365 research outputs found
Femto-Photography of Protons to Nuclei with Deeply Virtual Compton Scattering
Developments in deeply virtual Compton scattering allow the direct
measurements of scattering amplitudes for exchange of a highly virtual photon
with fine spatial resolution. Real-space images of the target can be obtained
from this information. Spatial resolution is determined by the momentum
transfer rather than the wavelength of the detected photon. Quantum photographs
of the proton, nuclei, and other elementary particles with resolution on the
scale of a fraction of a femtometer is feasible with existing experimental
technology.Comment: To be published in Physical Review D. Replaces previous version with
minor changes in presentatio
A-dependence of nuclear transparency in quasielastic A(e,e'p) at high Q^2
The A-dependence of the quasielastic A(e,e'p) reaction has been studied at
SLAC with H-2, C, Fe, and Au nuclei at momentum transfers Q^2 = 1, 3, 5, and
6.8 (GeV/c)^2. We extract the nuclear transparency T(A,Q^2), a measure of the
average probability that the struck proton escapes from the nucleus A without
interaction. Several calculations predict a significant increase in T with
momentum transfer, a phenomenon known as Color Transparency. No significant
rise within errors is seen for any of the nuclei studied.Comment: 5 pages incl. 2 figures, Caltech preprint OAP-73
Partisan Views of the Economy
In this paper it is argued that political parties may have incentives to adopt a partisan view on the working of the economic system. Our approach is based on a dynamical spatial voting model in which political parties are policy oriented. This model revolves around two interrelated issues x and y. The policy maker sets x directly. There exist two views on the relationship between x and y. Model uncertainty confronts policy makers with the problem of the selection of a model to base their actions on. We show that if voters have imperfect information about the working of the economic system that model selection contains a strategic element. Policy makers are inclined to adopt a view on the working of the economic system which fits in with their preferences.
There is no inherent logic that places monetarists to the right of New Economists. They have different models of economic mechanism, but they need not have different political values. A conservative can be a Keynesian and a liberal a monetarist. These combinations are in fact surprisingly rare.
James Tobin, 1974,The New Economics One Decade Older, p. 62.
I am greatly indebted to Peter Broer, Ben Heydra, Jos Jansen and Wilko Letterie for many helpful suggestions. Furthermore, I would like to thank an anonymous referee for his comments
Decomposition of the QCD String into Dipoles and Unintegrated Gluon Distributions
We present the perturbative and non-perturbative QCD structure of the
dipole-dipole scattering amplitude in momentum space. The perturbative
contribution is described by two-gluon exchange and the non-perturbative
contribution by the stochastic vacuum model which leads to confinement of the
quark and antiquark in the dipole via a string of color fields. This QCD string
gives important non-perturbative contributions to high-energy reactions. A new
structure different from the perturbative dipole factors is found in the
string-string scattering amplitude. The string can be represented as an
integral over stringless dipoles with a given dipole number density. This
decomposition of the QCD string into dipoles allows us to calculate the
unintegrated gluon distribution of hadrons and photons from the dipole-hadron
and dipole-photon cross section via kT-factorization.Comment: 43 pages, 14 figure
Donor states in modulation-doped Si/SiGe heterostructures
We present a unified approach for calculating the properties of shallow
donors inside or outside heterostructure quantum wells. The method allows us to
obtain not only the binding energies of all localized states of any symmetry,
but also the energy width of the resonant states which may appear when a
localized state becomes degenerate with the continuous quantum well subbands.
The approach is non-variational, and we are therefore also able to evaluate the
wave functions. This is used to calculate the optical absorption spectrum,
which is strongly non-isotropic due to the selection rules. The results
obtained from calculations for Si/SiGe quantum wells allow us to
present the general behavior of the impurity states, as the donor position is
varied from the center of the well to deep inside the barrier. The influence on
the donor ground state from both the central-cell effect and the strain arising
from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure
Government Spending Cycles: Ideological or Opportunistic?
ands. The time series analysis, covering the period 1953â1993, allows for different types of government spending. In general, spending is inspired by ideological and opportunistic motives: all government expenditure categories show an upward drift during election times and the partisan motives behind government spending are clearly revealed: left-wing cabinets attach greater importance to social security and health care than right-wing cabinets and right-wing cabinets value expenditure on infrastructure and defense more than left-wing parties.
