76 research outputs found
Neutron-Neutron Fusion
The neutron-neutron fusion process, , at very low neutron
energies is studied in the framework of pionless effective field theory that
incorporates dibaryon fields. The cross section and electron energy spectrum
for this process are calculated up to next-to-leading order. We include the
radiative corrections of calculated for the one-body
transition amplitude. The precision of our theoretical estimates is found to be
governed essentially by the accuracy with which the empirical values of the
neutron-neutron scattering length and effective range are currently known. Also
discussed is the precision of theoretical estimates of the transition rates of
related electroweak processes in few-nucleon systems.Comment: 14 pages, 4 figures, minor correction, accepted for publication in
Phys. Lett.
Light-year Scale Radio Cores in Four LINER Galaxies
The LINER galaxies NGC 2911, NGC 3079, NGC 3998, and NGC 6500 were observed
at 5 GHz with the European VLBI Network at a resolution of 5 milliarcsecond and
found to possess flat-spectrum, variable, high-brightness temperature ( K) radio cores. These radio characteristics reinforce the view that
these LINERs host central engines associated with active galactic nuclei.Comment: 6 page
Effects of the magnetic moment interaction between nucleons on observables in the 3N continuum
The influence of the magnetic moment interaction of nucleons on
nucleon-deuteron elastic scattering and breakup cross sections and on elastic
scattering polarization observables has been studied. Among the numerous
elastic scattering observables only the vector analyzing powers were found to
show a significant effect, and of opposite sign for the proton-deuteron and
neutron-deuteron systems. This finding results in an even larger discrepancy
than the one previously established between neutron-deuteron data and
theoretical calculations. For the breakup reaction the largest effect was found
for the final-state-interaction cross sections. The consequences of this
observation on previous determinations of the ^1S_0 scattering lengths from
breakup data are discussed.Comment: 24 pages, 6 ps figures, 1 png figur
The two-nucleon system at next-to-next-to-next-to-leading order
We consider the two-nucleon system at next-to-next-to-next-to-leading order
(N^3LO) in chiral effective field theory. The two-nucleon potential at N^3LO
consists of one-, two- and three-pion exchanges and a set of contact
interactions with zero, two and four derivatives. In addition, one has to take
into account various isospin-breaking and relativistic corrections. We employ
spectral function regularization for the multi-pion exchanges. Within this
framework, it is shown that the three-pion exchange contribution is negligibly
small. The low-energy constants (LECs) related to pion-nucleon vertices are
taken consistently from studies of pion-nucleon scattering in chiral
perturbation theory. The total of 26 four-nucleon LECs has been determined by a
combined fit to some np and pp phase shifts from the Nijmegen analysis together
with the nn scattering length. The description of nucleon-nucleon scattering
and the deuteron observables at N^3LO is improved compared to the one at NLO
and NNLO. The theoretical uncertainties in observables are estimated based on
the variation of the cut-offs in the spectral function representation of the
potential and in the regulator utilized in the Lippmann-Schwinger equation.Comment: 62 pp, 13 fig
Baryon-baryon interactions in the SU6 quark model and their applications to light nuclear systems
Interactions between the octet-baryons (B8) in the spin-flavor SU6 quark
model are investigated in a unified coupled-channels framework of the
resonating-group method (RGM). The interaction Hamiltonian for quarks consists
of the phenomenological confinement potential, the color Fermi-Breit
interaction with explicit flavor-symmetry breaking (FSB), and effective-meson
exchange potentials of scalar-, pseudoscalar- and vector-meson types. The model
parameters are determined to reproduce the properties of the nucleon-nucleon
(NN) system and the low-energy cross section data for the hyperon-nucleon (YN)
interactions. The NN phase shifts and many observables for the NN and YN
interactions are nicely reproduced. Properties of these B8 B8 interactions are
analyzed through the G-matrix calculations. The B8 B8 interactions are then
applied to some of few-baryon systems and light Lambda-hypernuclei in a
three-cluster Faddeev formalism using two-cluster RGM kernels. An application
to the three-nucleon system shows that the quark-model NN interaction can give
a sufficient triton binding energy with little room for the three-nucleon
force. The hypertriton Faddeev calculation indicates that the attraction of the
Lambda N interaction in the 1S0 state is only slightly more attractive than
that in the 3S1 state. In the application to the alpha alpha Lambda system, the
energy spectrum of 9 Lambda Be is well reproduced using the alpha alpha RGM
kernel. The very small spin-orbit splitting of the 9 Lambda Be excited states
is also discussed. In the Lambda Lambda alpha Faddeev calculation, the NAGARA
event for 6 Lambda Lambda He is found to be consistent with the quark-model
Lambda Lambda interaction.Comment: 77 pages, 33 figures, review article to be published in Prog. Part.
