63 research outputs found
Electron Mass Operator in a Strong Magnetic Field and Dynamical Chiral Symmetry Breaking
The electron mass operator in a strong magnetic field is calculated. The
contribution of higher Landau levels of virtual electrons, along with the
ground Landau level, is shown to be essential in the leading log approximation.
The effect of the electron dynamical mass generation by a magnetic field is
investigated. In a model with N charged fermions, it is shown that some
critical number N_{cr} exists for any value of the electromagnetic coupling
constant alpha, such that the fermion dynamical mass is generated with a
doublet splitting for N < N_{cr}, and the dynamical mass does not arise at all
for N > N_{cr}, thus leaving the chiral symmetry unbroken.Comment: 4 pages, REVTEX4, 3 figure
Polarization change due to fast winds from accretion disks
A fraction of the radiation produced by an accretion disk may be Thomson
scattered by a wind flowing away from the disk. Employing a simple
plane-parallel model of the wind, we calculate the polarization of the
scattered radiation and find that its sign depends on the wind velocity,
beta=v/c. In the case, 0.12 < beta < 0.78, the polarization is parallel to the
disk normal, i.e., it is orthogonal to the standard Chandrasekhar's
polarization expected from accretion disks. The velocity of an
electron-positron wind is likely to saturate near the equilibrium value beta_*
of order 0.5 for which the accelerating radiation pressure is balanced by the
Compton drag. Then the change of polarization by the wind is most pronounced.
This may help to reconcile the standard accretion disk model with the optical
polarimetric observations of non-blazar AGNs.Comment: accepted for publication in ApJ Letter
Mass for the graviton
Can we give the graviton a mass? Does it even make sense to speak of a
massive graviton? In this essay I shall answer these questions in the
affirmative. I shall outline an alternative to Einstein Gravity that satisfies
the Equivalence Principle and automatically passes all classical weak-field
tests (GM/r approx 10^{-6}). It also passes medium-field tests (GM/r approx
1/5), but exhibits radically different strong-field behaviour (GM/r approx 1).
Black holes in the usual sense do not exist in this theory, and large-scale
cosmology is divorced from the distribution of matter. To do all this we have
to sacrifice something: the theory exhibits {*prior geometry*}, and depends on
a non-dynamical background metric.Comment: 12 pages, plain LaTeX. Major revisions: (1) Inconsistency in
equations of motion fixed. (2) More discussion of the problems associated
with quantization. (3) Many more references adde
Dynamical Chiral Symmetry Breaking in QED in a Magnetic Field: Toward Exact Results
We describe a (first, to the best of our knowledge) essentially soluble
example of dynamical symmetry breaking phenomenon in a 3+1 dimensional gauge
theory without fundamental scalar fields: QED in a constant magnetic field.Comment: 4 pages, 1 figure, REVTeX. Final version accepted for publication in
Physical Review Letter
Kruppel-like factor 4 signals through microRNA-206 to promote tumor initiation and cell survival
Tumor cell heterogeneity poses a major hurdle in the treatment of cancer. Mammary cancer stem-like cells (MaCSCs), or tumor-initiating cells, are highly tumorigenic sub-populations that have the potential to self-renew and to differentiate. These cells are clinically important, as they display therapeutic resistance and may contribute to treatment failure and recurrence, but the signaling axes relevant to the tumorigenic phenotype are poorly defined. The zinc-finger transcription factor Kruppel-like factor 4 (KLF4) is a pluripotency mediator that is enriched in MaCSCs. KLF4 promotes RAS-extracellular signal-regulated kinase pathway activity and tumor cell survival in triple-negative breast cancer (TNBC) cells. In this study, we found that both KLF4and a downstream effector, microRNA-206 (miR-206), are selectively enriched in the MaCSC fractions of cultured human TNBC cell lines, as well as in the aldehyde dehydrogenase-high MaCSC sub-population of cells derived from xenografted human mammary carcinomas. The suppression of endogenous KLF4 or miR-206 activities abrogated cell survival and in vivo tumor initiation, despite having only subtle effects on MaCSC abundance. Using a combinatorial approach that included in silico as well as loss- and gain-of-function in vitro assays, we identified miR-206-mediated repression of the pro-apoptotic molecules programmed cell death 4 (PDCD4) and connexin 43 (CX43/GJA1). Depletion of either of these two miR-206-regulated transcripts promoted resistance to anoikis, a prominent feature of CSCs, but did not consistently alter MaCSC abundance. Consistent with increased levels of miR-206 in MaCSCs, the expression of both PDCD4 and CX43 was suppressed in these cells relative to control cells. These results identify miR-206 as an effector of KLF4-mediated prosurvival signaling in MaCSCs through repression of PDCD4 and CX43. Consequently, our study suggests that a pluripotency factor exerts prosurvival signaling in MaCSCs, and that antagonism of KLF4-miR-206 signaling may selectively target the MaCSC niche in TNBC
The Schwarzschild black hole as a point particle
The description of a point mass in general relativity (GR) is given in the
framework of the field formulation of GR where all the dynamical fields,
including the gravitational field, are considered in a fixed background
spacetime. With the use of stationary (not static) coordinates non-singular at
the horizon, the Schwarzschild solution is presented as a point-like field
configuration in a whole background Minkowski space. The requirement of a
stable -causality stated recently in [J.B.Pitts and W.C.Schieve, Found.
