310 research outputs found
Acceleration of colliding shells around a black hole: Validity of the test particle approximation in the Banados-Silk-West process
Recently, Banados, Silk and West (BSW) showed that the total energy of two
colliding test particles has no upper limit in their center of mass frame in
the neighborhood of an extreme Kerr black hole, even if these particles were at
rest at infinity in the infinite past. We call this mechanism the BSW mechanism
or BSW process. The large energy of such particles would generate strong
gravity, although this has not been taken into account in the BSW analysis. A
similar mechanism is seen in the collision of two spherical test shells in the
neighborhood of an extreme Reissner-Nordstr\"om black hole. In this paper, in
order to draw some implications concerning the effects of gravity generated by
colliding particles in the BSW process, we study a collision of two spherical
dust shells, since their gravity can be exactly treated. We show that the
energy of two colliding shells in the center of mass frame observable from
infinity has an upper limit due to their own gravity. Our result suggests that
an upper limit also exists for the total energy of colliding particles in the
center of mass frame in the observable domain in the BSW process due the
gravity of the particles.Comment: 19 pages, 2 figures, title change
Half-integral Erd\H{o}s-P\'osa property of directed odd cycles
We prove that there exists a function
such that every digraph contains either directed odd cycles where every
vertex of is contained in at most two of them, or a vertex set of size
at most hitting all directed odd cycles. This extends the half-integral
Erd\H{o}s-P\'osa property of undirected odd cycles, proved by Reed [Mangoes and
blueberries. Combinatorica 1999], to digraphs.Comment: 16 pages, 5 figure
A unified half-integral Erd\H{o}s-P\'{o}sa theorem for cycles in graphs labelled by multiple abelian groups
Erd\H{o}s and P\'{o}sa proved in 1965 that there is a duality between the
maximum size of a packing of cycles and the minimum size of a vertex set
hitting all cycles. Such a duality does not hold if we restrict to odd cycles.
However, in 1999, Reed proved an analogue for odd cycles by relaxing packing to
half-integral packing. We prove a far-reaching generalisation of the theorem of
Reed; if the edges of a graph are labelled by finitely many abelian groups,
then there is a duality between the maximum size of a half-integral packing of
cycles whose values avoid a fixed finite set for each abelian group and the
minimum size of a vertex set hitting all such cycles.
A multitude of natural properties of cycles can be encoded in this setting,
for example cycles of length at least , cycles of length modulo ,
cycles intersecting a prescribed set of vertices at least times, and cycles
contained in given -homology classes in a graph embedded on a
fixed surface. Our main result allows us to prove a duality theorem for cycles
satisfying a fixed set of finitely many such properties.Comment: 28 pages, 4 figure
Production of Magnetic Turbulence by Cosmic Rays Drifting Upstream of Supernova Remnant Shocks
We present results of 2D and 3D PIC simulations of magnetic turbulence
production by isotropic cosmic-ray ions drifting upstream of SNR shocks. The
studies aim at testing recent predictions of a strong amplification of short
wavelength magnetic field and at studying the evolution of the magnetic
turbulence and its backreaction on cosmic rays. We observe that an oblique
filamentary mode grows more rapidly than the non-resonant parallel modes found
in analytical theory, and the growth rate of the field perturbations is much
slower than is estimated for the parallel plane-wave mode, possibly because in
our simulations we cannot maintain omega << Omega_i, the ion gyrofrequency, to
the degree required for the plane-wave mode to emerge. The evolved oblique
filamentary mode was also observed in MHD simulations to dominate in the
nonlinear phase. We thus confirm the generation of the turbulent magnetic field
due to the drift of cosmic-ray ions in the upstream plasma, but as our main
result find that the amplitude of the turbulence saturates at about dB/B~1. The
backreaction of the turbulence on the particles leads to an alignment of the
bulk-flow velocities of the cosmic rays and the background medium, which is an
essential characteristic of cosmic-ray modified shocks. It accounts for the
saturation of the instability at moderate field amplitudes. Previously
published MHD simulations have assumed a constant cosmic-ray current and no
energy or momentum flux in the cosmic rays, which excludes a backreaction of
the generated magnetic field on cosmic rays, and thus the saturation of the
field amplitude is artificially suppressed. This may explain the continued
growth of the magnetic field in the MHD simulations. A strong magnetic field
amplification to amplitudes dB >> B0 has not been demonstrated yet.Comment: revised version; accepted to ApJ; 36 pages, 13 figure
Implant isolation of ZnO
We study ion-irradiation-induced electrical isolation in n-type single-crystal ZnO epilayers. Emphasis is given to improving the thermal stability of isolation and obtaining a better understanding of the isolation mechanism. Results show that an increase in the dose of 2 MeV ¹⁶Oions (up to ∼2 orders of magnitude above the threshold isolation dose) and irradiation temperature (up to 350 °C) has a relatively minor effect on the thermal stability of electrical isolation, which is limited to temperatures of ∼300–400 °C. An analysis of the temperature dependence of sheet resistance suggests that effective levels associated with irradiation-produced defects are rather shallow (<50 meV). For the case of implantation with keV Cr, Fe, or Niions, the evolution of sheet resistance with annealing temperature is consistent with defect-induced isolation, with a relatively minor effect of Cr, Fe, or Ni impurities on the thermal stability of isolation. Results also reveal a negligible ion-beam flux effect in the case of irradiation with 2 MeV 16Oions, supporting high diffusivity of ion-beam-generated defects during ion irradiation and a very fast stabilization of collision cascade processes in ZnO. Based on these results, the mechanism for electrical isolation in ZnO by ion bombardment is discussed
