344 research outputs found
Rescattering in Meson Photoproduction off Few Body Systems
Exclusive reactions induced at high momentum transfer in few body systems
provide us with an original way to study the production and propagation of
hadrons in cold nuclear matter. In very well defined parts of the phase space,
the reaction amplitude develops a logarithmic singularity. It is on solid
ground since it depends only on on-shell elementary amplitudes and on low
momentum components of the nuclear wave function. This is the best window to
study the propagation of exotic configurations of hadrons such as, for
instance, the onset of color transparency. It may appear earlier in meson
photo-production reactions, more particularly in the strange sector, than in
more classical quasi elastic scattering of electrons. More generally, those
reactions provide us with the best tool to determine the cross section of the
scattering of various hadrons (strange particles, vector mesons) from the
nucleon and to access the production of possible exotic states.Comment: 15 pages; 11 figures During the review process of the paper, the
following changes have been implemented: 1- The title has been changed, 2-
The abstact and the first paragraph of the introduction have been rephrased
for consistency; 3- Figure 10 has been added; 4- The Appedix has been
considerably expanded: it gives the full expressions of the elementary
photoproduction amplitudes in terms of Pauli spinors and matrice
Calculations of the Exclusive Processes 2H(e,e'p)n, 3He(e,e'p)2H and 3He(e,e'p)(pn) within a Generalized Glauber Approach
The exclusive processes 2H(e,e'p)n, 3He(e,e'p)2H and 3He(e,e'p)(pn), have
been analyzed using realistic few-body wave functions and treating the final
state interaction (FSI) within a Generalized Eikonal Approximation (GEA), based
upon the direct calculation of the Feynman diagrams describing the rescattering
of the struck nucleon with the nucleons of the A-1 system. The approach
represents an improvement of the conventional Glauber approach (GA), in that it
allows one to take into account the effects of the nuclear excitation of the
system on the rescattering of the struck nucleon. Using realistic
three-body wave functions corresponding to the AV18 interaction, the results of
our parameter free calculations are compared with available experimental data.
It is found that in some kinematical conditions FSI effects represent small
corrections, whereas in other kinematics conditions they are very large and
absolutely necessary to provide a satisfactory agreement between theoretical
calculations and experimental data. It is shown that in the kinematics of the
experimental data which have been considered, covering the region of missing
momentum and energy with p_m < 0.6 GeV/c and E_m < 100 MeV in the perpendicular
kinematics, the GA and GEA predictions differ only by less than 3-4 %.Comment: Typos detected and removed while Proof reading. Physical Review C. in
Pres
Large Q2 Electrodisintegration of the Deuteron in Virtual Nucleon Approximation
The two-body break up of the deuteron is studied at high kinematics,
with main motivation to probe the deuteron at small internucleon distances.
Such studies are associated with the probing of high momentum component of the
deuteron wave function. For this, two main theoretical issues have been
addressed such as electromagnetic interaction of the virtual photon with the
bound nucleon and the strong interaction of produced baryons in the final state
of the break-up reaction. Within virtual nucleon approximation we developed a
new prescription to account for the bound nucleon effects in electromagnetic
interaction. The final state interaction at high kinematics is calculated
within generalized eikonal approximation (GEA). We studied the uncertainties
involved in the calculation and performed comparisons with the first
experimental data on deuteron electrodisintegration at large . We
demonstrate that the experimental data confirm GEA's early prediction that the
rescattering is maximal at of recoil nucleon production relative to
the momentum of the virtual photon. Comparisons also show that the forward
recoil nucleon angles are best suited for studies of the electromagnetic
interaction of bound nucleons and the high momentum structure of the deuteron.
Backward recoil angle kinematics show sizable effects due to the
-isobar contribution. The latter indicates the importance of further
development of GEA to account for the inelastic transitions in the intermediate
state of the electrodisintegration reactions.Comment: 22 pages, 9 figure
The space-time structure of hard scattering processes
Recent studies of exclusive electroproduction of vector mesons at JLab make
it possible for the first time to play with two independent hard scales: the
virtuality Q^2 of the photon, which sets the observation scale, and the
momentum transfer t to the hadronic system, which sets the interaction scale.
They reinforce the description of hard scattering processes in terms of few
effective degrees of freedom relevant to the Jlab-Hermes energy range.Comment: 4 pages; 5 figure
Short-Distance Structure of Nuclei
One of Jefferson Lab's original missions was to further our understanding of
the short-distance structure of nuclei. In particular, to understand what
happens when two or more nucleons within a nucleus have strongly overlapping
wave-functions; a phenomena commonly referred to as short-range correlations.