Constructive comments by Frans van Winden, Wilko Letterie, Peter Cornelisse, Arie Ros, André de Moor, Harry ter Rele and an anonymous referee are gratefully acknowledged
Nuclear DVCS at small-x using the color dipole phenomenology
Using the high energy color dipole formalism, we study the coherent and
incoherent nuclear DVCS process in the small-x regime. We consider simple
models for the elementary dipole-hadron scattering amplitude that captures main
features of the dependence on atomic number A, on energy and on momentum
transfer t. Using the obtained amplitudes we make predictions for the nuclear
DVCS cross section at photon level in the collider kinematics.Comment: 6 pages, 5 figures. Version to be published in European Physical
Journal
Snipe taxonomy based on vocal and non-vocal sound displays: the South American Snipe is two species
We analysed breeding sounds of the two subspecies of South American Snipe Gallinago paraguaiae paraguaiae and Gallinago paraguaiae magellanica to determine whether they might be different species: loud vocalizations given on the ground, and the tailâgenerated Winnow given in aerial display. Sounds of the two taxa differ qualitatively and quantitatively. Both taxa utter two types of ground call. In G. p. paraguaiae, the calls are bouts of identical sound elements repeated rhythmically and slowly (about five elements per second (Hz)) or rapidly (about 11 Hz). One call of G. p. magellanica is qualitatively similar to those of G. p. paraguaiae but sound elements are repeated more slowly (about 3 Hz). However, its other call type differs strikingly: it is a bout of rhythmically repeated sound couplets, each containing two kinds of sound element. The Winnow of G. p. paraguaiae is a series of sound elements that gradually increase in duration and energy; by contrast, that of G. p. magellanica has two or more kinds of sound element that roughly alternate and are repeated as sets, imparting a stuttering quality. Sounds of the related Puna Snipe (Gallinago andina) resemble but differ quantitatively from those of G. p. paraguaiae. Differences in breeding sounds of G. p. paraguaiae and G. p. magellanica are strong and hold throughout their geographical range. Therefore we suggest that the two taxa be considered different species: G. paraguaiae east of the Andes in much of South America except Patagonia, and G. magellanica in central and southern Chile, Argentina east of the Andes across Patagonia, and Falklands/Malvinas.Fil: Miller, Edward H.. Memorial University Of Newfoundland; CanadĂĄFil: Areta, Juan Ignacio. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Jaramillo, Alvaro. San Francisco Bay Bird Observatory; Estados UnidosFil: Imberti, Santiago. AsociaciĂłn Ambiente Sur, Rio Gallegos; ArgentinaFil: Matus, Ricardo. KilĂłmetro 7 Sur; Chil
Could saturation effects be visible in a future electron-ion collider?
We expect to observe parton saturation in a future electron - ion collider.
In this letter we discuss this expectation in more detail considering two
different models which are in good agreement with the existing experimental
data on nuclear structure functions. In particular, we study the predictions of
saturation effects in electron - ion collisions at high energies, using a
generalization for nuclear targets of the b-CGC model, which describes the
HERA quite well. We estimate the total, longitudinal and charm structure
functions in the dipole picture and compare them with the predictions obtained
using collinear factorization and modern sets of nuclear parton distributions.
Our results show that inclusive observables are not very useful in the search
for saturation effects. In the small region they are very difficult to
disentangle from the predictions of the collinear approaches . This happens
mainly because of the large uncertainties in the latter. On the other hand, our
results indicate that the contribution of diffractive processes to the total
cross section is about 20 % at large A and small Q^2, allowing for a detailed
study of diffractive observables. The study of diffractive processes becomes
essential to observe parton saturation.Comment: 7 pages 5 figure
Inclusive electron scattering from nuclei at xâ1
The inclusive A(e,eâČ) cross section for xâ1 was measured on 2H, C, Fe, and Au for momentum transfers Q2 from 1 to 6.8 (GeV/c)2. The scaling behavior of the data was examined in the region of transition from y scaling to x scaling. Throughout this transitional region, the data exhibit Ο scaling, reminiscent of the Bloom-Gilman duality seen in free nucleon scattering
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