickness. Unlike thin films of linear BCPs, islands/holes were not observed, and instead mixtures of parallel and perpendicular morphologies emerge after annealing. The lamellar periodicity (L₀) of the morphologies is found to be thickness dependent, increasing L₀ with decreasing film thickness for blade coated films. Films coated out of tetrahydrofuran (THF) resulted in a single well-defined lamellar periodicity, verified through atomic force microscopy (AFM) and grazing incidence small-angle X-ray scattering (GISAXS), which increases dramatically from the bulk value (30.6 nm) and continues to increase as the film thickness decreases. The largest observed L₀ was 65.5 nm, and this closely approaches the estimated upper limit of 67 nm corresponding to a fully extended backbone in a bilayer arrangement. Films coated out of propylene glycol methyl ether acetate (PGMEA) resulted in a mixture of perpendicular lamellae and a smaller, likely cylindrical morphology. The lamellar portion of the film shows the same thickness dependence as the lamellae observed in the THF coated films. The scaling of the lamellar L₀ with respect to film thickness follows predictions for confined semiflexible polymers with weak excluded volume interactions and can be related to models for confinement of DNA. Spin coated films shows the same reduction in periodicity, although at very different film thicknesses. This result suggests that the material has shallow free-energy barriers to transitioning between different L₀ and morphologies, a property that could be taken advantage of for patterning diverse structures with a single material

### Nuclear Hadronization: Within or Without?

Nuclei are unique analyzers for the early stage of the space-time development
of hadronization. DIS at medium energies is especially suitable for this task
being sensitive to hadronization dynamics, since the production length is
comparable with the nuclear size. This was the driving motivation to propose
measurements at HERMES using nuclear targets, and to provide predictions based
on a pQCD model of hadronization [1]. Now when the first results of the
experiment are released [2,3], one can compare the predictions with the data.
The model successfully describes with no adjustment the nuclear effects for
various energies, zh, pT, and Q2, for different flavors and different nuclei.
It turns out that the main source of nuclear suppression of the hadron
production rate is attenuation of colorless pre-hadrons in the medium. An
alternative model [4] is based upon an ad hoc assumption that the colorless
pre-hadron is produced outside the nucleus. This model has apparent problems
attempting to explain certain features of the results from HERMES. A good
understanding of the hadronization dynamics is important for proper
interpretation of the strong suppression of high-pT hadrons observed in heavy
ion collisions at RHIC. We demonstrate that the production length is even
shorter in this case and keeps contracting with rising pT.Comment: Latex 34 p. Based on talks given by B.Z.K. at the Fourth
International Conference on Perspectives in Hadronic Physics, Trieste, Italy,
May 12-16, 2003; and at the EuroConference on Hadron Structure Viewed with
Electromagnetic Probes, Santorini, Greece, October 7-12, 200

### Efficient Resolution of Anisotropic Structures

We highlight some recent new delevelopments concerning the sparse
representation of possibly high-dimensional functions exhibiting strong
anisotropic features and low regularity in isotropic Sobolev or Besov scales.
Specifically, we focus on the solution of transport equations which exhibit
propagation of singularities where, additionally, high-dimensionality enters
when the convection field, and hence the solutions, depend on parameters
varying over some compact set. Important constituents of our approach are
directionally adaptive discretization concepts motivated by compactly supported
shearlet systems, and well-conditioned stable variational formulations that
support trial spaces with anisotropic refinements with arbitrary
directionalities. We prove that they provide tight error-residual relations
which are used to contrive rigorously founded adaptive refinement schemes which
converge in $L_2$. Moreover, in the context of parameter dependent problems we
discuss two approaches serving different purposes and working under different
regularity assumptions. For frequent query problems, making essential use of
the novel well-conditioned variational formulations, a new Reduced Basis Method
is outlined which exhibits a certain rate-optimal performance for indefinite,
unsymmetric or singularly perturbed problems. For the radiative transfer
problem with scattering a sparse tensor method is presented which mitigates or
even overcomes the curse of dimensionality under suitable (so far still
isotropic) regularity assumptions. Numerical examples for both methods
illustrate the theoretical findings

### Inclusive production of charged pions in p+C collisions at 158 GeV/c beam momentum

The production of charged pions in minimum bias p+C interactions is studied
using a sample of 377000 inelastic events obtained with the NA49 detector at
the CERN SPS at 158 GeV/c beam momentum. The data cover a phase space area
ranging from 0 to 1.8 GeV/c in transverse momentum and from -0.1 to 0.5 in
Feynman x. Inclusive invariant cross sections are given on a grid of 270 bins
per charge thus offering for the first time a dense coverage of the projectile
hemisphere and of the cross-over region into the target fragmentation zone.Comment: 31 pages, 30 figures, submitted to European Journal of Physic

### A Toy Model for Testing Finite Element Methods to Simulate Extreme-Mass-Ratio Binary Systems

Extreme mass ratio binary systems, binaries involving stellar mass objects
orbiting massive black holes, are considered to be a primary source of
gravitational radiation to be detected by the space-based interferometer LISA.
The numerical modelling of these binary systems is extremely challenging
because the scales involved expand over several orders of magnitude. One needs
to handle large wavelength scales comparable to the size of the massive black
hole and, at the same time, to resolve the scales in the vicinity of the small
companion where radiation reaction effects play a crucial role. Adaptive finite
element methods, in which quantitative control of errors is achieved
automatically by finite element mesh adaptivity based on posteriori error
estimation, are a natural choice that has great potential for achieving the
high level of adaptivity required in these simulations. To demonstrate this, we
present the results of simulations of a toy model, consisting of a point-like
source orbiting a black hole under the action of a scalar gravitational field.Comment: 29 pages, 37 figures. RevTeX 4.0. Minor changes to match the
published versio

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