7,583 research outputs found
Unquenching the gluon propagator with Schwinger-Dyson equations
In this article we use the Schwinger-Dyson equations to compute the
nonperturbative modifications caused to the infrared finite gluon propagator
(in the Landau gauge) by the inclusion of a small number of quark families. Our
basic operating assumption is that the main bulk of the effect stems from the
"one-loop dressed" quark loop contributing to the full gluon self-energy. This
quark loop is then calculated, using as basic ingredients the full quark
propagator and quark-gluon vertex; for the quark propagator we use the solution
obtained from the quark gap equation, while for the vertex we employ suitable
Ans\"atze, which guarantee the transversality of the answer. The resulting
effect is included as a correction to the quenched gluon propagator, obtained
in recent lattice simulations. Our main finding is that the unquenched
propagator displays a considerable suppression in the intermediate momentum
region, which becomes more pronounced as we increase the number of active quark
families. The influence of the quarks on the saturation point of the propagator
cannot be reliably computed within the present scheme; the general tendency
appears to be to decrease it, suggesting a corresponding increase in the
effective gluon mass. The renormalization properties of our results, and the
uncertainties induced by the unspecified transverse part of the quark-gluon
vertex, are discussed. Finally, the dressing function of the gluon propagator
is compared with the available unquenched lattice data, showing rather good
agreement.Comment: 40 pages, 17 figure
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Enhanced Optical 13C Hyperpolarization in Diamond Treated by High-Temperature Rapid Thermal Annealing
Methods of optical dynamic nuclear polarization open the door to the replenishable hyperpolarization of nuclear spins, boosting their nuclear magnetic resonance/imaging signatures by orders of magnitude. Nanodiamond powder rich in negatively charged nitrogen vacancy defect centers has recently emerged as one such promising platform, wherein 13C nuclei can be hyperpolarized through the optically pumped defects completely at room temperature. Given the compelling possibility of relaying this 13C polarization to nuclei in external liquids, there is an urgent need for the engineered production of highly “hyperpolarizable” diamond particles. Here, a systematic study of various material dimensions affecting optical 13C hyperpolarization in diamond particles is reported on. It is discovered surprisingly that diamond annealing at elevated temperatures ∼1720 °C has remarkable effects on the hyperpolarization levels enhancing them by above an order of magnitude over materials annealed through conventional means. It is demonstrated these gains arise from a simultaneous improvement in NV− electron relaxation/coherence times, as well as the reduction of paramagnetic content, and an increase in 13C relaxation lifetimes. This work suggests methods for the guided materials production of fluorescent, 13C hyperpolarized, nanodiamonds and pathways for their use as multimodal (optical and magnetic resonance) imaging and hyperpolarization agents
q-Form fields on p-branes
In this paper, we give one general method for localizing any form (q-form)
field on p-branes with one extra dimension, and apply it to some typical
p-brane models. It is found that, for the thin and thick Minkowski branes with
an infinite extra dimension, the zero mode of the q-form fields with q<(p-1)/2
can be localized on the branes. For the thick Minkowski p-branes with one
finite extra dimension, the localizable q-form fields are those with q<p/2, and
there are also some massive bound Kaluza-Klein modes for these q-form fields on
the branes. For the same q-form field, the number of the bound Kaluza-Klein
modes (but except the scalar field (q=0)) increases with the dimension of the
p-branes. Moreover, on the same p-brane, the q-form fields with higher q have
less number of massive bound KK modes. While for a family of pure geometrical
thick p-branes with a compact extra dimension, the q-form fields with q<p/2
always have a localized zero mode. For a special pure geometrical thick
p-brane, there also exist some massive bound KK modes of the q-form fields with
q<p/2, whose number increases with the dimension of the p-brane.Comment: 14 pages, 2 figures, published versio
de Sitter Thick Brane Solution in Weyl Geometry
In this paper, we consider a de Sitter thick brane model in a pure geometric
Weyl integrable five-dimensional space-time, which is a generalization of
Riemann geometry and is invariant under a so-called Weyl rescaling. We find a
solution of this model via performing a conformal transformation to map the
Weylian structure into a familiar Riemannian one with a conformal metric. The
metric perturbations of the model are discussed. For gravitational
perturbation, we get the effective modified Pschl-Teller
potential in corresponding Schrdinger equation for
