2,091 research outputs found
Effective field theory approach to Casimir interactions on soft matter surfaces
We utilize an effective field theory approach to calculate Casimir
interactions between objects bound to thermally fluctuating fluid surfaces or
interfaces. This approach circumvents the complicated constraints imposed by
such objects on the functional integration measure by reverting to a point
particle representation. To capture the finite size effects, we perturb the
Hamiltonian by DH that encapsulates the particles' response to external fields.
DH is systematically expanded in a series of terms, each of which scales
homogeneously in the two power counting parameters: \lambda \equiv R/r, the
ratio of the typical object size (R) to the typical distance between them (r),
and delta=kB T/k, where k is the modulus characterizing the surface energy. The
coefficients of the terms in DH correspond to generalized polarizabilities and
thus the formalism applies to rigid as well as deformable objects.
Singularities induced by the point particle description can be dealt with using
standard renormalization techniques. We first illustrate and verify our
approach by re-deriving known pair forces between circular objects bound to
films or membranes. To demonstrate its efficiency and versatility, we then
derive a number of new results: The triplet interactions present in these
systems, a higher order correction to the film interaction, and general scaling
laws for the leading order interaction valid for objects of arbitrary shape and
internal flexibility.Comment: 4 pages, 1 figur
Teeth in the Tiger: Organizational Standing as a Critical Component of Fair Housing Act Enforcement
Transport of Large Scale Poloidal Flux in Black Hole Accretion
We report on a global, three-dimensional GRMHD simulation of an accretion
torus embedded in a large scale vertical magnetic field orbiting a
Schwarzschild black hole. This simulation investigates how a large scale
vertical field evolves within a turbulent accretion disk and whether global
magnetic field configurations suitable for launching jets and winds can
develop. We find that a "coronal mechanism" of magnetic flux motion, which
operates largely outside the disk body, dominates global flux evolution. In
this mechanism, magnetic stresses driven by orbital shear create large-scale
half-loops of magnetic field that stretch radially inward and then reconnect,
leading to discontinuous jumps in the location of magnetic flux. In contrast,
little or no flux is brought in directly by accretion within the disk itself.
The coronal mechanism establishes a dipole magnetic field in the evacuated
funnel around the orbital axis with a field intensity regulated by a
combination of the magnetic and gas pressures in the inner disk. These results
prompt a reevaluation of previous descriptions of magnetic flux motion
associated with accretion. Local pictures are undercut by the intrinsically
global character of magnetic flux. Formulations in terms of an "effective
viscosity" competing with an "effective resistivity" are undermined by the
nonlinearity of of the magnetic dynamics and the fact that the same turbulence
driving mass motion (traditionally identified as "viscosity") can alter
magnetic topology.Comment: 45 pages, 17 figures, 1 movie; ApJ accepted; updated version contains
several new figures and a movie detailing the operation of the coronal
mechanism. The movie and a version of the paper with high resolution figures
can be found at http://www.astro.virginia.edu/~krb3u/0906.2784
High Jet Efficiency and Simulations of Black Hole Magnetospheres
This article reports on a growing body of observational evidence that many
powerful lobe dominated (FR II) radio sources likely have jets with high
efficiency. This study extends the maximum efficiency line (jet power
25 times the thermal luminosity) defined in Fernandes et (2010) so as to span
four decades of jet power. The fact that this line extends over the full span
of FR II radio power is a strong indication that this is a fundamental property
of jet production that is independent of accretion power. This is a valuable
constraint for theorists. For example, the currently popular "no net flux"
numerical models of black hole accretion produce jets that are 2 to 3 orders of
magnitude too weak to be consistent with sources near maximum efficiency.Comment: To appear in ApJ Letter
Multiple start codons and phosphorylation result in discrete Rad52 protein species
The sequence of the Saccharomyces cerevisiae RAD52 gene contains five potential translation start sites and protein-blot analysis typically detects multiple Rad52 species with different electrophoretic mobilities. Here we define the gene products encoded by RAD52. We show that the multiple Rad52 protein species are due to promiscuous choice of start codons as well as post-translational modification. Specifically, Rad52 is phosphorylated both in a cell cycle-independent and in a cell cycle-dependent manner. Furthermore, phosphorylation is dependent on the presence of the Rad52 C terminus, but not dependent on its interaction with Rad51. We also show that the Rad52 protein can be translated from the last three start sites and expression from any one of them is sufficient for spontaneous recombination and the repair of gamma-ray-induced doublestrand breaks
Recombination-Mediated Telomere Maintenance in Saccharomyces cerevisiae Is Not Dependent on the Shu Complex
In cells lacking telomerase, telomeres shorten progressively during each cell division due to incomplete end-replication. When the telomeres become very short, cells enter a state that blocks cell division, termed senescence. A subset of these cells can overcome senescence and maintain their telomeres using telomerase-independent mechanisms. In Saccharomyces cerevisiae, these cells are called ‘survivors’ and are dependent on Rad52-dependent homologous recombination and Pol32-dependent break-induced replication. There are two main types of survivors: type I and type II. The type I survivors require Rad51 and maintain telomeres by amplification of subtelomeric elements, while the type II survivors are Rad51-independent, but require the MRX complex and Sgs1 to amplify the C1–3A/TG1–3 telomeric sequences. Rad52, Pol32, Rad51, and Sgs1 are also important to prevent accelerated senescence, indicating that recombination processes are important at telomeres even before the formation of survivors. The Shu complex, which consists of Shu1, Shu2, Psy3, and Csm2, promotes Rad51-dependent homologous recombination and has been suggested to be important for break-induced replication. It also promotes the formation of recombination intermediates that are processed by the Sgs1-Top3-Rmi1 complex, as mutations in the SHU genes can suppress various sgs1, top3, and rmi1 mutant phenotypes. Given the importance of recombination processes during senescence and survivor formation, and the involvement of the Shu complex in many of the same processes during DNA repair, we hypothesized that the Shu complex may also have functions at telomeres. Surprisingly, we find that this is not the case: the Shu complex does not affect the rate of senescence, does not influence survivor formation, and deletion of SHU1 does not suppress the rapid senescence and type II survivor formation defect of a telomerase-negative sgs1 mutant. Altogether, our data suggest that the Shu complex is not important for recombination processes at telomeres
Cancellation of Sudakov Logarithms in Radiative Decays of Quarkonia
We study infrared QCD effects in radiative quarkonia decays. We examine the
endpoint region z -> 1 of the photon spectrum. We point out a cancellation
mechanism for the corrections in , , in
the short-distance coefficient for the color-singlet Fock state in the
quarkonium. The cancellation is due to the coherence of the color radiation,
and applies even though logarithmic contributions are present in the jet
distributions associated with the decay. We comment on the implications of our
results for the modeling of hadronization in the endpoint region and for the
role of color-octet states in the quarkonium.Comment: 15 pages, 1 figur
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