Herein, we review the results of the (e,e'), (e,e'p) and (e,e'pN) reactions
that have been used at Jefferson Lab to probe this short-distance structure as
well as provide an outlook for future experiments.Comment: 16 pages, 8 figures, for publication in Journal of Physics
A mechanism for the Double-Spin Asymmetry in Electromagnetic Production at HERMES
We calculate the contribution of meson and pomeron exchanges to the
double-spin asymmetry in -meson electromagnetic production at HERMES
energies. We show that the observed double-spin asymmetries, which are large,
can be explained by the interference between the natural parity -secondary
Reggeon and the unnatural parity anomalous exchanges.Comment: 7 pages, 3 figures, Late
Structural Analysis and Photocurrent Spectroscopy of CCCs on 99.99% Aluminum
A characterization of chromate conversion coatings (CCCs) formed in the presence and in the absence of accelerator (ferro-ferricyanide redox couple) has been performed by various techniques (transmission electron microscopy, TEM, glow discharge optical emission spectrometry, GDOES, X-ray absorption near-end structure, XANES, and photon correlation spectroscopy). The results of a detailed investigation on morphological, compositional, and solid-state properties of freshly converted aluminum samples at different immersion times (30 s-90 min) are reported. The TEM and GDOES data suggest the presence of iron-cyanide species only in the external layer of CCC of nearly constant thickness. The XANES data suggest the presence of both Cr(VI) and Cr(III) species with a ratio Cr(VI)/Cr(III) close to 1:2. This ratio remains constant with the conversion time and seems slightly affected by the composition of conversion solution. The photoelectrochemical study suggests an insulating or slightly p-type behavior for CCC layers. A bandgap value of about 2.55 eV has been estimated, regardless of the conversion solution, although some differences in the photocurrent spectra have been observed for coatings formed in the presence or absence of accelerator. The location of electronic energy levels of the Al/CCC/electrolyte interface has been derived which could account for the different kinetics of coating formation in the presence of accelerator
The reaction cross section
The one- and two-step mechanisms of the reaction in
the range of incident proton kinetic energy 1.13-3.0 GeV have been
investigated. A remarkable peculiarity of the two-step mechanism which
incorporates subprocesses and is the so
called velocity matching providing the presence of all intermediate particles
nearly to the on-mass-shell. The differential cross section has been calculated
using a realistic model for the hypertritium wave function. The
maximum value of the cross section is estimated as 1nb/sr. The
contribution of the one-step mechanism with the elementary process into the cross section has been found to be two - three orders of
magnitude smaller in comparison with the two-step mechanism.Comment: 10 pages, Latex, 3 Postscript figure
Attenuation and modification of the ballast water microbial community during voyages into the Canadian Arctic
Aim: Ballast water is a major vector of non-indigenous species introductions world-wide. Our understanding of population dynamics of organisms entrained in ballast is largely limited to studies of zooplankton and phytoplankton. Bacteria are more numerous and diverse than zooplankton or phytoplankton, yet remain comparatively understudied. We apply a metagenomics approach to characterize changes in the microbial ballast water community over the course of three voyages on one ship, and assess the effects of ballast water exchange (BWE), spring/summer sampling month and time since voyage start. Location: Quebec City and Deception Bay, Quebec, and the coastal marine region offshore of eastern Canada. Methods: We used universal primers to Ion Torrent sequence a fragment of the bacterial 16S ribosomal DNA for samples collected over three voyages of one ship between Quebec City and Deception Bay in June, July and August 2015. We compared richness (total number of species in the community) and diversity (accounts for both species abundance and evenness) using linear mixed-effects analysis and compared community composition using non-metric multidimensional scaling and permutational multivariate analysis of variance. Initial comparisons were between months. Subsequent analyses focused on each month separately. Results: Ion Torrent sequencing returned c. 2.9 million reads and revealed monthly differences in diversity and richness, and in community structure in ballast water. June had higher richness and diversity than either July or August, and showed most clearly the effect of BWE on the microbial community. Main conclusions: Our results suggest that environmental conditions associated with different spring/summer sampling months drive differences in microbial diversity in ballast water. This study showed that BWE removes some components of the freshwater starting microbial community and replaces them with other taxa. BWE also changed proportional representation of some microbes without removing them completely. It appears that some taxa are resident in ballast tanks and are not removed by BWE. © 2017 John Wiley & Sons Lt
Photoproduction of the Eta-Prime Mesons as a New Tool to Probe Baryon Resonances
We examine eta prime photoproduction, as a novel tool to study baryon
resonances around 2 GeV, of particular interest to the quark shell model, which
predicts a number of them. We find important roles of the form factors at the
strong vertices, and show that the N^*(2080) can be probed efficiently by this
reaction.Comment: Will be published in Phys. Rev.
- …