5,342 research outputs found
Hadronic Spectra and Light-Front Wavefunctions in Holographic QCD
We show how the string amplitude defined on the fifth dimension in
AdS space can be precisely mapped to the light-front wavefunctions of
hadrons in physical spacetime. We find an exact correspondence between the
holographic variable and an impact variable , which represents the
measure of transverse separation of the constituents within the hadrons. In
addition, we derive effective four dimensional Schr\"odinger equations for the
bound states of massless quarks and gluons which exactly reproduce the AdS/CFT
results and give a realistic description of the light-quark meson and baryon
spectrum as well as the form factors for spacelike . Only one parameter
which sets the mass scale, , is introduced.Comment: 4 pages, REVTex4, 2 figures. References added. To appear in Phys.
Rev. Let
Constraining the CDM and Galileon models with recent cosmological data
The Galileon theory belongs to the class of modified gravity models that can
explain the late-time accelerated expansion of the Universe. In previous works,
cosmological constraints on the Galileon model were derived, both in the
uncoupled case and with a disformal coupling of the Galileon field to matter.
There, we showed that these models agree with the most recent cosmological
data. In this work, we used updated cosmological data sets to derive new
constraints on Galileon models, including the case of a constant conformal
Galileon coupling to matter. We also explored the tracker solution of the
uncoupled Galileon model. After updating our data sets, especially with the
latest \textit{Planck} data and BAO measurements, we fitted the cosmological
parameters of the CDM and Galileon models. The same analysis framework
as in our previous papers was used to derive cosmological constraints, using
precise measurements of cosmological distances and of the cosmic structure
growth rate. We showed that all tested Galileon models are as compatible with
cosmological data as the CDM model. This means that present
cosmological data are not accurate enough to distinguish clearly between both
theories. Among the different Galileon models, we found that a conformal
coupling is not favoured, contrary to the disformal coupling which is preferred
at the level over the uncoupled case. The tracker solution of the
uncoupled Galileon model is also highly disfavoured due to large tensions with
supernovae and \textit{Planck}+BAO data. However, outside of the tracker
solution, the general uncoupled Galileon model, as well as the general
disformally coupled Galileon model, remain the most promising Galileon
scenarios to confront with future cosmological data. Finally, we also discuss
constraints coming from Lunar Laser Ranging experiment and gravitational wave
speed of propagation.Comment: 22 pages, 17 figures, published version in A&
Hadronic Spectrum of a Holographic Dual of QCD
We compute the spectrum of light hadrons in a holographic dual of QCD defined
on which has conformal behavior at short distances and
confinement at large interquark separation. Specific hadrons are identified by
the correspondence of string modes with the dimension of the interpolating
operator of the hadron's valence Fock state. Higher orbital excitations are
matched quanta to quanta with fluctuations about the AdS background. Since only
one parameter, the QCD scale , is used, the agreement with the
pattern of physical states is remarkable. In particular, the ratio of Delta to
nucleon trajectories is determined by the ratio of zeros of Bessel functions.Comment: 4 pages, REVTeX4, 2 figures. Version published in Phys. Rev. Let
Experimental constraints on the uncoupled Galileon model from SNLS3 data and other cosmological probes
The Galileon model is a modified gravity theory that may provide an
explanation for the accelerated expansion of the Universe. This model does not
suffer from instabilities or ghost problems (normally associated with
higher-order derivative theories), restores local General Relativity -- thanks
to the Vainshtein screening effect -- and predicts late time acceleration of
the expansion. In this paper, we derive a new definition of the Galileon
parameters that allows us to avoid having to choose initial conditions for the
Galileon field, and then test this model against precise measurements of the
cosmological distances and the rate of growth of cosmic structures. We observe
a small tension between the constraints set by growth data and those from
distances. However, we find that the Galileon model remains consistent with
current observations and is still competitive with the \Lambda CDM model,
contrary to what was concluded in recent publications.Comment: 19 pages, 15 figures, accepted to Astronomy and Astrophysic
First experimental constraints on the disformally coupled Galileon model
The Galileon model is a modified gravity model that can explain the late-time
accelerated expansion of the Universe. In a previous work, we derived
experimental constraints on the Galileon model with no explicit coupling to
matter and showed that this model agrees with the most recent cosmological
data. In the context of braneworld constructions or massive gravity, the
Galileon model exhibits a disformal coupling to matter, which we study in this
paper. After comparing our constraints on the uncoupled model with recent
studies, we extend the analysis framework to the disformally coupled Galileon
model and derive the first experimental constraints on that coupling, using
precise measurements of cosmological distances and the growth rate of cosmic
structures. In the uncoupled case, with updated data, we still observe a low
tension between the constraints set by growth data and those from distances. In
the disformally coupled Galileon model, we obtain better agreement with data
and favour a non-zero disformal coupling to matter at the level.
This gives an interesting hint of the possible braneworld origin of Galileon
theory.Comment: 9 pages, 6 figures, updated versio
Nonperturbative QCD Coupling and its function from Light-Front Holography
The light-front holographic mapping of classical gravity in AdS space,
modified by a positive-sign dilaton background, leads to a nonperturbative
effective coupling . It agrees with hadron physics data
extracted from different observables, such as the effective charge defined by
the Bjorken sum rule, as well as with the predictions of models with built-in
confinement and lattice simulations. It also displays a transition from
perturbative to nonperturbative conformal regimes at a momentum scale
GeV. The resulting function appears to capture the essential
characteristics of the full function of QCD, thus giving further
support to the application of the gauge/gravity duality to the confining
dynamics of strongly coupled QCD. Commensurate scale relations relate
observables to each other without scheme or scale ambiguity. In this paper we
extrapolate these relations to the nonperturbative domain, thus extending the
range of predictions based on .Comment: 32 pages, 7 figures. Final version published in Phys. Rev.
A new liver perfusion and preservation system for transplantation Research in large animals
A kidney perfusion machine, model MOX-100 (Waters Instruments, Ltd, Rochester, MN) was modified to allow continuous perfusion of the portal vein and pulsatile perfusion of the hepatic artery of the liver. Additional apparatus consists of a cooling system, a membrane oxygenator, a filter for foreign bodies, and bubble traps. This system not only allows hypothermic perfusion preservation of the liver graft, but furthermore enables investigation of ex vivo simulation of various circulatory circumstances in which physiological perfusion of the liver is studied. We have used this system to evaluate the viability of liver allografts preserved by cold storage. The liver was placed on the perfusion system and perfused with blood with a hematocrit of approximately 20% and maintained at 37°C for 3 h. The flows of the hepatic artery and portal vein were adjusted to 0.33 mL and 0.67 mL/g of liver tissue, respectively. Parameters of viability consisted of hourly bile output, oxygen consumption, liver enzymes, electrolytes, vascular resistance, and liver histology. This method of liver assessment in large animals will allow the objective evaluation of organ viability for transplantation and thereby improve the outcome of organ transplantation. Furthermore, this pump enables investigation into the pathophysiology of liver ischemia and preservation. © 1990 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
Attractive instability of oppositely charged membranes induced by charge density fluctuations
We predict the conditions under which two oppositely charged membranes show a
dynamic, attractive instability. Two layers with unequal charges of opposite
sign can repel or be stable when in close proximity. However, dynamic charge
density fluctuations can induce an attractive instability and thus facilitate
fusion. We predict the dominant instability modes and timescales and show how
these are controlled by the relative charge and membrane viscosities. These
dynamic instabilities may be the precursors of membrane fusion in systems where
artificial vesicles are engulfed by biological cells of opposite charge
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