10,944 research outputs found
Color screening in a constituent quark model of hadronic matter
The effect of color screening on the formation of a heavy quark-antiquark
() bound state--such as the meson--is studied using a
constituent-quark model. The response of the nuclear medium to the addition of
two color charges is simulated directly in terms of its quark constituents via
a string-flip potential that allows for quark confinement within hadrons yet
enables the hadrons to separate without generating unphysical long-range
forces. Medium modifications to the properties of the heavy meson, such as its
energy and its mean-square radius, are extracted by solving Schr\"odinger's
equation for the pair in the presence of a (screened)
density-dependent potential. The density dependence of the heavy-quark
potential is in qualitative agreement with earlier studies of its temperature
dependence extracted from lattice calculations at finite temperature. In the
present model it is confirmed that abrupt changes in the properties of the
-meson in the hadronic medium ({\it plasma}), correlate strongly with
the deconfining phase transition.Comment: 7 pages, 3 figures, submitted to PRC for publication, uses revtex
Requirements for contractility in disordered cytoskeletal bundles
Actomyosin contractility is essential for biological force generation, and is
well understood in highly organized structures such as striated muscle.
Additionally, actomyosin bundles devoid of this organization are known to
contract both in vivo and in vitro, which cannot be described by standard
muscle models. To narrow down the search for possible contraction mechanisms in
these systems, we investigate their microscopic symmetries. We show that
contractile behavior requires non-identical motors that generate large enough
forces to probe the nonlinear elastic behavior of F-actin. This suggests a role
for filament buckling in the contraction of these bundles, consistent with
recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene
Short-patch correction of C/C mismatches in human cells
We examined whether the human nucleotide excision repair complex, which is specialized on the removal of bulky DNA adducts, also displays a correcting activity on base mismatches. The cytosine/cytosine (C/C) lesion was used as a model substrate to monitor the correction of base mismatches in human cells. Fibroblasts with different repair capabilities were transfected with shuttle vectors that contain a site-directed C/C mismatch in the replication origin, accompanied by an additional C/C mismatch in one of the flanking sequences that are not essential for replication. Analysis of the vector progeny obtained from these doubly modified substrates revealed that C/C mismatches were eliminated before DNA synthesis not only in the repair-proficient background, but also when the target cells carried a genetic defect in long-patch mismatch repair, in nucleotide excision repair, or when both pathways were deleted. Furthermore, cells deficient for long-patch mismatch repair as well as a cell line that combines mismatch and nucleotide excision repair defects were able to correct multiple C/C mispairs, placed at distances of 21-44 nt, in an independent manner, such that the removal of each lesion led to individual repair patches. These results support the existence of a concurrent short-patch mechanism that rectifies C/C mismatche
KIC 10080943: a binary star with two γ Doradus/δ Scuti hybrid pulsators. Analysis of the g modes
We use 4 yr of Kepler photometry to study the non-eclipsing spectroscopic binary KIC 10080943. We find both components to be γ Doradus/δ Scuti hybrids, which pulsate in both p and g modes. We present an analysis of the g modes, which is complicated by the fact that the two sets of l = 1 modes partially overlap in the frequency spectrum. Nevertheless, it is possible to disentangle them by identifying rotationally split doublets from one component and triplets from the other. The identification is helped by the presence of additive combina- tion frequencies in the spectrum that involve the doublets but not the triplets. The rotational splittings of the multiplets imply core rotation periods of about 11 and 7 d in the two stars. One of the stars also shows evidence of l = 2 modes
The cool-star spectral catalog: A uniform collection of IUE SWP-LOs
Over the past decade and a half of its operations, the International Ultraviolet Explorer has recorded low-dispersion spectrograms in the 1150-2000 A interval of more than 800 stars of late spectral type (F-M). The sub-2000 A region contains a number of emission lines that are key diagnostics of physical conditions in the high-excitation chromospheres and subcoronal 'transition zones' of such stars. Many of the sources have been observed a number of times, and the available collection of SWP-LO exposures in the IUE Archives exceeds 4,000. With support from the Astrophysics Data Program, we have assembled the archival material into a catalog of IUE far-UV fluxes of late-type stars. In order to ensure uniform processing of the spectra, we: (1) photometrically corrected the raw vidicon images with a custom version of the 1985 SWP ITF; (2) identified and eliminated, sharp cosmic-ray 'hits' by means of a spatial filter; (3) extracted the spectral traces with the 'optimal' (weighted-slit) strategy; and (4) calibrated them against a well-characterized reference source, the DA white dwarf G191-B2B. Our approach is similar to that adopted by the IUE Project for its 'Final Archive', but our implementation is specialized to the case of chromospheric emission-line sources. We measured the resulting SWP-LO spectra using a semi-autonomous algorithm that establishes a smooth continuum by numerical filtering, and then fits the significant emissions (or absorptions) by means of a constrained Bevington-type multiple-Gaussian procedure. The algorithm assigns errors to the fitted fluxes - or upper limits in the absence of a significant detection - according to a model based on careful measurements of the noise properties of the IUE's intensified SEC cameras. Here, we describe the 'visualization' strategies we adopted to ensure human-review of the semi-autonomous processing and measuring algorithms; the derivation of the noise model and the assignment of errors; and the structure of the final catalog as delivered to the Astrophysics Data System
Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions
A study of two-dimensional QCD on a spatial circle with Majorana fermions in
the adjoint representation of the gauge groups SU(2) and SU(3) has been
performed. The main emphasis is put on the symmetry properties related to the
homotopically non-trivial gauge transformations and the discrete axial symmetry
of this model. Within a gauge fixed canonical framework, the delicate interplay
of topology on the one hand and Jacobians and boundary conditions arising in
the course of resolving Gauss's law on the other hand is exhibited. As a
result, a consistent description of the residual gauge symmetry (for
SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum
of the model is determined analytically in the limit of a small circle. There,
the Born-Oppenheimer approximation is justified and reduces the vacuum problem
to simple quantum mechanics. The issue of fermion condensates is addressed and
residual discrepancies with other approaches are pointed out.Comment: 44 pages; for hardcopies of figures, contact
[email protected]
Two dimensional QCD with matter in adjoint representation: What does it teach us?