Kaluza-Klein (KK) modes of the graviton. There is only one bound state, which
is a normalizable massless zero mode and represents a stable 4-dimensional
graviton. Furthermore, there exists a mass gap between the massless mode and
continuous KK modes. We also find that the model is stable under the scalar
perturbation in the metric. The correction to the Newtonian potential on the
brane is proportional to , where is the de Sitter
parameter of the brane. This is very different from the correction caused by a
volcano-like effective potential.Comment: 24 pages, 13 figures, published versio
Localization of gravity on a de Sitter thick braneworld without scalar fields
In this work we present a simple thick braneworld model that is generated by
an intriguing interplay between a 5D cosmological constant with a de Sitter
metric induced in the 3-brane without the inclusion of scalar fields. We show
that 4D gravity is localized on this brane, provide analytic expressions for
the massive Kaluza-Klein (KK) fluctuation modes and also show that the spectrum
of metric excitations displays a mass gap. We finally present the corrections
to Newton's law due to these massive modes. This model has no naked
singularities along the fifth dimension despite the existence of a mass gap in
the graviton spectrum as it happens in thick branes with 4D Poincare symmetry,
providing a simple model with very good features: the curvature is completely
smooth along the fifth dimension, it localizes 4D gravity and the spectrum of
gravity fluctuations presents a mass gap, a fact that rules out the existence
of phenomenologically dangerous ultralight KK excitations in the model. We
finally present our solution as a limit of scalar thick branes.Comment: 11 pages in latex, no figures, title and abstract changed, a new
section and some references adde
Dynamics of gravity driven three-dimensional thin films on hydrophilic-hydrophobic patterned substrates
We investigate numerically the dynamics of unstable gravity driven
three-dimensional thin liquid films on hydrophilic-hydrophobic patterned
substrates of longitudinal stripes and checkerboard arrangements. The thin film
can be guided preferentially on hydrophilic longitudinal stripes, while fingers
develop on adjacent hydrophobic stripes if their width is large enough. On
checkerboard patterns, the film fingering occurs on hydrophobic domains, while
lateral spreading is favoured on hydrophilic domains, providing a mechanism to
tune the growth rate of the film. By means of kinematical arguments, we
quantitatively predict the growth rate of the contact line on checkerboard
arrangements, providing a first step towards potential techniques that control
thin film growth in experimental setups.Comment: 30 pages, 12 figure
Mass hierarchy, mass gap and corrections to Newton's law on thick branes with Poincare symmetry
We consider a scalar thick brane configuration arising in a 5D theory of
gravity coupled to a self-interacting scalar field in a Riemannian manifold. We
start from known classical solutions of the corresponding field equations and
elaborate on the physics of the transverse traceless modes of linear
fluctuations of the classical background, which obey a Schroedinger-like
equation. We further consider two special cases in which this equation can be
solved analytically for any massive mode with m^2>0, in contrast with numerical
approaches, allowing us to study in closed form the massive spectrum of
Kaluza-Klein (KK) excitations and to compute the corrections to Newton's law in
the thin brane limit. In the first case we consider a solution with a mass gap
in the spectrum of KK fluctuations with two bound states - the massless 4D
graviton free of tachyonic instabilities and a massive KK excitation - as well
as a tower of continuous massive KK modes which obey a Legendre equation. The
mass gap is defined by the inverse of the brane thickness, allowing us to get
rid of the potentially dangerous multiplicity of arbitrarily light KK modes. It
is shown that due to this lucky circumstance, the solution of the mass
hierarchy problem is much simpler and transparent than in the (thin)
Randall-Sundrum (RS) two-brane configuration. In the second case we present a
smooth version of the RS model with a single massless bound state, which
accounts for the 4D graviton, and a sector of continuous fluctuation modes with
no mass gap, which obey a confluent Heun equation in the Ince limit. (The
latter seems to have physical applications for the first time within braneworld
models). For this solution the mass hierarchy problem is solved as in the
Lykken-Randall model and the model is completely free of naked singularities.Comment: 25 pages in latex, no figures, content changed, corrections to
Newton's law included for smooth version of RS model and an author adde
A Possible Detection of Occultation by a Proto-planetary Clump in GM Cephei
GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an
abrupt variable and to have a circumstellar disk with very active accretion.
