383 research outputs found
Coherent Vector Meson Photoproduction with Nuclear Breakup in Relativistic Heavy Ion Collisions
Relativistic heavy ions are copious sources of virtual photons. The large
photon flux gives rise to a substantial photonuclear interaction probability at
impact parameters where no hadronic interactions can occur. Multiple
photonuclear interactions in a single collision are possible. In this letter,
we use mutual Coulomb excitation of both nuclei as a tag for moderate impact
parameter collisions. We calculate the cross section for coherent vector meson
production accompanied by mutual excitation, and show that the median impact
parameter is much smaller than for untagged production. The vector meson
rapidity and transverse momentum distribution are very different from untagged
exclusive vector meson production.Comment: 14 pages, including 4 figure
Crystal structure of the spliceosomal 15.5 kD protein bound to a U4 snRNA fragment.
and Lin, 1991). It is thought that the U4/U6 interaction is made and broken in each cycle of splicing. The structural rearrangements of the U4 and U6 snRNAs are evolution
Interference in Exclusive Vector Meson Production in Heavy Ion Collisions
Photons emitted from the electromagnetic fields of relativistic heavy ions
can fluctuate into quark anti-quark pairs and scatter from a target nucleus,
emerging as vector mesons. These coherent interactions are identifiable by
final states consisting of the two nuclei and a vector meson with a small
transverse momentum. The emitters and targets can switch roles, and the two
possibilities are indistinguishable, so interference may occur. Vector mesons
are negative parity so the amplitudes have opposite signs. When the meson
transverse wavelength is larger than the impact parameter, the interference is
large and destructive.
The short-lived vector mesons decay before amplitudes from the two sources
can overlap, and so cannot interfere directly. However, the decay products are
emitted in an entangled state, and the interference depends on observing the
complete final state. The non-local wave function is an example of the
Einstein-Podolsky-Rosen paradox.Comment: 13 pages with 3 figures; submitted to Physical Review Letter
Eriophyid mites in classical biological control of weeds: Progress and challenges
A classical biological control agent is an exotic host-specific natural enemy, which is intentionally introduced to obtain long-term control of an alien invasive species. Among the arthropods considered for this role, eriophyid mites are likely to possess the main attributes required: Host specificity, efficacy, and long-lasting effects. However, so far, only a few species have been approved for release. Due to their microscopic size and the general lack of knowledge regarding their biology and behavior, working with eriophyids is particularly challenging. Furthermore, mites disperse in wind, and little is known about biotic and abiotic constraints to their population growth. All these aspects pose challenges that, if not properly dealt with, can make it particularly difficult to evaluate eriophyids as prospective biological control agents and jeopardize the general success of control programs. We identified some of the critical aspects of working with eriophyids in classical biological control of weeds and focused on how they have been or may be addressed. In particular, we analyzed the importance of accurate mite identification, the difficulties faced in the evaluation of their host specificity, risk assessment of nontarget species, their impact on the weed, and the final steps of mite release and post-release monitoring
Dissecting the role of conserved box C/D sRNA sequences in di-sRNP assembly and function
In all three kingdoms of life, nucleotides in ribosomal RNA (rRNA) are post-transcriptionally modified. One type of chemical modification is 2′-O-ribose methylation, which is, in eukaryotes and archaea, performed by box C/D small ribonucleoproteins (box C/D sRNPs in archaea) and box C/D small nucleolar ribonucleoproteins (box C/D snoRNPs in eukaryotes), respectively. Recently, the first structure of any catalytically active box C/D s(no)RNP determined by electron microscopy and single particle analysis surprisingly demonstrated that they are dimeric RNPs. Mutational analyses of the Nop5 protein interface suggested that di-sRNP formation is also required for the in vitro catalytic activity. We have now analyzed the functional relevance of the second interface, the sRNA interface, within the box C/D di-sRNP. Mutations in conserved sequence elements of the sRNA, which allow sRNP assembly but which severely interfere with the catalytic activity of box C/D sRNPs, prevent formation of the di-sRNP. In addition, we can observe the dimeric box C/D sRNP architecture with a different box C/D sRNP, suggesting that this architecture is conserved. Together, these results provide further support for the functional relevance of the di-sRNP architecture and also provide a structural explanation for the observed defects in catalysis of 2′-O-ribose methylation
Inclusive meson production in peripheral collisions of ultrarelativistic heavy ions
There exist several proposals to use Weizs\"{a}cker-Williams photons
generated by ultrarelativistic heavy ions to produce exotic particles in
fusion reactions. To estimate the background conditions for such
reactions we analyze various mechanisms of meson production in very peripheral
collisions of ultrarelativistic heavy ions at RHIC and LHC energies. Besides
fusion they include also electromagnetic interactions
and strong nucleon-nucleon interactions in grazing collisions. All these
processes are characterised by low multiplicities of produced particles.