We analyse the highly excited states in
with adjoint matter by using such general methods as dispersion relations,
duality and unitarity. We find the Hagedorn-like spectrum where parameters and can be expressed in
terms of asymptotics of the following matrix elements f_{n_{\{k\}}} \sim \la
0|Tr(\bar{\Psi}\Psi)^{k}|n_{k}\ra. We argue that the asymptotical values
do not depend on (after appropriate normalization). Thus,
we obtain and in case of
Majorana fermions in the adjoint representation. The Hagedorn temperature is
the limiting temperature in this case. We also argue that the chiral condensate
\la 0|Tr(\bar{\Psi}\Psi) |0\ra is not zero in the model. Contrary to the 't
Hooft model, this condensate does not break down any continuous symmetries and
can not be considered as an order parameter. Thus, no Goldstone boson appears
as a consequence of the condensation. We also discuss a few apparently
different but actually tightly related problems: master field, condensate,
wee-partons and constituent quark model in the light cone framework.Comment: uuencoded Z-compressed file for figs at the end. Revised version to
appear in Nuclear Physics B. More detail disscusion about the condensate and
discrete chiral symmetry breaking phenomenon in the mode
Local Magnetization in the Boundary Ising Chain at Finite Temperature
We study the local magnetization in the 2-D Ising model at its critical
temperature on a semi-infinite cylinder geometry, and with a nonzero magnetic
field applied at the circular boundary of circumference . This model
is equivalent to the semi-infinite quantum critical 1-D transverse field Ising
model at temperature , with a symmetry-breaking field
applied at the point boundary. Using conformal field theory methods
we obtain the full scaling function for the local magnetization analytically in
the continuum limit, thereby refining the previous results of Leclair, Lesage
and Saleur in Ref. \onlinecite{Leclair}. The validity of our result as the
continuum limit of the 1-D lattice model is confirmed numerically, exploiting a
modified Jordan-Wigner representation. Applications of the result are
discussed.Comment: 9 pages, 3 figure
Hamiltonian approach to the bound state problem in QCD_2
Bosonization of the two-dimensional QCD in the large N_C limit is performed
in the framework of Hamiltonian approach in the Coulomb gauge. The generalized
Bogoliubov transformation is applied to diagonalize the Hamiltonian in the
bosonic sector of the theory, and the composite operators creating/annihilating
bosons are obtained in terms of dressed quark operators. The bound state
equation is reconstructed as a result of the generalized Bogoliubov
transformation, and the form of its massless solution, chiral pion, is found
explicitly. Chiral properties of the theory are discussed.Comment: 9 pages, LaTeX2
A unified solution for the orbit and light-time effect in the V505 Sgr system
The multiple system V505 Sagittarii is composed of at least three stars: a
compact eclipsing pair and a distant component, which orbit is measured
directly using speckle interferometry. In order to explain the observed orbit
of the third body in V505 Sagittarii and also other observable quantities,
namely the minima timings of the eclipsing binary and two different radial
velocities in the spectrum, we thoroughly test a fourth-body hypothesis - a
perturbation by a dim, yet-unobserved object. We use an N-body numerical
integrator to simulate future and past orbital evolution of 3 or 4 components
in this system. We construct a suitable chi^2 metric from all available
speckle-interferometry, minima-timings and radial-velocity data and we scan a
part of a parameter space to get at least some of allowed solutions. In
principle, we are able to explain all observable quantities by a presence of a
fourth body, but the resulting likelihood of this hypothesis is very low. We
also discuss other theoretical explanations of the minima timings variations.
Further observations of the minima timings during the next decade or
high-resolution spectroscopic data can significantly constrain the model
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