Identification and structural characterization of a mutant KRAS‐G12V specific TCR restricted by HLA‐A3
Mutations in KRAS are some of the most common across multiple cancer types and are thus attractive targets for therapy. Recent studies demonstrated that mutant KRAS generates immunogenic neoantigens that are targetable by adoptive T‐cell therapy in metastatic diseases. To expand mutant KRAS‐specific immunotherapies, it is critical to identify additional HLA‐I allotypes that can present KRAS neoantigens and their cognate T‐cell receptors (TCR). Here, we identified a murine TCR specific to a KRAS‐G12V neoantigen (7VVVGAVGVGK16) using a vaccination approach with transgenic mice expressing HLA‐A*03:01 (HLA‐A3). This TCR demonstrated exquisite specificity for mutant G12V and not WT KRAS peptides. To investigate the molecular basis for neoantigen recognition by this TCR, we determined its structure in complex with HLA‐A3(G12V). G12V‐TCR CDR3β and CDR1β formed a hydrophobic pocket to interact with p6 Val of the G12V but not the WT KRAS peptide. To improve the tumor sensitivity of this TCR, we designed rational substitutions to improve TCR:HLA‐A3 contacts. Two substitutions exhibited modest improvements in TCR binding avidity to HLA‐A3 (G12V) but did not sufficiently improve T‐cell sensitivity for further clinical development. Our study provides mechanistic insight into how TCRs detect neoantigens and reveals the challenges in targeting KRAS‐G12V mutations
STM observation of electronic wave interference effect in finite-sized graphite with dislocation-network structures
Superperiodic patterns near a step edge were observed by STM on
several-layer-thick graphite sheets on a highly oriented pyrolitic graphite
substrate, where a dislocation network is generated at the interface between
the graphite overlayer and the substrate. Triangular- and rhombic-shaped
periodic patterns whose periodicities are around 100 nm were observed on the
upper terrace near the step edge. In contrast, only outlines of the patterns
similar to those on the upper terrace were observed on the lower terrace. On
the upper terrace, their geometrical patterns gradually disappeared and became
similar to those on the lower terrace without any changes of their periodicity
in increasing a bias voltage. By assuming a periodic scattering potential at
the interface due to dislocations, the varying corrugation amplitudes of the
patterns can be understood as changes in LDOS as a result of the beat of
perturbed and unperturbed waves, i.e. the interference in an overlayer. The
observed changes in the image depending on an overlayer height and a bias
voltage can be explained by the electronic wave interference in the ultra-thin
overlayer distorted under the influence of dislocation-network structures.Comment: 8 pages; 6 figures; Paper which a part of cond-mat/0311068 is
disscussed in detai
Pulmonary Macrophages Attenuate Hypoxic Pulmonary Vasoconstriction via beta(3)AR/iNOS Pathway in Rats Exposed to Chronic Intermittent Hypoxia
Chronic intermittent hypoxia (IH) induces activation of the sympathoadrenal system, which plays a pivotal role in attenuating hypoxic pulmonary vasoconstriction (HPV) via central beta(1)-adrenergic receptors (AR) (brain) and peripheral beta(2)AR (pulmonary arteries). Prolonged hypercatecholemia has been shown to upregulate beta(3)AR. However, the relationship between IH and beta(3)AR in the modification of HPV is unknown. It has been observed that chronic stimulation of beta(3)AR upregulates inducible nitric oxide synthase (iNOS) in cardiomyocytes and that IH exposure causes expression of iNOS in RAW264.7 macrophages. iNOS has been shown to have the ability to dilate pulmonary vessels. Hence, we hypothesized that chronic IH activates beta(3)AR/iNOS signaling in pulmonary macrophages, leading to the promotion of NO secretion and attenuated HPV. Sprague-Dawley rats were exposed to IH (3-min periods of 4-21% O-2) for 8 h/d for 6 weeks. The urinary catecholamine concentrations of IH rats were high compared with those of controls, indicating activation of the sympathoadrenal system following chronic IH. Interestingly, chronic IH induced the migration of circulating monocytes into the lungs and the predominant increase in the number of proinflammatory pulmonary macrophages. In these macrophages, both beta(3)AR and iNOS were upregulated and stimulation of the beta(3)AR/iNOS pathway in vitro caused them to promote NO secretion. Furthermore, in vivo synchrotron radiation microangiography showed that HPV was significantly attenuated in IH rats and the attenuated HPV was fully restored by blockade of beta(3)AR/iNOS pathway or depletion of pulmonary macrophages. These results suggest that circulating monocyte-derived pulmonary macrophages attenuate HPV via activation of beta(3)AR/iNOS signaling in chronic IH
Neutralino Dark Matter in Mirage Mediation
We study the phenomenology of neutralino dark matter (DM) in mirage mediation
scenario of supersymmetry breaking which results from the moduli stabilization
in some string/brane models. Depending upon the model parameters, especially
the anomaly to modulus mediation ratio determined by the moduli stabilization
mechanism, the nature of the lightest supersymmetric particle (LSP) changes
from Bino-like neutralino to Higgsino-like one via Bino-Higgsino mixing region.
For the Bino-like LSP, the standard thermal production mechanism can give a
right amount of relic DM density through the stop/stau-neutralino
coannihilation or the pseudo-scalar Higgs resonance process. We also examine
the prospect of direct and indirect DM detection in various parameter regions
of mirage mediation. Neutralino DM in galactic halo might be detected by near
future direct detection experiments in the case of Bino-Higgsino mixed LSP. The
gamma ray flux from Galactic Center might be detectable also if the DM density
profile takes a cuspy shape.Comment: One reference adde
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