and events are simulated by corresponding Monte Carlo codes,
RELDIS and FRITIOF. In each of these processes a certain fraction of pions is
produced close to the mid-rapidity region that gives a background for the
events. The possibility of selecting mesons produced in
fusion events via different cut procedures is
demonstrated.Comment: 27 pages with 4 eps-figures included, uses axodraw.sty Tab.2 and 3
correcte
Assessment of the Trophic Status by Monitoring of Reservoir’s Water Quality
Continuous long-term monitoring of aquatic systems is important for understanding their complete
evolution in order to monitor changes in the trophic status and water quality. The continuous
monitoring during a period of 20 years, by sampling once a month at two locations, the water
quality of reservoir “Grlište”, which is used for the water supplying town Zaječar (Eastern Serbia),
is observed and developmental stages in the life of the reservoir were determinated. It
should be noted that the obtained results were used also in the purpose of finding a cause of cyanobacteria
bloom, as a consequence of algal production. Limiting factors of algal production usually
were nitrogen and phosphorus, however, in this study, obtained results of subtraction between
trophic state index, calculated through total chlorophyll a (TSIChl-a) and trophic state index, calculated
through total phosphorus (TSITP), indicated that limiting factor of algal production was light.
On the basis of the concentrations of dissolved oxygen (DO), total phosphorus (TP) and chlorophyll
a (Chl-a) in the surface and in the bottom, it was concluded that the reservoir passed through
four development phases during the examined period. Results of long-term monitoring showed
that in the first years after the formation of the reservoir, the highest trophicity was detected (hypereutrophic
status), but later the reservoir mostly maintained eutrophic status
Particle emission following Coulomb excitation in ultrarelativistic heavy-ion collisions
We study nuclear reactions induced by virtual photons associated with
Lorentz-boosted Coulomb fields of ultrarelativistic heavy ions. Evaporation,
fission and multifragmentation mechanisms are included in a new RELDIS code,
which describes the deexcitation of residual nuclei formed after single and
double photon absorption in peripheral heavy-ion collisions. Partial cross
sections for different dissociation channels, including the multiple neutron
emission ones, are calculated and compared with data when available. Rapidity
and transverse momentum distributions of nucleons, nuclear fragments and pions,
produced electromagnetically, are also calculated. These results provide
important information for designing large-rapidity detectors and zero-degree
calorimeters at RHIC and LHC. The electromagnetic dissociation of nuclei
imposes some constrains on the investigation of exotic particle production in
gamma-gamma fusion reactions.Comment: 26 LaTeX pages including 8 figures, uses epsf.st
Coulomb Effects on Electromagnetic Pair Production in Ultrarelativistic Heavy-Ion Collisions
We discuss the implications of the eikonal amplitude on the pair production
probability in ultrarelativistic heavy-ion transits. In this context the
Weizs\"acker-Williams method is shown to be exact in the ultrarelativistic
limit, irrespective of the produced particles' mass. A new equivalent
single-photon distribution is derived which correctly accounts for the Coulomb
distortions. As an immediate application, consequences for unitarity violation
in photo-dissociation processes in peripheral heavy-ion encounters are
discussed.Comment: 13 pages, 4 .eps figure
Lymphocyte subsets and the role of Th1/Th2 balance in stressed chronic pain patients
Background: The complex regional pain syndrome (CRPS) and fibromyalgia (FM) are chronic pain syndromes occurring in highly stressed individuals. Despite the known connection between the nervous system and immune cells, information on distribution of lymphocyte subsets under stress and pain conditions is limited. Methods: We performed a comparative study in 15 patients with CRPS type I, 22 patients with FM and 37 age- and sex-matched healthy controls and investigated the influence of pain and stress on lymphocyte number, subpopulations and the Th1/Th2 cytokine ratio in T lymphocytes. Results: Lymphocyte numbers did not differ between groups. Quantitative analyses of lymphocyte subpopulations showed a significant reduction of cytotoxic CD8+ lymphocytes in both CRPS (p < 0.01) and FM (p < 0.05) patients as compared with healthy controls. Additionally, CRPS patients were characterized by a lower percentage of IL-2-producing T cell subpopulations reflecting a diminished Th1 response in contrast to no changes in the Th2 cytokine profile. Conclusions: Future studies are warranted to answer whether such immunological changes play a pathogenetic role in CRPS and FM or merely reflect the consequences of a pain-induced neurohumoral stress response, and whether they contribute to immunosuppression in stressed chronic pain patients. Copyright (c) 2008 S. Karger AG, Basel
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