Nucl. Phy
Quantitative imaging of 124I and 86Y with PET
The quantitative accuracy and image quality of positron emission tomography (PET) measurements with 124I and 86Y is affected by the prompt emission of gamma radiation and positrons in their decays, as well as the higher energy of the emitted positrons compared to those emitted by 18F. PET scanners cannot distinguish between true coincidences, involving two 511-keV annihilation photons, and coincidences involving one annihilation photon and a prompt gamma, if the energy of this prompt gamma is within the energy window of the scanner. The current review deals with a number of aspects of the challenge this poses for quantitative PET imaging. First, the effect of prompt gamma coincidences on quantitative accuracy of PET images is discussed and a number of suggested corrections are described. Then, the effect of prompt gamma coincidences and the increased singles count rates due to gamma radiation on the count rate performance of PET is addressed, as well as possible improvements based on modification of the scanner’s energy windows. Finally, the effect of positron energy on spatial resolution and recovery is assessed. The methods presented in this overview aim to overcome the challenges associated with the decay characteristics of 124I and 86Y. Careful application of the presented correction methods can allow for quantitatively accurate images with improved image contrast
The impact of oxygen on the transcriptome of recombinant S. cerevisiae and P. pastoris - a comparative analysis
Background: Saccharomyces cerevisiae and Pichia pastoris are two of the most relevant microbial eukaryotic platforms for the production of recombinant proteins. Their known genome sequences enabled several transcriptomic profiling studies under many different environmental conditions, thus mimicking not only perturbations and adaptations which occur in their natural surroundings, but also in industrial processes. Notably, the majority of such transcriptome analyses were performed using non-engineered strains. In this comparative study, the gene expression profiles of S. cerevisiae and P. pastoris, a Crabtree positive and Crabtree negative yeast, respectively, were analyzed for three different oxygenation conditions (normoxic, oxygen-limited and hypoxic) under recombinant protein producing conditions in chemostat cultivations. Results: The major differences in the transcriptomes of S. cerevisiae and P. pastoris were observed between hypoxic and normoxic conditions, where the availability of oxygen strongly affected ergosterol biosynthesis, central carbon metabolism and stress responses, particularly the unfolded protein response. Steady state conditions under low oxygen set-points seemed to perturb the transcriptome of S. cerevisiae to a much lesser extent than the one of P. pastoris, reflecting the major tolerance of the baker's yeast towards oxygen limitation, and a higher fermentative capacity. Further important differences were related to Fab production, which was not significantly affected by oxygen availability in S. cerevisiae, while a clear productivity increase had been previously reported for hypoxically grown P. pastoris. Conclusions: The effect of three different levels of oxygen availability on the physiology of P. pastoris and S. cerevisiae revealed a very distinct remodelling of the transcriptional program, leading to novel insights into the different adaptive responses of Crabtree negative and positive yeasts to oxygen availability. Moreover, the application of such comparative genomic studies to recombinant hosts grown in different environments might lead to the identification of key factors for efficient protein production
A trio of gamma-ray burst supernovae: GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu
We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu. For GRB 130215A/SN 2013ez, we also present optical spectroscopy at t − t0 = 16.1 d, which covers rest-frame 3000–6250 Å. Based on Fe ii λ5169 and Si ii λ6355, our spectrum indicates an unusually low expansion velocity of ~4000–6350 km s-1, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A/SN 2013fu, we used our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we took advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Mészáros (2001, ApJ, 552, L35), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P = 12 ms and a magnetic field of B = 1.1 × 1015 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs
Geographical and temporal distribution of SARS-CoV-2 clades in the WHO European Region, January to June 2020
We show the distribution of SARS-CoV-2 genetic clades over time and between countries and outline potential genomic surveillance objectives. We applied three available genomic nomenclature systems for SARS-CoV-2 to all sequence data from the WHO European Region available during the COVID-19 pandemic until 10 July 2020. We highlight the importance of real-time sequencing and data dissemination in a pandemic situation. We provide a comparison of the nomenclatures and lay a foundation for future European genomic surveillance of SARS-CoV-2.Peer reviewe
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