Phys., v. 34, 211 (2004)] is used essentially as a criterion for testing
configurations.Comment: LATEX, 8 pages, no figure
Magnetic Catalysis: A Review
We give an overview of the magnetic catalysis phenomenon. In the framework of
quantum field theory, magnetic catalysis is broadly defined as an enhancement
of dynamical symmetry breaking by an external magnetic field. We start from a
brief discussion of spontaneous symmetry breaking and the role of a magnetic
field in its a dynamics. This is followed by a detailed presentation of the
essential features of the phenomenon. In particular, we emphasize that the
dimensional reduction plays a profound role in the pairing dynamics in a
magnetic field. Using the general nature of underlying physics and its
robustness with respect to interaction types and model content, we argue that
magnetic catalysis is a universal and model-independent phenomenon. In support
of this claim, we show how magnetic catalysis is realized in various models
with short-range and long-range interactions. We argue that the general nature
of the phenomenon implies a wide range of potential applications: from certain
types of solid state systems to models in cosmology, particle and nuclear
physics. We finish the review with general remarks about magnetic catalysis and
an outlook for future research.Comment: 37 pages, to appear in Lect. Notes Phys. "Strongly interacting matter
in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A.
Schmitt, H.-U. Yee. Version 2: references adde
Nonstationary random acoustic and electromagnetic fields as wave diffusion processes
We investigate the effects of relatively rapid variations of the boundaries
of an overmoded cavity on the stochastic properties of its interior acoustic or
electromagnetic field. For quasi-static variations, this field can be
represented as an ideal incoherent and statistically homogeneous isotropic
random scalar or vector field, respectively. A physical model is constructed
showing that the field dynamics can be characterized as a generalized diffusion
process. The Langevin--It\^{o} and Fokker--Planck equations are derived and
their associated statistics and distributions for the complex analytic field,
its magnitude and energy density are computed. The energy diffusion parameter
is found to be proportional to the square of the ratio of the standard
deviation of the source field to the characteristic time constant of the
dynamic process, but is independent of the initial energy density, to first
order. The energy drift vanishes in the asymptotic limit. The time-energy
probability distribution is in general not separable, as a result of
nonstationarity. A general solution of the Fokker--Planck equation is obtained
in integral form, together with explicit closed-form solutions for several
asymptotic cases. The findings extend known results on statistics and
distributions of quasi-stationary ideal random fields (pure diffusions), which
are retrieved as special cases.Comment: 54 pages, 8 figures, to appear in J. Phys. A: Math. Theo
High-quality gene assembly directly from unpurified mixtures of microarray-synthesized oligonucleotides
To meet the growing demand for synthetic genes more robust, scalable and inexpensive gene assembly technologies must be developed. Here, we present a protocol for high-quality gene assembly directly from low-cost marginal-quality microarray-synthesized oligonucleotides. Significantly, we eliminated the time- and money-consuming oligonucleotide purification steps through the use of hybridization-based selection embedded in the assembly process. The protocol was tested on mixtures of up to 2000 oligonucleotides eluted directly from microarrays obtained from three different chip manufacturers. These mixtures containing <5% perfect oligos, and were used directly for assembly of 27 test genes of different sizes. Gene quality was assessed by sequencing, and their activity was tested in coupled in vitro transcription/translation reactions. Genes assembled from the microarray-eluted material using the new protocol matched the quality of the genes assembled from >95% pure column-synthesized oligonucleotides by the standard protocol. Both averaged only 2.7 errors/kb, and genes assembled from microarray-eluted material without clonal selection produced only 30% less protein than sequence-confirmed clones. This report represents the first demonstration of cost-efficient gene assembly from microarray-synthesized oligonucleotides. The overall cost of assembly by this method approaches 5¢ per base, making gene synthesis more affordable than traditional cloning
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