Our monitoring observations in 2009-2011 revealed the star to show sporadic
flare events, each with brightening of < 0.5 mag lasting for days. These
brightening events, associated with a color change toward the blue, should
originate from an increased accretion activity. Moreover, the star also
underwent a brightness drop of ~1 mag lasting for about a month, during which
the star became bluer when fainter. Such brightness drops seem to have a
recurrence time scale of a year, as evidenced in our data and the photometric
behavior of GM Cep over a century. Between consecutive drops, the star
brightened gradually by about 1 mag and became blue at peak luminosity. We
propose that the drop is caused by obscuration of the central star by an
orbiting dust concentration. The UX Orionis type of activity in GM Cep
therefore exemplifies the disk inhomogeneity process in transition between
grain coagulation and planetesimal formation in a young circumstellar disk.Comment: In submission to the Astrophysical Journal, 4 figure
Effects of temperature on thick branes and the fermion (quasi-)localization
Following Campos's work [Phys. Rev. Lett. 88, 141602 (2002)], we investigate
the effects of temperature on flat, de Sitter (dS), and anti-de Following
Campos's work [Phys. Rev. Lett. \textbf{88}, 141602 (2002)], we investigate the
effects of temperature on flat, de Sitter (dS), and anti-de Sitter (AdS) thick
branes in five-dimensional (5D) warped spacetime, and on the fermion
(quasi-)localization. First, in the case of flat brane, when the critical
temperature reaches, the solution of the background scalar field and the warp
factor is not unique. So the thickness of the flat thick brane is uncertain at
the critical value of the temperature parameter, which is found to be lower
than the one in flat 5D spacetime. The mass spectra of the fermion Kaluza-Klein
(KK) modes are continuous, and there is a series of fermion resonances. The
number and lifetime of the resonances are finite and increase with the
temperature parameter, but the mass of the resonances decreases with the
temperature parameter. Second, in the case of dS brane, we do not find such a
critical value of the temperature parameter. The mass spectra of the fermion KK
modes are also continuous, and there is a series of fermion resonances. The
effects of temperature on resonance number, lifetime, and mass are the same
with the case of flat brane. Last, in the case of AdS brane, {the critical
value of the temperature parameter can less or greater than the one in the flat
5D spacetime.} The spectra of fermion KK modes are discrete, and the mass of
fermion KK modes does not decrease monotonically with increasing temperature
parameter.Comment: 24 pages, 15 figures, published versio
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High contrast dual-mode optical and 13C magnetic resonance imaging in diamond particles
Multichannel imaging -- the ability to acquire images of an object through
more than one imaging mode simultaneously -- has opened interesting new
perspectives in areas ranging from astronomy to medicine. Visible optics and
magnetic resonance imaging (MRI) offer complementary advantages of resolution,
speed and depth of penetration, and as such would be attractive in combination.
In this paper, we take first steps towards marrying together optical and MR
imaging in a class of biocompatible particulate materials constructed out of
diamond. The particles are endowed with a high density of quantum defects
(Nitrogen Vacancy centers) that under optical excitation fluoresce brightly in
the visible, but also concurrently electron spin polarize. This allows the
hyperpolarization of lattice 13C nuclei to make the particles over three-orders
of magnitude brighter than in conventional MRI. Dual-mode optical and MR
imaging permits immediate access to improvements in resolution and
signal-to-noise especially in scattering environments. We highlight additional
benefits in background-free imaging, demonstrating lock-in suppression by
factors of 2 and 5 in optical and MR domains respectively. Ultimate limits
could approach as much as two orders of magnitude in each domain. Finally,
leveraging the ability of optical and MR imaging to simultaneously probe
Fourier-reciprocal domains (real and k-space), we elucidate the ability to
employ hybrid sub-sampling in both conjugate spaces to vastly accelerate
dual-image acquisition, by as much as two orders of magnitude in practically
relevant sparse-imaging scenarios. This is accompanied by a reduction in
optical power by the same factor. Our work suggests interesting possibilities
for the simultaneous optical and low-field MR imaging of targeted diamond
nanoparticles
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